PERRIN ET AL.: STOMACH CONTENTS OF PORPOISE AND YELLOWFIN TUNA 



Table 7. — Stomach contents of yellowfin tuna, spotted porpoise, and spinner porpoise taken in a single net haul: Haul 6. 



' Whole squid counted in 12 full stomachs, not saved; beaks not counted or saved. 



digestion of the stomach contents indicated 

 that they had not fed at the same time as the 

 tuna and/or the spotted porpoise. 



DISCUSSION 



Interpretation of the feeding habits of animals 

 from their stomach contents is complicated by 

 two factors. First, material from the stomachs 

 of the primary prey may be mistaken as food 

 items of the predator. This is especially true 

 when, as in the present study, many stomachs 

 contain squid, which are active piscivores in 

 their own right. However, if we are concerned 

 only with the general depth at which feeding 

 takes place, it makes less difference whether 

 the fish remains found in the gut are primary 

 or secondary in origin. When two predators 

 show consistent differences in the probable 

 depth origin of their stomach contents, there are 

 at least three alternative interpretations: the 

 two predators feed at different depth ranges 

 (gathering the material found directly or 

 through some secondary predator), they feed at 

 the same depths but at different times of the 

 day, thus taking advantage of diurnal migration 

 of some prey species, or they feed at the same 

 depth with one only eating prey items that were 



recent predators at another depth. The first 

 explanation seems most probable. 



The other complicating factor is less easy to 

 resolve. The depth distributions of many pelagic 

 prey items are imperfectly known. This requires 

 that much caution be exercised when interpret- 

 ing food habits of predators from data based on 

 rare or little-known prey. 



Depth Distribution of Fish 



Exocoetidae 



Oxijporhamphus micro'pterus and species of 

 both Exocoetus and Cypselurus are all constant 

 inhabitants of epipelagic waters. Exocoetids 

 were extremely common in the stomachs of tuna 

 and S. atte)inata. Only one exocoetid otolith was 

 found in all 46 stomachs of S. longirostns ex- 

 amined. 



Scombridae 



The frigate mackerel, Auxis sp., is another 

 common fish of surface waters and is of rela- 

 tively large size. Its occurrence in specimen 

 stomachs parallels that of the flyingfish, often 

 being found in S. attemiata and yellowfin tuna 



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