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Anarhichas lupus is a boreo-Arctic species with biological characteristics often associated with vulnerability to over-exploitation. Although not commercially targeted in the North Sea, A. lupus is a bycatch species in mixed demersal fisheries. Here we provide an overview of the status of A. lupus in the North Sea, as observed from commercial landings and fishery-independent trawl survey data. A. lupus was once common across much of the central and northern North Sea but, since the 1980s, have declined in abundance, demographic characteristics (reduced size) and geographical range, with the shallower and more southerly parts of its range most impacted. A. lupus is still relatively frequent in the northern North Sea, where fishing intensity, though decreasing, is high. Bycatch through fishing remains a potential threat and, considering the likely impacts of predicted climate change on cold-water species, risks of further regional depletion and/or range contraction remain. Whether or not A. lupus is able to re-establish viable populations in former habitat in UK coastal waters is unknown. Given the lack of data, the precautionary principle would suggest that manageable pressures be minimised where the species and its habitat are at risk of further impacts, and more regular assessments of population status be undertaken. This article is protected by copyright. All rights reserved.

Fecundity, maturity and the relationship between growth and maturity of common wolffish Anarhichas lupus were studied in Icelandic waters. A total of 788 female common wolffish were sampled in two areas: one in the relatively warm sea west of Iceland and the other in the colder sea east of Iceland. No difference was detected in fecundity of common wolffish between areas. The time from the onset of the cortical alveolus stage until spawning, was on average, 10 years in the east and 8 years in the west area. Common wolffish in the east area reached cortical alveolus stage, on average, at a greater age but similar size compared to common wolffish in the west area. Similarly, common wolffish started spawning, on average, at greater age and larger size in the east than in the west area. Common wolffish grew faster in the west than in the east area. Spawning common wolffish grew faster than common wolffish at the cortical alveolus stage in both areas. The relationship between growth and maturity for common wolffish in Icelandic waters appeared to be related to temperature, characterized by fast growth and early maturation in the west and slower growth and delayed maturation in the east.

Wolffish Anarhichas lupus (Anarhichadidae) hatch from large demersal eggs (c. 5-6 mm) after a long incubation period (c. 800-1000 degree days) at 20-25 mm L T with a juvenile appearance: a small yolk sac, pigmented but partly transparent skin densely supplied with mucous cells, well developed body musculature and a cartilaginous skeleton. The results of histomorphological studies illustrate the apparent functionality of tissues and organs at hatching. The mucosa of the oral cavity and various parts of the digestive channel are well differentiated and the cellular characteristics are described within each region. A stomach with gastric glands is established before hatching. The pancreas has well developed endo- and exocrine tissue and the hepatocytes are vacuolized. The kidney tissue has numerous tubuli and haematopoietic tissue in the bilobed head region. The spleen, thymus and thyroid glands also appear mature. Cones are dominant in the visual cell layer of the retina.

No significant differences in mean mass between groups were found at any time in spotted wolffish Anarhichas minor, mean (± S.D.) initial mass 76 (± 21) g, reared at salinities of 12, 17, 25 and 34‰ for 12 weeks at 8° C. Salinity did not have a significant effect on daily feeding rates, total food consumption, food conversion efficiency and protein efficiency ratio. Growth trajectories varied between groups, but no overall difference in growth was found. Plasma osmolality and plasma chloride levels decreased with salinity in a 48 h abrupt exposure trial, and in the growth experiment the low salinity groups (12 and 17‰) exhibited significantly lower values compared with 25 and 34‰. The decrease was moderate and concentrations were well within the range described for other marine species. The results indicate that the spotted wolffish is a strong osmoregulator which could be reared at various salinity levels.

Mature common wolffish, Anarhichas lupus L., previously held at 8°C for several months, were exposed to 4, 8 or 12°C throughout the breeding season (November-January). Regardless of temperature, some females ovulated in early-November, but thereafter ovulation was delayed in the 8 and 12°C groups compared with the 4°C group. Temperature experienced by the adult fish also affected egg development and survival even though eggs were incubated at a common temperature of 6°C. The percentages of normally cleaved eggs (at the 32-cell stage) and egg survival to the eyed stage were significantly lower in the 12°C group (8%) than in the 4°C (73%) and 8°C (48%) groups. The differences in egg survival were also significant between the 4 and 8°C groups. Egg survival was also influenced by the length of time the broodfish had been exposed to the different temperatures: at 4 and 12°C there were significant positive and negative relationships, respectively, between egg survival and the number of days for which the female had been exposed to the given temperature prior to ovulation. Temperature experienced by the female did not affect egg size, relative egg weight, fertilization rate, relative fecundity or the number of ovulating individuals. The results indicate that both the timing of ovulation and egg quality of common wolffish are affected by the temperature experienced during the breeding season.

The von Bertalanffy growth parameters for common wolf-fish Anarhichas lupus in the North Sea were: male: L∞ = 111.2 cm, t 0 = -0.43 and K=0.12; and female: L∞=115.1 cm, t 0 = -0.39 and K=0.11, making this the fastest growing stock reported. Resting metabolic rates (RMR ± S.E.) and maximum metabolic rates (MMR S.E.) for six adult common wolf-fish (mean weight, 1.39 kg) at 5° C were 12.18 ± 1.6 mg O 2 kg -1 h -1 and 70.65 ± 7.63 mg O 2 kg -1 h -1 respectively, and at 10° C were 25.43 ± 1.31 mg O 2 kg -1 h -1 and 113.84 ± 16.26 mg O 2 kg -1 h -1 Absolute metabolic scope was 53% greater at 10° C than at 5° C. The diet was dominated by Decapoda (39% overall by relative occurrence), Bivalvia (20%) and Gastropoda (12%). Sea urchins, typically of low energy value, occupied only 7% of the diet. The fast growth probably resulted from summer temperatures approximating to the optimum for food processing and growth, but may have been influenced by diet, and reduced competition following high fishing intensity.

Temperature treatment of common wolffish Anarhichas lupus during vitellogenesis affected the time of final maturation; ovulation in fish held at 8 and 12° C from mid-April to October was about four and five weeks delayed, compared with a 4° C group. Fish in the 8° C group had significantly larger eggs than those in the 4° C and the 12° C groups, and a significantly higher egg production than fish in the 12° C group. Temperature treatment did not affect either fertilization rate or relative fecundity, but absolute fecundity was significantly lower in the 12° C group than the other groups due to poor growth of the fish at high temperature. This did not affect the numbers of spawning individuals. There was a trend towards lower egg survival to the eyed stage in the 12° C group compared to the 4 and 8° C groups, although the effect was not statistically significant. The results indicate that both the timing of final maturation and investment in ovarian growth in common wolffish are affected by temperature experienced during vitellogenesis.