Table 3. — Estimated sizes of four fish prey species of harbor seals in Southern New 

 England, based on regression equations relating otolith length (OL) to fish fork length (FL). 



Species 



Regression 

 equation 



Estimated prey size 

 (FL, mm) 



r 2 



Range 



Mean 



Melanogrammus aeglefinus 

 Merluccius bilinearis 

 Pollachius virens 

 Urophycis chuss 



FL = 3.4(OL) - 9.32 0.97 45 110-310 230 



FL = 22.4(OL) - 1.44 0.98 34 30-460 170 



FL = 4.9(OL) - 22.58 0.95 5 160-310 280 



FL = 25.0(OL) + 0.63 0.96 2 340 



Discussion 



Analyzing stomach contents from stranded ani- 

 mals to determine prey preference or selection does 

 yield a partial list of prey species exploited; however, 

 several apparent biases prohibit the realization of ac- 

 curate quantitative results. Therefore, the utility of 

 this method is questionable 



The limited number of stomachs containing food 

 was likely due to the weakened condition of seals 

 prior to stranding and their inability to obtain food. 

 The stomachs that did contain food all came from 

 stranded animals, and therefore may not reflect on 

 what a healthy seal was feeding. The stranded seals 

 were generally animals with debilitating conditions 

 like lungworm and heartworm, and may not have 

 been able to feed in usual feeding areas, or secure 

 usual prey, and thus were probably less selective 

 about prey items. 



For example, the shells found in the two stomachs 

 may represent prey items desirable only to a disease- 

 weakened seal. The size and number of these shells 

 suggest that they were not ingested incidentally. 

 Comparing the stomach contents to a "condition 

 index", such as length vs. girth or blubber thick- 

 ness, might indicate whether the stranded animals 

 are less selective about prey species than healthy 

 ones. 



The abundance of squid beaks found in the 

 stomachs suggests that squid are an important part 

 of the diet of harbor seals along coastal New 

 England; however, our own finding of squid beaks 

 in 56% of 63 stomachs may be inflated. Boulva and 

 McLaren (1979) found squid remains in 20.6% of 279 

 stomachs examined from eastern Canada, and Pit- 

 cher (1980b) similarly found cephalopod beaks in 

 21.1% of 351 harbar seals collected in the Gulf of 

 Alaska. Seals have been shown to retain, then re- 

 gurgitate, cephalopod beaks rather than pass them 

 through their digestive tract (Miller 1978 3 ; Pitcher 



1980b). Retention of squid beaks will tend to over- 

 represent the utilization of squid as a prey species 

 (Pitcher 1980a). The retention of beaks during a 

 period of fasting prior to death may also account for 

 the large percentage (41%) of stomachs containing 

 squid beaks and no other type of prey remains. 



Large fish may be underrepresented if the heads 

 (i.e, otoliths) are not eaten (Boulva and McLaren 

 1979; Brown and Mate 1983). Pitcher (1980b) sug- 

 gested that seals often fragment large fish while 

 eating them, usually discarding the head. 



Finally, the relationship between the time when 

 prey was eaten and when the stomach was collected 

 may determine what types of prey remains will be 

 recovered (Frost and Lowry 1980; Pitcher 1980a; 

 Brown and Mate 1983). For example, the low num- 

 ber of sand lance otoliths found in the stomachs may 

 not accurately represent the importance of sand 

 lance as a prey species of harbor seals in southern 

 New England because otoliths of the size of the ones 

 recovered are very small and delicate and may not 

 remain for long in the seal stomachs once freed from 

 the skull (Smith and Gaskin 1974). 



Thus, using only frequency of occurrence as a 

 measure of prey preference or selection may be mis- 

 leading by overemphasizing the importance of some 

 species. For example, based on number, cephalopods 

 were the major prey item; however fewer otoliths 

 representing fish of greater weight may show that 

 fish indeed are more improtant. The full importance 

 of fish or squid in the diet of seals can be accurately 

 described only if quantitative assessments such as 

 weight or volume of food items in the stomachs can 

 be determined (Rae 1973; Frost and Lowry 1980). 



In summary, given a large sample of animals the 

 analysis of stomach contents from stranded seals 

 does provide information on the types of prey 

 selected. However, the analysis of stomach contents 

 from stranded seals greatly overemphasizes cephal- 

 opod remains while likely underrepresenting most 



3 Miller, L. K. 1978. Energetics of the northern fur seal in rela- 

 tion to climate and food resources of the Bering Sea. Marine Mam- 

 mal Commission, Final Report, Contract MM5AC025. (Available 



National Technical Information Service, Springfield, VA 22151 as 

 PB-275 296, 32 p.) 



219 



