Abstract
The observations of basking sharks (Cetorhinus maximus) along the coast of South France are neither regular nor abundant, despite annual phytoplankton growth. We have studied the evolution of the chlorophyll-a concentration of the Gulf of Lion, the unique bioregion of non-coastal Mediterranean bloom, and the northwest bloom bioregion. The spring of 2017 was characterized by many observations in a short period, most of them made on the west side of the Gulf of Lion. These observations correlate significantly with a regional concentration in chlorophyll-a lower than usual. They are also associated with a higher phytoplanktonic development in the Gulf of Lion than in the Liguro-Provencal basin, where the annual bloom occurs. A relationship may exist with the distance to the zone of bloom end. Those elements conducted to a hypothesis of "profitable sharks". These animals would come every year to feed themselves offshore the French coast. However, due to the premature extinction of the bloom in the spring of 2017, the animals searched for sectors with the highest energetic profitability from the zone of bloom termination. They left again in the last weeks following its end. The implication of global warming in the 2017 environmental anomaly phenomenon seems likely but is not demonstrated.