Sexual size dimorphism in three North Sea gadoids

Existing biological data on whiting Merlangius merlangus, cod Gadus morhua and haddock Melanogrammus aeglefinus from a long‐term international survey were analysed to address sexual size dimorphism (SSD) and its effect on their ecology and management. Results show that SSD, with larger females of the same age as males, is a result of higher growth rates in females. A direct consequence of SSD is the pronounced length‐dependent female ratio that was found in all three gadoids in the North Sea. Female ratios of the three species changed from equality to female dominance at specific dominance transition lengths of c. 30, 35 and 60 cm for M. merlangus, G. morhua and M. aeglefinus, respectively. An analysis by area for M. merlangus also revealed length dependence of female ratios. SSD and length‐dependent female ratios under most circumstances are inseparable. Higher overall energy demand as well as a higher energy uptake rate must result from the observed SSD and dimorphism in growth rates. Potential processes related to feeding, locomotion and physiology are proposed that could balance the increased energy investment of females. Potential consequences of SSD and length dependency of female ratios are the reduction of the reproductive potential of a stock due to size‐selective fishing and biased assessment of the true size of the female spawning stock that could distort decisions in fisheries management.     

Climate,plankton and cod

While a few North Atlantic cod stocks are stable, none have increased and many have declined in recent years. Although overfishing is the main cause of most observed declines, this study shows that in some regions, climate by its influence on plankton may exert a strong control on cod stocks, complicating the management of this species that often assumes a constant carrying capacity. First, we investigate the likely drivers of changes in the cod stock in the North Sea by evaluating the potential relationships between climate, plankton and cod. We do this by deriving a Plankton Index that reflects the quality and quantity of plankton food available for larval cod. We show that this Plankton Index explains 46.24% of the total variance in cod recruitment and 68.89% of the variance in total cod biomass. Because the effects of climate act predominantly through plankton during the larval stage of cod development, our results indicate a pronounced sensitivity of cod stocks to climate at the warmer, southern edge of their distribution, for example in the North Sea. Our analyses also reveal for the first time, that at a large basin scale, the abundance of Calanus finmarchicus is associated with a high probability of cod occurrence, whereas the genus Pseudocalanus appears less important. Ecosystem‐based fisheries management (EBFM) generally considers the effect of fishing on the ecosystem and not the effect of climate‐induced changes in the ecosystem state for the living resources. These results suggest that EBFM must consider the position of a stock within its ecological niche, the direct effects of climate and the influence of climate on the trophodynamics of the ecosystem.     

Marking otoliths of Baltic cod (Gadus morhua Linnaeus, 1758) with tetracycline and strontium chloride

Identified were suitable dosages of tetracycline hydrochloride (TET) in three single treatments and three combined treatments of TET with 2 mg/kg strontium chloride (STR) in wild western Baltic cod (Gadus morhua), in terms of (a) obtainable mark qualities (visibility of fluorescent bands), (b) growth assessment, and (c) induced mortality rates. Isotonic NaCl solution was injected in a control group (25 cod per treatment). The results provide the basis for imperative age validation studies of Baltic Sea cod. Cod originating from pound nets near Fehmarn Island were kept in swimming net cages at the harbor of Warnemünde for 1.5 months. Mean initial total length was 28(±3) cm (salinity: 13, water temperature: 13 to 8°C). Overall average growth of surviving cod was 0.8 mm/day. In single TET treatments, lowest mortality rates and best mark quality were observed for TET concentrations of 100 compared to 50 and 25 mg/kg wet mass. Mortality rates of the 100 mg/kg treatment group were remarkably lower than in the control group emphasizing the antibiotic effect of TET. By contrast, the double treatment in the TET‐STR groups resulted in a binding interaction between both markers in the fish body causing either the antibiotic potency being inhibited or TET and STR forming a non‐beneficial chelate (increased mortality), and decreased incorporation of TET in the otolith (reduced visibility of TET bands). Consequently, TET (short‐term marker) and STR (long‐term marker) should not be injected together. Our results demonstrate that the binding interactions between these substances known from homoiotherm animals also apply for poikilotherms such as fish.     

Energy acquisition and retention by age‐0 and age‐1 paddlefish Polyodon spathula (Walbaum, 1792) in relation to size,growth, and rearing conditions in two Great Plains reservoirs and hatchery ponds

The objective of this study was to investigate lipid accumulation and storage in age‐0 and age‐1 paddlefish Polyodon spathula (Walbaum, 1792) in relation to age, stock, year, and growth. Juvenile paddlefish were collected from three locations in North Dakota and Montana, USA, during July and August of 2011 and 2012 and proximate analysis was used to determine lipid content. RNA/DNA ratios were used as an index of growth rates. Differences in age‐based lipid accumulation and storage in juvenile paddlefish suggest a split allocation between growth and lipid storage, with growth being the highest initial priority and emphasis on energy storage occurring at a larger size, later in life. Differences in lipid allocation between stocks indicate that allocation is influenced by hatchery/wild rearing conditions. Differences within and between year‐classes are consistent with field evidence observed in 2012 of a strong 2011 year‐class, and indicate that during productive times, paddlefish may allocate energy to both body growth and lipid reserves, and that allocation differs among years. The lack of a relationship between RNA/DNA ratio and lipid does not support a physiologically exclusive allocation strategy between growth and lipid. Evidence from this and other studies suggests rather that an emphasis on growth, some energy storage, and a large rostrum size in relation to overall fish length in age‐0 and age‐1 fish, may be adaptive in avoiding predation while accruing necessary energy reserves for overwintering. Although this study also provides reference information regarding proximate composition of wild and hatchery origin juvenile paddlefish, much more study is needed into the relationships among growth, low and high lipid groups, lipid allocation in juvenile paddlefish as well as the existence and timing of allocation changes between growth and storage. To aid in understanding paddlefish survival and year‐class strengths, these relationships also need to be linked to inter‐annual differences in early rearing environments for age‐0 and age‐1 fish.     

Growth and mortality of exploited fishes in China's coastal seas and their uses for yield‐per‐recruit analyses

Given the scarcity of information suitable for other forms of fish stock assessments, the growth, and mortality of 10 important marine exploited fishes in China's coastal seas were estimated, based on published length‐frequency data and the ELEFAN approach and software. The resulting parameters, complemented with growth curves from FishBase were then used for yield‐per‐recruit analyses. These 10 species were both traditional commercial species (Trichiurus lepturus, Larimichthys polyactis, Scomber japonicus) and less commercial interest species (Coilia nasus, Engraulis japonicus, Decapterus maruadsi, Psenopsis anomala, Coilia mystus, Setipinna tenuifilis, Harpadon nehereus). The yield‐per‐recruit analyses indicated that the fishing mortality experienced by these 10 fish species was very high, especially in view of the extremely small cod‐end mesh sizes deployed by Chinese trawlers, of about 10 mm or less (vs. 25 mm as minimum legal size). More precisely, all 10 species suffered from growth overfishing, which was very pronounced in large species. Indeed, enlarging cod‐end mesh size, by increasing mean sizes at first capture, would increase yield‐per‐recruit for all 10 fish species by a factor of up to 2.5. Obviously, such increases in mesh size would have to be in tandem with other management measures designed to ensure the sustainability of China's marine fisheries resources.     

Effect of spatial differences in growth on distribution of seasonally co‐occurring herring Clupea harengus stocks

The mechanisms most likely to determine the distribution of the two major herring Clupea harengus stocks in their common early summer feeding ground in the eastern North Sea, Skagerrak and Kattegat were investigated through analysis of acoustic survey data from six consecutive years. No change was detected in biomass of North Sea autumn spawning C. harengus (NSAS) over time, whereas the biomass of western Baltic spring spawning C. harengus (WBSS) declined severely. Analyses of centre of abundance by stock showed no change in NSAS distribution, whereas the WBSS changed to a more western distribution over time. Contrary to previous perception of the juvenile migration, NSAS were found to leave the study area at the age between 1 and 2 years and WBSS 1 year olds were encountered in the Skagerrak. The estimated parameters of von Bertalanffy growth equations showed marked differences between areas with fish in the eastern part of the area having the lowest size at age at all ages. Further, their growth conditions appeared to deteriorate progressively over the period studied. Both NSAS and WBSS showed the highest condition in the North Sea and Skagerrak while condition was substantially lower in age Kattegat. The westward movement of spring spawners over time suggests that growth rate and possibly density of conspecifics influence the migration pattern and distribution of C. harengus in the area. In contrast, there was no evidence to suggest that distribution was constant over time within stocks or that distribution reflected size‐dependent limitations on migration distance.     

Population ecological parameters and stock assessment of Russian sturgeon Acipenser gueldenstaedti Brandt & Ratzeburg, 1833 in the Southern Caspian Sea

Russian sturgeon (Acipenser gueldenstaedti Brandt & Ratzeburg, 1833) is one of the major commercial sturgeon species, but there is no adequate information and previous‐published about population dynamics and stock assessment of this species in the southern Caspian Sea. This paper examines the age structure, growth parameters, maturity, age at first capture, optimum length, natural and fishing mortality and amount of biomass in the southern Caspian Sea (Iranian waters), during a two decades time series period from 1990–1991 to 2008–2009. For a pooled data, the growth parameters were estimated as L_∞ = 214.0 cm, K = 0.054, t₀ = ?4.5 year. Size at fifty percent sexual maturity was at FL = 118 cm for females and 113 cm for males. The age at first capture (t_c) estimated to be 12.1 years. In the catch composition, bulk of the catch (75%) belonged to 13–17 years old. The instantaneous coefficient of natural mortality (M) was estimated as 0.120 year^?1 and the instantaneous coefficient of fishing mortality (F) varied during the 19‐year period between 0.130 to 0.505 year^?1. The biomass showed a descending trend from 1,941.2 mt in 1990–1991 collapsed to about 55 mt in 2004–2005, and then decreased to the lowest level and represented 18.5 mt in 2008–2009. The result revealed that, the stock of Russian sturgeon is being over‐fished. We concluded that to manage the sturgeons stocks, a coordinated regional and international effort are needed to provide immediate implementation of stock enhancement and management in the Caspian Sea.     

Sexual maturation of cod (Gadus morhua L.) in the southern Baltic (1990–2006)

Baltic cod, like other species, is susceptible to inter‐annual fluctuations in sexual maturation, depending on the length, age, sex, extent of the habitat area, and stock abundance of the cod population. Maturity is one of the biological indicators used to detect changes in a stock that can be caused by fishing. To address these issues specifically for the eastern Baltic cod stock, long‐term data (1990–2006) from Polish research vessels in the southern Baltic were examined. To date, the ICES has used the same maturity ogives over extended periods and assumed invariant sex ratios for the assessment of eastern Baltic cod. The combined maturity ogives calculated in the present study were markedly lower, particularly for age groups 2–4 (5), in all periods, than those used in the ICES assessment. Moreover, the proportion of females increased with length and age, suggesting that annual verification of the sex ratio is needed. The present study also revealed that the total length (L_50%) and the age (mean age‐at‐maturity; MAM_50%) at which 50% achieved first sexual maturity were higher for females than for males in the study period. The long‐term mean L_50% and MAM_50% for females were 43.9 cm and 4.3 years, respectively, and for males 34.8 cm and 3.4 years. There was also a spatial difference between calculated maturity ogives, with slightly lower L_50% (range: 1.4–8.6 cm) in the Gdańsk Basin than in the Bornholm Basin. The increasing trend in fishing mortality observed in 1993–2004 (ICES data) did not translate into a temporal trend in calculated maturity ogives. However, changes in L_50% and MAM_50% reflected recruitment variations (ICES data). The significance of these findings is discussed in the context of the environment and recruits abundance.     

Geographic variation in fitness‐related traits of the bladderwrack Fucus vesiculosus along the Baltic Sea‐North Sea salinity gradient

In the course of the ongoing global intensification and diversification of human pressures, the study of variation patterns of biological traits along environmental gradients can provide relevant information on the performance of species under shifting conditions. The pronounced salinity gradient, co‐occurrence of multiple stressors, and accelerated rates of change make the Baltic Sea and its transition to North Sea a suitable region for this type of study. Focusing on the bladderwrack Fucus vesiculosus, one of the main foundation species on hard‐bottoms of the Baltic Sea, we analyzed the phenotypic variation among populations occurring along 2,000 km of coasts subjected to salinities from 4 to >30 and a variety of other stressors. Morphological and biochemical traits, including palatability for grazers, were recorded at 20 stations along the Baltic Sea and four stations in the North Sea. We evaluated in a common modeling framework the relative contribution of multiple environmental drivers to the observed trait patterns. Salinity was the main and, in some cases, the only environmental driver of the geographic trait variation in F. vesiculosus. The decrease in salinity from North Sea to Baltic Sea stations was accompanied by a decline in thallus size, photosynthetic pigments, and energy storage compounds, and affected the interaction of the alga with herbivores and epibiota. For some traits, drivers that vary locally such as wave exposure, light availability or nutrient enrichment were also important. The strong genetic population structure in this macroalgae might play a role in the generation and maintenance of phenotypic patterns across geographic scales. In light of our results, the desalination process projected for the Baltic Sea could have detrimental impacts on F. vesiculosus in areas close to its tolerance limit, affecting ecosystem functions such as habitat formation, primary production, and food supply.     

Forecasting future recruitment success for Atlantic cod in the warming and acidifying Barents Sea

Productivity of marine fish stocks is known to be affected by environmental and ecological drivers, and global climate change is anticipated to alter recruitment success of many stocks. While the direct effects of environmental drivers on fish early life stage survival can be quantified experimentally, indirect effects in marine ecosystems and the role of adaptation are still highly uncertain. We developed an integrative model for the effects of ocean warming and acidification on the early life stages of Atlantic cod in the Barents Sea, termed SCREI (Simulator of Cod Recruitment under Environmental Influences). Experimental results on temperature and CO₂ effects on egg fertilization, egg and larval survival and development times are incorporated. Calibration using empirical time series of egg production, temperature, food and predator abundance reproduces age‐0 recruitment over three decades. We project trajectories of recruitment success under different scenarios and quantify confidence limits based on variation in experiments. A publicly accessible web version of the SCREI model can be run under www.oceanchange.uni-bremen.de/ ;SCREI. Severe reductions in average age‐0 recruitment success of Barents Sea cod are projected under uncompensated warming and acidification toward the middle to end of this century. Although high population stochasticity was found, considerable rates of evolutionary adaptation to acidification and shifts in organismal thermal windows would be needed to buffer impacts on recruitment. While increases in food availability may mitigate short‐term impacts, an increase in egg production achieved by stock management could provide more long‐term safety for cod recruitment success. The SCREI model provides a novel integration of multiple driver effects in different life stages and enables an estimation of uncertainty associated with interindividual and ecological variation. The model thus helps to advance toward an improved empirical foundation for quantifying climate change impacts on marine fish recruitment, relevant for ecosystem‐based assessments of marine systems under climate change.     

The recent population expansion of boarfish,Capros aper (Linnaeus, 1758): interactions of climate,growth and recruitment

The objectives of this study were to evaluate whether temperature changes in the Northeast Atlantic influence the growth and recruitment dynamics of boarfish, Capros aper. Two geographically separate areas were examined, ‘north’ at the northern distribution range west of Ireland and ‘south’ on the main fishing grounds south of Ireland. No significant differences in length‐at‐age were observed between the two areas. Interannual otolith growth patterns were similar between the two areas with distinct years of faster and slower growth. In the ‘north’, no significant relationship between adult growth and temperature was observed, while growth in the ‘south’ was positively related to temperature up to approximately 16°C growth rates were suppressed in the years with temperatures above that. Recruitment showed a positive correlation with adult growth the previous year for the Spanish recruitment index only, suggesting spatial connectivity between the Celtic Sea and the Bay of Biscay. The age distributions were similar in both areas and despite the boarfish's longevity of >30 years, are dominated by the age classes corresponding to the years with high recruitment, suggesting that increased recruitment is responsible for the observed stock expansion.     

Recent trends in the abundance of plaice Pleuronectes platessa and cod Gadus morhua in shallow coastal waters of the Northeastern Atlantic continental shelf – a review

Shallow, near-shore water habitats on the continental shelf of the Northeast Atlantic have been productive fishing areas in the past. Here, we review the present knowledge about (i) recent trends in the abundance of plaice and cod in these habitats and (ii) hypotheses regarding the factors responsible for any trends. At present, only a few studies exist on the trends of abundance of plaice or cod, namely from the Bay of Biscay, the North Sea and the Skagerrak/Kattegat. They suggest a declining abundance in coastal, shallow areas and – at least for plaice – a latitudinal gradient with an erosion of the southern distribution boundary in the Bay of Biscay and deepening of stocks in the North Sea. In contrast, no trend in shallow water abundance of plaice similar to a decline in deep-water stocks during the 1970s and their slow recovery during the 2000s is apparent in the Skagerrak/Kattegat. Although shallow habitats fundamentally differ from deeper areas by the prevalence of juvenile stages, the declining trends coincide with decreasing abundance/landings and spatial stock relocations in the deeper areas. Whether this indicates a common trend pointing at connectivity between shallow and deep water remains open. Fundamental differences exist in the suggested causes of the trends in different geographical areas. High fishing pressure together with low local recruitment apparently prevents the recovery of overexploited plaice and cod stocks in the Skagerrak/Kattegat. In contrast, the responses of juveniles and adult fish to increasing seawater temperature are the main hypotheses for changes in distribution and abundance of both fish species in the North Sea/Bay of Biscay. However, temperature alone cannot explain the observed decline of fish in coastal areas, and the causes may be more complex, involving nutrient loading, primary productivity or food availability, although at present, knowledge of these factors is insufficient.     

Temperature-dependent development rates of cod Gadus morhua eggs

Temperature development relationships were determined for batches of Irish Sea cod Gadus morhua eggs incubated in flow-through incubators. Hatching began 16·4 days after fertilization (DAF) at 6° C, 10·3 DAF at 8° C, 9·4 DAF at 10° C and 7·4 DAF at 12° C. Egg mortality increased at the higher temperatures, but survival was >80%. Results were compared with published data at four comparable stage end points: the end of blastula, the end of gastrula, the point of growth of the embryo completely surrounding the yolk and the point when 50% of the eggs were hatched. All the studies showed a curvilinear relationship between age at stage and temperature. There was a 12 day inter-study difference in time to 50% hatch at 2° C and 4 day difference at 10° C. There were no consistent trends that differentiated eastern v. western, or northern v. southern populations. A single model for cod egg incubation time from fertilization to 50% hatch was derived based on data from six cod populations, but it is recommended that individual stock relationships should be used where possible.     

Multiple ice‐age refugia in Pacific cod,Gadus macrocephalus

Pleistocene ice‐ages greatly influenced the historical abundances of Pacific cod, Gadus macrocephalus, in the North Pacific and its marginal seas. We surveyed genetic variation at 11 microsatellite loci and mitochondrial (mt) DNA in samples from twelve locations from the Sea of Japan to Washington State. Both microsatellite (mean H = 0.868) and mtDNA haplotype (mean h = 0.958) diversities were large and did not show any geographical trends. Genetic differentiation between samples was significantly correlated with geographical distance between samples for both microsatellites (F_ST = 0.028, r² = 0.33) and mtDNA (F_ST = 0.027, r² = 0.18). Both marker classes showed a strong genetic discontinuity between northwestern and northeastern Pacific populations that likely represents groups previously isolated during glaciations that are now in secondary contact. Significant differences appeared between samples from the Sea of Japan and Okhotsk Sea that may reflect ice‐age isolations in the northwest Pacific. In the northeast Pacific, a microsatellite and mtDNA partition was detected between coastal and Georgia Basin populations. The presence of two major coastal mtDNA lineages on either side of the Pacific Ocean basin implies at least two ice‐age refugia and separate postglacial population expansions facilitated by different glacial histories. Northward expansions into the Gulf of Alaska were possible 14–15 kyr ago, but deglaciation and colonization of the Georgia Basin probably occurred somewhat later. Population expansions were evident in mtDNA mismatch distributions and in Bayesian skyline plots of the three major lineages, but the start of expansions appeared to pre‐date the last glacial maximum.     

Climate change and fishing: a century of shifting distribution in North Sea cod

Globally, spatial distributions of fish stocks are shifting but although the role of climate change in range shifts is increasingly appreciated, little remains known of the likely additional impact that high levels of fishing pressure might have on distribution. For North Sea cod, we show for the first time and in great spatial detail how the stock has shifted its distribution over the past 100 years. We digitized extensive historical fisheries data from paper charts in UK government archives and combined these with contemporary data to a time‐series spanning 1913–2012 (excluding both World Wars). New analysis of old data revealed that the current distribution pattern of cod – mostly in the deeper, northern‐ and north‐easternmost parts of the North Sea – is almost opposite to that during most of the Twentieth Century – mainly concentrated in the west, off England and Scotland. Statistical analysis revealed that the deepening, northward shift is likely attributable to warming; however, the eastward shift is best explained by fishing pressure, suggestive of significant depletion of the stock from its previous stronghold, off the coasts of England and Scotland. These spatial patterns were confirmed for the most recent 3½ decades by data from fisheries‐independent surveys, which go back to the 1970s. Our results demonstrate the fundamental importance of both climate change and fishing pressure for our understanding of changing distributions of commercially exploited fish.     

New data on the longevity of coastal cod Gadus morhua Linnaeus, 1758 in the White Sea

The cod, Gadus morhua, is a common and abundant demersal fish in the White Sea coastal zone. The published data on the maximum age of White Sea cod still requires additional documentation, because some authors have noted difficulties in age determination by otoliths of large specimens. To obtain accurate data on the longevity of this species in the White Sea, an image analysis of thin‐sections of otoliths was made for age estimation. Research surveys in the Chupa Inlet and adjacent waters of the Kandalaksha Bay were conducted in June to August from 2007 to 2013. The five largest specimens of a total of 3564 captured fish were selected for age determination. Age of these largest individuals ranged between 7 and 12 years, total length and weight varied from 60.2 to 77.0 cm and from 2.4 to 6.1 kg, respectively. According to the data, maximal White Sea cod longevity is greater than in previously published data, and demonstrates similar longevity to conspecifics from the Baltic and North seas, whereas its life cycle is much shorter than cod from the Northeast Arctic, Iceland, Greenland, Newfoundland and Labrador stocks.     

The stress hormone corticosterone in a marine top predator reflects short‐term changes in food availability

In many seabird studies, single annual proxies of prey abundance have been used to explain variability in breeding performance, but much more important is probably the timing of prey availability relative to the breeding season when energy demand is at a maximum. Until now, intraseasonal variation in prey availability has been difficult to quantify in seabirds. Using a state‐of‐the‐art ocean drift model of larval cod Gadus morhua, an important constituent of the diet of common guillemots Uria aalge in the southwestern Barents Sea, we were able to show clear, short‐term correlations between food availability and measurements of the stress hormone corticosterone (CORT) in parental guillemots over a 3‐year period (2009–2011). The model allowed the extraction of abundance and size of cod larvae with very high spatial (4 km) and temporal resolutions (1 day) and showed that cod larvae from adjacent northern spawning grounds in Norway were always available near the guillemot breeding colony while those from more distant southerly spawning grounds were less frequent, but larger. The latter arrived in waves whose magnitude and timing, and thus overlap with the guillemot breeding season, varied between years. CORT levels in adult guillemots were lower in birds caught after a week with high frequencies of southern cod larvae. This pattern was restricted to the two years (2009 and 2010) in which southern larvae arrived before the end of the guillemot breeding season. Any such pattern was masked in 2011 by already exceptionally high numbers of cod larvae in the region throughout chick‐rearing period. The findings suggest that CORT levels in breeding birds increase when the arrival of southern sizable larvae does not match the period of peak energy requirements during breeding.     

Resource‐driven colonization by cod in a high Arctic food web

1. Climate change is commonly associated with many species redistributions and the influence of other factors may be marginalized, especially in the rapidly warming Arctic.
2. The Barents Sea, a high latitude large marine ecosystem in the Northeast Atlantic has experienced above‐average temperatures since the mid‐2000s with divergent bottom temperature trends at subregional scales.
3. Concurrently, the Barents Sea stock of Atlantic cod Gadus morhua, one of the most important commercial fish stocks in the world, increased following a large reduction in fishing pressure and expanded north of 80°N.
4. We examined the influence of food availability and temperature on cod expansion using a comprehensive data set on cod stomach fullness stratified by subregions characterized by divergent temperature trends. We then tested whether food availability, as indexed by cod stomach fullness, played a role in cod expansion in subregions that were warming, cooling, or showed no trend.
5. The greatest increase in cod occupancy occurred in three northern subregions with contrasting temperature trends. Cod apparently benefited from initial high food availability in these regions that previously had few large‐bodied fish predators.
6. The stomach fullness in the northern subregions declined rapidly after a few years of high cod abundance, suggesting that the arrival of cod caused a top‐down effect on the prey base. Prolonged cod residency in the northern Barents Sea is, therefore, not a certainty.

     

Effects of varying natural mortality and selectivity on the assessment of eastern Baltic cod (Gadus morhua Linnaeus, 1758) stock

In 2014, the International Council for the Exploration of the Sea (ICES) was unexpectedly unable to provide an analytical assessment of eastern Baltic cod stock; factors such as data issues, assessment methodology, and the ecological situation of cod were indicated as the reasons for this failure. Some evidence suggests that the natural mortality (M) of cod could increase substantially in forthcoming years and that the selectivity could change. In this paper, age‐structured and stock‐production assessment models were applied to simulate the dynamics of cod stock; in the models, both constant and increasing natural mortalities were permitted. In the age‐structured model, the effects of selectivity related to the cod size on the cod assessment were also analysed. In addition, stock with characteristics similar to Baltic cod stock and increasing natural mortality was generated and assessed with the age‐structured model using both constant and increasing M. It was shown that models with increasing natural mortality of cod in recent years perform much better than models with constant natural mortality in terms of the distribution of residuals and retrospective patterns. The models with size‐dependent selectivity did not perform better than other standard assessments. The assessment of generated stock (where natural mortality was increasing) with constant natural mortality in the assessment model showed a poor distribution of residuals and strong retrospective patterns, similar to the ICES assessment with constant M. The conducted simulations strongly suggest that the main reason for the poor recent cod assessment is the increase in natural mortality, which is not considered in the assessment methodology.     

Sex‐specific food intake in whiting Merlangius merlangus

In this study, the topic of sexual growth dimorphism in whiting Merlangius merlangus is examined. To understand the magnitude and underlying mechanisms, North Sea International Bottom Trawl Survey (IBTS) data and two additional datasets from the third quarter of 2007 and the first quarter of 2012 were analysed. Merlangius merlangus displays distinct differences in growth parameters between males and females, with females reaching a higher asymptotic length (L_∞) than males. To identify the mechanisms which lead to higher growth in females, the quantity and the quality of the diet of M. merlangus in the North Sea were investigated to compare the sex‐specific energy uptake levels. The diet composition did not differ between the sexes, but females had higher stomach content masses than males of the same total length (L_T), and showed lower proportions of empty stomachs. Moreover, female M. merlangus had higher liver and empty stomach masses compared with males of the same size, which indicates additional sex‐specific differences in the metabolic costs and energy allocation patterns. Finally, interannual differences were found in the stomach contents, the share of empty stomachs and liver masses of M. merlangus in the North Sea.