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

Population Sizes and Trends

 

Estimates of the size of the breeding part of the population for Atlantic cod are available from two sources.  The first set of estimates is derived from Virtual Population Analysis, or VPA, an analysis reliant on commercial fishery catch data and incorporating assumptions concerning the magnitude of natural, or non-fishing, mortality.  Increasingly, VPAs also include estimates of the proportional representation of mature fish by age.  Regarding information on breeders, one of the outputs of a VPA model is an estimate of the number of spawning individuals.  The second means of estimating the size of Atlantic cod breeding populations is to use the catch rates of cod of reproductive age as determined from the annual research surveys conducted by the Department of Fisheries and Oceans (DFO).

The primary utility of VPA estimates of abundance is that they allow one to express breeding population size in the same units (numbers of individuals) for each stock or population.  The main weaknesses associated with VPA estimates of abundance are that they rely upon accurate reporting of commercial catch data, they do not account for the illegal practices of discarding and catch misreporting, and they depend upon reliable estimates of mortality due to natural causes.  The primary strength associated with research survey estimates of the size of the breeding population is that the data are obtained from random samples of cod caught throughout the geographical area of each stock.  Thus, they are unbiased and do not depend upon the validity of assumptions concerning natural mortality and the accuracy of commercial fishery data.  Unfortunately, differences among the survey areas in the catchability of cod (the proportion of cod available to be caught by the survey gear that is actually caught by that gear), coupled with differences in survey gear, prevent one from comparing the absolute values of survey catch rates among populations.  In addition, survey estimates can be unduly variable in some areas in some years, leading one to conclude that survey catch rates may provide more reliable estimates of abundance trends for some stocks (e.g., Southern Gulf of St. Lawrence cod) than for others (e.g., St. Pierre Bank cod).

For the population trends described below, the ages of fish in each breeding population were > 5 years for all stocks, except > 4 years for Western Scotian Shelf/Bay of Fundy cod and > 3 years for Georges Bank cod.

Generation time, as usually applied by COSEWIC (Appendix C; Organization and Procedures Manual, May 2003), is the average age of parents in the current cohort.  However, COSEWIC notes that among species for which generation time varies under threat, generation time should be that estimated for the species during the pre-disturbance state.  Under these circumstances, for exploited species, age at maturity can be estimated as (age at first reproduction + 1/M), where M is the instantaneous rate of mortality due to natural events, and age at first reproduction is approximated by the age at which 50% of the adults are mature.  M is thought to be 0.2 for cod in an unfished state (Smedbol et al. 2002).

I estimated the rate of decline from the slope of the linear regression of loge abundance (Nt) versus time (t, in years), as recommended by the Marine Fishes Specialist Subcommittee at its January 2002 meeting and as adopted by DFO in its recent compilation of cod stock decline rates (Smedbol et al. 2002).  The resulting regression equation is ln(Nt) = α + β*t.  The percentage decline over t years can be calculated as (1−exp(β*t))*100.  This method is illustrated below for the Newfoundland and Labrador population.

DFO has estimated area of occupancy for each of the stocks it manages in two ways (Smedbol et al. 2002).  One, called the design-weighted area of occupancy, or DWAO, most closely approximates the definition of area of occupancy used by COSEWIC.  The second is defined as the minimum area containing 95% of the cod, or D95.  Both indices are based on the fisheries-independent research survey data collected annually by DFO.

Because of its concordance with COSEWIC's definition of area of occupancy, only the analyses of the DWAO data will be reported here.  Although this index should provide reliable estimates of the rate of decline in area of occupancy, the estimates of area of occupancy used in the analyses represent under-estimates of the actual areas of occupancy.

The status report recommends the identification of four populations of Atlantic cod (Figure 5).

Figure 5.  Distribution of Atlantic cod in Canadian waters.

Figure 5.  Distribution of Atlantic cod in Canadian waters.  Boundaries of the Arctic, Newfoundland & Labrador, Laurentian North, and Maritimes populations are delineated by heavy lines.  The depth contour is at 200 metres.

 

Arctic Population

Cod in this population are those inhabiting waters within and to the east of Baffin Island, Nunavut (marine waters are in NAFO Divisions 0A, 0B).  Although little is known about cod inhabiting the marine waters in this area, they may be the ancestral source of relict landlocked populations of cod known or suspected to inhabit 7 lakes that receive intermittent tidal intrusions of salt water (Table 1).  The best-studied of these lake populations is that inhabiting Ogac Lake, a salt, meromictic lake on Baffin Island that receives influxes of seawater only during the highest summer tides (McLaren 1967; Patriquin 1967).  The estimated abundance of reproductive cod in Ogac Lake was approximately 500 individuals in 1962 (Patriquin 1967).

Although there are no genetic data available for cod in this population, lake-dwelling individuals are considerably larger on average than those found elsewhere in Canadian waters.  In 1952, Storrs McCall and Arthur Dawson of McGill University captured a considerable number of cod from Ogac Lake, including one that measured 135 cm (25.2 kg) and one measuring 141 cm (Bruemmer 1966).  In the 1960s, Patriquin (1967) reported sizes at maturity of 65 and 85 cm for Ogac Lake males and females, respectively, lengths that are 20 to 40 cm greater than sizes at maturity reported elsewhere (Brander 1994).  The maximum size of cod captured in Ogac Lake in 1965 was 144 cm, and large-sized cod continued to exist in Ogac Lake in the 1980s and 1990s (see table below).  Cod in excess of 1 metre in length have also been captured in the Nettilling Fjord Lake, near Pangnirtung in Cumberland Sound.  One cod captured there in August 1998 is reported to have been 138 cm long, 25 kg in weight, and 13+ years in age (personal communication from Margaret Treble, DFO, Winnipeg, to David Hardie, Dalhousie University, Halifax).

Although there are no data on temporal trends in abundance, increased angling pressure has been identified as a concern by local inhabitants.  Indeed, studies since the 1960s have remarked on the extraordinary ease with which large numbers of cod can be captured from these lakes.  For example, in Nettilling Fjord Lake, 500 kg of cod (n=25), or 0.5 tonnes, were caught in only 1 hour of handlining (20 hooks fished for 1 hour) in August, 1985 (Lewis 1989).  In 1986, 240 kg of cod (n=104) were removed from the same lake by 24 hooks fished for 1 hour (Lewis 1989).

LakeDateLength
(mean + SD)
Maximum length (estimated age)Source
Ogac Lake19 July 199949.9 + 16.7 cm; n=23111 cm (8+ yr)DFO Winnipeg
Ogac LakeFeb/ Mar 198764.3 + 17.3 cm; n=21107 cm (ages NA)DFO Winnipeg
Nettilling FjordSept 198561.0 + 10.3 cm; n=25102 cm (16+ yr)Lewis (1989)
Nettilling Fjord6-7 Aug 198662.0 + 9.3 cm; n=10477 cm (17 yr)Lewis (1989)
Nettilling FjordAug 198955.1 + 8.4 cm; n=100102 cm (ages NA)DFO Winnipeg


Table 1.  Reported locations of landlocked Atlantic cod in Nunavut.
LakeLocationSource
Ogac Lake (also known as Ogak Lake and Ney Harbour)

CONFIRMED
Across Frobisher Bay, 117 km from Iqaluit at the head of Ney Harbour (62°51’N, 67°20’W; NTS map 025J14).McLaren (1967); Amarak Hunters and Trappers' Office (HTO), Iqaluit
Qasigialiminiq

CONFIRMED
First lake on south arm of Nettilling Fjord; across from Pangnirtung and deeper into Cumberland Sound
(66°02’N, 68°12’W;
NTS map 026K00).
Pangnirtung HTO; Margaret Treble (DFO, Winnipeg); Dan Pike (North Atlantic Marine Mammal Commission, Tromsö, Norway, formerly of DFO, Iqaluit)
Tariuja

CONFIRMED
First lake on north arm of Brown Inlet, just west from Nettilling Fjord (65°29’N, 67°20’W; NTS map 026G00).Margaret Treble
Lake(s) on Broughton Island (now Qikiqtarjuaq)

CONFIRMED
North side of Cumberland Peninsula, 177 km from Pangnirtung (67°34’N, 63°54’W; NTS map 26P).Dan Pike
Lake at north mouth of Frobisher Bay

CONFIRMED
At old DFO field camp on the Beekman Peninsula, across from Brevoort Island, north of Cyrus Field Bay.Dan Pike.  The Annual Report of the FRBC (Arctic Biological Station) from 1951-52 (Calanus) mentions Ogac Lake cod and Beekman Pensinsula cod.
Lake in the vicinity of Burwell, NU

UNCONFIRMED
Burwell, in the years 1947-1950?Personal communication to Corey Morris (DFO, St. John's) from Kathleen Martin (DFO, Winnipeg).
Resolution Island, NU

UNCONFIRMED
1950?Personal communication to Corey Morris (DFO, St. John's) from Kathleen Martin (DFO, Winnipeg).

Research surveys of the marine waters off eastern Baffin Island (NAFO Divisions 0A and 0B) are infrequent and those that have been undertaken have targeted Greenland halibut (Reinhardtius hippoglossoides).  The most recent surveys have caught exceedingly few Atlantic cod.  Among 66 tows conducted in NAFO Division 0A in 1999, only 3 cod were caught (all in a single tow) (Treble et al. 2000); among 48 tows conducted in 2001, no cod were captured (Treble 2002).  Similarly, among 64 tows conducted in Division 0B in 2000, only 1 cod was caught (Treble al. 2001); none was caught among the 36 tows conducted in 0B in 2001 (Treble 2002).

The suggestion that cod in Arctic marine waters exist at very low densities is supported by other information.  The Pangnirtung Fisheries Consortium reports that cod are absent in Cumberland Sound.  The Iqaluit Hunters and Trappers Organization reports the same for Frobisher Bay.  Low densities of cod in Arctic marine waters is also suggested by McLaren's (1967) report that no cod were taken during surveys of Frobisher Bay by M/V Calanus in the 1950s.