Aspects of the Biology and Fishery for Monkfish (Lophius piscatorius Linnaeus 1758) in Waters Around the Shetland Isles, Northeastern Atlantic.

Through the development of a targeted fishery, mainly by demersal trawl, the fishery for monkfish, also known as the monk or anglerfish increased in Scotland from 6100 tonnes in 1986 to a peak of 25000 tonnes (live wt.) in 1996. This was partly in response to the decline in traditional demersal whitefish species. Lophius piscatorius Linneaus, 1758, and the congener L. budegassa are caught and sold together but the latter only comprises 1% of the catch in Shetland (Scotland, UK). During the mid-1990s the total value of landings of monkfish into Scotland exceeded those of cod Gadus morhua and of whiting Merlangius merlangus. Landings of monkfish have declined since 1996 but quayside prices are greater than those for the majority of other species. Available data have shown that, of the Lophius fisheries worldwide, the fishery in the Northeastern Atlantic is the largest.
In this study the biology of, and fishery for L. piscatorius was investigated primarily in the area around the Shetland Islands and several new findings have emerged. It was found that catch rates in the commercial fishery were generally between 5 and 15 individuals per hour towing time although there were differences between grounds and between seasons. There was evidence of a negative trend in fish size over time on one of the main fishing grounds during the course of this study. A bigger-deeper trend was found and at inshore areas a negative trend between CPUE and depth was also found. Some inshore areas may act as nursery areas.
A tagging study was undertaken and recapture locations indicated a pattern of
offshore migration during the autumn and winter. Some movement between different inshore grounds also occurred. The mean growth rate obtained from recaptured fish was 9cm/year, similar to the 8-12 cm/year estimated from cohorts identified in length-frequency distributions. These tended to be higher than growth rates predicted from age determinations using otoliths. Lengths-at-ages given in this study tended to be higher than those previously reported.
Analysis of the diet suggested opportunistic feeding, with both seasonal and
geographical variations recorded. Gadoids were important, particularly during the
winter and spring but sandeel (Ammodytidae spp.) dominated the diet during the
summer. In situ feeding behaviour, including a direct opportunistic attack on a cod,
was described for the first time and several behaviours, previously unrecorded in the
species, e.g. recess creation using digging and scraping behaviours, breath holding
and spine raising in relation to the proximity of potential prey, were identified.
“Walking” behaviour using the pair fins was also described. The autumn offshore migration may be related to changes in prey availability and / or the reproductive cycle. Estimated lengths at maturity (L50%) for males and females
in the Shetland area were 58 cm (4+ years) and 98 cm (11+ years) respectively. The bigger-deeper trend suggests a deep-water spawning and mature females were scarce in the fishery on shelf areas. The spawning season was estimated to occur mainly between December and the end of May.
In light of the current findings several key areas have emerged that warrant further research. In relation to the management of Lophius, because of its morphology, biology and in light of the current findings (e.g. possible inshore nursery areas, an autumn offshore migration in conjunction with previously suggested spawning areas), it is suggested that a management approach incorporating effort limitation and/or restricted areas may warrant further consideration, as it is clear that effective management cannot easily be achieved by measures to increase gear selectivity in the mixed demersal trawl fishery of which the monkfish fishery is part.