Research Document 1998/131

The status of Thorny skate (Raja radiata), a non-traditional species in NAFO Divisions 3L, 3N, 3O and subdivision 3Ps

By D. Kulka and F. K. Mowbray

Abstract

The exploitation of thorny skate has increased in recent years following the decline in the traditional groundfish resources in the waters around Newfoundland. Prior to 1994, skate in Canadian waters were taken only as bycatch, most of which were discarded. However with the establishment of a directed fishery for skate on the southwestern Grand Bank and southern St. Pierre Bank in 1994, domestic landings have averaged about 3,000 t annually. This paper represents the third assessment of this "new" fishery. This evaluation has shown that biomass indices, following a decline to their lowest historic level may have stabilized in recent years. However, a change in survey gear (in the fall of 1995) with different catch characteristics has created a discontinuity in the time series and prevents a comparison between the two periods. Survey data from the fall was examined for the first time. It was found that the spring survey previously used to estimate biomass and abundance may not include a substantial portion of the population. On average, fall survey estimates of biomass were 40% higher. In addition, differences in spring and fall distributions and seasonal change in bycatch rates of skate from Grand Bank slope fisheries suggests that thorny skate migrate toward the shelf edge in the winter, a portion of the population out of the surveyed area. They re-aggregate onto the bank in midsummer and throughout the fall. Analysis of lengths taken during the research surveys have covered a consistent range since 1985 with main modes occurring at 15 to 32 cm and 65 to 83 cm, the latter comprising mature fish. A recent increase in proportion of larger skates in the catches is noted. A spatial analysis of skate size shows a degree of segregation by size, the largest fish tending to aggregate along the southwestern shelf edge with a mix of sizes in the bank aggregation to the north. A comparison of distribution of skate from surveys data with commercial grounds shows that the Canadian fleet fishes about 1/3^rd of the area of high concentration of skate in the spring, primarily along the shelf edge where the skate are largest. The Spanish fleet fishes a separate area on the Southeast Shoal outside 200 miles in the fall taking smaller fish. A relationship of both skate density and size with temperature and depth was observed, denser concentrations, in particular larger fish, were associated with warmer, deeper locations.

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