Fisheries ecology of common wolffish, Anarhichas lupus, in the North Sea
The common wolffish (Anarhichas lupus), a benthivorous species, is widely dispersed, although not abundant in the North Sea north of 54ºN latitude. Despite being widely distributed in sub Arctic and cold North Atlantic waters, little is known of the ecology of this species in the North Sea. This thesis aimed to provide ecological information for this species at the southern limit of its range in the Northeast Atlantic. North Sea common wolffish are regarded as sedentary in habit and mainly inhabited depths ranging from 60 m to 150 m. The bottom temperatures where this species were found in the North Sea ranged between 3ºC and 12ºC. The common wolffish is a by-catch species for the North Sea bottom trawl fishery. The long-term catch data and CPUE (catch per unit effort) indicated that the stock abundance of North Sea common wolfish has been decreasing over the last decades. However, it is suggested from VPA (virtual population analysis) and Yield-per-recruit analysis, that the North Sea common wolfish stock is still in a safe condition but has been overexploited. The means of total mortality (Z), natural mortality (M) and fishing mortality (F) of this species for 1996-1998 were 0.47, 0.17 and 0.30, respectively. Both resting metabolic rate (RMR) and maximum metabolic rate (MMR) were low for this species. RMR and MMR were measured for six adult common wolffish (mean weight, 1.39kg) at 5ºC and 10ºC. At 5ºC the mean RMR± SE was 12.18 ± 1.6 mgO(_2),kg(^-1)h(^-1), and mean MMR ± SE was 70.65 ± 7.63 mgO(_2)kg(^-1)h(^-1). At 10ºC the mean RMR±SE was 25.43 ±1.31 mgO(_2) kg (^-1)h(^-1), and mean MMR ± SE was 113.84 ± 16.26 mgO(_2) kg(^-1)h(^-1). Absolute metabolic scope was 53% greater at 10ºC than at 5ºC.The main food for common wolffish in the North Sea were Decapoda comprising 39 % of the overall diet by occurrence, with hermit crabs (Paguridae) being the dominant species. Other important dietary components were Bivalvia (20 %) and Gastropoda (12 %). Sea urchins (Echinidae), which are important prey of common wolffish in eastern Newfoundland, Gulf of Maine and Iceland, but typically of low energy value, occupied only 7 % of the diet. During the autumn/winter season, wolffish consumed less food compared with the remainder of the year. From this study, von Bertalanffy growth parameters for common wolffish in the North Sea were calculated to be: for male, L(_oo)= 111.2 cm, t(_o) = -0.43 and = 0.12; and for female, L(_oo)= 115.1 cm, t(_o)= -0.39 and K = 0.11, making North Sea common wolffish the fastest growing stock of this species reported. The fast growth of North Sea common wolffish may be a result of the higher energy content of the diet or greater metabolic scope available for food processing. This increased metabolic scope being associated with life in the warmer North Sea, in comparison to those stocks found elsewhere in the range of this typically sub-Arctic species. The maximum age found for this species in the North Sea was 18 years old. Median length at sexual maturity of male and female common wolffish is at 51.9 cm and 50.4 cm respectively, with the age of sexual maturity being achieved at 4-5 years in the North Sea. The spawning season of North Sea common wolffish is from October through to February. The fecundity varied with the body weight of females, from several thousand to 12,000 eggs. Ripe egg sizes ranged from 5 mm to 6.5 mm. Spermatozoa were found in mature male individuals throughout the whole year with the highest peak in October.