The Algerian coast as a seasonal corridor for the basking shark Cetorhinus maximus: Leveraging three decades of citizen science and bycatch data in the southwestern Mediterranean

Abstract

The basking shark (Cetorhinus maximus), an endangered filter-feeding elasmobranch, is subject to significant anthropogenic pressures in the Mediterranean Sea. Despite its ecological importance, data from the North African coast are limited. This study presents the first multi-decadal analysis (1990-2025) of C. maximus occurrences in the Algerian Basin (GSA 04). A robust dataset (N=27), compiled from fisheries records, direct sightings, and citizen science contributions (33.3% of records), was used to investigate the environmental drivers of its distribution. Statistical analyses revealed a significant increase in observation frequency since 2019 (Mann-Kendall τ=0.362, p<0.001). Occurrences exhibited a clear seasonal pattern, peaking in June. Bycatch accounted for the majority of records (63.0%), primarily associated with driftnet fisheries in the western sector. A strong positive correlation was found with sea surface temperature (Spearman’s ρ=0.68, p=0.021). In contrast, a negative relationship with primary productivity (ρ=-0.61, p=0.038) suggests a “trophic lag” whereby shark presence coincides with post-bloom conditions. Sea surface salinity (SSS) was not a significant predictor (ρ=0.12, p=0.45), indicating a broad tolerance to the hydrographic gradients of the Algerian Current. These findings highlight the Algerian coast as an important seasonal migratory corridor. The observed overlap between shark presence and intensive fishing activity underscores the urgent need for conservation measures. We recommend integrating citizen science into regional monitoring programs and implementing seasonal fishing gear restrictions to reduce bycatch mortality and support the long-term persistence of this species in the Mediterranean.