Modeling the effects of temperature on the survival and growth of North Sea cod (Gadus morhua) through the first year of life

Temperature and body size are widely agreed to be the primary factors influencing vital rates (e.g., growth, mortality) in marine fishes. We created a biophysical individual‐based model which included the effects of body size and temperature on development, growth and mortality rates of eggs, larvae and juveniles of Atlantic cod (Gadus morhua L.) in the North Sea. Temperature‐dependent mortality rates in our model were based on the consumption rate of predators of cod early‐life stages. The model predicted 35%, 53% and 12% of the total mortality to occur during the egg, larval and juvenile stages, respectively. A comparison of modeled and observed body size suggested that the growth of survivors through their first year of life is high and close to the growth rates in ad libitum feeding laboratory experiments. Furthermore, our model indicates that experiencing warmer temperatures during early life only benefits young cod (or theoretically any organism) if a high ratio exists between the temperature coefficients for the rate of growth and the rate of mortality. During the egg stage of cod, any benefit of developing more rapidly at warmer temperatures is largely counteracted by temperature‐dependent increases in predation pressure. In contrast, juvenile (age‐0) cod experiences a higher cumulative mortality at warmer temperatures in the North Sea. Thus, our study adds a new aspect to the ‘growth–survival’ hypothesis: faster growth is not always profitable for early‐life stages particularly if it is caused by warmer temperatures.     

Growth changes in bighand thornyhead <Emphasis Type="Italic">Sebastolobus macrochir</Emphasis> off the Pacific coast of northern Honshu,Japan

ABSTRACT:     The biomass of bighand thornyhead Sebastolobus macrochir was increased by the high recruitment success of the 1999–2002 year classes off the Pacific coast of northern Honshu, Japan. In this study, the growth of bighand thornyhead was examined over a 9-year period from 1996 to 2004 in this area. The growth of the 1999 year class and the 2000–2002 year classes was reduced at 3 and 2 years old, respectively, while the 1999–2002 year classes were smaller than the 1993–1998 year classes. In 2-, 3- and 4-year-old fish, the relationship between abundance and mean standard length was expressed by negative linear regressions, while fish became smaller when abundance of the year class was larger. Mean bottom temperatures were stable at depths of 350–900 m; variations in water temperature were small in the main distribution area of bighand thornyhead. We discuss the factors affecting the growth of bighand thornyhead via changes in the demersal fish community and feeding habits.     

When reproductive value exceeds economic value: an example from the Newfoundland cod fishery

The Northern Cod (Gadus morhua) fishery supported the removal of approximately 200 000 tonnes per year for centuries until collapse in the late 1980s and closure in 1992. Long recovery times, on the order of a decade or more, are now known to be regular concomitant of steep population declines in fish populations. We investigated reproductive value as an alternative to economic value in assessing fisheries sustainability. Our analysis showed that in cod, price‐driven heavy fishing (i.e. fishing mortality positively related to body mass and its dollar value) dramatically impaired reproductive capacity by sacrificing future egg production of large fish. Management based upon current value (either in terms of biomass or equivalently dollar value) substantially underestimates the value of large individuals to the stock; this drove the Northern Cod stock towards collapse by failing to protect individuals with high future value. Our results provide a general explanation for the erosion, collapse and prolonged recovery of a long‐lived species where reproductive value increases rapidly with increasing size. Failure to compute and communicate future value relative to current dollar value to resource harvesters leads to unrealistic perceptions of sustainable resource use.     

Environmentally driven forecasts of northern rock sole (Lepidopsetta polyxystra) recruitment in the eastern Bering Sea

Northern rock sole recruitment in the eastern Bering Sea has been hypothesized to (a) depend on wind‐driven surface currents linking spawning and nursery areas, (b) be density‐dependent, and (c) be negatively impacted by cold bottom temperatures over a large nursery area during the first summer of life. A suite of models was developed to test these hypotheses. Data included 32 years of recruitment and spawning biomass estimates derived from a stock assessment model and wind and temperature indices customized to the environmental exposure of age‐0 northern rock sole in the eastern Bering Sea. The predictive ability of the models was evaluated, and the models were used to forecast recruitment to age‐4 for recent year classes which are poorly retained by the standard multi‐species bottom trawl survey gear. Models which included wind and temperature indices performed better than a naïve forecast based on the running mean. The best‐performing model was a categorical model with wind and temperature thresholds, which explained 49% of the variation in recruitment. Ricker models performed more poorly than models without a spawning biomass term, providing no evidence that recruitment is related to stock size. The models forecast higher recruitment for the most recent year classes (2015–2018) than for prior year classes with observed poor recruitment (2006–2013). These environment‐based recruitment forecasts may improve recruitment estimates for the most recent year classes and facilitate study of the effects of future climate change on northern rock sole population dynamics.     

Discrete spatial dynamics in a marine broadcast spawner: Re-evaluating scales of connectivity and habitat associations in Atlantic cod (Gadus morhua L.) in coastal Newfoundland

We examined the spatial dynamics of Atlantic cod, Gadus morhua, through its life history on the northeast and southeast coasts of Newfoundland in order to delineate dispersal and critical habitat requirements as potential determinants of appropriate spatial scales for management. Hydroacoustic surveys across the northeast Newfoundland shelf in 1990, 1995, and 2000 showed the distribution of the remaining large aggregations of cod to be concentrated in inshore embayments, which is contrary to historic patterns. Inshore surveys on the south coast identified local spawning aggregations at specific geographic locations, to which adults homed in subsequent years. Dispersal of eggs and larvae to the offshore occurred during spring, but was localized (within bay) in the warm summer months. Abundance of age-0 cod increased with proximity to spawning location inshore, suggesting self-recruitment within local embayments. Age-2–3 juveniles actively selected structurally complex habitat at scales of less than tens of kilometers, with home ranges typically less than 10 ha. Juveniles were associated with specific habitat types, especially at age 0, in both lab and field experiments. Our results suggest that self-recruitment and critical life-history events (e.g., spawning, settlement, and maturity) may occur within local embayments, which currently assume much greater importance to overall cod stock dynamics than in the past. Given evidence for limited dispersal, and the inshore presence of critical habitat for early life-history stages, spatial management tools that consider local dynamics may provide a greater degree of protection for coastal Newfoundland cod than previously thought.     

Exploring the temperature optima and growth rates of Atlantic cod at the south‐easterly limit of its range

Four size groups of juvenile farmed Celtic Sea cod (2.7–41.8 g) were reared at a range of constant temperatures (8–19°C). The optimum temperature for growth (T_opt.G) decreased from 15.1°C for ~3 g fish to 12.5°C for ~42 g fish. A comparison of these results with those published for a more northerly population (Icelandic) suggests that there is no significant difference in the optimal temperature for growth of cod stocks within the size range studied. In contrast, the growth rates of Celtic Sea cod were lower than those derived from these established models (for a northerly stock) for small juveniles (<5 g), but similar for larger fish (>40 g). Thus, while T_opt.G appears fixed across the range, there may be high plasticity in local growth performance throughout the Holarctic distribution. Some possible explanations for these differences are considered.     

Discrimination between Icelandic cod (Gadus morhua L.) populations from adjacent spawning areas based on otolith growth and shape

Studies on Atlantic cod (Gadus morhua L.) have demonstrated extensive variation in life history characteristics among populations exposed to different environmental conditions. In Iceland, cod sampled from adjacent spawning areas, within the main spawning area at the southwest coast, exhibit variation in vital life history characteristics. In this study, we investigated the stock structure of Icelandic cod on the main spawning grounds to discriminate between these adjacent spawning groups by using otolith growth and shape. Otolith growth was based on annual increment width estimated for the major age groups (6–7 year olds) of the spawning stock. Otolith shape was determined using Fourier analysis and compared among cod sampled from the different spawning areas with stepwise canonical discriminant analysis (CDA). We found significant differences in growth and otolith shape between adjacent spawning groups of cod where those sampled close to the coast differed from those that were sampled further out on the bank and continental shelf. Our results suggest that these cod may belong to different populations and the large and fast growing cod spawning in the coastal area may need special protection given their significance to the overall productivity of the stock.     

Temperature‐based targeting in a multispecies fishery under climate change

Temperature controls important physiological processes in fish and determines aspects of their niches. In an effort to inform selective fishing and spatiotemporal management in the U.S. Northeast Multispecies fishery, we used 16 years of data from the Northeast Fisheries Science Center Spring and Fall Scientific Trawl Surveys to determine if bottom temperature can be used to differentiate the distribution of Atlantic cod (Gadus morhua) from other species within the fishery management plan (FMP). We identified two separate regimes in spring temperatures and used empirical cumulative distribution functions to calculate biomass availability by temperature for each species. We applied a bagged approach to find optimum thermal threshold values that maximize the difference in cod biomass from each of the other species. For our study area, 38% to 54% of the species considered were well separated from cod by temperature in spring, whereas only 17% were separable in the fall. This study suggests that temperature targeting can be used seasonally to separate cod from many other species in the FMP including top catches and no‐retention species. The use of temperature targeting may allow fishermen to better meet multiple quotas while avoiding choke species. Our results also suggested increasing thermal overlap between cod and species inhabiting higher median temperatures (e.g., spiny dogfish, Squalus acanthias) under the current warming temperature regime. These results indicate that the ability to selectively fish in the US Northeast Multispecies fishery will become more difficult under a warming ocean.     

Effects of Added Shelter and Stocking Density on Growth of Sleepy Cod Oxyeleotris lineolatus in Ponds

Sleepy cod Oxyeleotris lineolatus is a species of freshwater goby in demand in Australian markets by consumers of Asian origin. It is related to marble goby Oxyeleotris marmoratus , the most expensive freshwater food fish in Asia, which is cultured throughout southeast Asia in ponds and cages. The performance of sleepy cod in culture conditions was investigated to assess the viability of farming them in northern Australia. Sleepy cod fingerlings (62.8 ± 0.8 mm total length and 2.56 ± 0.095 g) were stocked into experimental ponds at 32,857 fish/ha, and grown out for 8 mo. Shelter was provided in each of three replicate ponds and was absent in three control ponds. The provision of shelter in juvenile growout was found to be of no benefit, although fish in ponds provided with shelter weighed slightly more per unit length than fish in ponds without shelter. Cannibalism was not a problem in growout, and survival was close to 100%. After the shelter trial was completed, fish were graded into large and small classes (three replicates of each), and grown out without shelter at the same density for 158 d. Following that, fish were again graded, and the largest 30% retained from growout at a density of 8,857 fish/ha (large, 198 ± 6.44 g) or 10,000 fish/ha (small, 48.9 ± 1.27 g). These were grown out for 188 d. Growth of selected stock at low densities was slower than earlier growth rates, although smaller fish gained weight more rapidly than larger fish. Growth rates were better than the only published data for marble goby. Further investigation into high density culture and different genotypes of sleepy cod needs to be undertaken to determine the viability of pond culture.     

The Irish sea trout enhancement programme: a review of the broodstock and ova production programmes

The Irish sea trout, Salmo trutta L., broodstock programme was instigated in response to the collapse of sea trout stocks in the west coast of Ireland in 1989 and 1990. Wild sea trout kelts and post-smolts were successfully reconditioned and used as broodstock to produce eyed ova for distribution to affected fisheries. From 1991 to 1999, a total of 8.2 million green ova from four separate stocks were produced. The number of females stripped increased from 34 in 1991 to a peak of 1435 in 1997. Green ova survival to the eyed ova stage ranged from 65 to 96%. The eyed ova produced were distributed to 23 affected fisheries along the west coast. The average cost per eyed ova decreased from €2.02 at the start of the programme to €0.04 at the end. The growth rates recorded for broodstock held in captivity were comparable with those recorded for wild sea trout. Relationships between fish size, egg size and number of eggs produced were examined for each stock and for each stock in each year. Significant positive relationships were found between fish size and egg number and fish size and egg size, with the exception of the first year in which fish were stripped, when a negative correlation between fish size and egg size was found.     

Evaluating spatio‐temporal variability in the habitat quality of Atlantic cod (Gadus morhua) in the Gulf of Maine

Reduced abundance and contracted spatial distribution of Atlantic cod, Gadus morhua, in the Gulf of Maine (GOM) may indicate large spatio‐temporal variation in their habitat quality. Season‐specific Habitat Suitability Index (HSI) models were developed to quantify such variation in the offshore GOM management area. Data used were non‐zero cod catch rates with calibrations from the Northeast Fisheries Science Center (NEFSC) spring and fall bottom trawl surveys over the period 1982–2013 and key physical environmental variables including depth, bottom temperature, bottom salinity and sediment types. Significant declines were found in the average HSI across the study area in the springs of early 2000s and 2010s. These low average HSI values coincide with reduced age‐1 recruitment of GOM cod stock after the mid‐1990s. Moreover, the western coastal areas of the GOM generally exhibited higher average HSI values than the eastern coastal areas, whereas the offshore areas always had the lowest average HSI. Relatively higher cod survey catch rates in the western GOM may imply positive influences of environmental controls on the distribution of GOM cod.     

Age and growth of sea perch (Helicolenus percoides) from two adjacent areas off the east coast of South Island,New Zealand

A validated ageing methodology using otolith thin sections is presented for sea perch (Helicolenus percoides) in two adjacent areas off the east coast of South Island, New Zealand. Oxytetracycline marking of two adult fish held in captivity for 1 year suggested that a single dark growth increment formed during winter, commencing May or June. The annual formation of growth increments was confirmed by the observed progression of year classes in comparable samples from three different years, and the progression of length modes in several consecutive years that matched the estimated growth curves. Otolith interpretation was difficult because of a central dense region and the presence of many fine bands within each growth increment, but the latter could be counted, and it is considered that a validated ageing procedure is presented. Von Bertalanffy growth parameters were calculated for east coast South Island (ECSI) and Chatham Rise males and females. In both areas the growth rate of males is slightly but significantly faster than for females. Growth is relatively slow throughout life. From the maximum observed ages for ECSI and Chatham Rise fish of 35 and 59 years, the respective natural mortality rates are estimated to be 0.12 and 0.07 year^?1, but the most plausible value for the species is likely to be at the lower end of this range. Some stock differentiation between the two areas is indicated, based primarily on differences in length-frequency distributions implying different patterns of year class strengths, but also on apparent differences in growth rates between areas. This supports the present management regime of separate commercial catch quotas.     

Changes in spawning stock structure strengthen the link between climate and recruitment in a heavily fished cod (Gadus morhua) stock

Atlantic cod (Gadus morhua) is one of the commercially most important fish species in the North Atlantic and plays a central role in several ecosystems. Fishing pressure has been heavy over a prolonged period and the recent decades have shown dramatic decline in abundance of many stocks. The Arcto‐Norwegian (or North‐east Arctic) cod stock in the Barents Sea is now the largest stock of Atlantic cod. Recruitment to this stock has varied extensively during the last 60 yr. There is evidence for fluctuations in climate, particularly sea temperature, being a main cause for this variability, higher temperatures being favourable for survival throughout the critical early life stages. Our studies of time series present compelling evidence for a strengthening of the climate–cod recruitment link during the last decades. We suggest this is an effect of the age and length composition of the spawning stock having changed distinctly. The age of the average spawner has decreased by more than 3 yr from between 10 and 11 in the late 1940s to 7–8 in the 1990s, average length from just above 90 cm to around 80 cm. The number of age classes contributing to the spawning stock has also decreased, while the number of length groups present increased slightly. Significant decrease in age of spawners has frequently been described for other heavily fished stocks worldwide. We therefore find it likely that the proposed mechanism of increased influence of climate on recruitment through changes in the spawning stock age and size composition is of a general nature and might be found in other systems.     

Use of tag data to compare growth rates of Atlantic coast striped bass stocks

Migratory stocks of Atlantic coast striped bass, Morone saxatilis (Walbaum), range primarily from North Carolina (NC) northward to Canadian waters. Between 1986 and 2000, 267 045 wild striped bass were tagged and released from NC to Massachusetts as part of the Cooperative Striped Bass Tagging Program. Direct measurements of growth of individual fish can be obtained from tag data and are useful for understanding the dynamics of fish populations. Growth rates from regressions of length‐increment vs. time‐at‐liberty were estimated for striped bass tagged and released in three southern states [NC, Virginia (VA) and Maryland (MD)] and three northern states (New Jersey, New York and Rhode Island). Striped bass tagged in waters of northern states grew faster (significantly steeper regression slopes) than those tagged in southern areas. Migratory patterns, stock mixing, and unmeasured biotic and abiotic influences on growth precluded conclusions that observed growth patterns are stock‐specific. These results, however, indicate latitudinal differences in growth rates, and should be considered in future research and management of Atlantic coast striped bass.     

Greenland cod (Gadus morhua): modeling recruitment variation during the second half of the 20th century

Two approaches were used to qualify observed variability in Greenland cod (Gadus morhua) recruitment. In the first analysis, we used the linear trend of the Greenland cod recruitment time series and climatic variables, such as air temperatures from the Denmark Strait and wind conditions off East Greenland and Southwest Greenland, to explain the interannual variation in cod recruitment off Greenland. The model resulting from this ‘trend/environmental approach’, explained 79% of the interannual variation in cod recruitment off Greenland. In the second, analytical approach, the ‘regime approach’, multiple linear regression models were used, with the input data being the time series of cod recruitment and spawning stock biomass (SSB) from Iceland and Greenland, sea surface and air temperatures around Greenland, and zonal wind components between Iceland and Greenland. Model results indicated that, during the decades between 1950 and 1990, there were three different cause–effect regimes which significantly influenced the variability of cod recruitment. The three regimes included: (a) the 1950s and 1960s, a regime with favorable sea surface temperatures and a self‐sustaining cod stock off Greenland with high SSB that produced a series of above‐average, strong year classes; (b) the 1970s and 1980s, a regime of declining SSB and recruitment, with recruitment dependent on advection from Iceland; and (c) the 1990s, when the advective potential for recruitment from the Icelandic cod stock was the only available source for replenishment of the Greenland cod stocks, because cod recruitment in Greenland waters was negligible. The three models explained 76–77% of the observed interannual variation in cod recruitment off Greenland. Both approaches suggested that advective factors were the dominant influences for cod recruitment in the ‘Iceland–Greenland System’.     

Interannual variation in pelagic juvenile rockfish (Sebastes spp.) abundance – going with the flow

We report results from 28 yr of a midwater trawl survey of pelagic juvenile rockfish (Sebastes spp.) conducted off the central California coast. The fishery‐independent survey is designed to provide pre‐recruit indices of abundance for use in groundfish stock assessments. Standardized catch rate time series for 10 species were developed from delta‐generalized linear models that include main effects for year, station, and calendar date. Results show that interannual fluctuations of all 10 species are strongly coherent but highly variable, demonstrating both high‐ and low‐frequency components. A similarly coherent result is observed in the size composition of fish, with large fish associated with elevated catch rates. In contrast, spatial and seasonal patterns of abundance show greater species‐specific differences. A comparison of the shared common trend in pelagic juvenile rockfish abundance, derived from principal components analysis, with recruitments from five rockfish stock assessments shows that the time series are significantly correlated. An examination of oceanographic factors associated with year‐to‐year variability indicates that a signature of upwelled water at the time of the survey is only weakly related to abundance. Likewise, basin‐scale indices (the Multivariate El Niño‐Southern Oscillation Index, the Pacific Decadal Oscillation, the North Pacific Gyre Oscillation, and the Northern Oscillation Index) are poorly correlated with abundance. In contrast, sea level anomalies in the months preceding the survey are well correlated with reproductive success. In particular, equatorward anomalies in the alongshore flow field following the spawning season are associated with elevated survival and poleward anomalies with poor survival.     

Long‐term growth patterns in a pond‐dwelling population of crucian carp,Carassius carassius: environmental and density‐related factors

Abstract Scale samples from crucian carp, Carassius carassius (L.), collected over a 10‐year period from a population in an ornamental pond were used to examine patterns in growth as a function of environmental factors, including water temperatures. Back‐calculated standard lengths (SL) at age differed between year classes in most cases. Annual SL increments were related to temperature and age using a non‐linear growth model. Growth declined with increasing age, whereas annual SL increments increased with increasing water temperature. The best‐fit model was with the total number of summer days when air temperature exceeded 20 °C (using water temperature equivalents of air temperature). A growth model including water temperature, age, year class, relative condition and rainfall was a better fit than other models. Year‐class strength was positively correlated with water degree‐days, and year‐class strength negatively affected annual SL increments. The results indicate that within a strong year class of crucian carp, the growth of individual fish is reduced compared with weak year classes, suggesting that density negatively affects growth in ponds where resources are limited.     

The influence of wind and temperature on the catch rate of the American lobster (Homarus americanus) during spring fisheries off eastern Canada

The effects of wind and temperature on catch rate of American lobster (Homarus americanus) in the Baie des Chaleurs and off Cape Breton Island in Eastern Canada were investigated. Data on lobster catch and the number of trap hauls were available through a fishermen's volunteer logbook program, bottom temperatures were measured from thermistors either moored nearby or placed inside lobster traps and wind measurements were obtained from local airports. In the Baie des Chaleurs and off the east coast of Cape Breton, a positive and significant correlation was found between the mean temperature change during the 24 h prior to the traps being hauled and the change in the average catch of lobsters per trap haul. Catch rates rose with increasing bottom temperatures and fell with declining bottom temperatures. Higher correlations between changes in temperature and catch rates occurred at sites where the temperature variance was greater. The short‐term fluctuations in lobster catch rates corresponding to temperature changes are hypothesized to result from behavioral changes affecting lobster activity. In both study areas, the large temperature variability was mainly forced by alongshore winds producing upwelling and downwelling, consistent with a classical Ekman response. The effect of the winds on lobster catch is shown to be principally due to their influence on ocean bottom temperatures. Along the south coast of Cape Breton, no relationship was found between catch rates and either temperature or wind, perhaps because lower lobster abundance resulted in a lower signal‐to‐noise ratio. The results of this study qualitatively support the observations by fishermen of a wind‐induced effect on lobster catch rates.     

Variation in fish condition between Atlantic cod (Gadus morhua) stocks, the effect on their productivity and management implications

A comparative analysis of the fish condition (Fulton’s K) of 10 cod stocks in the north Atlantic in relation to the temperature of their habitat, growth rates and their reproductive potential is presented. It is shown that the cod stocks in the north Atlantic display different levels of mean condition, which is partly due to the different temperature regimes of their habitats. Cod living in colder waters, e.g. Greenland, Labrador and Grand Bank stocks, were found to be in poorer condition than cod living in warmer waters, e.g. North Sea and Irish Sea stocks.

Poor condition causes reduced productivity in terms of slow growth and low recruitment potential. Stocks in better condition display significantly higher weights at age 4 than stocks in poor condition. The a coefficients (function’s slopes) obtained from standardised Ricker’s recruitment–spawning stock biomass (SSB) relationships were defined as indicators for the recruitment potential of stocks. These a coefficients were found to be positively correlated with the mean condition factor of the 10 stocks analysed. This indicates that stocks consisting of individuals in poor condition appear to be very susceptible to reduced recruitment at low SSB, while the stocks that consist of fish in good condition seem to behave more robustly with a higher probability of good recruitment at low SSB. The positive effect of the cod condition on their reproductive potential generally implies that the stocks in good condition in the temperate regions of the northeast and west Atlantic can sustain higher exploitation rates than stocks in poor condition in the colder regimes of the northwest Atlantic (Greenland, Labrador and Grand Bank). This is confirmed by the positive relationship established between the estimated biological management reference points F_med and the mean cod condition factors, as well as by the recent status of these stocks.     

Effects of successive size grading on the growth of juvenile European sea bass (Dicentrarchus labrax L.) at three temperatures

Juvenile European sea bass from the same fish stock were selected by successive size grading processes using 2, 3 and 4 mm bar graders at 79, 96 and 99 days post hatching, thus forming three groups (n=300) consisting of similar‐sized fish that differed by time of each group formation. The growth patterns of fish groups were studied at three temperatures during 5 weeks of rearing. Three‐way anova followed by the Tukey multiple comparison test (P<0.05) showed a high dependence of growth on the temperature applied. The smallest size and weight of fish were detected in all groups reared at 19 °C compared with fish held at 21 and 23 °C respectively. Differences in coefficients of variation of lengths were small and insignificant between groups and temperatures. Growth in the length of sea bass juveniles during the test period was a linear function of time and no differences were observed in growth rate among groups at a particular temperature. Growth rates of fish were 0.71 ± 0.02, 0.62 ± 0.01 and 0.52 ± 0.02 mm day^?1 at 23, 21 and 19 °C respectively. These results indicated that the variations in body size of juveniles in the test period were not the result of differences in the growth potential of individuals.