Body condition decline in Western European fishes

The body condition of small pelagic fish has been declining since the early 2000s in several ecosystems, including the Mediterranean Sea and the Bay of Biscay, probably in response to bottom-up control. Possible trends in body condition of demersal and benthic species in the same regions have so far received little attention. Using a North East Atlantic case study (Bay of Biscay and Celtic Sea), we computed Le Cren's relative body condition index for 264,000 fish individuals from 17 species, based on individual length and weight data collected between 2002 and 2022. Using linear models, a decreasing trend in body condition was evidenced for 12 species, while only two species showed an increase. Anchovy (Engraulis encrasicolus), plaice (Pleuronectes platessa), megrim (Lepidorhombus whiffiagonis) and sardine (Sardina pilchardus) have suffered the greatest declines. For these species, individual weight at equivalent size decreased by 3-5 % per decade. The variability between individuals' body condition was mainly explained by sex and sampling season. Among the other variables tested, stock biomass and chlorophyll-a concentration had significant effects on a majority of species, while sea surface temperature did not explain much variability. The declining trend of body condition at the fish community scale could then be partly explained by bottom-up control, but probably also involved complex interactions at the ecosystem scale. Beyond the possible consequences on fish provisioning quality, the prevalence of declining body condition of various fish should be considered as a warning on possible important changes in the overall functioning of marine food-webs.