Distribution and relative abundance of sperm whales in relation to key environmental features, squid landings and the distribution of other cetacean species in the Gulf of California, Mexico
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Sperm whales (Physeter macrocephalus) feed predominantly on meso- and bathypelagic cephalopods for which effective sampling methods have not been developed. The Gulf of California is one of the very few areas where sperm whales might feed on a commercially fished species of squid (jumbo squid, Dosidicus gigas), presenting a unique opportunity to investigate the impacts of variations in jumbo squid abundance on sperm whale distribution. This study examines sperm whale distribution and relative abundance in relation to the distribution of D. gigas, other cetacean species and key environmental features over spatial scales ranging from a few kilometers to a several hundreds of kilometers. Data were collected during two field seasons in spring–summer 1998 and 1999 using non-invasive techniques. Landing statistics show that the jumbo squid fishery collapsed in 1998 and started recovering in early 1999. Despite this collapse in 1998, sperm whales remained abundant during both years, but there were strong differences in their aggregative behavior. In 1998, sperm whales were roughly evenly distributed, while in 1999, there were three super-aggregations (~55×75 km across), which were stable for over a month. During both 1998 and 1999, sperm whales were uniformly distributed with respect to mean depth, slope and sea surface temperature over spatial scales of ~10, 19, and 37 km segments and over areas of ~70×90 km. There was no close association between sperm whale distribution and the distribution of jumbo squid landings in 1998. In 1999, about two-thirds of the individuals were found in areas of possibly high jumbo squid biomass. There was a significant correlation between the occurrence of sperm whales and that of bottlenose dolphins (Tursiops truncatus), despite the fact that they usually inhabit different water depths. This is the first study which was able to relate sperm whale distribution and relative abundance to the abundance of their main prey items. It suggests that sperm whales change their distribution in response to a decline in jumbo squid but that they do not leave the Gulf of California. However, this study encompassed only 2 years and further investigations are needed to gain an understanding of what may trigger large-scale movements.
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