Waring et al Incidental take of marine mammals off northeast United States 



357 



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LENGTH CLASSES (CM) 



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Figure 5 



Length distribution of incidentally captured pilot whales 

 measured by observers aboard foreign fishing vessels off 

 the northeastern United States, 1986-88. Size at attain- 

 ment of sexual maturity is from Perrin and Reilly (1984). 



Incidental take of common dolphins and 

 pilot whales relative to time of day and fishery 



Significantly more common dolphins were captured 

 during the Loligo fishery at night than during other 

 time periods throughout the day. Possibly the diurnal 

 movement upward at night would concentrate Loligo 

 nearer the surface, thereby also concentrating common 

 dolphins feeding on this prey. This would result in an 

 increased likelihood of common dolphins capture due 

 to a narrowing of the fishing area within the water 

 column. 



If common dolphins follow squids downward during 

 daylight hours, then this would account for the lack of 

 surface-feeding observations during the daytime. Con- 

 versely, if common dolphins feed principally at night, 

 then common dolphins might become spatially separ- 

 ated from squid during daylight hours, resulting in a 

 decreased likelihood of incidental capture. Although the 

 reasons that common dolphins are caught at night in 

 the DWF Loligo fishery are not readily apparent, it 

 does seem that the day/night differences in capture are 

 also related to a behavioral phenomenon of the dol- 

 phins, and not simply fishing practices. 



Conversely, the take of pilot whales was significant- 

 ly greater during daylight hours. Pilot whales were 

 observed on numerous occasions in active pursuit and 

 opportunistically feeding in and around the mouth of 

 the net during haulback operations. Similar observa- 

 tions have been reported for the Pacific bottlenose 

 dolphin and other species in Leatherwood (1975). The 

 large size and widespread opening of pelagic mackerel 



trawls might serve to corral larger delphinid species 

 such as pilot whales. Current data do not allow con- 

 elusive determination of whether the high incidence of 

 pilot whale mortality during daylight hours is behavi- 

 oral or the result of fishing practices by the Nether- 

 lands, GDR, and Poland. 



Food habits 



Common dolphins have been reported to feed on a wide 

 variety of epipelagic and mesopelagic schooling fin- 

 fishes and squids (Collett et al. 1981, Fiscus 1982, 

 Fiscus and Niggol 1965, Fitch and Brownell 1968, 

 Jones 1981, Nishiwaki 1972, Norris and Prescott 1961, 

 Major 1986). Prey items collected from stomachs of 

 common dolphins captured in the DWF indicate that 

 Loligo and Atlantic mackerel are important prey items 

 for dolphins during midwinter in the shelf waters of 

 the mid-Atlantic. 



Information on the dietary habits of pilot whales is 

 limited, but they are considered teuthophagous, feeding 

 principally on squid with fish as an alternative. Atlan- 

 tic cod and Greenland turbot Reinhardtius hippoglos- 

 soides. which were taken off Newfoundland by over- 

 wintering pilot whales when squid were not available, 

 are the only finfish prey items reported from the north- 

 west Atlantic (Sergeant 1962, Mercer 1975). 



We have not examined any stomachs of pilot whales 

 taken in the DWF squid fisheries. However, it seems 

 likely, based on qualitative examination of the relative 

 abundance and co-occurrence of pilot whales and Loligo 

 and known preferences for squid from the literature. 



