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Alternate Title

Abundance and Seasonal Occurrence of Cetaceans in Outer Continental Shelf and Slope Waters of the North-Central and Northwestern Gulf of Mexico

Abstract

Eight aerial line-transect surveys of outer continental shelf and continental slope waters (range 100-2,000 m deep) were conducted seasonally from summer 1992 through spring 1994 in the north-central and northwestern Gulf of Mexico to study the seasonal occurrence and spatial distribution of cetaceans and to estimate their abundances. The surveys sampled an 85,815 km2 study area, resulting in 49,960 km of effort and sightings of at least 18 cetacean species and 365 cetacean groups. Eight species identified in four seasons included bottlenose dolphin (Tursiops truncatus), pantropical spotted dolphin (Stenella attenuata), Risso's dolphin (Grampus griseus), dwarf sperm whale (Kogia sima) and pygmy sperm whale (Kogia breviceps), sperm whale (Physeter macrocephalus), short-finned pilot whale (Globicephala macrorhynchus), rough-toothed dolphin (Steno bredanensis), and Atlantic spotted dolphin (Stenella frontalis). Clymene dolphin (Stenella clymene), striped dolphin (Stenella coeruleoalba), and beaked whales (Mesoplodon spp.) were sighted in three seasons. The number of species sighted seasonally ranged from 10 in fall to 15 in winter. The overall estimated abundance (number of animals) of five species, which accounted for 71% of the identified group sightings, were as follows: bottlenose dolphin, 2,890 (coefficient of variation [CV] = 0.20); pantropical spotted dolphin, 5,097 (CV = 0.24); Risso's dolphin, 1,237 (CV = 0.28); dwarf-pygmy sperm whale, 176 (CV = 0.31 ); and sperm whale, 87 (CV = 0.27). Melon-headed whales (Peponocephala electra) were sighted less frequently but were abundant (2,561; CV = 0.74) because of large group sizes. Common species were widely distributed spatially but occurred in different water depth ranges. In general, species abundance estimates varied seasonally, but the precision of estimates was usually poor (CV > 0.30) and provided little power to detect significant seasonal differences.

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