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During male–male competition, evolution can favor alternative reproductive tactics. This often results in a dominant morph that holds a resource, such as a nest for egg laying, which competes with a smaller sneaker morph that reproduces by stealing fertilizations. The salinity environment can influence male growth rates, for example, via osmoregulatory costs, which in turn may influence the use of sneaker tactics for small males competing for mating opportunities. Salinity can also affect sperm directly; however, little is known of how salinity influences sneaker tactics through sperm performance. We sampled males of the invasive round goby (Neogobius melanostomus) from two environments, a freshwater river and a brackish estuary. This fish has two male morphs: nest‐holding dark males and non‐nest‐holding light males. We examined the role of water salinity of 0, 8, and 16 on sperm performance and found that for estuarine males, a salinity of 0 reduced sperm velocity compared to a salinity of 8 and 16. Riverine males had low velocity in all salinities. Sperm viability also decreased by over 30% in 0 salinity, compared to 8 and 16, for fish from both environments. Gobies produce ejaculate contents in specialized glands that could in theory shield sperm in an adverse environment. However, gland contents did not improve sperm performance in our tests. Body mass and age estimates indicate that riverine males invested more in somatic growth compared to estuarine males. Estuarine light morph males had a high enough gonadosomatic index to indicate sneaker tactics. We propose that when sperm performance is low, such as for the riverine males, sneaker tactics are ineffective and will be selected against or phenotypically suppressed. Instead, we interpret the increased investment in somatic growth found in riverine males as a life‐history decision that is advantageous when defending a nest in the next reproductive season., For a male fish too small to defend mating resources such as a female or her eggs, siring offspring can be achieved by sneaking into another male's nest and ejaculating there. In the invasive round goby, this alternative tactic requires sperm to function long enough to fertilize the eggs. This fish has spread into a range of salinities, but for some populations, fresh water decreases sperm velocity and lifespan, and severely limits the use of these alternative reproductive tactics to the point where they may disappear from use.

Practical and applied knowledge of local fishers can help to improve our understanding of target species ecology and fisheries management decisions. In the Western Baltic Sea (WBS), the spatio-temporal distribution of cod is still largely unknown despite decades of research. We studied changes in cod distribution by obtaining information on temporal depth and habitat use of cod from commercial gillnet fishers using semi-directed interviews supplemented by at-sea observer data. Linear and non-linear regression analyses revealed significant relationships between depth use of cod and sea surface temperature (SST) as well as thermal stratification. Moreover, habitat use was related to SST and residence depth of cod. Areas deeper than 15 m were favoured from late December until March during low SST and a mixed water column (spawning) and also from July until August during high SST and strong thermal stratification (summer aestivation). Shallower areas were favoured during the rest of the year. The depth and habitat use displayed distinct seasonal up- and downslope movements of cod. This study highlights the importance of shallow-water and structured habitats for cod in the WBS and the value of local knowledge held by fishers for a better understanding of the distributional dynamics of important marine resource populations.

In this study, logbook, landings, and vessel monitoring system data of German brown shrimp (Crangon crangon L.) fishers were combined to analyse spatial and temporal distribution patterns. Landings per unit effort (LPUE) were standardized to the efficiency level of a reference vessel. The standardization altered the spatial–temporal patterns of the LPUE as the vessels differed by a factor of up to 4.7. LPUE data were separated into two size classes according to sieve fractions. Small brown shrimps were characterized by LPUE values increasing early in the year in shallow waters, especially in estuaries in July. Subsequently, peak LPUE values are reached at the end of September in all depths, but with variable spatial patterns between years. LPUE data indicate a general outward migration between early summer and winter. In winter, large shrimp shows a clear increase in LPUE with depths mainly in regions adjacent to the Dutch coast and a reversed pattern close to the Danish coast. The large size class in winter—which consists mainly of mature or egg-bearing females—showed high LPUE values along two persistent diagonal bands in the German Bight. These patterns could lead to local recruitment overfishing, if fishermen are aware of such patterns.

Daily formation of fish otolith micro-increments is frequently assumed, however applying inferences about timing of life history events and formation of otolith macro-structures requires further validation of the periodicity of micro-increment formation. We analysed micro-increments from Western Baltic cod (WBC, Gadus morhua) otoliths marked with tetracycline-hydrochloride as part of an age validation study to test the assumption of daily formation of micro-increments. We found that the number of counted micro-increments consistently underestimated the age of cod aged 1 and older. Time at liberty was also underestimated, especially for fish at liberty during winter. In contrast, micro-increment counts of otoliths from wild-caught young-of-the-year (YOY) cod could be used to realistically estimate timing of hatch and translucent zone formation. Under ambient conditions, settlement did not correspond to any visible pattern within the otoliths, but could be inferred from the prey switch observed from stomach content analyses. We therefore conclude that micro-increments can be assumed to form on a daily basis until the first winter, and can therefore be used to investigate early life history of YOY WBC. However, the periodicity of micro-increment formation appears to vary seasonally in older individuals, with the number of micro-increments formed during the winter period being particularly low.

Aim: The few fish species able to reproduce across wide osmotic ranges either plastically acclimate sperm performance to, or are locally adapted to, different salinities. The invasive round goby (Neogobius melanostomus) is spreading in Eurasia and the Americas, into both fresh and brackish water. We aim to understand if reproduction in different salinities is affected by an ability to acclimate. Location: Brackish and freshwater systems of northern Europe and the Baltic Sea. Methods: We cross-exposed round gobies of freshwater and brackish origin to 0 and 16 practical salinity units (PSU), and the fish were given nest boxes in which to spawn. After 4 weeks, we measured their sperm performance in both 0 and 16 PSU; fertilization success of each egg clutch was measured through visual analysis of eggs. Clutches were split and allowed to develop in both 0 and 16 PSU salinity, and reproductive success (zygote development) was measured 20 ± 1 days later. Responses were analysed using generalized mixed models. Results: After a month, the fish showed no plasticity in sperm performance to their acclimation salinity, regardless of their origin. Sperm velocity was highest in the salinity similar to the males’ origin. Significantly lower fertilization success was measured for individuals that reproduced outside their salinity of origin despite recurring spawning events in all treatment groups. Among fertilized eggs, zygote development was similar regardless of salinity treatment of either eggs or parents. Main Conclusions: Short-term acclimation to new salinities does not affect sperm performance in the round goby. Alternative hypotheses such as local adaption should be further investigated. Limits to the species’ reproductive success, and therefore invasion processes, are likely dependent on environment-phenotype matches. Fish of brackish origin spawned successfully in freshwater, pointing to an increased risk of introducing populations of brackish origin into freshwater.

The mackerel (Scomber scombrus) stock is one of the commercially most important pelagic species in the Northeast Atlantic, being targeted by various nations. Environmental and stock size changes caused a stock expansion after 2007, initiating a yet unresolved dispute between the European Union, United Kingdom, Iceland, Greenland and the Faroe Islands. We analyze the impacts of different Total Allowable Catches (TACs) on the mackerel stock and possible adaptations in simulated fleet behavior by applying an age-structured bio-economic optimization and simulation model, FishRent. We implement dynamic seasonal migration patterns, which alter the spatial extension of mackerel depending on biomass levels, assuming density dependent mechanisms. If true, this is an important consideration because it may provide the possibility for major fishing parties (the European Union, Norway and the UK) to exclude minor parties (Iceland) from fishing mackerel by selecting a high enough fishing pressure and therefore keeping the stock from migrating further north. This can, for instance, be achieved by setting a corresponding TAC. When applying a TAC under a business-as-usual scenario or higher, the stock biomass decreased close to BMSY on the long-term. In this case, the Danish fleets would benefit, increasing their profit up to 80%. All other fleets were negatively affected with a profit decreasing between 50% and 100% depending on the scenario. When applying a TAC according to the stock advice, the stock biomass was estimated to increase on the long-term. This benefitted the Irish and British fleets, but effects on the Icelandic fleet were neutral.

This study presents the diet composition of western Baltic cod Gadus morhua based on 3150 stomachs sampled year-round between 2016 and 2017 using angling, gillnetting and bottom trawling, which enhanced the spatio-temporal coverage of cod habitats. Cod diet composition in shallow areas (20 m), the contribution of herring Clupea harengus decreased and round goby Neogobius melanostomus occurred as a new prey species. Statistical modelling revealed significant relationships between diet composition, catch depth, fish length and season. Generalized additive modelling identified a negative relationship between catch depth and stomach content weight, suggesting reduced food intake in winter when cod use deeper areas for spawning and during peak summer when cod tend to avoid high water temperatures. The results of this study highlight the importance of shallow coastal areas as major feeding habitats of adult cod in the western Baltic Sea, which were previously unknown because samples were restricted to deeper trawlable areas. The results strongly suggest that historic stomach analyses overestimated the role of forage fish and underestimated the role of invertebrate prey. Eventually, this study shows the importance of a comprehensive habitat coverage for unbiased stomach sampling programmes to provide a more reliable estimation of top predator diet, a key information for food web analyses and multispecies models.

BONUS XWEBS Deliverable D2.1

Single species stock assessment models are, and will remain, the workhorse of fisheries management. However, they are incapable of assessing the structure and functioning of the ecosystem the fisheries operate in. This study describes the trophic structure of the southern North Sea and the flows between the nodes of its food-web. It is based on the outputs of an Ecopath food-web model of the North Sea south of the Skagerrak (ICES area VIb and c), parametrized representing the year 1991. The study also compares the southern to a whole North Sea Ecopath model (whole ICES area IV) parametrized for the same year, 1991. The two dominant flows of biomass led from primary producers to detritus, and from there into benthos. The southern North Sea differed from the whole North Sea representation in its fish community composition, primarily attributable to the biogeography of the species. Flatfish were caught more and roundfish less in the south, even with a nominally identically gear, to wit, otter trawlers. The largely different fish and catch compositions call for a specific, local management of the shallow southern North Sea's fisheries, which is based on ecosystem boundaries, rather than politically defined areas. Beyond fished stocks, food-web network indicators suggest that both systems functioned similarly, if compared to a global set of Ecopath models. They also deem the 1991 North Sea and its southern sub-part a densely woven, mature and resilient food-web.

Salt marshes provide favourable feeding conditions for fish thanks to a variety of food sources including terrestrial prey, which can be negatively affected by livestock grazing. We evaluated the seasonal effect of sheep grazing on fish diets in a Wadden Sea salt marsh. Consumed quantities and frequencies of terrestrial prey differed between predators. While terrestrial prey was unimportant for herring and sand goby, common goby and three-spined sticklebacks consumed it immediately during winter, when it became available through marsh flooding. In contrast to the relative stomach content weights, the stomachs content compositions differed between grazing treatments. Sticklebacks consumed terrestrial prey more often and in higher amounts in the ungrazed than in the grazed marsh. Using random forest models, the importance of grazing management for fish compared to other influencing factors was analysed based on sticklebacks. Its most important terrestrial prey was the energy-rich and probably easy to catch amphipod Orchestia gammarellus. Both the ratio of fishes feeding on Orchestia and the consumed amount of Orchestia depend mainly on the accessibility, determined by the flooding height and duration. The abundance of Orchestia was the second most important factor determining its consumed amounts. Orchestia densities are significantly reduced by sheep grazing. Therefore, sheep grazing has a negative impact on fish feeding opportunities in winter, when marine prey on the tidal flats is scarce. To improve the feeding habitat value of Wadden Sea salt marshes for fish it is advisable to keep lower elevated marshes with large creeks ungrazed.

Diel vertical migration (DVM) is a common behaviour and often relates with the diurnal feeding periodicity. Nonetheless, sprat feeding behaviour and daily ration (DR) estimation are usually based on daytime stomach contents from deeper layers. Our study provides a new approach for DR estimation, taking into consideration the DVM associated feeding periodicity. We analysed sprat DVM by hydroacoustics and collected stomach contents in the Central Baltic Sea at different depths. The main outcome is that feeding in the deep during the day represents only 16–39% of the DR. DRs were 1.4-times higher using our approach compared to the established approach. The underestimation by the established approach was mainly caused by ignoring the effect of higher temperatures from the upper layers on the gastric evacuation rate. Our results have major implications not only for DR estimates but also on the interpretation of prey selectivity. The study emphasizes the importance of adapting the sampling design to the vertical feeding dynamics to avoid a biased picture of predator prey interactions.

Brown shrimps (Crangon crangon) occur in high densities in the southern North Sea and support a large, but so far unmanaged fishery with >500 vessels. Cohort-based stock assessment is not possible, and catch per unit effort from scientific surveys and commercial landings are the only variables collected. Landings per unit effort are currently used to approximate the state of stock and to trigger catch restrictions, but, although decisive in interpreting unit catches or landings, factors affecting catch rates are rarely understood. Using data from two long-term (1997–2010) scientific surveys conducted in autumn and winter, respectively, in the southern North Sea and from a vertically resolving stow net deployed at two stations in the German Wadden Sea (2005–2007), we investigate the effects of season, reproductive state, size, tidal state, daylight, and water depth on catch rates of brown shrimp. Log-linear random intercept models revealed an influence of all factors examined on the catch rate. Depth had a clear effect on the composition of size and reproductive state, supporting the hypothesis that brown shrimp utilize selective tidal stream transport to migrate to depths preferred during certain periods within their life cycle. The vertical distribution of brown shrimp across the water column revealed that on average two thirds to three quarters of the brown shrimps were located above reach of the standard shrimp catching gear. Our findings indicate that multiple factors and interactions affect catch rates of brown shrimp and, thus, need to be accounted for when interpreting unit catches or landings for management purposes. We suggest that brown shrimps are not primarily demersal, and that stock size estimates solely relying on beam trawl data may underestimate the true density of shrimps per area.

Spatially explicit bio-economic models that are age-structured and dynamic become increasingly important, being used for different purposes including spatial management measure evaluation. One of the reasons why those complex models are still rare is the extensive data need. FishRent incorporates highly resolved economic information of multiple fleets at the same time linking this to a detailed age-structured biology of multiple species simultaneously. Additionally, it follows the European Data Collection Framework (DCF) data structure, hence the data is relatively easy to implement We adapted the temporal (annual) version of the pelagic FishRent model to be spatially explicit and incorporated seasonal migration patterns of North Sea herring. During this process, we showed the effects of increasing the spatial resolutions on simulated stock biomass and simulated fleet behaviour. When interested in the general, annual population development over the years, a relatively low resolution might suffice. Spatial effects of the fleet behaviour are, however, better captured with a higher resolution. Further, we closed the major spawning grounds at different resolutions. By doing so, we illustrated the need to incorporate a dynamic behaviour of fishing fleets and to increase fleets’ flexibility by increasing the amount of accessible areas for each fleet.

Inundated salt marshes are valued as nekton habitat due to their low predator and high prey abundance. But habitat quality depends also on abiotic properties. We measured hydrological (temperature, oxygen concentration) and hydrodynamic (flow velocity, turbidity, sediment grain size) parameters in four German salt-marsh creeks and compared these with the adjacent tidal flat and subtidal area. Water temperatures in the salt-marsh creeks varied with season, daytime and over the tides. In contrast to deeper subtidal areas, the small water body in the marsh creeks adapted rapidly to air temperatures, resulting in relatively cool water temperatures in winter and relatively warm temperatures in summer. Despite high summer temperatures, the oxygen concentration in the creeks remained year round within the tolerance range of most fishes and crustaceans (above 5 mg/L), probably due to tidal mixing and primary production. Furthermore, the water body in the salt-marsh creeks was sheltered by the vegetated marsh surface from wind and waves and thus characterized by low flow velocities (0.15 ms−1), as well as lower turbidity and finer sediments than in the subtidal. As the specific set of habitat characteristics in the marsh creeks could explain utilization of this habitat by nekton, we performed a constrained redundancy analysis including temperature and turbidity as explanatory variables. Turbidity explained almost 21% of the variation in nekton species occurrence, while temperature accounted for only 2% of the variation. Compared to larger animals, small-sized nekton, which dominated the salt-marsh nekton community, is more capable to deal with extreme temperatures and will profit more from reduced currents than larger animals. Despite the limited accessibility for aquatic organism, intertidal salt-marsh creeks appear to provide favourable habitat for small nekton due to increased growth potential during seasonally higher temperatures, reduced swimming effort, and increased prey visibility and availability in the muddy sediments.

Thermoregulation in aquatic ectotherms is a complex behavioral pattern that is affected by various biotic and abiotic factors with one being salinity. Especially in coastal and estuarine habitats, altering levels of salinity involve osmoregulatory adjustments that affect total energy budgets and may influence behavioral responses towards temperature. To examine the effect of salinity on behavioral thermoregulation in a marine evertebrate ectotherm, we acclimated juvenile and sub-adult common brown shrimp ( Crangon crangon , L.) to salinities of 10, 20 and 30 PSU and investigated their thermal preference in an annular chamber system using the gravitational method for temperature preference determination. Thermal preference of individual brown shrimp was considerably variable and brown shrimp selected a wide range of temperatures in each level of salinity as well as within individual experimental trials. However, salinity significantly affected thermal preference with the shrimp selecting higher temperatures at 10 and 20 PSU when compared to 30 PSU of salinity. Body size had no effect on thermal selection and did not interact with salinity. Temperature preference differed by sex and male shrimp selected significantly higher temperatures at 10 PSU when compared to females. The results show that salinity strongly affects thermal selection in brown shrimp and confirms the strong interrelation of temperature and salinity on seasonal migratory movements that has been previously derived from observations in the field. In the field, however, it remains unclear whether salinity drives thermal selection or whether changes in temperature modify salinity preference.

A recent increase in the construction of Offshore Wind Farms (OWFs) has initiated numerous environmental impact assessments and monitoring programs. These focus on sea mammals, seabirds, benthos or demersal fish, but generally ignore any potential effects OWFs may have on the pelagic ecosystem. The only work on the latter has been through modelling analyses, which predict localised impacts like enhanced vertical mixing leading to a decrease in seasonal stratification, as well as shelf-wide changes of tidal amplitudes. Here we provide for the first-time empirical bio-physical data from an OWF. The data were obtained by towing a remotely operated vehicle (TRIAXUS ROTV) through two non-operating OWFs in the summer stratified North Sea. The undulating TRIAXUS transects provided high-resolution CTD data accompanied by oxygen and chlorophyll- a measurements. We provide empirical indication that vertical mixing is increased within the OWFs, leading to a doming of the thermocline and a subsequent transport of nutrients into the surface mixed layer (SML). Nutrients were taken up rapidly because underwater photosynthetically active radiation (PAR) enabled net primary production in the entire water column, especially within submesoscale chlorophyll- a pillars that were observed at regular intervals within the OWF regions. Video Plankton Recorder (VPR) images revealed distinct meroplankton distribution patterns in a copepod-dominated plankton community. Hydroacoustic records did not show any OWF effects on the distribution of pelagic fish. The results of a pre-OWF survey show however, that it is difficult to fully separate the anthropogenic impacts from the natural variability.

Geolocation models are essential tools to derive detailed information on the movement patterns of demersal fish equipped with data storage tags recording temperature and depth. However, modelled horizontal positions are accompanied by an often unknown error which needs to be assessed to support the appropriate interpretation. We conducted five validation experiments in the southern Baltic Sea comprising a) simulated tracks, b) stationary, nearshore moored tags, c) temperature data from an automated offshore measuring station, d) a temperature-depth probe mounted to an otter board of a commercial vessel and e) tags mounted to the conductivity-temperature-depth probe (CTD) and the otter board of a survey vessel to obtain temperature-depth data from known locations. Using depth-temperature data from each experiment, we calculated modelled positions using an adapted R-package HMMoce model framework. These experiments allowed us to compare true and modelled positions to quantify model performance and assess its potential efficacy for future geolocation studies on tagged cod. Correctly modelled start and end positions were a good indicator for all validation methods; modelled and true tracks were generally similar with differences in daily positions varying on average between 2 and 19 km. The performance of the geolocation model was improved when temperature-depth profiles indicated heterogeneity in the oceanographic environment, particularly in stratified water columns. The validation methods confirmed that the modified HMMoce framework could be used as a geolocation tool for Eastern Baltic cod when an uncertainty of less than 20 km from the true position is acceptable. Since all presented methods are cost-effective, transparent, open source and reproducible, they can be routinely applied to understand expected model uncertainties and thus make more robust inference about movements of tagged fish.

The Eastern Baltic cod abundance started rapidly to increase in the mid-2000s as evidenced by analytical stock assessments, due to increased recruitment and declining fishing mortality. Since 2014, the analytical stock assessment is not available, leaving the present stock status unclear and casting doubts about the magnitude of the recent increase in recruitment. Earlier studies identified main factors impacting on cod reproductive success to be related to the loss of two out of three spawning areas in the 1980s caused by lack of major Baltic inflows with a concurrent reduction in salinity and oxygen. Other important factors include prey availability for first-feeding larvae, egg predation by sprat and herring and cannibalism on juveniles, all in one way or the other related to the prevailing hydrographic conditions. These factors cannot explain increased reproductive success in the last decade, as the period was characterized by an absence of large-scale Baltic inflows since 2003 and persistent anoxic conditions in the bottom water of the deep Baltic basins. This questions the perception of the increased recruitment in later years and challenges our present understanding of cod recruitment dynamics in the Baltic Sea. In this contribution, we review evidence from the recent literature supplemented by information from latest research cruises to elucidate whether cod reproductive success indeed has increased during the last decade, and we suggest the key processes responsible for the recent dynamics in cod recruitment and outline directions for future research.

Ectothermic organisms rely on behavioral thermoregulation to modulate their body temperature and the final thermal preferendum (FTP) paradigm (FTPP) postulates that a given species holds a species-specific FTP. The FTPP, however, has been challenged by a considerable amount of recent findings on vertebrate ectotherms, indicating that the FTP is not as static as originally postulated but can be modulated by various factors. In contrast to vertebrates, evidence from invertebrate ectotherms is far more limited. To test for the validity of the FTPP and to study whether behavioral thermoregulation in a marine invertebrate varies seasonally, thermal preferenda of the common brown shrimp (Crangon crangon, L.) were determined through a 14 month period. Thermal preferenda of males and females of different size groups were measured in an annular chamber system (4–25 °C) using the gravitational method for thermal preference testing. The results revealed that brown shrimp behaviorally selected a wide range of temperatures and did not show a single or constant thermal preferendum. Thermal preference was modulated following a seasonal pattern suggesting the existence of distinct seasonal preference zones rather than a species-specific preference temperature. Brown shrimp selected low temperatures during winter, however, a difference of 5 °C in preferred temperature was observed between a cold and a relatively mild one. Highest thermal preferenda did not coincide with field water temperatures but were observed after field water temperatures exceeded the annual temperature peak. Besides season, body size of brown shrimp significantly affected thermal selection and small individuals selected higher temperatures compared to large ones. A significant interaction of body size and water temperature at which the shrimp were caught was detected. The study further provides evidence that cohort identity as well as female reproductive state may affect thermal preference as well. The results of the study reveal that thermal selection in brown shrimp is complex, not static and that preference temperature can be modulated by various factors, representing the first evidence for a marine invertebrate ectotherm.

Small mesh codends applied in the brown shrimp (Crangon crangon L.) fishery in the North Sea cause large bycatch of fish, invertebrates and undersized shrimp. We investigated the most obvious and enforceable mitigation measure: the increase of mesh size beyond 20 mm, which was the medium mesh size of the fleet before 2016. The predicted selectivity from different codend designs was applied in a yield-per-recruit model able to reproduce the seasonal pattern of landings and biomass by sex, size, and age. The model enables assessment and quantifying population responses to possible management actions, also considering other influential variables such as natural mortality, which vastly decreased during the 1990s to the contemporary low level due to the decrease of cod in the Southern North Sea. Model results show that the selectivity of 20 mm codends leads to the poorest fishery and population indexes. Any increase in mesh size within the range considered and in the present low-predator situation would lead to a larger shrimp stock and higher catches. Increasing mesh size reduces catches of undersized and small commercial-sized shrimps entering the fishery mainly in summer time, thus leading to surplus catches in autumn and spring. Model results reveal that an increase of mesh size diminishes growth overfishing and allows the formation of a larger stock that may better cope a fluctuating natural mortality and the steadily upwards creeping fishing intensity.

The authors (MS, AK, SS, AT) gratefully acknowledge financial support through the European Commission’s MYFISH project (Maximising yield of fisheries while balancing ecosystem, economic and social concerns, FP7/2007-2013 under grant agreement n°289257; http://myfishproject.eu). In marine ecosystems, maximum sustainable yield considerations are affected by any substantial changes that occur in the top and bottom compartments of the food-web. This study explores how the southern North Sea’s fisheries may need to adjust their fishing efforts to maintain optimum yields of sole, plaice, cod and brown shrimps under increased marine mammal populations and a reduced primary productivity. We constructed plausible scenarios of ongoing food-web changes using the results of Bayesian age-structured population models to estimate carrying capacities of harbour porpoises (Phocoena phocoena) and grey seals (Halichoerus grypus). Losses in primary productivity were predicted by lower trophic level ecosystem models. These scenarios were implemented in a food-web model of the southern North Sea. For each scenario, we sought mixed-fleet fishing efforts that would deliver maximum yields of sole, plaice, cod and brown shrimp combined. We also did so for a baseline run with unaltered mammal and primary production, and compared the differences in optimal fishing strategies, predicted yields, and states of the stocks between the scenarios. We found stocks and yields to be far more sensitive to changes in primary productivity than to increased marine mammal predation. The latter predominantly impacted cod, and even benefitted brown shrimps compared to the baseline run. Under 30% reduced primary productivity, fishing efforts had to be reduced by 50% to still provide maximum yields, whereas the marine mammal scenario induced no need to adjust the fishing regime. This draws attention to the potential gains of incorporating bottom-up processes into long-term management considerations, while marine mammal predation may be less of a concern, in particular for flatfish fisheries in the North Sea, and may even benefit shrimp trawlers because of reduced predation on shrimp from fish predators. Publisher PDF

Predator populations are often affected by the abundance of their prey, but pronounced effects on predatory fish have mainly been demonstrated in ecosystems where a key predator depends largely on one key prey species. The North Sea food web has a comparatively high level of complexity with a high diversity of forage fish, and hence strong effects are less likely to occur. However, in the early 2000s within large parts of the North Sea, several forage fish stocks simultaneously suffered from successive years of recruitment failure together with decreasing stock abundances. Whiting Merlangius merlangus is a major fish predator in the North Sea ecosystem and is known to be almost exclusively piscivorous. We hypothesised that shortages in forage fish should lead to negative effects on growth or condition of a predator that relies on a few dominant prey fish species. In our study, we combined 6 different North Sea data sets on abundance of forage fish and length-at-age, condition and stomach contents of M. merlangus to analyse contrasting periods with high and low forage fish availability. We found a simultaneous decrease in forage fish availability and M. merlangus length-at-age in the period from 2000-2007 and a subsequent parallel increase in prey abundance and length-at-age after 2007. In the period of low forage fish availability, mean stomach content mass was on average 60% less than in the reference periods. Additionally, a bioenergetics calculation revealed that even smaller differences in the stomach contents than those observed would have been sufficient to explain the observed differences in length-at-age. Our findings emphasize the need to incorporate predator-prey interactions in assessment models and management strategies.

The southern North Sea is the stage of conflicting objectives of beam trawlers targeting flatfish; of shrimp trawlers fishing for brown shrimp with bycatch of juvenile flatfish; and of demersal trawlers, the main fishery on cod, a key predator of shrimp and other groups. To expose trade-offs between the fleets’ objectives and to explore what a possible variant of a multispecies maximum sustainable yield (MSY) could look like, we parameterized an ecosystem model and subjected it to a range of different fishing effort levels of the three fleets. Long-term projections highlighted multiple fishing regimes that lead to catches of at least 30% of all focal stocks' single species MSYs at the same time. Trade-offs between the yields of shrimp fishers and demersal trawlers made higher simultaneous yields impossible. Besides optimizing multispecies catches, we identified effort regimes that satisfied a set of descriptors of good environmental status (GES). We found that GES can only be obtained through low efforts of beam and demersal trawlers, which cannot be aligned with our multispecies MSY variant without accepting trade-offs in fishing yields and/or conservation goals.

Highlights: • Juvenile fish somatic-, biochemical- and otolith-based condition indices are compared. • RNA/DNA, otolith increments, and somatic condition explained > 70% growth variability. • Response times of proxies differ after food deprivation and re-feeding. • RNA/DNA most rapid response, otolith increments explain length but not mass growth. • Caution suggested if condition indices are applied to fish in patchy prey environments. Abstract: Reliable estimates of short- and longer-term in situ growth and condition of organisms are critical if one hopes to understand how the environment regulates survival. This study reports the first comparison of somatic- (K), biochemical- (RNA–DNA ratio, RD) and otolith- (increment widths, OIW) based indices of condition of a young juvenile fish. Measurements were made on European sprat (Sprattus sprattus) that had i) known differences in somatic growth rate caused by providing different, constant prey ration levels, ii) been fed ad libitum at 7, 11, 15, 18 and 22 °C, and iii) been deprived of prey for either 4, 8 or 12 days and re-fed for 8 days. All three proxies explained significant amounts (70 to 90%) of the variability in measured growth rate. In fish experiencing a change in their feeding level and concomitant change in mass-at-length (K), RD tracked changes in both length and mass while OIW only tracked changes in length. Values of OIW and RD were highest at 18 °C suggesting that this is the optimal temperature for growth in these juveniles. During food deprivation, RD and OIW rapidly decreased and reached their lowest values within ~ 4 days. Upon re-feeding, RD increased most rapidly, K was most variable and the response time in OIW was slowest (two-times slower than RD). These patterns reflected preferential allocation of food energy to restore body mass in recently re-fed fish prior to fish increasing both mass and length. These results indicate that the sensitivity and applicability of growth proxies depend on the recent feeding history, that proxies have different response times, and that caution be taken when inferring growth and condition in early life stages of fishes that forage in patchy prey environments.

The concept of co-location of marine areas receives an increased significance in the light of sustainable development in the already heavily used offshore marine realm. Within this study, different spatial co-location scenarios for the coupling of offshore aquacultures and wind farms are evaluated in order to support efficient and sustainable marine spatial management strategies. A Geographic Information System (GIS) and multi-criteria evaluation (MCE) techniques were combined to index suitable co-sites in the German exclusive economic zone of the North Sea. The MCE was based on criteria such as temperature, salinity or oxygen. In total, 13 possible aquaculture candidates (seaweed, bivalves, fish and crustaceans) were selected for the scenario configuration. The GIS modelling framework proved to be powerful in defining potential co-location sites. The aquaculture candidate oarweed (Laminaria digitata) revealed the highest suitability scores at 10–20 m depth from April to June, followed by haddock (Melanogrammus aeglefinus) at 20–30 m depth and dulse (Palmaria palmata) and Sea belt (Saccharina latissima) at 0–10 m depth between April and June. In summary, results showed several wind farms were de facto suitable sites for aquaculture since they exhibited high suitability scores for Integrated Multi-Trophic Aquaculture (IMTA) systems combining fish species, bivalves and seaweeds. The present results illustrate how synergies may be realised between competing needs of both offshore wind energy and offshore IMTA in the German EEZ of the North Sea. This might offer guidance to stakeholders and assist decision-makers in determining the most suitable sites for pilot projects using IMTA techniques.

Sprat, Sprattus sprattus , is a small pelagic fish species with a wide distribution along the European continental shelf, the Mediterranean Sea, the Black Sea and the Baltic Sea. The Baltic Sea is the coldest area of the species distribution range. Even here, sprat is still numerous and holds a key role in the ecosystem. However, the population is sensitive to small changes in environmental conditions. In this paper, we set up a bioenergetic model of Baltic sprat and evaluate how variability and seasonal changes in the physical environment affects the physiology and growth of individual sprat. The model is dynamic and seasonally resolved. It is parameterised based on sprat from the Bornholm Basin. Our model results suggest that the optimal temperature for sprat growth is 17.5 °C, which is only observed in the Central Baltic Sea in the top water layer during a short period in summer. During 4½ winter months individual sprat do not grow and utilize stored energy from the previous growth season. We analysed the maximum attainable individual body size as a function of temperature. The model predicted reduced maximum body sizes with increasing temperature. The model can be used for studying climate change scenarios on individual growth, egg production and condition of Baltic sprat.

The brown shrimp (Crangon crangon) fishery is of great socio-economic importance to coastal communities on the North Sea. The fishery is exploited by beam trawlers often using codends with very small mesh sizes, leading to concerns about catch rates of undersized shrimp. However, little information is available on codend size selection, making it difficult to provide scientifically based advice on alternative codend designs. Therefore, this study establishes a predictive framework for codend size selection of brown shrimp, based on a large selectivity dataset from 33 different codend designs tested during four experimental fishing cruises, during which more than 350,000 brown shrimp were length measured. Predictions by the framework confirm concerns about the exploitation pattern in the fishery, because the retention probability of undersized shrimp reaches 95% with the currently applied designs. The framework predictions allow the exploration of obtainable exploitation patterns depending on codend design. For example, increasing codend mesh size to 25–29 mm would reduce the retention rate of undersized shrimp to a maximum of 50%, depending on codend mesh type. Predictive framework for codend size selection of brown shrimp (Crangon crangon) in the North Sea beam-trawl fishery

Designing effective management plans requires understanding fishers' behaviour under that plan, because fishers change their behaviour in response to economic and management incentives, which in turn will lead to different fishery outcomes. This study presents a modelling framework for management strategy evaluations which takes into account the response of fishers to management schemes. Based on the upcoming discard ban, two discard prevention strategies were tested for the North Sea saithe fishery, where fleet segments have either no or a generally low quota for cod. Costs and benefits were assessed under the current management, a non-flexible system, where fleet segments had to stop fishing once the cod quota was reached and a flexible system where quota of saithe could be used to cover over-quota catch of cod at a ratio 1:5. The flexible scenario was beneficial both in protecting the North Sea saithe and cod stock and in increasing net profits of fleet segments in the long term. The avoidance behaviour of fleet segments to over-quota catch led to a high SSB level of saithe and cod in the long term, ensuring high long-term catches and profits. A non-flexible scenario had a negative impact on the saithe stock, because mainly juvenile saithe before spawning were caught reducing the spawning-stock biomass in the longer term. A non-flexible scenario was costly in terms of up to 29% lower net profits for individual fleet segments generating little economic incentive to be compliant.

Temperature is of critical importance for ectotherms, having vast impacts on physiology, behavior and distribution. In many species, however, the behavioral component of temperature selection is not well understood. This study addresses the thermoregulatory behavior of the common brown shrimp (Crangon crangon, L.), which is a central component of the Wadden Sea ecosystem and an important fishery resource of high commercial value. To investigate whether brown shrimp are thermosensitive and perform behavioral thermoregulation, we examined the short- and long-term thermoregulatory behavior by using the acute and gravitational method for temperature preference testing. For the acute method, female adult brown shrimp were acclimated to 5 temperatures between 9 °C and 19 °C for two weeks. For the gravitational method, the shrimp were acclimated to 3 temperatures within the same range. Hereafter, thermal preferenda were determined in an annular shaped preference chamber. Acute and gravitational thermal preference experiments revealed brown shrimp to be thermosensitive and perform behavioral thermoregulation. Using the acute method, a positive correlation of acclimation and preferred temperature was observed, resulting in a final thermal preferendum of 15.9 °C. In experiments using the gravitational method, preference temperature was heavily modulated by the photoperiod, with brown shrimp selecting temperatures more precisely during the scotophase than the photophase. Determined at dark exclusively, however, no effect of acclimation temperature on gravitational preference after 24 and 48 h was observed. Preferenda ranged between 13.5-15.0 °C after 24 h and 12.0-14.9 °C after 48 h, respectively, and no significant differences between both methods were detected. Based on these findings, 20-24 h of gradient exposure can be considered sufficient to obtain thermal preferenda that are unaffected by the animal’s prior thermal history. Thermal preferenda determined in the present study were higher than the average temperature experienced by brown shrimp in the field. Still, thermal preferenda were considerably lower than previously reported optimum temperatures for brown shrimp.

The North Sea brown shrimp fishery is currently regulated neither with quotas nor with effort management. The current paradigm of non-management was based on an analysis of the total predation by cod and whiting in relation to commercial catches for the period 1970–1995 and the estimated total dominance of natural mortality. However, since this period, the North Sea ecosystem has undergone pronounced changes with overfishing and climate change causing a substantial decline in predator stocks, namely cod and whiting. In addition, both predators have shifted their range of distribution causing a reduced overlap with brown shrimp. Here, we extend the previous assessment of brown shrimp predation for the years 1996–2011 using updated stock assessment and predator distribution data. For the first time, predation estimates are used together with commercial landings to partition independent estimates of total mortality into fishing and predation mortality. We demonstrate that the decline of key predators of brown shrimp in combination with a shift in the distributional range of the predators has caused a new situation, in which the fishery has become the main mortality source of adult brown shrimp (>50 mm). Average landings since 2000 have been ∼40% higher than in the 1980s and 1990s, indicating that humans have at least partly taken over the share previously taken by juvenile whiting and cod. We discuss that this situation is likely to continue, because three marine mammal species have built up a combined population of over 80 000 individuals, which hunt for potential brown shrimp predators mainly in the distribution area of brown shrimp. The application of two yield-per-recruit models of different complexity indicates potential growth overfishing of brown shrimp and reopens the discussion of management.

Life cycle closure for species inhabiting areas with daily varying currents but directed net water transport requires specific behavior to minimize losses due to advection of passive drifting life stages. Variations in swimming activity of different-sized Crangon crangon (15–65 mm total length) were therefore monitored under constant laboratory conditions immediately after being caught in the German Wadden Sea. Activity of shrimps of different sizes, caught at different seasons, always peaked at times corresponding with ebb tide in the habitat from where they were taken. This behavior was maintained for several days if no external stimuli were present but shifted to night activity if a light–dark cycle was provided. The observed behavior/activity pattern was included in a coupled hydrodynamic and individual-based model (IBM) and the shift in the location of a shrimp cohort was monitored over time. Performance of ebb tide activity not only allowed the shrimps to reach the preferred deeper winter and spawning areas but also allowed them to migrate against the dominating current from eastern nurseries to more western located spawning areas. Passively drifting larvae released at these locations and later larval and juvenile stages that perform flood tide transport can reach the nurseries again. This links the nurseries and adult spawning grounds and closes the migration triangle.

The determination of in situ growth rates in crustaceans is a major obstacle for stock assessment, especially in short lived small shrimp species such as Crangon crangon. Here a new method is presented, which uses a condition index based on dry weight to predict growth increments. The method is based on a simple equation relating dry weights of shrimp and exuvia directly after moult with length measurements before and after moult. The parameter estimation of the model requires controlled moult experiments in the laboratory. In total 272 individuals moulted successfully within the first 2 days after capture, hence the increments reflected the feeding situation of the prior feeding period in the field. The results confirmed the validity of the method with data from two different years and demonstrated that in C. crangon the model parameters differ significantly between the two sexes. As a side product also moult intervals were estimated as a function of length and temperature. Moult increments were relatively shorter and more variable than previously found.

We examined factors affecting the fishing effort in the German brown shrimp (Crangon crangon) fishery, including shrimp and fuel price, catch per unit effort (cpue), number of days with strong wind, fishing port, and season. Time-series analysis (TS) using rational transfer functions (ARIMAX, an extension of autoregressive integrated moving average (ARIMA) modelling) were applied to model mean monthly boat effort (MBE, in hours), aggregated from the logbook data of five German ports for the years 2000–2008. We addressed two major areas: (i) whether cyclic effort patterns and the effect of external factors such as wind differ regionally, and (ii) whether external factors such as cpue, shrimp and fuel prices have a significant influence on the effort pattern. The estimated ARIMAX models accounted for 52–77% of the variance in MBE if analysed separately for each port, indicating a stable seasonal pattern of fishing effort without major interannual variations or significant trends. In spite of considerable variability in the external factors, none of cpue, shrimp or fuel price had a significant effect on the fishing effort. We interpreted this stability as a lack of alternative target species and discuss the theory that a response of a fishery to these external factors might only occur once certain critical threshold values have been exceeded, which was not the case during the investigated period. The models indicated differences in the seasonal patterns of the different harbours, with shorter trips and an earlier start for the main fishing season in the southwest (SW) regions, whereas in the northeast (NE) harbours, trips were longer, peak effort was shifted by 1–2 months and the spring season was of greater importance. The relationships of these regional patterns to the shrimp life cycle and coast topography, as well as implications for future management, are discussed.

There is growing interest in bioeconomic models as tools for understanding pathways of fishery behaviour in order to assess the impact of alternative policies on natural resources. A model system is presented that combines stochastic age-structured population dynamics with complex fisheries economics. Explicitly, the economic response of fleet segments to changes in stock development is analysed by applying observed values and stochastic recruitment. The optimization of net profits determines the fishing effort and the investment and disinvestment behaviour of fleet segments, which, in turn, affect the level of catch rates and discards. This tool was applied to the North Sea saithe fishery, where ICES re-evaluated the existing EU–Norway management plan, focusing on biological reference points only. Two scenarios were tested with alternative harvest control rules and then contrasted with one unregulated scenario with no quotas and driven by optimizing the net profit of the whole fleet. The model showed the success of both harvest control rules in rebuilding the stock and the associated costs to the fleets in terms of maximal 21% reduction in net profits, 21% reduction in crew wages and 11% reduction in fleet size in the midterm (2007–2015). In the long term (2022), successful stock recovery coincided with net profits almost equalling that of the unrestricted fishery. The model is highly sensitive to the parameter values but can be used strategically, providing a qualitative understanding of the anticipated relative changes.

There is growing interest in bio-economic models as tools for understanding pathways of fishery behaviour, in order to assess the impact on natural resources. Based on 'FishRent', a modelling approach is presented that integrates economics of the fleet, the impact of fishing on stock development and their spatio-temporal interplay. The simulation of species seasonal movements in combination with both observed values and stochastic recruitment allowed analysing the economic response of fleet segments to changes in stock distribution and development. Optimisation of net profits determines the effort adjustment and spatial allocation of fleet segments, which in turn affects the level of catch rates. Effort tended to concentrate where fish abundance was high, but also where fishing costs were low. In simulations with the current management plan spawning stock of North Sea saithe (Pollachius virens) declined below its precautionary reference point. In response fishing far from home ports became expensive and 40% of the initial effort was shifted to areas closer to home ports, but as areas of high fish concentrations were located by the modelled fleet segments catch rates remained high. Changes in seasonal/annual stock distribution, the stock decline and costs influenced the change in fishing effort distributions leading to overestimated catch per unit of effort values that masked the decline of stock abundance.

Brown shrimp (Crangon crangon, L.) are subjected to a huge annual temperature range, and certain thermal conditions during winter have been identified to affect the brown shrimp population. Despite that, little is known about its thermal biology with regard to critically low temperatures. In the present study, we determined the critical thermal minima (CTmin) and the critical lethal minima (CLmin) of male and female brown shrimp of different body sizes in laboratory-based experiments. For the CTmin trials, shrimp were acclimated to 4.0, 9.0, and 14.0 °C and exposed to a cooling rate of −0.2 °C min−1. In the CLmin trials, brown shrimp were exposed to a cooling rate of −1.0 °C day−1 without prior thermal acclimation. Acclimation temperature significantly affected the temperature tolerance of brown shrimp (p

Prey selectivity of Baltic herring Clupea harengus and sprat Sprattus sprattus was studied in the Bornholm Basin, Baltic Sea, in June 2001. A total of 165 sprat stomachs (10 to 15 cm total length, TL) and 214 herring stomachs (12 to 30 cm TL) were analysed. The diel vertical distribution of zooplankton prey was analysed by multi-net samples; clupeid distributions were estimated by hydroacoustic measurements. These measurements enabled us to describe the diel feeding rhythm and to estimate vertically resolved selectivity indices for the 2 most important zoo-planktivores in the Baltic Sea. Diet composition of herring and sprat were similar (mainly copepods and cladocerans), resulting in strong competition. Possibly to reduce this competition, both species were partly specializing on certain prey species (sprat: Podon spp.; herring: Evadne nordmanni and Temora longicornis) and copepodite stages (sprat: adult [C6] males of Pseudocalanus acuspes; herring: C6 females of P. acuspes). Sprat and, to some extent, herring exhibited a marked shift in prey preference between day and night. Sprat mainly selected T. longicornis during the day and Podon spp. during the night, while herring mainly selected T. longicornis during the day and E. nordmanni during parts of the night. A comparison of the field stomach contents with the estimated gastric evacuation predicted by parameters based on laboratory experiments indicated that sprat fed during the night, while herring did not or only to a minor extent. Comparison of our zooplankton sampling scheme with commonly used sampling designs revealed that investigations which consider both time and stage are needed to fully understand the feeding selectivity dynamics of herring and sprat in the Baltic Sea. However, the objective of a selectivity study should determine the most appropriate zooplankton sampling scheme.