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Recovery Strategy for Northern Wolffish and Spotted Wolffish, and Management Plan for Atlantic Wolffish in Canada [Final]

3. Population Abundance

3.1 Global Range

Because wolffish are the target of a significant directed fishery in parts of the north Atlantic, primarily in the northeast Atlantic off Greenland (Möller and Rätz 1999) but not in Canadian waters, it is assumed that there is some sort of aggregating phase to their life history and that they form sufficiently dense concentrations there to make a directed commercial fishery viable. If these denser concentrations in the northeast Atlantic extend over an area at least as great as their area of occupancy in Canadian waters, this would suggest that the population of wolffish could be more abundant in other parts of the Atlantic. However, because of different survey gears and protocols used in different parts of the range of wolffish, relative population abundance among various parts of its range cannot be determined at this time.

3.2 Population Sizes and Trends in Eastern Canadian Waters

Biomass and abundance estimates for wolffish at the centre of their abundance, the Grand Banks to Labrador Shelf, were derived from Newfoundland and Labrador regional fall research surveys (Simpson and Kulka 2002) conducted between 1977 and 2001 (Grand Bank, Northeast Newfoundland Shelf and South Labrador Shelf) and spring research surveys between 1971 and 2001 (Grand Bank and St. Pierre Banks only). Neither these fall or spring surveys cover the Gulf of St. Lawrence, the Scotian Shelf or the Northern Labrador Shelf into Davis Strait, although all but the Davis Strait is surveyed at other times with different gears. Thus, the spring and fall series are not comparable and neither covers the entire range of wolffish species in Canadian Atlantic waters. The fall survey series is the best measure of wolffish relative abundance as it extends over the area where all three species are at the center of their distribution (Simpson and Kulka 2002). Thus, the fall survey is used to describe trends in abundance. Although surveys on the Northern Labrador Shelf have been infrequent, the wolffish species there appear to have undergone a similar if not greater pattern of decline from the early 1980s to the mid-1990s, as described below for areas directly to the south.

The magnitude of the Newfoundland and Labrador fall indices after 1995 is not comparable to that of the pre-1995 period due to a change in gear type used during the surveys. Catchability conversion factors between the Engel (pre-1995) and Campelen (current) trawl gear are not available for wolffish species. That gear change is delineated in Figure 4 by a gray vertical bar to distinguish the two series. The area surveyed in the fall is divided into two areas based on distinct distribution characteristics (described above) and habitats. The northern area (2J+3K in Figure 4) covers the Southern Labrador Shelf and the Northeast Newfoundland Shelf and the southern area covers the Grand Bank (3LNO in Figure 4). In both the northern and southern parts of the survey, the indices declined by more than 90% for all three species, since the 1980s, (Figure 4).

For the Grand Banks to Labrador Shelf, A. denticulatus underwent the most significant decline of the three species (Figure 4a), greatest in the north (2J3K) and steepest between 1984 and 1994. A. denticulatus underwent a less precipitous decline in the south (3LNO). Note that the southern area was not surveyed in the fall prior to 1981. As a result of different decline rates between north and south, after 1991, A. denticulatus actually had a higher abundance in 3LNO than in 2J3K whereas, prior to that time, abundance to the north was about 5-6 times greater. Since 1995, the indices for A. denticulatus, both north and south, have been stable.

A. minorunderwent as nearly a dramatic decline as A. denticulatus (Figure 4b). However, in contrast, biomass was approximately equal in the northern and southern areas prior to the decline (Figure 4b). The decline rate was about the same in both areas, unlike A. denticulatus, and thus it retained about equal proportions of biomass between areas over the period of decline. Since 1995, the indices and particularly abundance has undergone a substantial increase, more than doubling numbers of wolffish between 1995 and 2001. This suggests recent recruitment and improved survival. However, it should be noted that since 1993, deep strata (and inshore strata) have been successively added to the surveys. What portion of the increase in the indices is attributable to an increased survey area is uncertain (a subject for future research).

Overall, the observed decline in A. lupus biomass was not as great as for the other species, but was on a similar scale in the north (2J3K) where most of the decline occurred for this species (Figure 4c). To the south (3LNO), the indices tended to be stable between 1981 and 1994. However, the fish that were located on the shelf edge of the Grand Bank did decline slightly, whereas the concentration on the southern bank actually increased slightly. After 1994 (and the change in survey gear to Campelen), the indices for A. lupus increased steadily, particularly to the south, in terms of biomass.

Spring surveys (starting in 1971) covered only the Grand Banks but the spatially restricted series is longer than that of the fall series. This index shows a fluctuating pattern over the longer term and if the spring survey indices reflect the entire population, this would suggest that the levels observed at the start of the fall survey series in 1977 represent a peak in the population (spring numbers were lower prior to 1977). The biomass and abundance spring indices for all three species of wolffish fluctuated over the survey period, increasing during the 1970s, declining in the early 1980s, increasing in the late 1980s and declining again in the early 1990s (Simpson and Kulka 2002). Since 1996, the spring abundance and biomass indices have increased. For both spring and fall surveys, the magnitude of the indices after 1995 are not comparable to earlier years due to the change in survey gear.

The relative size (total biomass/total number) of A. denticulatus increased during 1981-1991 in the north (2J3K), but declined thereafter (Simpson and Kulka 2002). For A. minor, the relative size of fish in the north was greater than in the south. Associated with the decline in abundance and biomass, the relative size of A. lupus also declined in the northern area (2J3K). Throughout the entire survey periods, relatively larger A. lupus were captured in the southern areas (particularly on the bank) than to the north. The relative size of all three wolffish species taken in the surveys is smaller in recent years (since 1995) across all areas. This is likely a result of changing to the Campelen survey gear that has a higher catchability for smaller fish. However, a proportionately greater increase in abundance than in biomass after 1995 observed in all three species, to differing extents, suggests that there may be recruitment in recent years as well.

The index of abundance for A. lupus from the summer survey of the Scotian Shelf and Bay of Fundy has been highly variable from year to year, and had no apparent trends until the latter half of the 1980s (McRuer et al. 2001). he index for this area increased to its highest values in the time series in the early 1990s, and has since remained above average. The biomass index (mean weight per tow) declined over the same period and is presently close to its lowest level in the series (McRuer et al. 2001). Examination of length frequencies from the Scotian Shelf summer survey indicates that the increase in abundance of A. lupus since 1986 was due to a greater proportion of smaller fish. The number of immature fish (<=55cm) since 1985 has been above average, while the number of mature fish declined throughout the period and is presently near the lowest observed in the series (McRuer et al. 2001).

The indices of abundance and biomass for A. lupus from the fall surveys of the southern Gulf of St. Lawrence also increased to above average values after 1987, but have declined to average values in recent years (McRuer et al. 2001). As was seen on the Scotian Shelf, the number of immature A. lupus increased in the southern Gulf fall survey. Mature fish from this survey were also more prevalent, contributing to the increased abundance after 1987, but have declined to low levels in the most recent surveys (McRuer et al. 2001).

The indices of abundance and biomass for A. lupus from the summer research survey of the northern Gulf of St. Lawrence (Div. 4RS), available since 1990, show an increasing trend (McRuer et al. 2001).

Population trends and changes in distribution are not available for A. denticulatus or A. minor in the Gulf of St. Lawrence or the Scotian Shelf as they are rare in these areas. There are no surveys in the Davis Strait (all three species are rare) and thus indices are not available for those areas either.

3.3 Percentage of Global Population in Eastern Canadian Waters

Different survey gears are used in different parts of the world (and in different parts of eastern Canada) to quantify population size and examine changes over time. Therefore, relative proportions of populations occurring in various parts of the range of the wolffish species in the Atlantic cannot be determined, although the Canadian Atlantic component certainly represents a significant proportion of the global population. Percentages of the global populations in eastern Canadian waters are not known at the present time.