Sea temperature shapes differences in European sardine (Sardina pilchardus) growth in two adjacent areas in the NW Mediterranean Sea.

Sardina pilchardus population in the NW Mediterranean Sea has experienced an important decline in biomass in the last decades. In the Gulf of Lions, a shift in growth performance in 2008 led to the collapse of the sardine fishery that persists today. While several hypotheses have been proposed, their basis remain unclear. This study investigates the influence of key environmental factors (sea surface temperature and primary production) along with biological traits (size and age at first maturity, spawning period and body condition) on sardine growth. Firstly, we analyse the growth performance of sardines collected monthly during 2017 and 2018 in two close areas (90 km apart), with distinct temperature conditions. Growth differences were observed that would be linked to temperature. The higher temperature in the South would favour faster juvenile growth, resulting in larger sizes-at-age 1 and older classes. Biological traits were similar between areas, therefore, would not explain the observed differences in growth performance. Secondly, this study examines temperature and chlorophyll-a trends (2002-2022) to provide insight into the ongoing NW Mediterranean sardine crisis. A temperature shift identified in 2007 would explain the reported changes in growth in 2008. Additionally, differences in the phenology of the phytoplankton growing period between areas were identified. The North area is experiencing an earlier onset of oligotrophic conditions, which would result in a reduction of food availability for sardine juveniles in the spring-summer growing period, which would be an additional stressor for sardines in the North area.