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Variations in physical conditions caused by climate change are likely to have large influences on marine organisms, including phytoplankton. Here, we investigated associations between satellite-derived chlorophyll a data from the Barents Sea and 2 key abiotic factors: sea surface temperature and sea-ice concentration. Specifically, we investigated how climate variability, through the measured physical factors, associated with phytoplankton phenology between 1998 and 2014. Associations between sea surface temperature and phytoplankton bloom dynamics differed depending on the area. The spring phytoplankton bloom occurred earlier and had higher magnitude in warm compared to cold years in the northern part of the Barents Sea, but there was no significant association in the southern part. In seasonally ice-covered regions, the association between the timing of the sea-ice retreat and the phytoplankton peak was nonlinear: sea-ice retreat time before mid-May was not associated with bloom timing, whereas the phytoplankton bloom occurred before or immediately following the ice retreat when the ice retreated after mid-May. Although drivers that are relatively constant across years, such as insolation, probably influenced the spatial gradient in chlorophyll, a space-for-time substitution captured the predicted effects of sea-ice retreat on the timing and magnitude of the phytoplankton bloom quite well.

Understanding the drivers (internal and external) that determine the productivity of marine ecosystems is challenging. For example, the correct estimate of recruitment is essential to estimate fish stock abundance. In this Theme Section, 5 papers explore the effect of fishing and cli- mate on population structure across sub-Arctic ecosystems. The studies focus on how temperature- and fishing-induced changes in spatial and demographic population structure affect recruitment and population growth rate. The results suggest common patterns, but also highlight differences in the relative importance of fishing and climate among the populations and ecosystems examined.

Environmental change linked to climate and human activities may affect top predators via the food chain in marine systems. Understanding the functional link between resources and predator responses (e.g. foraging effort, reproductive success, population growth) is of crucial importance in the context of an ecosystem-based approach to fisheries. In the Benguela upwelling ecosystem located off the South African coast, both climate and overfishing may have modified the spatio-temporal availability of sardines and anchovies over the past 2 decades. These fish are prevalent prey items for 3 seabirds: the African penguin Spheniscus demersus, Cape gannet Morus capensis and Cape cormorant Phalacrocorax capensis. Species-specific responses of these 3 sympatric seabirds to the fluctuations in their shared prey are poorly understood. Here we used generalized additive modelling to examine the ‘local population response’ (i.e. number of breeders) of the 3 seabirds to spatio-temporal variations in prey availability from 1987 to 2007. We showed that prey fluctuations influence seabird local population responses, presumably by modulating recruitment, breeding decision and breeding performance. We also identified speciesspecific population responses to prey availability: positive in African penguins and Cape gannets, and null in Cape cormorants. These patterns could be explained by seabirds partitioning prey in time via separated breeding periods, and space via accessing different foraging grounds due to specific feeding techniques. These responses also suggest different degrees of dependency upon pelagic resources among predators, which are likely due to contrasting foraging ecologies (e.g. dietary plasticity, foraging techniques, foraging ranges). 2017-11-14

In many marine species, large females tend to produce more robust offspring than small females. However, knowledge on maternal influences in decapod crustaceans is limited. This is unfortunate since many decapod populations are being intensively harvested and show signs of 'juvenescence', i.e. the loss of large (and presumably old) individuals. In this study, we quantified maternal influences in European lobster Homarus gammarus from Skagerrak, southern Norway. His- torical lobster catches in Skagerrak were substantial but the stock has suffered a major decline over the past 30 to 40 yr and is currently red listed as near threatened according to the IUCN (International Union for Conservation of Nature) criteria. We studied eggs and larvae from wild-caught ovigerous females ranging in carapace length from 79 to 152 mm (n = 45). Mean egg size increased while sib- ling size variation decreased significantly with increasing maternal size. Mean larval size at hatching was closely linked to both maternal size and mean egg size. A laboratory experiment showed a weak but significant nonlinear increase in pelagic larval survival with increasing mean egg size in the absence of food. These findings suggest that maternal influences on offspring quality could be a sig- nificant source of variation in lobster recruitment. Consequently, maternal influences could be an important source of error in fisheries science and management if they are assumed to be absent or unimportant.

Tagging-recovery studies have been used successfully on many occasions, and the data analysed in a variety of ways with respect to fish movement. Here we provide a synoptic account of useful statistical techniques for delimiting area utilization and movement patterns of a population of individuals. We focus specifically on kernel smoothing for population utilization distribution and gen- eralized additive models for temporal movement patterns. Furthermore, we use these 2 techniques to re-analyse data from tagging studies on coastal Atlantic cod Gadus morhua, shedding new light on one of the underlying mechanisms determining the population structure of the coastal cod along the Norwegian Skagerrak coast. Cod were found to use small areas. On average 25% of a population used areas smaller than 10 km 2 , while 95% of a population used areas smaller than 160 km 2 . The movements displayed were fjord-limited and predominantly within the bounds of the population's home range. By re-analysing existing data on neutral genetic markers, we show that the cross- distance of the area utilized was smaller than the distance over which pair-wise genetic differences appear.

The stock of Hokkaido spring spawning herring Clupea pallasii collapsed in the middle of the 20th century. In the first half of the 20th century, large amounts of spawning herring were caught by set nets at coastal spawning grounds off the coast of Hokkaido, Japan. Using the catch data, we analyzed the mean length at age with respect to sea-surface temperature and density- dependent growth during the years 1910 to 1954 by generalized additive modeling (GAM). This stock is distributed at the southern boundary of the distribution range of Pacific herring, and we thus hypothesized that high temperatures have a negative effect on growth. Our study shows that length of Hokkaido spring spawning herring is highly dependent on growth during the first years of life and on temperatures preceding and during the feeding season. Higher temperatures during winter have a negative effect on growth. We found only weak indications of density-dependent growth in the stock of Hokkaido spring spawning herring.

Coastal marine species with pelagic egg and larval stages, such as the Atlantic cod Gadus morhua, can be structured into genetically distinct local populations on a surprisingly small geographic scale considering their dispersal potential. Mechanisms responsible for such small-scale genetic structure may involve homing of adults to their natal spawning grounds, but also local reten- tion of pelagic eggs and larvae. For example, spawning within sheltered fjord habitats is expected to favour local retention of early life stages. Here, we studied the distribution of cod eggs along inshore- offshore transects in 20 Norwegian fjords. The general pattern exhibited across all fjords was a higher concentration of cod eggs inside the fjords than further offshore. In particular, fjords with shal- low sills (model threshold 37 m) show an abrupt reduction in egg density over the sill. This study pro- vides empirical support for an offspring retention hypothesis, which may help to explain the mainte- nance of local population structure in pelagic marine systems.

Journal home page: http://www.int-res.com/journals/meps/ We tested whether the long-term fluctuations in abundance of 0-group cod Gadus morhua and pollack Pollachius pollachius sampled from 1919 to 1994 at 38 stations along the Norwegian Skagerrak coast might be related to the North Atlantic Oscillation (NAO). The short- and long-term fluctuations of both species exhibited different spatial structures and were probably of different origins. The former, which were previously shown to be related to biotic interactions, displayed signifcant highest similarities across the smallest scales (0t o 21 km). In contrast, the long-term fluctuations were similar at a larger scale (equal to or larger than the Norwegian Skagerrak coast) and were probably due to extrinsic factors. We have shown that the climatic conditions along the Norwegian Skagerrak coast and the fluctuations of Calanus finmarchicus in the Skagerrak were closely associated with the NAO. Long-term fluctuations of cod and pollack were, however, not associated linearly or non-linearly with the NAO or C. finmarchicus. Trends of cod and pollack appeared to be related to the long-term fluctuations in the bottom sea grass coverage, mainly constituted by Zostera marina, and it cannot be dismissed that fishing has contributed to the recent decline of these fish populations. We also conclude that the postsettlement period is crucial for these populations because of competition for space.