Cod Productivity Research Articles

Evaluating the potential impacts of grey seal predation and fishery bycatch/discards on cod productivity on the Western Scotian Shelf and in the Bay of Fundy

Abstract The recovery of many groundfish stocks in the Northwest Atlantic has been impeded by elevated mortality among older/larger individuals. The sources of groundfish mortality are generally uncertain, though predation by growing grey seal herds and unreported fishing are thought to be drivers of mortality for Atlantic cod (Gadus morhua) on the Western Scotian Shelf and Bay of Fundy (4X5Y), as well as in nearby ecosystems. We developed a statistical catch-at-age model for 4X5Y cod that accounted for grey seal predation and bycatch/discards and tested the model across a broad range of consumption and discard scenarios. We found that most cod mortality was unexplained by seal predation and fishing when cod comprised a small to moderate portion of the grey seal diet. Predation and fishing accounted for slightly more than half of recent cod mortality when cod was highly prevalent in the seal diet (45% by weight) but explained little historical mortality. Bycatch was a minor source of mortality prior to 2019 and accounted for 0.%1–8.1% of recent mortality. Increased sampling of seal diets and monitoring of emerging seal colonies is necessary for reducing uncertainties around seal predation effects in the Northwest Atlantic.

Disentangling conditional effects of multiple regime shifts on Atlantic cod productivity.

Regime shifts are increasingly prevalent in the ecological literature. However, definitions vary and detection methods are still developing. Here, we employ a novel statistical algorithm based on the Bayesian online change-point detection framework to simultaneously identify shifts in the mean and (or) variance of time series data. We detected multiple regime shifts in long-term (59–154 years) patterns of coastal Norwegian Atlantic cod (>70% decline) and putative drivers of cod productivity: North Atlantic Oscillation (NAO); sea-surface temperature; zooplankton abundance; fishing mortality (F). The consequences of an environmental or climate-related regime shift on cod productivity are accentuated when regime shifts coincide, fishing mortality is high, and populations are small. The analyses suggest that increasing F increasingly sensitized cod in the mid 1970s and late 1990s to regime shifts in NAO, zooplankton abundance, and water temperature. Our work underscores the necessity of accounting for human-induced mortality in regime shift analyses of marine ecosystems.

MAIN RESULTS OF RESEARCH OF THE LABORATORY OF ICHTHYOLOGY OF THE MMBI RAS IN THE ARCTIC SEAS DURING 2015–2019.

The main results of researches of laboratory of ichthyology of the MMBI RAS in the period from 2015 to 2019 are presented. Data on the ichthyofauna of bays and fjords of North-Eastern Greenland, spatial distribution of some species of the Cottidae, Liparidae and Zoarcidae families in the Arctic seas of Russia, as well as on the dynamics of fish diversity in the coastal and sub-coastal zones of Eastern Murmansk are provided. Quantitative data on Barents sea Polar cod productivity, bioenergetic and adaptations of boreal and Arctic fish species were obtained.Aspects of the morphology of the thymus of lumpfish larvae used in aquaculture to control parasitic crustaceans are considered.

Multi-decadal responses of a cod (Gadus morhua) population to human-induced trophic changes, fishing, and climate

Understanding how human impacts have interacted with natural variability to affect populations and ecosystems is required for sustainable management and conservation. The Baltic Sea is one of the few large marine ecosystems worldwide where the relative contribution of several key forcings to changes in fish populations can be analyzed with empirical data. In this study we investigate how climate variability and multiple human impacts (fishing, marine mammal hunting, eutrophication) have affected multi-decadal scale dynamics of cod in the Baltic Sea during the 20th century. We document significant climate-driven variations in cod recruitment production at multi-annual timescales, which had major impacts on population dynamics and the yields to commercial fisheries. We also quantify the roles of marine mammal predation, eutrophication, and exploitation on the development of the cod population using simulation analyses, and show how the intensity of these forcings differed over time. In the early decades of the 20th century, marine mammal predation and nutrient availability were the main limiting factors; exploitation of cod was still relatively low. During the 1940s and subsequent decades, exploitation increased and became a dominant forcing on the population. Eutrophication had a relatively minor positive influence on cod biomass until the 1980s. The largest increase in cod biomass occurred during the late 1970s, following a long period of hydrographically related above-average cod productivity coupled to a temporary reduction in fishing pressure. The Baltic cod example demonstrates how combinations of different forcings can have synergistic effects and consequently dramatic impacts on population dynamics. Our results highlight the potential and limitations of human manipulations to influence predator species and show that sustainable management can only be achieved by considering both anthropogenic and naturally varying processes in a common framework.

Multi-decadal responses of a cod (Gadus morhua) population to human-induced trophic changes, exploitation and climate variability

Understanding how human impacts have interacted with natural variability to affect populations and ecosystems is required for sustainable management and conservation. The Baltic Sea is one of the few large marine ecosystems worldwide where the relative contribution of several key forcings to changes in fish populations can be analyzed with empirical data. In this study we investigate how climate variability and multiple human impacts (fishing, marine mammal hunting, eutrophication) have affected multi-decadal scale dynamics of cod in the Baltic Sea during the 20th century. We document significant climate-driven variations in cod recruitment production at multi-annual timescales, which had major impacts on population dynamics and the yields to commercial fisheries. We also quantify the roles of marine mammal predation, eutrophication, and exploitation on the development of the cod population using simulation analyses, and show how the intensity of these forcings differed over time. In the early decades of the 20th century, marine mammal predation and nutrient availability were the main limiting factors; exploitation of cod was still relatively low. During the 1940s and subsequent decades, exploitation increased and became a dominant forcing on the population. Eutrophication had a relatively minor positive influence on cod biomass until the 1980s. The largest increase in cod biomass occurred during the late 1970s, following a long period of hydrographically related above-average cod productivity coupled to a temporary reduction in fishing pressure. The Baltic cod example demonstrates how combinations of different forcings can have synergistic effects and consequently dramatic impacts on population dynamics. Our results highlight the potential and limitations of human manipulations to influence predator species and show that sustainable management can only be achieved by considering both anthropogenic and naturally varying processes in a common framework.

Hierarchical modelling of temperature and habitat size effects on population dynamics of North Atlantic cod

Abstract Mantzouni, I., Sørensen, H., O'Hara, R. B., and MacKenzie, B. R. 2010. Hierarchical modelling of temperature and habitat size effects on population dynamics of North Atlantic cod. – ICES Journal of Marine Science, 67: 833–855. Understanding how temperature affects cod (Gadus morhua) ecology is important for forecasting how populations will develop as climate changes in future. The effects of spawning-season temperature and habitat size on cod recruitment dynamics have been investigated across the North Atlantic. Ricker and Beverton and Holt stock–recruitment (SR) models were extended by applying hierarchical methods, mixed-effects models, and Bayesian inference to incorporate the influence of these ecosystem factors on model parameters representing cod maximum reproductive rate and carrying capacity. We identified the pattern of temperature effects on cod productivity at the species level and estimated SR model parameters with increased precision. Temperature impacts vary geographically, being positive in areas where temperatures are <5°C, and negative for higher temperatures. Using the relationship derived, it is possible to predict expected changes in population-specific reproductive rates and carrying capacities resulting from temperature increases. Further, carrying capacity covaries with available habitat size, explaining at least half its variability across stocks. These patterns improve our understanding of environmental impacts on key population parameters, which is required for an ecosystem approach to cod management, particularly under ocean-warming scenarios.

The origins of intensive marine fishing in medieval Europe: the English evidence

The catastrophic impact of fishing pressure on species such as cod and herring is well documented. However, the antiquity of their intensive exploitation has not been established. Systematic catch statistics are only available for ca.100 years, but large-scale fishing industries existed in medieval Europe and the expansion of cod fishing from the fourteenth century (first in Iceland, then in Newfoundland) played an important role in the European colonization of the Northwest Atlantic. History has demonstrated the scale of these late medieval and post-medieval fisheries, but only archaeology can illuminate earlier practices. Zooarchaeological evidence shows that the clearest changes in marine fishing in England between AD 600 and 1600 occurred rapidly around AD 1000 and involved large increases in catches of herring and cod. Surprisingly, this revolution predated the documented post-medieval expansion of England's sea fisheries and coincided with the Medieval Warm Period--when natural herring and cod productivity was probably low in the North Sea. This counterintuitive discovery can be explained by the concurrent rise of urbanism and human impacts on freshwater ecosystems. The search for 'pristine' baselines regarding marine ecosystems will thus need to employ medieval palaeoecological proxies in addition to recent fisheries data and early modern historical records.

Productivity and fitness in a fjord cod population: an ecological and evolutionary approach

Two modeling approaches were used to evaluate the combined responses in a fish population to environmental fluctuations and exploitation. Short-term effects of growth and survival of year classes of cod and the effects of fishing were evaluated by using a modified version of a dynamic ecosystem model (DYNECOMAS II), originally developed for extensive mariculture purposes in Masfjorden, western Norway. Simulations indicated that the availability of zooplankton and density-dependent predation and cannibalism were key factors for the carrying capacity for cod in the fjord. On a short-term scale, there was a strong-density-dependent regulation of cod productivity, individual growth and survival, while fisheries had little impact. Long-term effects on the fitness of cod in relation to fishery and environmental variability were evaluated by means of a life history model. The intrinsic rate of increase r was chosen as a measure of fitness. The impact on cod fitness of maturation age, variable survival at egg, larval and 0-group stage (reflecting variable recruitment to age 1) and the degree of density-dependent growth were studied for different entry ages to the fishery for both moderate and high fishing intensities. A modified Euler–Lotka equation was parameterized using available empirical data on growth, age-dependent fishing mortality and fecundity for cod from Masfjorden and DYNECOMAS II model-predicted natural mortality for juveniles (predation mortality). The solution ( r) was found by iteration for combinations of maturing age, fishing mortality and survival probability until 1 year of age. The predictions obtained were that (a) the lower the survival probability until 1 year of age, the lower the fitness; (b) the higher the maturing age the lower the fitness; (c) an early entry to even a moderate fishery resulted in r<0 for low survival probability to age 1; (d) if a high degree of density-dependent growth was associated with an early entry to an intensive fishery ( F=1) from age 1 year and onwards, the surviving individuals from an average year class may not produce enough replace spawners regardless of maturing age, since r<0.