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

Appendix 1: Cabot Strait

Cabot Strait (NAFO Division 4Vn):

The average age among individuals in the current cohort is 5.2 years, based on the survey catch rate data (Mohn et al. 2001; Diane Beanlands, DFO, Dartmouth, personal communication).  Based on data from the 1970s and 1980s for 4T cod, age at 50% maturity is about 4.5 years for southern Cabot Strait cod (Trippel et al. 1997). 

Figure 16.  Temporal variation in the estimated number of mature individuals in the Southern Gulf of St. Lawrence cod stock (NAFO Division 4T)


Figure 16.  Temporal variation in the estimated number of mature individuals in the Southern Gulf of St. Lawrence cod stock (NAFO Division 4T)


Figure 16.  Temporal variation in the estimated number of mature individuals in the Southern Gulf of St. Lawrence cod stock (NAFO Division 4T).

Thus, in an unfished state, generation time is estimated to be 9.5 years, yielding a three-generation time period of 28.5 years.  The survey data and VPA-based estimates of abundance are those reported by Mohn et al. (2001) and communicated to me by Diane Beanlands.  The earliest year for which survey data are available is 1970.  VPA estimates of abundance extend back to 1981.

The rate of decline experienced by Cabot Strait cod differs with the time period and with the type of abundance data used in the analysis.  In the past 20 years, it has declined 95%, based on VPA data.  The research survey data, which extend back to 1970 (Figure 17), suggest that the size of the Cabot Strait cod stock has remained stable over a 3-generation time frame.

Age (yr) at maturityGeneration
time (yr)

Data Source

Data type

Time period

Rate of change
4.59.5Surveycatch rate
(# per tow)
1972-2001+4%
  VPAnumbers of spawners1981-2000−95%

Area of occupancy:

Between 1970 and 2001 (the range of the reported data), area of occupancy remained constant at approximately 12,000 km2  (Smedbol et al. 2002).

Threats to recovery posed by fishing and marine mammal predation:

Since 1994, exploitation rates have remained very low at approximately 2% (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%).  McLaren et al. (2001) report that there is little information with which to judge the effects of seals on Cabot Strait cod.

Eastern Scotian Shelf (NAFO Divisions 4VsW):

The average age among individuals in the current cohort is 5.8 years, based on the survey catch rate data (Mohn et al. 1998; Diane Beanlands, DFO, Dartmouth, personal communication; Paul Fanning, DFO, Dartmouth, personal communication).  Based on data from the 1980s and 1990s, age at 50% maturity is about 4 years for eastern Scotian Shelf cod (Trippel et al. 1997).  Thus, in an unfished state, generation time is estimated to be 9 years, yielding a three-generation time period of 27 years.  The survey data are those reported by Mohn et al. (1998).  Abundance data for the mature part of the population, as estimated by VPA, are available from Mohn et al. (1998).  The earliest year for which survey data are available is 1970.  VPA estimates of abundance extend back to 1970 and end in 1997.

Figure 17.  Temporal variation in the estimated number of mature individuals in the Cabot Strait cod stock (NAFO Division 4Vn)


Figure 17.  Temporal variation in the estimated number of mature individuals in the Cabot Strait cod stock (NAFO Division 4Vn).

The 3-generation rate of decline experienced by Eastern Scotian Shelf cod ranges between 75 and 92% (Figure 18).  Indeed, in the five years since a VPA was last undertaken for this stock, survey data indicate a decline in catch rate of one order of magnitude (1998-2002 data).

Figure 18.  Temporal variation in the estimated number of mature individuals in the Eastern Scotian Shelf cod stock (NAFO Divisions 4VsW)


Figure 18.  Temporal variation in the estimated number of mature individuals in the Eastern Scotian Shelf cod stock (NAFO Divisions 4VsW)


Figure 18.  Temporal variation in the estimated number of mature individuals in the Eastern Scotian Shelf cod stock (NAFO Divisions 4VsW).

Age (yr) at maturityGeneration
time (yr)

Data Source

Data type

Time period

Rate of change
49Surveycatch rate
(# per tow)
1975-2002−92%
  VPAnumbers of spawners1970-1997−75%
        

Area of occupancy:

Between 1970 and 2000 (the range of the reported data), area of occupancy declined from approximately 65,000 km2 to approximately 35,000 km2, a rate of decline of roughly 46% (Smedbol et al. 2002).

Threats to recovery posed by fishing and marine mammal predation:

Since 1994, exploitation rates have remained very low at less than 2% (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%).  McLaren et al. (2001) concluded that "the overall evidence that grey seals may be preventing recovery of stocks on the Scotian Shelf is weak".

 

Western Scotian Shelf/Bay of Fundy (NAFO Division 4X):

The average age among individuals in the current cohort is 3.7 years, based on the survey catch rate data (Donald Clark, DFO, St. Andrews, personal communication).  Based on data from the 1970s and 1980s for 4X cod, age at 50% maturity is about 2.5 years (Trippel et al. 1997).  Thus, in an unfished state, generation time is estimated to be 7.5 years, yielding a three-generation time period of 22.5 years.  The survey data and VPA-based estimates of abundance are those reported by and communicated to me by Donald Clark.  The earliest year for which survey data are available is 1970.  VPA estimates of abundance also extend back to 1970.

The 3-generation rate of decline experienced by cod on the Western Scotian Shelf and Bay of Fundy ranged between 53 and 78% (Figure 19).

Age (yr) at maturityGeneration
time (yr)

Data Source

Data type

Time period

Rate of change
2.57.5Surveycatch rate
(# per tow)
1978-2001−53%
  VPAnumbers of spawners1979-2002−78%

Area of occupancy:

Between 1970 and 2000 (the range of the reported data), area of occupancy declined slightly from approximately 45,000 km2 to approximately 40,000 km2, a rate of decline of roughly 11% (Smedbol et al. 2002).


Figure 19.  Temporal variation in the estimated number of mature individuals in the Western Scotian Shelf/Bay of Fundy cod stock (NAFO Division 4X)

Figure 19.  Temporal variation in the estimated number of mature individuals in the Western Scotian Shelf/Bay of Fundy cod stock (NAFO Division 4X).

Threats to recovery posed by fishing and marine mammal predation:

Since 1994, exploitation rates have remained close to 30% (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 little information with which to judge the threat posed by seal predation to Western Scotian Shelf/Bay of Fundy cod (McLaren et al. 2001).

Georges Bank (NAFO Division 5Zj,m):

The average age among individuals in the current cohort is 4.9 years, based on the survey catch rate data (Hunt and Hatt 2002).  Based on data from the 1970s and 1980s, age at 50% maturity is about 2.5 years for Georges Bank cod (Hunt and Hatt 2002).  Thus, in an unfished state, generation time is estimated to be 7.5 years, yielding a three-generation time period of 22.5 years.  The survey and VPA abundance data are those reported by Hunt and Hatt (2002).  The earliest year for which survey data are available is 1978 for the U.S. NMFS (National Marine Fisheries Service) fall survey and 1986 for DFO's spring survey.  VPA estimates of abundance extend back to 1978.

The estimated rate of decline for Georges Bank cod differed considerably among the sources of abundance data.  Estimated rates of change over three generations ranged from a 68 to 70% decline for the longest of the time series to a 5% increase for the DFO spring survey data.  Although the VPA abundance data reveal a steady decline since the late 1970s, survey catch rate data available since the mid-1980s are highly variable (Figure 20).

Age (yr) at maturityGeneration
time (yr)

Data Source

Data type

Time period

Rate of change
2.57.5NMFS fall surveycatch rate
(# per tow)
1978-2001−68%
  DFO spring surveycatch rate
(# per tow)
1986-2002+5%
  VPAnumbers of spawners1979-2002−70%

Area of occupancy:

Between 1987 and 2001 (the range of the reported data), area of occupancy remained unchanged at approximately 14,000 km2 (Smedbol et al. 2002).

Threats to recovery posed by fishing and marine mammal predation:

Since 1999, exploitation rates have declined to approximately 10% (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 little information with which to judge the threat posed by seal predation to Georges Bank cod (McLaren et al. 2001).

Figure 20.  Temporal variation in the estimated number of mature individuals in the Georges Bank (Canadian portion) cod stock (NAFO Division 5Zj,m)

Figure 20.  Temporal variation in the estimated number of mature individuals in the Georges Bank (Canadian portion) cod stock (NAFO Division 5Zj,m).

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