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

Species Information

Name and classification
ClassActinopterygii
OrderGadiformes
FamilyGadidae
Latin binomialGadus morhua  Linnaeus 1758
Common namesEnglish -- Atlantic cod
 French -- morue franche
 Inuktitut -- ogac (Nunavut); ovak, ogac (Ungava Bay); uugak, ugak (Innu, Labrador) (McAllister et al. 1987)

 

Description

The Atlantic cod is a medium to large marine fish (Figure 1), inhabiting cold (10o to 15o C) and very cold waters (less than 0o to 5o C) in coastal areas and in offshore waters overlying the continental shelf throughout the Northwest and Northeast Atlantic Ocean.  Morphologically, the feature that distinguishes cod from most other fishes (a feature shared by other gadids) is the presence of three dorsal fins and two anal fins.  Otherwise, cod have the ‘classic’, streamlined, fusiform shape characteristic of fish that are able to sustain moderate speed over relatively long distances.  The colour of cod varies a great deal throughout Canadian waters, having been described by fishers as near-black, brown, and red, depending on the location of capture (Neis et al.1999).  The flesh of cod is composed of firm, non-oily, white tissue that deteriorates relatively slowly after death, and is easily preserved by drying, salting, or some combination thereof.

1


Figure 1.  Line drawing of Atlantic cod, or morue franche, Gadus morhua, by H.L. Todd.  Image reproduced with permission from the Smithsonian Institution, NMNH, Division of Fishes.

 

Populations

There are two approaches one could adopt in assigning Atlantic cod to an at-risk category.  The first, and the one adopted in 1998 when COSEWIC assigned cod to the Special Concern (Vulnerable) category, is to group all cod together and to assign a single designation to cod throughout their entire range in Canadian waters.  Such an approach could be defended scientifically if there was no evidence of population differentiation in genetic variation, life history, or ecology throughout the species' range in the Northwest Atlantic.  However, available genetic and ecological data are consistent with the hypothesis that Atlantic cod can be distinguished as separate populations, some of which can be combined and potentially recognized as Evolutionarily Significant Units (ESUs).

Heuristically, an ESU is defined by two criteria: 1) it must be substantially reproductively isolated from other conspecific units, and 2) it must represent an important component of the evolutionary legacy of the species (Waples 1991).  ESUs are generally more reproductively isolated over a longer period of time than are the populations within them.

Importantly, as Waples (1991) argued, the data required to designate ESUs should include information on genetic, ecological, and life history differences within and among putative ESUs.  The reason for this is that one type of information is often insufficient on its own.  For example, an absence of selectively-neutral, microsatellite variation between putative populations provides no information on the degree to which populations differ with respect to selectively important genetic variation.  And one’s interpretation of microsatellite variation must be tempered by potential sampling deficiencies (e.g., lack of information on temporal variability, insufficient sample sizes, use of extraordinarily variable loci).  Similarly, geographic differences in life history may simply reflect environmental, rather than genetic, variation.

Combining information from genetic, ecological, and life history research, there is substantial evidence of population differentiation among Atlantic cod in the Northwest Atlantic.  However, while differences among cod stocks are sufficiently high to warrant their separate treatment from a management perspective (Smedbol et al. 2002), it is not clear how populations might best be designated as ESUs, given that genetic and life history data are not available for all stocks.

Nonetheless, there is ever-increasing and substantive evidence of adaptive differences among cod at spatial scales considerably smaller than the geographical range of the species in Canada.  The limited degree of movement among populations required for adaptive differences to arise is suggested by a series of genetic and mark-recapture studies.  From the general perspective, these and other relevant studies include those on:

1.           genetic analyses of microsatellite variation (Bentzen et al. 1996; Ruzzante et al. 1996, 1997, 1998, 1999, 2000a,b, 2001; Pogson et al. 2001; Beacham et al. 2002; see Carr et al. [1995] and Carr and Crutcher [1998] for an alternative interpretation, based on mtDNA analyses)

2.           mark-recapture data (Templeman 1962; Taggart et al. 1995; Hunt et al. 1999; Brattey et al. 2001a,b)

3.           differences in migratory behaviour (Lear 1984; Chouinard et al. 2001; Lilly et al. 2001)

4.           spatio-temporal differences in spawning period and location (Myers et al. 1993; Hutchings et al. 1993)

5.           differences in otolith trace element composition (Campana et al. 1999)

6.           geographic variation in life history (Trippel et al. 1997; McIntyre and Hutchings in press)

7.           spatial differences in vertebral number (Templeman 1981), variation demonstrated to have a genetic basis in fishes (e.g., Billerbeck et al. 1997) and to be adaptively significant (e.g., Swain 1992)

8.           genetically-based differences in the production of antifreeze proteins (Goddard et al. 1999)

9.           genetically based differences in growth rate and food conversion efficiency (Purchase and Brown 2001)

10.      genetically based differences in the influence of light intensity on survival and growth rate in early life (Puvanendran and Brown 1998)

11.    geographical differences in recruitment, natural mortality and somatic growth (Swain and Castonguay 2000).

The challenge here may not be one of convincing individuals of the utility and scientific justification for identifying ESUs for Atlantic cod; rather, the difficulty will be in finding agreement on the geographical boundaries used to delineate the ESUs that are consistent with the biological data and with COSEWIC's assignment of extinction risk at levels below that of the species.  In this context, it is important to acknowledge that an ESU can contain multiple populations, each of which might be connected by some small degree of migration (McElhany et al. 2000).

Based on COSEWIC's guidelines for assigning status below the species level, and within the empirical and theoretical constructs of Evolutionarily Significant Units (Waples 1991), four Populations are identified in the present report and, when data are available, trends in the numbers of breeding individuals are described for each.  Each of the Populations includes cod found in more than one management unit, as delineated by NAFO (Northwest Atlantic Fishery Organization) divisions.  These divisions also identify the cod stocks managed by the Department of Fisheries and Oceans (DFO).

Arctic population

Cod in this population are those confined to coastal lakes along Frobisher Bay and Cumberland Sound, and those inhabiting the marine environment east and southeast of Baffin Island, Nunavut (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 (at least seven of which are known or suspected) inhabiting lakes that receive intermittent tidal intrusions of salt water.

Newfoundlandand Labrador population

Cod in this population inhabit the waters ranging from immediately north of Cape Chidley (the northern tip of Labrador) southeast to Grand Bank off eastern Newfoundland.  For management purposes, cod in this population are treated as three separate stocks by DFO:  (1) Northern Labrador cod (NAFO Divisions 2GH), (2) "Northern" cod, i.e., those found off southeastern Labrador, the Northeast Newfoundland Shelf, and the northern half of Grand Bank (NAFO Divisions 2J3KL), and (3) Southern Grand Bank cod (NAFO Divisions 3NO).  Approximately 75% to 80% of the Atlantic cod in Canadian waters were located within this Population in the early 1960s.

Laurentian North population:  Cod in this population combine the stocks identified for management purposes by DFO as (1) St. Pierre Bank (NAFO Division 3Ps) and (2) Northern Gulf of St. Lawrence (NAFO Divisions 3Pn4RS).  These stocks are located north of the Laurentian Channel, bordering the south coast of Newfoundland and Quebec, respectively.

Maritimes population

Cod in this population combine the stocks identified for management purposes as five separate stocks by DFO:  (1) Southern Gulf of St. Lawrence (NAFO Division 4T), (2) Cabot Strait (NAFO Division 4Vn), (3) Eastern Scotian Shelf (NAFO Divisions 4VsW, (4) Bay of Fundy/Western Scotian Shelf (NAFO Division 4X), and (5) cod found on the Canadian portion of Georges Bank (NAFO Division 5Zej,m).  Geographically, these stocks are located in the waters adjacent to the three Maritime provinces, extending from the southern Gulf of St. Lawrence south to the Canadian portion of Georges Bank.

Scientific basis for distinguishing the Newfoundland & Labrador, Laurentian North, and Maritimes populations

These populations can be distinguished from one another by a combination of different types of data.  These include age at maturity, maximum population growth rate (rmax), temporal trends in abundance, genetic variability at selectively neutral loci, and genetic differences among selectively important traits.

1.  Age at maturity

 

Age at maturity (represented as the age at which 50% of females are reproductive) differs among the populations, particularly between the Newfoundland & Labrador population and the other two populations.  To compare ages at maturity, I calculated the average age across all stocks within each population, weighting the average age for each stock by the highest estimated abundance of mature individuals in that stock (as determined by VPA abundance data; see POPULATION SIZES AND TRENDS below).  Cod are oldest at maturity in the Newfoundland & Labrador population in which both stocks mature at 6 years, and youngest in the Maritimes population in which 2 of the 5 stocks mature at 2.5 years.
PopulationNumber of stocksAge at maturity (range among stocks)Reference(s)
Newfoundland & Labrador26.0 (6-6 yr)Lilly et al. (1991); Trippel et al. (1997); Stansbury et al. (2001)
Laurentian North24.5 (4-6 yr)Brattey et al. (2001a); Yvon Lambert, personal communication
Maritimes54.3 (2.5-4.5 yr)Trippel et al. (1997); Doug Swain, personal communication; Hunt and Hatt (2002)

 

2.  Maximum population growth rate (rmax)

Based on data available in the mid-1990s, one can compare estimates of maximum population growth for the populations identified here.  The estimate for northern cod is available from Hutchings (1999); the remaining estimates are reported by Myers et al. (1997a; revised Table 1 [from which the present estimates of r were obtained] can be obtained from Ransom Myers, Department of Biology, Dalhousie University, Halifax, NS  B3H 4J1).  These estimates are subject to the caveat that rmax may have changed since the mid-1990s.  However, what is important here is the question of whether rmax is likely to differ among the populations, even for data restricted to the pre-collapse and immediate post-collapse periods for each stock. 

 

To compare rmax among populations, I calculated the average rmax across all stocks within each population, weighting the rmax estimate for each stock by the highest known abundance of mature individuals in that stock (as determined by VPA abundance data; see POPULATION SIZES AND TRENDS below).  As one would predict, based on their differences in age at maturity, the Newfoundland & Labrador population has a lower maximum population growth rate than its two more southerly counterparts.
PopulationNumber of stocksMaximum population growth rate, rmax (range among stocks)Reference(s)
Newfoundland & Labrador20.15 (0.13-0.35)Myers et al. (1997, revised Table 1); Hutchings (1999)
Laurentian North20.32 (0.29-0.39)Myers et al. (1997, revised Table 1)
Maritimes50.31 (0.24-0.67)Myers et al. (1997, revised Table 1)

3.  Temporal abundance trends

There is a concordance in the temporal abundance trends of the cod stocks that comprise the Newfoundland & Labrador population that is not evident throughout all parts of the ranges of the other populations.  All cod in the Newfoundland & Labrador Population have experienced steady declines across the entire time period of available data and are currently at historic low levels of abundance (see POPULATION SIZES AND TRENDS below).  In contrast, the current low levels of cod in parts of the other two populations have been experienced previously and are not, thus, unprecedented.

4.  Genetic differentiation at selectively neutral loci

There is strong evidence of genetic differentiation at selectively neutral loci among the populations.  Based on an analysis of 1,300 cod at 5 microsatellite loci, Ruzzante et al.'s (1998) estimates of genetic differentiation were either highest (Rst), or among the highest (Fst), when comparing cod sampled north and south of the Laurentian Channel, which separates the Newfoundland & Labrador and Laurentian North populations from the Maritimes population.  Pogson et al.'s (2001) analysis of 10 nuclear restriction-fragment-length-polymorphism (RFLP) loci also revealed significant genetic differences among cod sampled from the populations.  In addition, Pogson et al. (2001) reported a highly significant negative association between gene flow and geographic distance among cod sampled in the Canadian waters from the Newfoundland & Labrador and Maritimes populations, an association also reported by Beacham et al. (2002) from their analysis of 7 microsatellite loci and a pantophysin locus.  These negative associations imply that the greater the geographical separation of cod, the lower their genetic affinity.  Additional evidence for genetic differentiation among the populations at selectively neutral loci is summarized in the table below.

For the most part, the reduced gene flow that one can infer from these genetic studies are supported by tagging studies that have been conducted since the 1950s (Taggart et al. 1995).  Templeman (1962), for example, reported that "there do not appear to be any migration tracks or any considerable intermingling across [the Laurentian] Channel and stocks on each side of the Channel are thus separate."  Although recent tagging studies (Brattey et al. 2001a,b) have suggested that cod from the St. Pierre Bank stock of the Laurentian North population intermingle with those in the southern portion of the Newfoundland & Labrador population, the genetic research cited above and in the summary table below suggests that migrants are not particularly successful reproductively.

Summary of the evidence for genetic differentiation among the Newfoundland & Labrador, Laurentian North, and Maritimes populations, based on studies of selectively neutral loci and selectively important traits.
PopulationLaurentian NorthMaritimes
Newfoundland & Labrador1. Seven microsatellite loci and one pantophysin locus (Beacham et al. 2002)
2.  Five microsatellite loci (Ruzzante et al. 1998)
1.  Six microsatellite loci (Bentzen et al. 1996)
2.  Five microsatellite loci (Ruzzante et al. 1998)
3.  Ten nuclear restriction-fragment-length-polymorphism (RFLP) loci(Pogson et al. 2001)
4.  Influence of light on survival and growth (Puvanendran and Brown 1998).
5.  Growth rate and food conversion efficiency (Purchase and Brown 2001)
Laurentian NorthNA1.  Six microsatellite loci (Ruzzante et al. 2000)
2.  Five microsatellite loci (Ruzzante et al. 1998)
3.  Larval growth and survival (Hutchings et al. MS2002)

5.  Genetic differentiation among selectively important traits

In addition to differences at selectively neutral loci, there is increasing evidence that cod in the Northwest Atlantic differ from one another at loci that are under selection.  This inference is drawn from experiments in which differences among cod populations have been documented after the effects of the environment have been removed from the analysis.

Among reports in the published literature, Goddard et al. (1999) found that cod in the northern part of the Newfoundland & Labrador population produced higher levels of anti-freeze protein (which prevents ice crystals from forming in the blood at sub-zero degree temperatures) than cod further south in the same population, a difference that almost certainly would be magnified if one were to compare anti-freeze protein production in cod that never experience sub-zero degree water temperatures, e.g., those throughout most of the Laurentian North and Maritimes populations.  Comparing cod from the southern portion of the Maritimes population with those from the Newfoundland & Labrador population, genetic differences have also been documented in growth rate and food conversion efficiency (Purchase and Brown 2001), and in the influence of light intensity on survival and growth in early life (Puvanendran and Brown 1998).

There is additional evidence for genetic differences among traits of importance to fitness between cod from the Laurentian North and Maritimes populations (Hutchings et al. MS2002, unpublished data). To assess the genetic basis of phenotypic variation in Atlantic cod, colleagues at the Department of Fisheries and Oceans (Moncton) and at Memorial University of Newfoundland and I are conducting "common-garden" experiments in which cod from putatively different populations are reared under the same environmental conditions in the laboratory.  Statistically significant group-level differences in the average expression of a trait, or its interaction with an environmental variable, would suggest that population differences have a genetic and, for fitness-related traits, adaptive basis.

To date, we have undertaken experiments on cod sampled from the Laurentian North (St. Pierre Bank) and Maritimes populations (Southern Gulf of St. Lawrence, Western Scotian Shelf/Bay of Fundy).  Population differences in fitness-related traits for cod larvae and juveniles were quantified at two levels of food (1,000 and 4,500 prey/L) and temperature (7o and 11o C), four replicates per treatment. 

Preliminary analyses reveal significant main and interaction effects of temperature, food, and population on larval growth and survival.  Average growth rate differed significantly among populations (F=68.03, p<0.0001); larvae from the southernmost population were smallest at 43 days post-hatch whereas larvae from the northernmost population were largest, a pattern consistent with the hypothesis of adaptive variation in growth rate (see Purchase and Brown 2001).  A significant temperature×population interaction (p=0.0003) on growth suggests a genetic difference in plasticity among populations.  Significant population× food (p=0.008) and population×temperature interactions (p=0.025) also suggest that the effects of food and temperature on larval survival differ genetically among populations.

Given this evidence of genetically based differences in selectively important traits for cod sampled from the Laurentian North and Maritimes populations, it is reasonable to conclude that these differences will be maintained, if not increased, when cod from the Newfoundland and Labrador population are similarly analysed (these experiments will begin in May 2003).