Data from: Maiden voyage into death: are fisheries affecting seabird juvenile survival during the first days at sea?

The study of juvenile migration behavior of seabird species has been
limited so far by the inability to track their movements during long time
periods. Foraging and flying skills of young individuals are assumed to be
inferior to those of adults, making them more vulnerable during
long-distance migrations. In addition to natural oceanographic effects and
intrinsic conditions, incidental seabird harvest by human fisheries is one
of the main causes for worldwide seabird population declines, and it has
been hypothesized that juveniles are particularly vulnerable to bycatch
during their first weeks at sea after leaving the nest. We used solar
powered satellite tags to track the at sea movements of adults and
juveniles of Scopoli’s shearwater (Calonectris diomedea) after the autumn
departure from their breeding colony in Chafarinas Islands (southwestern
Mediterranean Sea). Eighty percent of juvenile tags stopped transmitting
during the first week at sea, within 50 km of their natal colony, in an
area with one of the highest concentrations of fishing activities in the
Mediterranean Sea. All adult birds tagged and only twenty per cent of
juveniles migrated into the Atlantic and southwards along the coast of
West Africa. The two age groups showed different habitat preferences, with
juveniles travelling farther from the coast, in windier and less
productive waters than adults. We conclude that Scopoli’s shearwater
juveniles are particularly vulnerable to mortality events, and we
highlight that fisheries, along with differential age-related behavior
skills between adults and juveniles, are likely causes of this mortality.
Overall, our study highlights the importance of conducting tracking
studies during the first stages of juvenile migration.