Northern wolffish and spotted wolffish recovery strategy: chapter 10

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12. Perspective on the Assessment and Designation of Wolffish Species

The mandate of the Wolffish Recovery Team is to develop and recommend a strategy and specific associated actions to conserve and promote the recovery of A. denticulatus and A. minor, designated as “threatened”, as well as A. lupus, designated as “special concern. Designations are based on draft COSEWIC Status Reports and thus the contents of those Reports underlie the actions put forth in this document. The following section contains a Recovery Team perspective on aspects of the draft Status Reports that have influenced the nature and content of some of the recommendation of this Recovery Strategy and Management Plan.

• The basis for the COSEWIC “threatened” designation was negative biomass and abundance trends derived from fall survey time series covering the Grand Bank, northeast Newfoundland and Labrador Shelves. The time frame of the decline was 1978 to 1994, from the start of the fall survey series (the available abundance trend data) to the last year that the Engel trawl survey gear was used. The draft Status Reports assumed that 1978 represented the baseline population size, the starting point with which to measure the magnitude of the decline for the three species, and that the ensuing 17 years to 1994 represents 3 generations, the period that COSEWIC criteria specifies for the determination of decline rate. Insufficient information was provided to support the use of 1978 as a reference population size or 17 years as 3 generations. The life history of all of the wolffish species for the northwest Atlantic is unknown and thus represents a data gap.
• The use of any single point from fisheries surveys as a population size reference point as was done for the draft status reports is potentially misleading because of uncertainties in almost all fishery survey results. Pooling multi-year averages, basing “normal” population size on cyclic patterns in population size and using generation times based on life history attributes is a more robust approach.
• The unpublished COSEWIC Status Reports for all three wolffish species indicate that “the general decline continues to the present”. However, the fall survey data illustrated in the Status Reports indicate stable or increasing trends, not declines, since the mid-1990s (refer also to Figure 4).
• Based on fall survey data, the difference in the index between 1978 and 1994 was used to define the population decline. Natural fluctuations in population size were not considered in the draft status reports. However, spring survey data for a portion of the distribution shows that the population of the wolffish species was lower prior to 1978. Those data suggest that wolffish species, like most other fish species undergo fluctuations in population size and that 1978 may represent a peak in population size.
• The issue of heterogeneity in the population structure of wolffish species is not addressed in the draft Status Reports. The Reports assume a single Atlantic population (DU) for each species, with fall survey trends off Newfoundland and Labrador representing the population trends for the Atlantic. However, the existence of considerable spatial variation in abundance trends in different areas (see Part 3 and 6) of the Atlantic suggests the possibility of multiple DU’s for each of the species. The draft status report did not examine available survey trends in the Gulf of St. Lawrence or the Scotian Shelf where trends were stable or increasing.
• The draft Status Reports use unweighted mean number/tow as the index of abundance but do not account for the stratified design upon which Canadian surveys, including the fall NL survey, are based. It should be demonstrated that the within-year variance of treating each tow as a random event is not significantly different from the stratified random variance if weighted mean number/tow is used.
• The draft Status Reports suggest that habitat degradation resulting from bottom trawling may have been a proximal cause in the decline of the wolffish, but little evidence was presented to support that supposition. Data explorations by the Recovery Team and Kulka et al. (2004) suggest that areas most heavily fished by bottom trawlers continue to have the highest abundance of wolffish; areas not fished by bottom trawlers on the inner shelf have experienced the greatest declines. On the Labrador Shelf, where the decline of wolffish abundance was the greatest, only 20% of the region was heavily fished in the 1980s, and only 5% in the 1990s. Yet the decline appears to be universal in the Labrador region. Hence, there is insufficient evidence to conclude that habitat damage from bottom trawling was the sole or main factor leading to the decline of the wolfish species.

• The draft Status Report suggests that mortality due to fishing may have been a proximal cause of the declines. While there is no direct measure of fishing mortality for the wolffishes, the index of exploitation (catch/relative biomass), that represents a maximum estimate of the proportion of the stock that was removed from the population (given that the biomass index is a minimum value) is very low (Table 20). In addition, virtually allA. denticulatus and about 50% of the other two species have been discarded over the years. Survival of these discarded fish may be higher than for other species, given anecdotal information (from observers and survey technicians) that wolffish are much livelier than other species when captured and may stand a better chance of surviving. If so, the actual mortality for all species could be lower than the catch statistics reflected in Table 20. However, discard survival represents a data gap requiring quantification. Also, there is no indication that exploitation indices increased during the decline, as might be expected if fishing mortality was a proximal cause of the decline. In total, there is insufficient evidence to conclude that fishing was the sole or main factor leading to the decline of the wolffish species.
• Although the wolffish species have experienced significant declines in abundance since the late 1970s, there remain millions of each species, capable of survival and reproduction. These wolffish continue to be spread across a reduced but wide area.
• The Status Reports do not deal with the issues associated with population overlap or extension into adjacent jurisdictions (e.g., trans-boundary distributions). Specifically, distributions of the wolffish species appear to be contiguous in the north with fish in Greenland waters, with fish occurring in the NAFO Regulatory area and to the south with fish in USA waters.

The Recovery Team is deeply concerned about the declines that have taken place in the wolffish populations since the late 1970s. However, considering all available information, the Team feels that the proximal cause(s) of the declines remain uncertain. The “threatened” designation is defined as applying to species likely to become “endangered” with imminent extinction if limiting factors are not reversed. For the purposes of proposing mitigating actions for species recovery, the Team has assumed that fishing is an important factor (despite the uncertain evidence), but also that other unidentified factors may be limiting wolffish recovery.