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

Appendix 1: Northern Labrador

Population trend data for each of the Atlantic cod management units recognized by the Department of Fisheries and Oceans

 

Northern Labrador(NAFO Divisions 2GH):

There are very few population abundance data available for cod in this area.  The available data are complicated by the fact that surveys prior to 1986 were line transect surveys (i.e., involving non-random sampling) whereas those conducted thereafter were stratified-random surveys.  The data reported here (Figure 10) are those reported by DFO in the publication authored by Smedbol et al. (2002).  Based on an estimated age at maturity of 5.25 years for the period 1947 to 1950 (Smedbol et al. 2002), northern Labrador cod have declined 95% over the past three generations, based on the available data.

 

Northeast NewfoundlandShelf, or “Northern” Cod (NAFO Divisions 2J3KL):

The average age among individuals in the current cohort is 6.2 years, based on the survey catch rate data (Lilly et al. 1991; Peter Shelton, DFO, Newfoundland, personal communication).  Based on data from the 1980s, age at 50% maturity is about 6years for northern cod (Lilly et al. 2001).  Thus, in an unfished state, generation time is estimated to be 6+(1/0.2) = 11years, yielding a three-generation time period of 33 years.

The survey catch rate data are those reported by Lilly et al. (1991) and by Peter Shelton (personal communication).  Abundance of the mature portion of the population, as estimated by VPA, are available from Baird et al. (1992) for the years 1962 to 1977 and from Bishop et al. (1993) for the years 1978 to 1992.  Spawning population size for the years 1993 through 2001, for which VPA-based estimates are unavailable, was extrapolated from the survey-based spawner biomass index provided by Lilly et al. (1991; Fig. 13).  To undertake this estimation, I first regressed the (log) survey catch rate data (numbers per tow for individuals age 5 years and older) against the (log) VPA abundance estimates for the same age classes for the years 1983 to 1992.  The correlation coefficient (r=0.74) associated with this regression was significant (p=0.01).  I then used this regression, incorporating the survey catch rate data from 1993 to 2001, to estimate numbers of individuals for the years 1993 to 2001.

Irrespective of the data source, the 3-generation rate of decline experienced by northern cod exceeded 95% (Figure 11).

Age (yr) at maturityGeneration
time (yr)
Data SourceData typeTime periodRate of change
611surveycatch rate
(# per tow)
1983-2001−99.9%
  VPAnumbers of spawners1968-2001−97%


Figure 11.  Temporal variation in the estimated number of mature individuals in the Northern cod stock (NAFO Divisions 2J3KL).

Figure 11.  Temporal variation in the estimated number of mature individuals in the Northern cod stock (NAFO Divisions 2J3KL).

Among other things, this has resulted in an extraordinary decline in the overall abundance of cod in Canadian waters.  In the early 1960s, approximately 75-80% of the cod in Canadian waters was located in NAFO areas 2J, 3K, and 3L (see Figure 12); in 2001, approximately 20% of Canada's cod were located in the same area.  Of note is the fact that I have used ages 5 years and older for my calculation of the size of the breeding population of northern cod.  The reason for doing so is based on the observation that significant numbers of cod 5 and 6 years of age in this stock are mature (Lilley et al. 2001).  In contrast, recent estimates of the size of the breeding population of northern cod by DFO (Smedbol et al. 2002) included only those fish aged 7 and older.  Thus, although the trends in abundance are the same, my estimates will over-estimate breeding population size while those presented by DFO can be considered under-estimates.

Figure 12.  Temporal variation in the estimated number of mature individuals in Canada’s Atlantic cod stocks, showing the size of the Northern cod stock (2J3KL), part of the Newfoundland & Labrador population, relative to the sizes of the other stocks.

Figure 12.  Temporal variation in the estimated number of mature individuals in Canada’s Atlantic cod stocks, showing the size of the Northern cod stock (2J3KL), part of the Newfoundland & Labrador population, relative to the sizes of the other stocks.

Area of occupancy:

Between 1983 and 2001 (the range of the reported data), area of occupancy declined from approximately 275,000 km2 to approximately 215,000 km2, a rate of decline of roughly 22% (Smedbol et al. 2002).

Threats to recovery posed by fishing and marine mammal predation:

DFO has estimated that, since 1999, fishing has been removing 4 to 63% of the available northern cod, depending on the fishing region (DFO 2002; Brattey and Healey MS2003) (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%).  For the northern cod stock as a whole, recent estimates of exploitation rate are in excess of 30% (Smedbol et al. 2002). Regarding predation of cod by seals, an independent Expert Panel concluded that "it is difficult to disagree with the most recent stock status assessment that 'there is a possibility that predation by seals is preventing the recovery of the cod stock'. This view is further supported by new (although highly uncertain) estimates that the consumption of cod >3 y old by hooded seals in the offshore substantially exceeds the estimated biomass" (McLaren et al. 2001).

Southern Grand Bank (NAFO Divisions 3NO):

The average age among individuals in the current cohort is 11.3 years, based on the spring survey catch rate data for 2001 (Don Stansbury, DFO, Newfoundland Region, personal communication).  Based on data from the 1970s and 1980s, age at 50% maturity is about 6yr for southern Grand Bank cod (Trippel et al. 1997; Stansbury et al. 2001).  Thus, in an unfished state, generation time is estimated to be 11years, yielding a three-generation time period of 33 years.  The survey data are those reported by Stansbury et al. (2001) and by Stansbury (personal communication).  Abundance data for the mature part of the population, as estimated by VPA, are available from Stansbury et al. (2001).  The earliest year for which survey data are available is 1984.  VPA estimates of abundance extend back to 1959.

Irrespective of the data source, the 3-generation rate of decline experienced by southern Grand Bank cod exceeded 95% (Figure 13).

Age (yr) at maturityGeneration
time (yr)
Data SourceData typeTime periodRate of change
611surveycatch rate
(# per tow)
1984-2001−95%
  VPAnumbers of spawners1968-2001−98%

 


Figure 13.  Temporal variation in the estimated number of mature individuals in the Southern Grand Bank cod stock (NAFO Divisions 3NO).

Figure 13.  Temporal variation in the estimated number of mature individuals in the Southern Grand Bank cod stock (NAFO Divisions 3NO).

Area of occupancy:

Between 1984 and 2000 (the range of the reported data), area of occupancy declined from approximately 90,000 km2 to approximately 75,000 km2, a rate of decline of roughly 17% (Smedbol et al. 2002).

Threats to recovery posed by fishing and marine mammal predation:

In the past 5 years, exploitation rates have been low, generally estimated to be less than 5% (Smedbol et al. 2002) (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%).  There is insufficient data to evaluate the threat to recovery posed by marine mammal predation on southern Grand Bank cod (McLaren et al. 2001).