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The relationship between spawning fish abundance and the number of offspring, the so-called stock-recruitment relationship, is crucial for fisheries management and conservation measures. Using the most comprehensive data set ever assembled, we quantify this relationship for 211 fish stocks worldwide, revealing a global pattern with a pervasive asymptotic shape that shows increasing recruitment reaching an upper limit for values around half to two-thirds of parental biomass. This corroborates previous theoretical and modeling results. However, parental biomass is a predictor for only 5% to 15% of the variance in recruitment, demonstrating the weak predictive power of the stock-recruitment relationship in marine fish populations. Thus, there is a need to move rapidly toward models that integrate environmental conditions and species interactions in fisheries stock assessment and management, as suggested by Johan Hjort 100 years ago. [ABSTRACT FROM AUTHOR]

Since 2007, the ecosystem of the Gulf of Lions has shifted to a different regime, characterised by a low anchovy ( Engraulis encrasicolus) and sardine ( Sardina pilchardus) biomass and a remarkably high sprat ( Sprattus sprattus) biomass. Surprisingly, the abundance and recruitment of anchovy and sardine remained high. To understand which processes (bottom-up or top-down control, etc.) could have caused this shift, we studied the changes in body condition, growth and size and age of anchovy, sardine and sprat over 1984-1985 and 1992-2012, using data from scientific surveys. The annual age structure of anchovy and sardine was estimated using Bayesian mixture models based on size frequency data with priors on the age-length relationship derived from independent otolith readings. The results indicated periods during which anchovy and sardine were in an average (1992-2004), good (2005-2007) or poor (2008-2012) overall state of condition. For sardine, the shift towards smaller fish observed during these past 4 years was explained by a combination of slower growth and the disappearance of older individuals (ages 2+). Despite the increase in biomass of sprat since 2008, indications were found that sprat was also smaller than in the past. As growth and condition decreased and overexploitation has not been documented or suspected for those three species in this area, we propose that the current decline in sardine and anchovy biomass could be due to qualitative and/or quantitative modifications in the planktonic production (i.e. a bottom-up control) or mass mortalities of adults due to an epidemic disease. [ABSTRACT FROM AUTHOR]

Atlantic bluefin tuna ( ABFT) has always displayed spectacular changes in its spatial distribution, but the underlying mechanism of such variations still remains obscure. This study focuses on this challenging issue by scrutinizing the intriguing 'Brazilian episode' during which a large quantity of ABFT (a temperate species) was caught during the 1960s in the equatorial Atlantic. To investigate this event, we applied a niche model to an extensive data set of catch and environmental variables from 1960 to 2009. ABFT exhibited a remarkably large ecological niche, which matches well with our current knowledge of ABFT. Our results also depicted a high probability of ABFT occurrence in the South Atlantic and, more interestingly, favorable environmental conditions in the western equatorial Atlantic during the 1960s, but not later. ABFT could thus have migrated from their northern spawning grounds to the South Atlantic during the 1960s through the western equatorial Atlantic, playing the role of an 'ecological bridge'. We argue that the rarity of ABFT in the southern Atlantic during the last four decades would result from the interaction of several processes, particularly oceanographic conditions, migratory behavior, density-dependence, exploitation levels and population structure. Examination of the catch data further indicated that the fish caught in the equatorial Atlantic were from the western stock and we concluded that the lack of rebuilding of this stock could result from a regime shift due to the combination of oceanographic changes in the equatorial Atlantic and overfishing in the North Atlantic in the 1960s and 1970s. [ABSTRACT FROM AUTHOR]

Focuses on the exploitation of the Atlantic bluefin tuna in the Mediterranean Sea. Increase in the efficiency of long-established fisheries; Details on the situation of the eastern stock of bluefin tuna; Assessment and management of eastern stock,.

Long-term time-series of bluefin tuna catches from ancestral Mediterranean and Atlantic trap fisheries are presented and analysed. The trap is a passive gear, little modified for centuries, that catches bluefin tuna Thunnus thynnus during their annual spawning migration. These features, together with preliminary analyses, lead us to suggest that long-term fluctuations in trap catches could reflect those in true abundance if they vary in the same manner all around the western Mediterranean and adjacent Atlantic. To test this hypothesis, we investigated 54 time-series more than 20 years long (the longest ones spanning four centuries) of trap catches along the western Mediterranean coasts of Italy, Sicily, Sardinia, Tunisia, Spain, and Morocco, and the adjacent Atlantic coasts of Spain, Portugal, and Morocco. Trends and cycles were identified using Eigen Vector Filtering and spectral analysis, and the synchrony between short- and long-term fluctuations in trap catches was studied with the modified correlogram of Koenig and Knops. The magnitude of fluctuations in trap catches is large, periods of great abundance being up to seven times bigger than those when abundance was low. More interesting was the occurrence of 100-year-long periodic fluctuations as well as 20-year cycles. These medium- to long-term fluctuations, representing more than 50% of the total variability in the time-series, were synchronous all around the western Mediterranean and adjacent North Atlantic. In contrast, short-term variability was synchronous at a local scale only. It is argued that long-term fluctuations in trap catches could be considered as a proxy for those of true abundance, and a synthetic time-series has been computed to depict them. Biological and ecological processes that could cause such long-term fluctuations are also discussed. [ABSTRACT FROM PUBLISHER]

Time series of 0- and 1-group cod sampled in the period 1919 to 1994 at 38 stations along the Norwegian Skagerrak coast were investigated using spatial and temporal analyses. Both groups displayed similar spatial structure and had similar spatial distributions. Spatial heterogeneity occurred on a mesoscale (differences between fjords) and on a local scale (both groups significantly more abundant at sheltered stations in the inner fjord). Temporal fluctuations exhibiting a cyclic component at around 2–2.5 years were spatially structured on a local scale. The cycle in abundance appears to be a result of biotic interaction, such as competition for space and food and/or cannibalism between cohorts. Similar long-term trends were also observed, although those for the 0-group were more pronounced. In contrast to the 2–2.5 years' cycle, long-term trends were related to events taking place on a scale equal to or larger than the Norwegian Skagerrak. The causes of these fluctuations are complex and probably result from factors such as biotic interactions, changes in seagrass coverage, and fishing. [ABSTRACT FROM PUBLISHER]

• Rhone river nutrients concentrations significantly decreased since 1990. • Rhone river inputs are the main driver of Chl- a in the Gulf of Lions shelf area. • Changes were observed in the environment and small pelagic fish population in mid 2000s. • Small pelagic community and environmental system of the Gulf of Lions presented the same patterns of variation. An important decrease in small pelagic fish condition and size has been observed in the most productive ecosystem of the Mediterranean Sea, the Gulf of Lions, since 2008, leading to an important fishery crisis. Previous studies suggested bottom-up control to be the most probable cause for these changes. Here, we investigate whether an environmental change might have caused such a situation. In the absence of zooplankton time series, this study aims at describing temporal changes in key abiotic factors for the planktonic and fish production of the Gulf of Lions, such as SST, meso-scale fronts, wind-induced coastal upwelling, river discharge, water stratification and deep convection and then at understanding potential link on Chl- a concentration as well as small pelagic fish populations. Our results indicate that the environmental conditions have broadly changed in the Gulf of Lion, with a major change in the mid-2000s, affecting the Chla concentration (which showed a regime shift in 2007), but also the SST, the upwelling and frontal activities, the Rhone river discharge (and particularly the N and P nutrients inputs) as well as the deep winter convection. Those changes could have affected the plankton production and consequently the small pelagic fish community that displayed similar patterns of variations as the environmental conditions. [ABSTRACT FROM AUTHOR]

The commercially important Atlantic bluefin tuna (Thunnus thynnus), a large migratory fish, has experienced notable recovery aided by accurate resource assessment and effective fisheries management efforts. Traditionally, this species has been perceived as consisting of eastern and western populations, spawning respectively in the Mediterranean Sea and the Gulf of Mexico, with mixing occurring throughout the Atlantic. However, recent studies have challenged this assumption by revealing weak genetic differentiation and identifying a previously unknown spawning ground in the Slope Sea used by Atlantic bluefin tuna of uncertain origin. To further understand the current and past population structure and connectivity of Atlantic bluefin tuna, we have assembled a unique dataset including thousands of genome‐wide single‐nucleotide polymorphisms (SNPs) from 500 larvae, young of the year and spawning adult samples covering the three spawning grounds and including individuals of other Thunnus species. Our analyses support two weakly differentiated but demographically connected ancestral populations that interbreed in the Slope Sea. Moreover, we also identified signatures of introgression from albacore (Thunnus alalunga) into the Atlantic bluefin tuna genome, exhibiting varied frequencies across spawning areas, indicating strong gene flow from the Mediterranean Sea towards the Slope Sea. We hypothesize that the observed genetic differentiation may be attributed to increased gene flow caused by a recent intensification of westward migration by the eastern population, which could have implications for the genetic diversity and conservation of western populations. Future conservation efforts should consider these findings to address potential genetic homogenization in the species. [ABSTRACT FROM AUTHOR]

A response by Jean-Marc Fromentin to a letter to the editor about his letter “The Fate of Atlantic Bluefin Tuna” in the March 12, 2010 issue is presented.

To limit climate warming to 2°C above preindustrial levels, most economic sectors will need a rapid transformation toward a net zero emission of CO2. Tuna fisheries is a key food production sector that burns fossil fuel to operate but also reduces the deadfall of large‐bodied fish so the capacity of this natural carbon pump to deep sea. Yet, the carbon balance of tuna populations, so the net difference between CO2 emission due to industrial exploitation and CO2 sequestration by fish deadfall after natural mortality, is still unknown. Here, by considering the dynamics of two main contrasting tuna species (Katsuwonus pelamis and Thunnus obesus) across the Pacific since the 1980s, we show that most tuna populations became CO2 sources instead of remaining natural sinks. Without considering the supply chain, the main factors associated with this shift are exploitation rate, transshipment intensity, fuel consumption, and climate change. Our study urges for a better global ocean stewardship, by curbing subsidies and limiting transshipment in remote international waters, to quickly rebuild most pelagic fish stocks above their target management reference points and reactivate a neglected carbon pump toward the deep sea as an additional Nature Climate Solution in our portfolio. Even if this potential carbon sequestration by surface unit may appear low compared to that of coastal ecosystems or tropical forests, the ocean covers a vast area and the sinking biomass of dead vertebrates can sequester carbon for around 1000 years in the deep sea. We also highlight the multiple co‐benefits and trade‐offs from engaging the industrial fisheries sector with carbon neutrality. [ABSTRACT FROM AUTHOR]

1. Trends and fluctuations in populations are determined by complex interactions between extrinsic forcing and intrinsic dynamics. As an example, the dynamics of many marine fish are characterized by age-structured dynamics forced by stochastic recruitment. 2. In this study we develop stochastic age-structured models for two case studies, the Atlantic bluefin tuna and the Atlantic cod. The former exemplifies intracohort interactions and density-dependent reproduction, the latter exemplifies density-dependent survival and intercohort interactions. 3. We use transfer functions and delay-coordinate models to study how the combination of age-structured interactions and stochastic recruitment can induce low-frequency variability. `Cohort resonance', as we dub this effect, can induce apparent trends in abundance and may be common in age-structured populations. 4. Our study complements the theory of structured populations that focuses on cycles and chaos (high-frequency dynamics). 5. The innate low-frequency fluctuations we describe can potentially mimic or cloak critical variation in abundance linked to environmental change, over-exploitation or other types of anthropogenic forcing. 6. From a management and conservation viewpoint, it will be important to find ways to separate anthropogenic forcing from cohort resonant effects and/or to understand the way they interact. [ABSTRACT FROM AUTHOR]

Animal mortality is difficult to observe in marine systems, preventing a mechanistic understanding of major drivers of fish population dynamics. In particular, starvation is known to be a major cause of mortality at larval stages, but adult mortality is often unknown. In this study, we used a laboratory food-deprivation experiment, on wild caught sardine Sardina pilchardus from the Gulf of Lions. This population is interesting because mean individual phenotype shifted around 2008, becoming dominated by small, young individuals in poor body condition, a phenomenon that may result from declines in energy availability. Continuous monitoring of body mass loss and metabolic rate in 78 captive food-deprived individuals revealed that sardines could survive for up to 57 days on body reserves. Sardines submitted to long-term caloric restriction prior to food-deprivation displayed adaptive phenotypic plasticity, reducing metabolic energy expenditure and enduring starvation for longer than sardines that had not been calorie-restricted. Overall, entry into critical fasting phase 3 occurred at a body condition of 0.72. Such a degree of leanness has rarely been observed over 34 years of wild population monitoring. Still, the proportion of sardines below this threshold has doubled since 2008 and is maximal in January and February (the peak of the reproductive season), now reaching almost 10 % of the population at that time. These results indicate that the demographic changes observed in the wild may result in part from starvation-related adult mortality at the end of the winter reproductive period, despite adaptive plastic responses. • Laboratory fasting experiment informs about natural mortality in the wild. • Sardines reduce their metabolic energy expenditures to endure long fasting. • Calorie-restricted sardines display better phenotypic plasticity to face fasting. • Entry into critical fasting phase 3 occurred at a body condition of 0.72 • Sardines in critical condition represent now ~10 % of the population in the wild. [ABSTRACT FROM AUTHOR]

Determining patterns of change in species richness and the processes underlying the dynamics of biodiversity are of key interest within the field of ecology, but few studies have investigated the dynamics of vertebrate communities at a decadal temporal scale. Here, we report findings on the spatio-temporal variability in the richness and composition of fish communities along the Norwegian Skagerrak coast having been surveyed for more than half a century. Using statistical models incorporating non-detection and associated sampling variance, we estimate local species richness and changes in species composition allowing us to compute temporal variability in species richness. We tested whether temporal variation could be related to distance to the open sea and to local levels of pollution. Clear differences in mean species richness and temporal variability are observed between fjords that were and were not exposed to the effects of pollution. Altogether this indicates that the fjord is an appropriate scale for studying changes in coastal fish communities in space and time. The year-to-year rates of local extinction and turnover were found to be smaller than spatial differences in community composition. At the regional level, exposure to the open sea plays a homogenizing role, possibly due to coastal currents and advection.