Feeding behavior of two ommastrephid squids Ommastrephes bartramii and Sthenoteuthis oualaniensis off Hawaii

The feeding behaviors of squids Ommastrephes bartramii (n = 315) and Sthenoteuthis oualaniensis (n = 280), collected during yearly 2 wk cruises from 1996 to 2001, were compared. Fish and squid prey dominated the stomach contents and were identified from otoliths and beaks, respectively. A total of 2561 otoliths and 424 beaks were recovered; 84% of O. bartramii stomachs contained otoliths while 54% contained cephalopod beaks. For S. oualaniensis, 75% of stomachs contained otoliths while 34% contained cephalopod beaks. Myctophidae were the most prevalent fish prey item for both species in frequency and abundance; however, O. bartramii fed on a wider range of prey items than S. oualaniensis at the family and species level. The myctophid Symbolophorus evermanni was a substantial prey item for both species. Both predators fed substantially on the squid families Onychoteuthidae and Enoploteuthidae, but O. bartramii fed on a wider range of squid prey items than S. oualaniensis. Levins' (1968; 'Evolution in changing environments', Princeton University Press) standardized measure of niche breadth was greater for O. bartramii than for S. oualaniensis when calculated using either otoliths (0.56 vs. 0.086) or beaks (0.22 vs. 0.097). MacArthur & Levins' (1967; Am Nat 101:377-385) measure of niche overlap of S. oualaniensis on O. bartramii was greater than that of O. bartramii on S. oualaniensis, for both fish otoliths (1.16 vs. 0.22) and squid beaks (0.93 vs. 0.46). These results suggested that O. bartramii is a more generalized predator than S. oualaniensis. Competition for food resources appears low between O. bartramii and S. oualaniensis where they overlap in the North Pacific.