Local diver knowledge reveals decline in scalloped hammerhead sharks (Sphyrna lewini) at seamounts in the southwestern Gulf of California

Abstract

The Gulf of California is a marginal sea that has seen widespread species declines due to fishing. The scalloped hammerhead shark (Sphyrna lewini) is a semi-pelagic species that form large schools at seamounts where they refuge during the day. El Bajo seamount off Espíritu Santo Island and Las Animas seamount off San José Island in the southern Gulf of California are long-established scuba-dive destinations for observing S. lewini. In this study local scuba-diver knowledge was used to determine changes in S. lewini school abundance over a 50 year period between 1970 and 2020. The abundance of S. lewini encountered per dive at El Bajo was reported to have significantly declined, falling by 97%, from an average of 150 sharks in the 1970 s to 5 sharks in the 2010 s and at Las Animas by 100%, from an average of 100 sharks in the 1970 s to 0 sharks in the 2010 s. A shifting baseline was evident and there was a significant relationship between the year participants had first dived the seamounts and the number they considered to be a ‘very abundant’ school of S. lewini (GAM p < 0.001). This study demonstrates the importance of collecting local ecological knowledge to document declines in species populations, especially for S. lewini which is currently listed as ‘Critically Endangered’ on the IUCN Red List.