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COSEWIC assessment and update status report on the Atlantic Cod in Canada
- Assessment Summary
- Executive Summary
- Species Information
- Population Sizes and Trends
- Newfoundland & Labrador Population
- Limiting Factors and Threats
- Special Significance of the Species
- Existing Protection or Other Status
- Summary of Status Report
- Technical Summary: Arctic Population
- Technical Summary: Newfoundland & Labrador Population
- Technical Summary: Laurentian North Population
- Technical Summary: Maritimes Population
- Literature Cited
- Biographical Summary of Contractor
- Authorities Consulted
- Appendix 1: Northern Labrador
- Appendix 1: St. Pierre Bank
- Appendix 1: Cabot Strait
Summary of Status Report
The report suggests that Atlantic cod in Canada be recognized as four populations, in accordance with known genetic, ecological, and demographic data, and in accordance with the guidelines detailed in Appendix F5 of COSEWIC’s Organization and Procedures Manual (Version 16, April 2002).
Regarding the assignment of risk, only the primary cause of the reduction in Atlantic cod (fishing) can be deemed reversible and understood. However, fishing has not ceased in any of the populations (although it is restricted in some parts of some Populations, such as Eastern Scotian Shelf). In the Laurentian North population, excessive fishing mortality has reduced the breeding part of the population, particularly in the Northern Gulf section of this population. For the Newfoundland & Labrador population, it is evident, based on harvest rates estimated by DFO, that fishing is delaying recovery in parts of this population's range.
There are several additional factors that may influence one's perception of risk.
The first concerns the downgrading of risk when there is a possibility of ‘rescue’, that is, immigration to the population under consideration from another population. It is not clear that such a downgrading is warranted when the neighbouring population(s) is(are) also at risk. In the present case, there are no data that would allow one to evaluate the levels of immigration or emigration into or out of the Arctic population. While cod movement from the eastern part of the Laurentian North population (i.e., St. Pierre Bank) into the southern part of the Newfoundland & Labrador population has been documented, it is not clear that these individuals contribute significantly, either demographically or genetically, to the latter Population. Indeed, the complete absence of recovery in the southern part of the Newfoundland & Labrador population would suggest that such contributions are minimal. Regarding the Maritimes population, rescue is possible from American populations of cod in the extreme southern part of the population, although these stocks are also at low levels.
A second factor that might influence perception of risk pertains to changes in the life history of cod in some parts of its Canadian range, notably for the Newfoundland & Labrador population. The exploitation rates experienced by cod in the late 1980s and early 1990s were sufficiently high to effect genetic selection responses to life history traits, possibly favouring earlier maturity (Hutchings 1999) and slower growth (Sinclair et al. 2002) as a consequence. From an evolutionary perspective, an extended number of reproductive events per lifetime is essential for genotypic, and population, persistence in Atlantic cod. In the early 1960s, when cod aged 10 years and older were contributing more than 50% of the eggs to the Northern cod stock (Hutchings and Myers 1994), part of the Newfoundland & Labrador population, the expected number of lifetime reproductive events may well have been as much as 10 on average, and certainly more than 5. Given the absence of cod aged 10 years on the Northeast Newfoundland Shelf in recent years, the expected number of lifetime reproductive events for cod in this area is clearly less than 5 and may well be as few as 2 or 3. This considerable reduction in lifetime breeding events can be expected to have rather serious consequences for the persistence of this and other similarly affected populations, irrespective of the absolute numbers of individuals in each population.
There is another reason not to be complacent about the apparently high numbers of mature cod in Canadian waters, despite (in some cases) extraordinary declines in abundance. Although the absolute numbers of cod in each population appears high, it is important to acknowledge that census estimates of mature individuals (Nc) do not reflect the actual numbers of individuals that contribute genes during spawning, as reflected by the effective population size (Ne). For broadcast spawning organisms such as Atlantic cod, it has been estimated that Ne is 2 to 5 orders of magnitude lower than Nc because of the increased variance in individual reproductive success associated with this type of mating system (Hedgecock 1994). Empirical support for this range in Ne/Nc has recently been forthcoming from studies of marine fishes.
For example, based on estimates of Ne from declines in heterozygosity and temporal fluctuations in allele frequency over 46-48 years, Hauser et al. (2002) concluded that effective population size in the broadcast spawning New Zealand snapper (Pagrus auratus) was 5 orders of magnitude less than census population sizes (which number in the millions). The effective number of female red drum (Sciaenops ocellatus) in the Gulf of Mexico, estimated from both mitochondrial (Turner et al. 1999) and microsatellite DNA (Turner et al. 2002), is 10-3 that of the female census population size. In other words, based on an Ne/Nc of 10-3, although one might count 1 million breeding individuals in a population, the number of individuals contributing genes to future generations would actually be 1,000.
When assessing risk for the Laurentian North population, it is important to note that the two stocks in this population have previously shown positive signs of recovery following severe reductions in fishing pressure. This is in contrast to cod in other areas, such as the Newfoundland & Labrador population and the Eastern Scotian Shelf component of the Maritimes population, where reductions in fishing pressure have not been associated with recovery. Also, current levels of abundance in the St. Pierre Bank portion of this population are not unprecedented.
For the Maritimes population, one factor that may mitigate extinction risk is the observation that the present abundance of the primary cod stock in this population (Southern Gulf of St. Lawrence) is not unprecedented. Recent declines in some parts of the population may be inflated because of unusually high abundance levels in the 1980s. Again, this is particularly true for Southern Gulf cod for which the long-term (1950-2002, about 5 generations) decline is 35% (Smedbol et al. 2002). In addition, both the mean age of spawners and the proportional representation of old fish in the Southern Gulf stock are at historically (1950-2001) high levels (Swain and Chouinard 2000; Smedbol et al. 2002).
In contrast, however, within one of the stocks in this population (Eastern Scotian Shelf) the proportional representation of large, old cod is very low (Paul Fanning, DFO, Dartmouth, personal communication). Indeed, in the absence of fishing, survey catch rates have declined precipitously between 1998 and 2002. Also of potential worry is the observation that length at 50% maturity has declined from 40 (1979-1984) to 33 cm (1997-2002) in Division 4W and from 42 (1979-1984) to 37 cm (1997-2002) in Division 4Vs (Paul Fanning, DFO, Dartmouth, unpublished data). For a species that typically does not reach maturity until it has attained a size of 50 cm (Brander 1994), these unexplained declines in size at maturity can be expected to have a negative influence on population recovery.
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