Aspects of the biology and fishery for monkfish Lophius piscatorius Linnaeus 1758 in waters around the Shetland Isles, Northeastern Atlantic
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 variation 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 paired fins, was also described. The autumn offshore migration may be related to changes in prey availability and / or the reproductive cycle.