Bayesian occupancy modelling of benthic Crustacea and the recovery of the European spiny lobster, Palinurus elephas

There is ever-greater need for information about changing marine biodiversity, but such information is sparse at large spatial or temporal scales. Records about distributions of species collected by volunteers can fill gaps in knowledge that cannot yet be addressed by more structured sampling. Bayesian occupancy models show great promise for estimating trends in occurrence of species through time. This study uses the Sparta occupancy model with records from the Seasearch programme from coastal waters of Britain and Ireland during the period 2000-2020, focussing on three species of Crustacea (Cancer pagurus, Homarus gammarus and Palinurus elephas). Populations of P. elephas crashed in the 1970s, but now appear to be re-establishing in south-west England. The Sparta model provides evidence about recovery that is more robust than anecdotal reports or simple counts of records. Estimates of occupancy are made at different spatial scales and compared among species and areas. Trends in occupancy are compared qualitatively with patterns in fisheries landings data. Occupancy by P. elephas has increased drastically since 2014, a pattern not seen in the other two species. For each species, occupancy varied among areas and in some areas, patterns in estimates of occupancy were similar to trends in landings from fisheries. Citizen science records are increasingly recognized to have value which has not yet been fully exploited. Greater use should be made of the Seasearch dataset in order to provide population trends for benthic marine taxa. Such analyses will broaden our understanding of and taxonomic coverage of changes in biodiversity.