Northern Cod Stock Research Articles

Equilibrium, transient dynamics and sustainable reference points under age-specific natural mortality rates and varying levels of population productivity: The case of the Northern cod stock

Scientific advisory bodies provide scientific advice for sustainable fisheries management based on the precautionary approach and maximum sustainable yield (MSY) reference points, such as spawning stock biomass (SSB) value Blim, and fishing mortality giving MSY, FMSY. The lack of a stock-recruitment function (SRF) to identify a clear breakpoint Blim has recently emerged in important stock collapses. It also precludes the use of equilibrium-based methods to analyze the sustainability of FMSY. Considering a hockey stick (HS) SRF, we propose here an equilibrium-based method that characterizes the equilibriums, their stability properties, transient dynamics, and changes in productivity (including age-specific natural mortality rates). We show that these relevant factors, not taken into account in standard methods, should play a central role in fisheries management and conservation. Considering the Northern cod stock (NCS) (Gadus morhua) by way of illustration, we properly estimate the HS and its associated Blim. We find that the HS fitted by the Fisheries Library in R underestimates Blim. Additionally, we determine the levels of productivity (medium-low or medium-high), and their corresponding growth rates of the SSB, which are consistent with the observed population dynamics. We find that the NCS was managed during the 1980s under myopic (unsustainable) harvest control rules, neglecting high age-specific natural mortality rates. We also find that recovery of the NCS remains a distant prospect, despite the current stable, positive equilibrium (sustainable FMSY).

Temporal evolution of critical traits and their relationship to cod stock collapse and recovery

The North Atlantic Fisheries Organization response to a precipitous decline of the Flemish Cap cod ( Gadus morhua) stock during the 1990s was the imposition of an 11 year moratorium on directed fishing for cod; recovery followed. Over the three decades that encompassed the precollapse, collapse, and recovery stages, we found that the cod stock status was characterized by four traits: spawning stock biomass, maturity-at-age, weight-at-age, and recruitment. The temporal evolution of these traits was consistent with a density-dependent conceptual model suggesting that phenotypic plasticity was at play during the rebuilding of the stock. The temporal progression of the broader fish community paralleled that of cod, underlying its key ecosystem position. The same demographic variables defined the state of the adjacent Northern Cod stock that underwent a similar pattern of decline, an intermittent moratorium but only partial recovery. This partial recovery is possibly related, in part, to declines of prey species brought about by excessive harvesting after the cod collapse and an apparent collapse of capelin, a major dietary component.

Indexing starvation mortality to assess its role in the population regulation of Northern cod

Starvation is a ubiquitous process in nature as all animals depend on finite resources to survive. Limited food resources can lead to starvation-induced mortality and, depending on the scale of the limitation, population-level consequences may emerge. However, attempts to model starvation-induced mortality are rare as it is difficult to isolate the effects of specific sources of natural mortality. Using commonly collected data on body condition, we develop a statistical approach to index starvation-induced mortality by estimating the proportion of individuals experiencing severe emaciation in a fish population. This approach accounts for seasonal patterns in body condition, allowing data collected at any time of year to be included. Using Canada’s Northern cod (Gadus morhua) stock as a case study, we found a positive association between rates of natural mortality estimated by an integrated population model and the index of mortality derived from proportions of cod in poor condition. This index is also associated with the availability of two key prey species, capelin (Mallotus villosus) and Northern shrimp (Pandalus borealis). Taken together, our results indicate that starvation-induced mortality represents an important component of the natural mortality experienced by Northern cod, supporting the idea that insufficient food contributed to the collapse of the stock in the early 1990s and, subsequently, limited its recovery. These implications highlight the practical relevance of explicitly considering prey availability (e.g., through body condition or prey abundance indices) to improve stock assessment modeling and aid the development of actionable ecosystem-based fisheries management strategies.

Life-stage-dependent supergene haplotype frequencies and metapopulation neutral genetic patterns of Atlantic cod, Gadus morhua, from Canada's Northern cod stock region and adjacent areas.

Among highly migratory fish species, nursery areas occupied by juveniles often differ from adult habitats. To better understand the spatial dynamics of Canada's Northern cod stock, juveniles caught off the east coast of Newfoundland and Labrador were compared to adults from the same region as well as individuals from other areas in Atlantic Canada using double-digest restriction site-associated DNA sequencing-derived single nucleotide polymorphisms. A reduced proportion of homozygotes with a chromosomal inversion located in linkage group 1 (LG1) was detected between juvenile and adult samples in the Northern cod stock region, potentially indicating age-dependent habitat use or ontogenetic selection for attributes associated with the many genes located in LG1. No selectively neutral genetic differences were found between samples from the Northern cod stock; nevertheless, significant differences were found between some of these samples and cod collected from St. Pierre Bank, Bay of Fundy, Browns Bank and the southern Scotian Shelf. Clustering analysis of variants at neutral loci provided evidence for three major genetic units: (a) the Newfoundland Atlantic Coast, (b) eastern and southern Gulf of St. Lawrence and Burgeo Bank and (c) the Bay of Fundy, Browns Bank and southern Scotian Shelf. Both adaptive and neutral population structure within the Northern cod stock should be considered by managers to promote demographic rebuilding of the stock.

Golden cod persist through climate change

This “golden cod” (left) from Gilbert Bay, off the coast of Labrador, Canada, is easily recognized by commercial harvesters because of its distinctive “golden” coloration; this fish is technically an Atlantic cod (Gadus morhua), though it is much darker than cod from the adjoining northern population (right). Atlantic cod is a population-rich species, incorporating tremendous intraspecific biodiversity, and is prized by North Atlantic fisheries. The northern cod stock, which collapsed and was placed under a fishing moratorium in 1992, has not recovered owing in part to colder-than-normal ocean temperatures in the Labrador Sea. Remarkably, while northern cod experienced high rates of natural mortality during the 1990s, the Gilbert Bay cod population grew. This golden cod spent its entire life (~17 years) in Gilbert Bay, living 6 months of the year just meters beneath sea ice in sub-zero temperatures. It therefore experienced much colder conditions, and for longer time periods, than other Atlantic cod. During summer, however, golden cod take advantage of the short growth season in the shallow waters that warm quickly within the protection of the bay.

Temperature influences on growth of unfished juvenile Northern cod (Gadus morhua) during stock collapse

AbstractThis study examined growth of unfished juvenile Northern cod (Gadus morhua) off Newfoundland concomitant with stock collapse in the cold early 1990s. Two unpublished data sets were examined, one from collapse‐period trapping sites along the northeast coast of Newfoundland and one from a post‐collapse inshore trawl survey. Cumulative surface and bottom temperatures were significant predictors of growth rates of the young fish with year‐classes born during collapse experiencing slower growth than those born during subsequent warming. Relationships between accrued temperature and growth were consistent across periods, with slow growth of collapse‐period fish reflecting slower accumulation of temperature‐at‐age. Temperature influences were spatially broad‐based with no significant differences in growth rates for fish captured along the entire northeast coast of Newfoundland. Predicted differences in growth rates for collapse versus recovery year‐classes were proportional to cumulative surface temperatures but not cumulative bottom temperatures. Although significant, temperature effects on growth were relatively unimportant at youngest ages. Overall, growth differences between periods were small but large differences occurred between slowest and fastest growing year‐classes. The results suggest initial responses to increasing temperatures were delayed following collapse. We conclude that although temperature was an important determinant of dampened productivity that it alone cannot account for the collapse and slow recovery of the stock. This is the first known study to directly quantitatively link temperature impacts to an unfished component of the Northern cod stock complex during collapse, removing need for implicit assumptions about whether or not cold conditions contributed to the collapse of this iconic fish stock.

Transient dynamics: Equilibrium, collapse, and extinction in age-structured models. The case of the Northern cod stock

Equilibrium-based methods are widely used in the literature on population ecology to analyze the sustainability of the management strategy. This approach fails to account for many factors such as age-specific natural mortality rates, the cohort survival rates, the possibility of extinction, the characterization of the equilibriums, their stability properties, and transient dynamics. We propose here a method, based on the theory of non-linear dynamic systems, that determines necessary and sufficient conditions for the stability of the equilibrium in age-structured models, using time-independent survival rates (autonomous) and age-specific natural mortality rates. In the case of a hockey stick stock-recruitment function, the method characterizes the equilibriums, their stability properties, and transient dynamics. Depending on the cohort survival rates, we find that there are two opposite scenarios: extinction or positive equilibrium. In the latter case, we also find the possibility of collapse with slow recovery. We demonstrate that the stock tends to equilibrium at an exponential rate in both scenarios. Considering the Northern cod (Gadus morhua) stock by way of illustration, we find that slow recovery of the stock could be expected at sufficiently low cohort survival rates, despite the fact that the condition for the stability of the equilibrium was met during the moratorium period (positive equilibrium). This result is consistent with the species’ lack of expected recovery. In contrast to equilibrium-based methods under constant natural mortality rates and the precautionary approach framework, we also find the possibility of extinction at sufficiently low cohort survival rates, even in the absence of harvesting.

A state-space stock assessment model for northern cod, including under-reported catches and variable natural mortality rates

A state-space assessment model for the northern cod (Gadus morhua) stock off southern Labrador and eastern Newfoundland is developed here. The model utilizes information from offshore trawl surveys, inshore acoustic surveys, fishery catch age compositions, partial fishery landings, and tagging. This is done using an approach that avoids the use of subjective data-weighting. Estimates of fishing mortality rates (F) are usually conditional on assumptions about natural mortality rates (M) in stock assessment models. However, by integrating much of the information on northern cod, it is possible to estimate F and M separately. It is also possible to estimate a change in the offshore survey catchability by including inshore acoustic biomass estimates. The proposed model also accounts for biased total catch statistics, which is a common problem in stock assessments. The main goal of the model is to provide realistic projections of the impacts of various levels of future fishery catches on the recovery of this stock. The projections incorporate uncertainty about M and catch. This is vital information for successful future fisheries. The model has been developed for the specific data sources available for northern cod, but it could be adapted to other stocks with similar data sources.

The role of harp seals, fisheries and food availability in driving the dynamics of northern cod

The Atlantic cod Gadus morhua population off Newfoundland collapsed in the early 1990s due to over-exploitation, and despite marked reduction in fishing effort the stock remains depressed. Harp seal Pagophilus groenlandicus predation has been repeatedly proposed as an explanation for this lack of recovery, but other hypotheses include reduced prey availability and/or food quality (i.e. lack of capelin Mallotus villosus), as well as fisheries catches and environ- mental effects. Using a bioenergeticsallometric model, we gauged the relative contributions of these drivers on the lack of recovery and dynamics of the northern cod stock. Biomass dynamics were best explained by a combination of fisheries removals and capelin availability, whereas seal consumption was found not to be an important driver of the northern cod stock. Prey availability was linked to reduced somatic condition during the 1990s and 2000s. We discuss evidence that suggests that cod may be experiencing depensatory dynamics, but not related to a 'predator pit effect'. Our study suggests that trophic control is bottom-up, and that a depressed capelin stock could be a serious impediment for cod rebuilding.

Mechanistic insights into the effects of climate change on larval cod

Understanding the biophysical mechanisms that shape variability in fisheries recruitment is critical for estimating the effects of climate change on fisheries. In this study, we used an Earth System Model (ESM) and a mechanistic individual-based model (IBM) for larval fish to analyze how climate change may impact the growth and survival of larval cod in the North Atlantic. We focused our analysis on five regions that span the current geographical range of cod and are known to contain important spawning populations. Under the SRES A2 (high emissions) scenario, the ESM-projected surface ocean temperatures are expected to increase by >1°C for 3 of the 5 regions, and stratification is expected to increase at all sites between 1950-1999 and 2050-2099. This enhanced stratification is projected to decrease large (>5μm ESD) phytoplankton productivity and mesozooplankton biomass at all 5 sites. Higher temperatures are projected to increase larval metabolic costs, which combined with decreased food resources will reduce larval weight, increase the probability of larvae dying from starvation and increase larval exposure to visual and invertebrate predators at most sites. If current concentrations of piscivore and invertebrate predators are maintained, larval survival is projected to decrease at all five sites by 2050-2099. In contrast to past observed responses to climate variability in which warm anomalies led to better recruitment in cold-water stocks, our simulations indicated that reduced prey availability under climate change may cause a reduction in larval survival despite higher temperatures in these regions. In the lower prey environment projected under climate change, higher metabolic costs due to higher temperatures outweigh the advantages of higher growth potential, leading to negative effects on northern cod stocks. Our results provide an important first large-scale assessment of the impacts of climate change on larval cod in the North Atlantic.

Detecting the presence of depensation in collapsed fisheries: The case of the Northern cod stock

Motivated by the evidence that many collapsed stocks have failed to recover despite the fact that fishing mortality has been reduced, or even when a moratorium is in effect, we develop a methodological approach using splines to analyze the stochastic population dynamics of fish stocks at low stock levels. Considering the aggregate Northern cod stock by way of illustration, we find that the species’ lack of recovery, despite the moratorium which still remains in force, is consistent with the hypothesis of depensatory population dynamics at low population sizes, as opposed to the compensation estimated by the conventional regression methods used in classic bioeconomic models.

The Newfoundland Fishery and Economy Twenty Years after the Northern Cod Moratorium

on July 2, 1992, Newfoundlanders received a terrible shock. The federal Minister of Fisheries and oceans, Newfoundland’s own John C. Crosbie, banned the commercial directed fishing of the northern cod stock, historically one of the great fisheries of the world. According to the initial estimates, which proved conservative, 9,000 fishermen and 10,000 plant workers would be displaced and eligible for emergency assistance payments of $225 per week.1 Nobody who saw the Minister’s press conference will ever forget the vision of angry fishermen trying to enter the room while the police hustled Crosbie down a back staircase. The fishermen were angry at losing their livelihood while being offered a derisory replacement income. Crosbie reported that since 1990, the spawning stock biomass had fallen by threequarters “due primarily to ecological factors” and a two-year moratorium would be sufficient for the stock to recover to its long-term average.2 More than 20 years have passed since that fateful day. The northern cod stock did not recover in two years, nor has it recovered in twenty. As might be expected, during the 20 years since the moratorium was first declared, there have been dramatic changes in

Tracking and Protecting Marine Species at Risk: Scientific Advances, Sea of Governance Challenges

The state of many marine species in the world's oceans is increasingly gloomy. The northern cod stock off Newfoundland, once thought to be inexhaustible, is now listed as endangered by the Committe...

National Government Responses to Marine Stewardship Council (MSC) Fisheries Certification: Insights from Atlantic Canada

Over the last decade, the proliferation of social and environmental certification programmes has attracted the attention of a growing number of political scientists interested in new forms of ‘private’ transnational governance. However, we still lack analyses on the nature and extent of different state responses to and involvement in new private transnational governance arrangements in particular sectors and in different jurisdictions. This article advances our understanding of the interactions between nation-state and private transnational modes of governance by analysing the role of national government authorities in Marine Stewardship Council (MSC) fisheries certification in Atlantic Canada, known more for the disastrous collapse of Northern cod stocks than good marine stewardship. Focusing on the 2008 certification of Northern shrimp (Pandalus borealis) fisheries off the Province of Newfoundland and Labrador, the analysis finds that the implementation and maintenance of MSC certification in this case depended on significant support from government authorities. The delicate legitimacy of both authorities faces a period of uncertainty in this case since some certified shrimp stocks appear to be in decline and perhaps also migrating northward off Newfoundland and Labrador.

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

AbstractThe 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.

Comparison of the response of Atlantic cod (Gadus morhua) in the high-latitude regions of the North Atlantic during the warm periods of the 1920s–1960s and the 1990s–2000s

Concern about future anthropogenic warming has lead to demands for information on what might happen to fish and fisheries under various climate-change scenarios. One suggestion has been to use past events as a proxy for what will happen in the future. In this paper a comparison between the responses of Atlantic cod (Gadus morhua) to two major warm periods in the North Atlantic during the 20th century is carried out to determine how reliable the past might be as a predictor of the future. The first warm period began during the 1920s, remained relatively warm through the 1960s, and was limited primarily to the northern regions (>60°N). The second warm period, which again covered the northern regions but also extended farther south (30°N), began in the 1990s and has continued into the present century. During the earlier warm period, the most northern of the cod stocks (West Greenland, Icelandic, and Northeast Arctic cod in the Barents Sea) increased in abundance, individual growth was high, recruitment was strong, and their distribution spread northward. Available plankton data suggest that these cod responses were driven by bottom-up processes. Fishing pressure increased during this period of high cod abundance and the northern cod stocks began to decline, as early as the 1950s in the Barents Sea but during the 1960s elsewhere. Individual growth declined as temperatures cooled and the cod distributions retracted southward. During the warming in the 1990s, the spawning stock biomass of cod in the Barents Sea again increased, recruitment rose, and the stock spread northward, but the individual growth did not improve significantly. Cod off West Greenland also have shown signs of improving recruitment and increasing biomass, albeit they are still very low in comparison to the earlier warming period. The abundance of Icelandic cod, on the other hand, has remained low through the recent warm period and spawning stock biomass and total biomass are at levels near the lowest on record. The different responses of cod to the two warm events, in particular the reduced cod production during the recent warm period, are attributed to the effects of intense fishing pressure and possibly related ecosystem changes. The implications of the results of the comparisons on the development of cod scenarios under future climate change are addressed.

Did over-reliance on commercial catch rate data precipitate the collapse of northern cod?

Abstract It has been suggested that a number of “lessons” can be learned from the collapse of the northern cod stock off Newfoundland and Labrador. However, not all purported lessons have been validated with available data. One lesson is thought to be that over-reliance on commercial catch rate data and an incorrect assumption regarding the functional relationship between catch rate and population size were major contributors to overestimating stock size, precipitating the collapse. The current study describes calibration approaches used in assessments, and evaluates alternative functional relationships between commercial catch rates and stock size. In addition, historical population size is re-estimated using only research vessel data and compared with estimates obtained based on both commercial catch rate and research vessel data. Calibration with commercial catch rate contributed to overestimating stock size in some years, but there is no evidence that the assumed functional relationship between commercial catch rate and population size was a significant factor in the collapse.

The Newfoundland fishery: ten years after the moratorium

Abstract Excessive overfishing combined with negative environmental conditions led to the decimation of the northern cod stock and its closure to commercial fishing in 1992. During the ensuing dozen years, the stock has not recovered. This paper considers the current state of the Newfoundland economy and the Newfoundland fishery and asks whether the lessons of the earlier day have been learned. The answer appears to be no. The main mover of the economy is exogenous to the fishery, the unemployment rate remains high, currently more than twice the Canadian rate, and the population of the province has fallen by ten percent. With respect to the fishery, although the decimated groundfish have been replaced by shellfish and the nominal value of landings has doubled since 1992, the fishery still has excess capacity, fishermen and fish plant workers generally earn low incomes, and the fishery remains heavily subsidized.

Offshore spawning and year-class strength of northern cod (2J3KL) during the fishing moratorium, 1994-1996

Two new surveys have been conducted in recent years to assess the status of the northern cod (Gadus morhua) stock (2J3KL). An offshore acoustic survey in spring sampled concentrations of spawning adults and estimated potential fecundity. A pelagic juvenile fish survey carried out in late summer measured cod year-class strength prior to settlement. Spawning biomass of offshore cod decreased from 11 700 t in 1994 to 2000 t in 1995 to 900 t in 1996 with a corresponding decrease in eggs spawned. This decline was mirrored in the distribution and abundance of pelagic juvenile cod. Natural mortality estimated between the egg and pelagic juvenile stages increased from –0.068·day–1 in 1994 to –0.084·day–1 in 1995 to –0.112·day–1 in 1996. The decline in year-class strength of cod appeared to result primarily from a reduction in the number of offshore spawners compounded by an increase in natural mortality during the egg and larval stages. The decline in spawners occurred 2 years after the implementation of a fishing moratorium and cannot be linked to fishing. The rapid decline in abundance of adult fish combined with increased natural mortality during the egg and larval drift is consistent with depensation.

Interpreting the collapse of the northern cod stock from survey and catch data

Sequential population analysis reconstructions of the northern cod (Gadus morhus) stock using survey and catch data formed the basis for the scientific advice for substantial reductions in catch quotas in the late 1980s. However, the model failed to provide acceptable reconstructions of the population in subsequent stock assessments when data for the 1990s were added. The model assumes constant age-independent instantaneous rate of natural mortality, accurate catch reporting, and a constant linear relationship between the survey index at age and the population size at age. It seems likely that one or more of these assumptions failed to hold around the time of the collapse. Diagnostic studies were carried out to determine the magnitude of the departure from the assumptions required to allow the model to fit the data. Information related to changes in natural mortality, fishing activity, and survey catchability is reviewed to evaluate the plausibility of departures from model assumptions of the magnitude estimated. It is concluded that unreported deaths caused by the offshore fishery may be most plausible as the main contributing factor to lack of model fit but that factors such as increased natural mortality, and possibly changes in survey catchability, also played a role.