Abundance, interaction and movement in a European lobster stock

Abstract

European lobsters form one of the most economically valuable portions of UK landings, yet they are little regulated, despite stocks being considered fully exploited. Biological and behavioural knowledge is lacking, managerial effort is low and understanding is often inferred from other species. To ensure continued productivity of this important fishery, improved data on fishing activity, population dynamics, catchability, recruitment, movement and distribution are urgently required. Through analysis of capture-mark-recapture data, fishery-independent catches, behavioural-interaction studies and acoustic telemetry tracking, this thesis aims to provide a basis for future research and management. Capture-mark-recapture (CMR) and fishery-independent catch data established estimates of density, proportionate distribution, movements and site-fidelity and catchability parameters. These revealed high site fidelity and catchability differences between sexes leading to female-skewed density estimates. If these findings are corroborated, the effect and causes of disproportionate sex ratios must be addressed. The mixed-species nature of UK shellfisheries led to studies recording the impact of inter-specific and intra-specific interactions on catchability and catch rates. Lobster presence significantly lowered catchability of crab species and occurrences of same-sex lobster pairings were lower than expected. Findings highlight both the inconsistency of using catch per unit effort (CPUE) as a direct index of abundance and the danger of analysing crab and lobster catch data in isolation from each other. The final study employed an acoustic telemetry array to quantify in situ lobster movement, providing unique information on short-term home-ranges and habitat-utilisation. There were both transient and resident portions of the population, not predictable by sex or size. Males had significantly larger home-ranges than females, which could explain their increased catchability estimated in the CMR study. In contrast to trap catch data, most lobsters were recorded using soft substrate outwith their home-range. Movement behaviour changed accordingly, from ‘searching’ behaviour on mixed and hard substrates to ‘exploratory’ behaviour on soft. This highlighted a potential connectivity between isolated rocky habitats. The present study reveals the importance of undertaking local lobster studies in order to elucidate behavioural traits and highlight sampling uncertainties that can have important impacts on methods of stock assessment. Findings provide an initial baseline for further data collection, allowing changes in the population to be monitored.