[show abstract][hide abstract] ABSTRACT: 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 (~55Ꮣ 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 ~70Ꮲ 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.
[show abstract][hide abstract] ABSTRACT: The northeastern section of the Alboran Sea is currently under consideration as a Special Area for Conservation under the European Union's Habitat Directive. Within this framework, the present study focuses on the distribution of cetaceans in this area and is part of the Spanish Ministry of the Environment's “Program for the Identification of Areas of Special Interest for the Conservation of Cetaceans in the Spanish Mediterranean”. Shipboard visual surveys were conducted in 1992 and from 1995 to 2001 in the north-eastern Alboran Sea, covering 14,409km. A total of 1,134 sightings of cetaceans were made. From the data collected, the distribution of seven species of odontocete was examined with respect to two physiographic variables, water depth and slope. Analyses of χ2 and fitting of GLMs demonstrated significant differences in distribution for all species, mainly with respect to depth. Kruskal–Wallis tests, factor analysis and discriminant function analysis showed that the species could be classified in two major groups, shallow-waters (short-beaked common dolphin and bottlenose dolphin) and deep-waters (striped dolphin, Risso's dolphin, long-finned pilot whale, sperm whale and beaked whale), respectively. Preferred habitats in terms of water depth were areas deeper than 600m for the deep-water group, and the shallower ranges from shore to 400m for the other. The distribution of cetaceans was further matched with that of their most common prey in order to establish which habitats could be considered important for their feeding. The resulting analysis highlighted two areas in the region as important habitats for the conservation of the most vulnerable species in the Mediterranean, the bottlenose and the common dolphin.
Deep-sea Research Part I-oceanographic Research Papers - DEEP-SEA RES PT I-OCEANOG RES. 01/2002; 49(11):2053-2073.
[show abstract][hide abstract] ABSTRACT: Factors responsible for aggregations of jumbo flying squid Dosidicus gigas, an important component of the marine food web and target of commercial fisheries off the Costa Rica Dome in the Eastern Tropical Pacific Ocean (ETP), were examined during 2 years of different extreme oceanographic conditions: fall 1997 El Niño and fall 1999 La Niña. A high abundance of squid occurred in association with the well-developed countercurrent ridge (upwelling) off the Costa Rica Dome during fall 1997, but not during fall 1999, when the countercurrent ridge was less developed. Two features of the well-developed countercurrent ridge were considered important for the occurrence of high jumbo flying squid concentrations. Firstly, subsurface chlorophyll a (chl a) maxima were formed along the countercurrent ridge, resulting in integrated chl a concentrations in the upper 100 m being relatively high considering the generally low productivity of the ETP during an El Niño event. Secondly, a strong salinity front formed along the North Equatorial Countercurrent, which is possibly responsible for retention of jumbo flying squid in the ridge. Large yellowfin tuna Thunnus albacares, which mainly feed on micronekton (small fishes, cephalopods and swimming crabs), as do jumbo flying squid, were also more highly concentrated along the countercurrent ridge during 1997 than during 1999. It was noted that skipjack tuna Katsuwonus pelamis and small yellowfin tuna, which mainly feed on zooplankton, were associated with the equatorial ridge in the ETP, indicating that prey faunal components may also play an important part in the close association of jumbo flying squid with the countercurrent ridge.
Marine Ecology-progress Series - MAR ECOL-PROGR SER. 01/2002; 231:151-166.
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