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Fishery Bulletin 92|4), 1994 



in all seasons except summer. The number of species 

 or species groups sighted in summer, fall, winter and 

 spring was 12, 10, 6, and 10, respectively. Seasonal 

 sighting rates of all cetacean herds ranged from 0.91 

 herds/100 km (winter) to 2.01 herds/100 km (fall) and 

 those of all animals sighted ranged from 16.80 animals/ 

 100 km (summer) to 52.25 animals/100 km (fall). 



The relative abundance of several species or spe- 

 cies groups varied seasonally (Table 4 ). Sighting rates 

 (herds and animals) of Risso's dolphins showed a dis- 

 tinct peak during spring. During April alone, 30% of 

 the Risso's dolphin herds and 48% of the animals 

 were sighted. Sighting rates of sperm whales and 

 Atlantic spotted dolphins peaked in the fall. Sight- 

 ing rates of bottlenose dolphins and pygmy and dwarf 

 sperm whales were highest during summer and fall. 

 SSC dolphins exhibited similar herd sighting rates 

 in each season, but the animal sighting rate was 

 much lower during summer. SSC dolphin herds av- 

 eraged only about 20 animals in summer but near 

 100 or more during other seasons. 



Cetaceans were sighted throughout the area sur- 

 veyed (Fig. 2). Each species or species group sighted 

 20 or more times had a wide spatial distribution and 

 was sighted in all three zones (Fig. 2, Table 5). Ten 

 species were sighted in the eastern zone, 13 in the 

 central zone, and nine in the western zone. Sighting 



rates of all herds sighted were generally similar in 

 the eastern and central zones (1.67 and 1.71 herds/ 

 100 km, respectively) and a little lower in the west- 

 ern zone ( 1.05 herds/100 km). However, because the 

 mean herd size of all cetaceans sighted from the east- 

 ern zone (35.8 animals) was larger than those of the 

 central (19.2 animals) and western (21.5 animals) 

 zones, the sighting rates of animals were more vari- 

 able: 59.5 animals/100 km (eastern), 32.7 animals/100 

 km (central), and 22.6 animals/100 km (western). 



Sighting rates of species or species groups sighted 

 20 or more times varied by zone (Table 5). Except for 

 Atlantic spotted dolphins and pantropical spotted dol- 

 phins, herd sighting rates were lowest in the west- 

 ern zone. Except for pantropical spotted dolphins, 

 the animal sighting rates were also lowest in the west- 

 ern zone. Sighting rates (herd and animal) of Risso's 

 dolphins and SSC dolphins were highest in the east- 

 ern zone. In the central zone, sighting rates of sperm 

 whales, bottlenose dolphins, Atlantic spotted dolphins, 

 and pygmy and dwarf sperm whales were highest; those 

 of pantropical spotted dolphins were lowest. 



Risso's dolphin sightings in the eastern part of the 

 study area were generally concentrated near the 200- 

 m isobath. Most of the sperm whale sightings (65%) 

 occurred southeast of the Mississippi River delta 

 along the 1,000-m isobath. Of 39 bottlenose dolphin 

 herds sighted, 19 were sighted in waters less than 

 100 m deep, at the head of the Mississippi Canyon 

 and on the Upper Mississippi Fan. A concentration 

 of pygmy and dwarf sperm whales occurred along 

 the western Mississippi Canyon. Most of the SSC 

 dolphin sightings occurred on the Upper Mississippi 

 Fan and in DeSoto Canyon. Four of the five pilot 

 whale herds sighted were encountered on the Upper 

 Mississippi Fan on 4 November 1989. The only ba- 

 leen whales sighted, one fin whale, Balaenoptera 

 physalus, in the fall and one Bryde's whale, B. edeni, 

 in the summer, were both sighted in waters about 

 200 m deep in the DeSoto Canyon (Fig. 2). 



The relative abundance of many species or species 

 groups was different from the Gulf stranding data 

 (Table 6). Compared with the stranding database, 

 the relative abundances of Risso's dolphins, pantropical 

 spotted dolphins, and Atlantic spotted dolphins were 

 greater in our study. The relative abundances of 

 balaenopterid whales, ziphiids, pilot whales, and pygmy 

 and dwarf sperm whales from the stranding data were 

 larger than those observed during our surveys. 



Discussion 



This study was the first to focus on cetaceans in the 

 oceanic Gulf of Mexico. We sighted at least 18 of the 

 29 cetacean species with one or more historical 



