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COSEWIC assessment and update status report on the Atlantic Cod in Canada

Limiting Factors and Threats

The primary factor responsible for the decline of Atlantic cod was overfishing (Hutchings and Myers 1994; Hutchings 1996; Myers et al. 1997b; Shelton and Lilly 2000; Hutchings and Ferguson 2000a,b; Bundy 2001; Fu et al. 2001; Smedbol et al. 2002).  In some areas, reductions in individual growth (Dutil and Lambert 2000; Drinkwater 2002), attributable to environmental effects or size-selective fishing mortality (Sinclair et al. 2002), may have exacerbated the rate of population decline; in some areas, increases in natural mortality may also have contributed to the decline (Dutil and Lambert 2000).  It is important to note, however, that there is no evidence to suggest that the rates of growth and natural mortality experienced by cod in the 1980s were unprecedented.  Although hypotheses invoking other factors have been posited, there are inadequate data that would allow for definitive tests of these hypotheses.

In theory, removal of the dominant source of anthropogenic mortality (fishing) should have resulted in population recovery.  However, with one exception (St. Pierre Bank cod in the northeastern portion of the range of the Gulf/Maritimes population), recovery has not been forthcoming in the decade since the fisheries were initially closed.  Empirical analyses of these issues suggest that factors other than fishing are of greater importance to recovery than fishing alone.

Recent work indicates that lack of recovery is not at all unusual among marine fish populations that experience 15-year rates of decline greater than 80% (Hutchings 2000, 2001a), even when associated with dramatic reductions in fishing mortality (Hutchings 2001b; Denney et al. submitted).  However, although the factors responsible for the lack of recovery in Atlantic cod are not well understood, recent analyses implicate some combination of the following:  below-average water temperatures (in some areas, in some years), fishing quotas, illegal fishing practices, bycatch, marine mammal predation, genetic changes to life history effected by fishing (see Hutchings 1999; Sinclair et al. 2002), and predation of eggs by pelagic fishes (Hutchings 1999, 2000, 2001b; Swain and Castonguay 2000; Swain and Sinclair 2000; McLaren et al. 2001; Smedbol et al. 2002).

The tremendous influence that even small catches (in the absolute sense) can have on recovery can be illustrated by considering the lack of recovery for northern cod (NAFO Divisions 2J3KL) in the Newfoundland & Labrador population.  Despite having experienced declines in excess of 97%, DFO accepted the advice of the Fisheries Resource Conservation Council (www.frcc.gc.ca) and allowed directed fishing on this stock from 1999 through 2002, a measure that has had a clear and deleterious impact on the recovery of Canada's most depleted cod stock, as illustrated by the exploitation rates presented below.

Although the quotas in recent years have been small relative to those set in the mid-1980s (5,600-9,000 tonnes compared with quotas exceeding 200,000 tonnes), the impact on northern cod has been high because the size of the population is correspondingly low.  This conclusion is based on mark-recapture estimates of exploitation rate analysed by DFO (DFO 2002; Brattey and Healey MS2003).  A summary of these estimates is provided in the table below (these data were obtained from Table 4 in Brattey and Healey MS2003, unpublished data, presented at DFO's Zonal Assessment Meeting, Halifax, 20 February 2003).

Tagging location (northeast coast of Newfoundland)Exploitation rates on northern cod tagged and recaptured in 1999Exploitation rates on northern cod tagged and recaptured in 2002
Notre Dame Bay30-63%12-20%
Bonavista Bay6-18%14-19%
Trinity Bay4-13%10-27%

Based on these estimates of exploitation rate, one can only conclude that the northern cod quotas reintroduced in 1999 have had a negative impact on the growth and recovery of northern cod.  This conclusion is based on two observations.  Firstly, prior to the cod collapses in the early 1990s, it was estimated that cod populations could increase in abundance at exploitation rates less than 18%.  Many of the exploitation rates reported above would, thus, permit little or no population growth.  Secondly, maximum rates of population increase for northern cod have been estimated at 10 to 30% per annum (Myers et al. 1997a; Hutchings 1999).  Given that the range in exploitation rate estimates typically falls within this 10-30% range of maximum population growth, population recovery will not occur, irrespective of the number of individuals in the population.

Based on the proceedings of a DFO workshop held in St. John’s, Newfoundland, in March 2000 (Swain and Castonguay 2000), the lack of recovery of Atlantic cod in Canadian waters south of Cape Chidley can be attributed to:

1.        Collapsed age structure, loss of spawning components (e.g., the spring-spawning component on the Eastern Scotian Shelf), and/or reduced area occupied by spawners;

2.        Below-average recruitment rate in some parts of the range (Southern Grand Bank, St. Pierre Bank, Eastern and Western Scotian Shelf), but not others (NE Newfoundland Shelf, Northern and Southern Gulf of St. Lawrence);

3.        Higher-than-expected natural mortality of adults in some parts of the range of each population;

4.        Decline in individual growth rate in some areas within each population.

If physical structure is critically important to the survival of juvenile cod, notably in the form of plants, bottom heterogeneity, and corals, then reductions in suitable habitat may also be affecting recovery.  The reduction in physical heterogeneity on the bottom, and the loss of potentially important deep-sea corals, can be attributed to the bottom-trawling gear used to catch groundfish (Collie et al. 1997, 2000; Kaiser and de Groot 2000).  In Canadian waters, it has been reported that bottom-trawling may have relatively little impact on invertebrate macrofauna inhabiting sandy bottoms (Kenchington et al. 2001), although similar studies on the potential effects of trawling on fishes and fish habitat have not been undertaken (but see Mortensen et al. submitted).

Regarding predation by marine mammals, an independent expert panel concluded that the recoveries of northern cod in the Newfoundland & Labrador population and that of northern Gulf cod in the Laurentian North population have been negatively affected by seal predation (McLaren et al. 2001).

The possibility that the intense fishing pressure experienced by cod in the late 1980s and early 1990s resulted in genetic changes to heritable life history traits cannot be discounted.  There is evidence to suggest that age at maturity and growth rate is lower in several cod stocks at present than it was prior to the stock collapses (Smedbol et al. 2002).  The observed changes in age at maturity cannot be explained as phenotypic responses (Hutchings 1999), leaving genetic responses to selection (selecting against late-maturing genotypes) as the most parsimonious explanation for the earlier maturity observed in some areas.  Similarly, smaller weights-at-age among cod in some areas can also be explained as a result of selection against fast-growing genotypes during periods of intensive fishing (see Sinclair et al. 2002).  Of course, in order to test these hypotheses of genetic change, one would have to compare, in a common-garden experiment, the ages at maturity and growth rates of random samples of genotypes before and after the selection event (i.e., fishing in the late 1980s and early 1990s).  The obvious unavailability of the former renders the testing of these hypotheses impossible.

In summary, the primary cause of the reduction of Atlantic cod throughout its range was over-exploitation; in some areas, the rate of decline may have been exacerbated by reductions in individual growth and apparent increases in natural mortality.  Identifiable threats to the recovery of Atlantic cod include directed fishing (a consequence of the setting of quotas), indirected fishing (a consequence of illegal fishing, catch misreporting, discarding), and bycatch from other fisheries for bottom-dwelling species.  Additional threats to recovery include altered biological ecosystems, and concomitant changes to the magnitude and types of species interactions (such as an increase in cod mortality attributable to seal predation).  These ecosystem-level changes appear to have resulted in increased mortality among older cod.  Selection against late maturity and rapid growth rate, induced by previously high rates of exploitation, may also be contributing to the higher mortality (effected by earlier maturity; Beverton et al. 1994) and slower growth observed in some areas today.