Contrasts in the movements and habitat use of juvenile and adult white sharks (Carcharodon carcharias) at Guadalupe Island, Mexico

Abstract

Although the effort in the study of white sharks in Mexico is rapidly elucidating adult biology, almost nothing is known about the juveniles. Current understanding of this life history is based largely on the incidental take of juveniles in nursery grounds in the Pacific coast of Baja California and some individuals tagged in the USA that have migrated to Mexican waters. Also, it is not known how or when they recruit to adult aggregation sites or how they learn to make seasonal migrations offshore. Five white sharks were manually tracked using ultrasonic transmitters with depth and temperature sensors between 2006 and 2007. Additional white sharks were tagged (N = 60) with long-lived coded transmitters and detected at listening stations located on the west coast of the United States of America and Guadalupe Island (GI) from August 2008 to October 2015. We found that: (1) juvenile white sharks remained close to the island throughout the day between the surface and 50 m depth in warm waters (from 14 to 20 °C), whereas the adults moved offshore into deep waters during the day and stayed close to the island during the night presenting a broader tolerance of colder waters (from 9 to 20 °C); (2) tagged white sharks had a positive correlation between total length and habitat range, and the core areas of adults were related to pinniped colonies; (3) adults patrolled in deep waters in November and December when the northern elephant seals (NESs) returned back to the island for pupping with their mean mass higher than during the winter post-breeding migration; (4) tagged juvenile white sharks remained near the island for 12–14 months before departing; and (5) tagged subadults undertook coastal migrations before starting their offshore migrations. The data collected in our study suggest that white shark juveniles arrive to GI from nursery grounds on the mainland after they have reached at least 180 cm TL; then, they remained around the island for several months, potentially taking advantage of the diversity of prey. In addition, they may start their first offshore migrations, coming back to their nursery grounds and GI before they reach maturity, while at GI juveniles stayed close to the shore and in shallow water to avoid adults, probably feeding on demersal prey and species that perform nocturnal migrations such as squid and mackerel. It is argued that the distribution of the large white sharks in GI is controlled by the availability of NES and that adult white sharks look for this prey in deep waters during the day in the vicinity of the seal colonies, taking advantage of the great visibility of GI waters. It is also possible that white sharks take advantage of NES in GI before they go to their pupping grounds to give birth in California and Baja California or to their offshore migration to the west.