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Table of contents

Areas occupied by each stock are taken from Myers et al. Mean bottom temperatures are from Brander and Myers et al.

Percentage of total biomass is the fraction of total biomass that comes from the age range used to estimate mean weight-at-age. The range of ages used in estimating total biomass and mean individual weight for each stock is given in columns 5 and 6, respectively, and the percentage of total biomass which the age groups included in the mean weight represent is in column 7. Data sources, in which are details of sampling and methods, are given in the right-hand column.

Trends in population biomass for each cod stock are represented by total biomass. This is the product of population number- and weight-at-age, the former being derived from catch-at-age using virtual population analysis VPA , the latter from sampling of commercial and research survey catches. This range was chosen to represent the most abundant and best sampled ages in the catch, i. Weight-at-age state is not intended to represent growth rate in this analysis, but changes in weight-at-age provide evidence of changes in growth. The open squares along the x -axis represent periods of prolonged decline in total biomass.

Data sources can be found in ICES The Canadian shelf stocks are the upper six on the left. The top panel on the left shows the depth-averaged 5-year means of temperature at Stn 27 on the Newfoundland shelf Colbourne and Anderson, The quality and the completeness of the time-series were erratic prior to , so almost all the data used are for subsequent years. Changes to VPAs, which are in many cases updated annually, alter the estimates of numbers and stock biomass, but for earlier years such retrospective changes are small and do not affect the main trends.

Mean annual bottom temperature for each stock is taken from Brander and Myers et al. The depth-averaged 5-year running mean of water-column temperature for the Newfoundland Shelf Stn 27 is from Colbourne and Anderson Total biomass of cod per unit area was calculated using estimates of habitat area from Myers et al. The distribution in the Barents Sea is truncated by a thermal limit and in the Baltic by a salinity limit.

The decline in biomass in these six stocks was in every case preceded by a decline in mean weight-at-age. The Canadian cod stocks share other common patterns of change in total biomass and weight-at-age. Both declined until about , followed by a substantial increase to the early or mids.

Weight-at-age increased in all Canadian stocks after , but total biomass only increased in some. The southernmost stock in the Northwest Atlantic is the shared stock on Georges Bank. It underwent a prolonged period of decline in total biomass from to , but not in weight-at-age.

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With the exception of the Celtic Sea, Northeast Atlantic cod stocks also declined after , but the declines do not share a common pattern and are much smaller than those among the Canadian stocks. In the Northeast Atlantic there have also been a number of periods of rapid increase in total biomass. On both sides of the North Atlantic, changes in mean weight-at-age are smaller in the warmer water stocks Georges Bank, North Sea, Irish Sea, Celtic Sea , and there are no prolonged periods of decline in weight-at-age in any of these areas, or in the Baltic.

As total population biomass is the product of number-at-age and individual weight-at-age, a change in weight-at-age will other things being equal result in an immediate and proportional change in biomass. However, a density-dependent relationship would cause weight-at-age to decline as biomass increased, so a negative relationship between weight-at-age and biomass may be regarded as evidence of density-dependent effects Lorenzen and Engberg, Seven of the significant relationships are positive. The only negative relationship is for the Eastern Baltic stock.

The models do not take temporal autocorrelation into account. All North Atlantic cod stocks have declined in total biomass since , with the exception of the stock in the Celtic Sea. Overfishing is a major cause of decline in every case, but there may be other contributing factors. Could a change in weight-at-age cause a lagged decline in stock biomass?

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I suggest two explanations and proposals for further modelling. First, the allocation of energy to routine metabolism, somatic growth, and reproduction may change Nisbet et al. Dynamic energy budget models would be suitable for exploring this process Nisbet et al. Second, when weight-at-age changes, the effect of fishing on the stock is altered in several ways. For example, a total allowable catch TAC intended to harvest a defined proportion of the stock will only do so if the weight-at-age has been correctly predicted.

An unanticipated decline in weight-at-age would cause fishing mortality to be greater than intended more small fish being caught. Moreover, fishers dump fish of poor condition highgrading , resulting in additional mortality Kulka, Even before the underlying processes are revealed, it would seem worthwhile to monitor persistent changes in mean weight-at-age, because a decline may provide early information of increased risk of stock decline. All stocks have experienced periods of increasing total biomass, which are generally attributed to one or more years of good recruitment.

For example, temperature has a direct effect on growth rate, but also an indirect effect by altering the production of food organisms ICES, Another reason why it is difficult to assign causes with confidence is the poor quality of field information on the contributing factors; for want of more precise, directly observed information, various proxies are used. Interannual changes in temperature may be represented by mean bottom temperature at a fixed station or by a value for the area occupied by cod during an annual fishing survey i.

Observed changes in weight-at-age have been attributed to all of these causes and to their combinations in different stocks.

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The prima facie evidence of patterns of change in weight-at-age and total biomass, which are similar to this temperature pattern, point to a causal connection. Temperature influences growth of cod, and evidence from experiments and in the field shows that the effect of temperature change is progressively greater at low temperatures Bjornsson and Steinarsson, ; Brander, ; Folkvord, This may explain why the warm-water stocks Georges Bank, North Sea, Irish Sea, Celtic Sea show less variability in weight-at-age than the cold-water stocks.

More detailed studies of the growth changes in individual Canadian cod stocks support the existence of a temperature effect, but also the effects of prey availability Krohn et al. Other published work Dutil et al. Unlike in the Northwest Atlantic, where the oceanographic and biological environment of the Canadian shelf is dominated by changes in the Labrador Slope Current, Northeast Atlantic stocks do not show common patterns of change in weight-at-age.

For some Northeast Atlantic stocks, there is evidence that temperature causes changes in weight-at-age Brander, ; Ottersen and Loeng, , but it is difficult to distinguish direct temperature effects from the associated effects on production or dynamics of forage species, such as capelin Michalsen et al. Interannual variability in capelin Mallotus villosus abundance seems to play a major role in changes in weight-at-age in the Arcto-Norwegian and Icelandic cod stocks and is important in the Northwest Atlantic too Rose and O'Driscoll, The Eastern Baltic is the only stock with a significant negative relationship between weight-at-age and stock abundance, but this has been ascribed to the closely coupled predator—prey relationships between cod and its principal prey species there, sprat Sprattus sprattus and herring Clupea harengus , rather than to density-dependence Gislason, Growth may be density-dependent in situations where there is competition for a limited supply of food or some other essential item, but direct evidence of such competition is difficult to obtain.

A convincing, process-based case for density-dependent growth in those stocks probably requires detailed information on aggregation, food availability, and spatial dynamics. A recent meta-analysis found evidence that density-dependent growth was a key mechanism in the regulation of fish populations Lorenzen and Engberg, However, among the nine marine stocks which they analysed, five did not show evidence of density-dependent growth, including the only cod stock North Sea. As their meta-analysis only included biomass, it did not consider alternative explanations for the changes observed in growth, such as prey abundance Krohn et al.

Thanks are due to Henrik Sparholt for useful discussions and to anonymous referees for suggesting improvements. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide.

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Sign In or Create an Account. Sign In. Advanced Search. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents. Trends in cod biomass and weight-at-age. Patterns of decline. Causes and consequences of changes in weight-at-age. Weight-at-age does not increase as biomass declines. Human Resources Development Canada. House of Commons. Standing Committee on Fisheries and Oceans. Chair: George S. Creating new wealth from the Sea.

Fish, Food and Allied Workers Union. Gien, Lan. Perceived impact of the fishery closure on health. Occasional Paper. Harris, Michael. Lament for an ocean : the collapse of the Atlantic cod fishery : a true crime story. Ommer, Rosemary. Sustainability of communities of fish and fishers in Canada. Sinclair, Peter R. Leaving and staying : Bonavista residents adjust to the moratorium. John's, Nfld. Occasional paper. Summit of the Sea Core Conference : St. John's, Newfoundland : Conference Proceedings.

Canning, Patricia M. Children and families in communities with high levels of economic uncertainty : summary report for parents and teachers. John's, Nfld: P. Canning, Peer review of some analyses of the status of northern cod conducted by Dr. Pynn, Ralph M.