The trophic ecology of two ommastrephid squid species, Ommastrephes bartramii and Sthenoteuthis oualaniensis, in the north Pacific sub-tropical gyre

Abstract

This paper examines the trophic ecology of the squids Ommastrephes bartramii and Sthenoteuthis oualaniensis, using stomach contents and stable isotopic techniques. Simple energetics models were constructed using some of the data collected. Samples for stomach contents were collected from 1996-2001 and 323 O. bartramii and 302 S. oualaniensis were captured. Fish and cephalopod remains dominated the stomach contents. Myctophids were found most abundantly in both squids, Symbolophorus evermanni was recovered at the highest percentage (7.5%) in O. bartramii, while M. lychnobium or spinosum, Lobianchia gemellerii, and Myctophum selenoides were all recovered at similar proportions (≃5%). Of the Myctophidae found in S. oualaniensis stomachs, S. evermanni was the most abundant (37%), followed by C. warmingii and H. proximum/rheinhardti (both ≃15%), and M. Iychnobium (5%). Beaks from Onychoteuthidae occurred most frequently (14%) in O. bartramii, while Histioteuthidae, Enoploteuthidae, and unidentified beaks all occurred at similar frequencies (10-12%). In S. oualaniensis, Enoploteuthidae occurred most frequently (17%) followed by Onychoteuthidae (10%). The diet of O. bartramii was more general while S. oualaniensis diet was more specialized on certain prey groups. From 1998-2001 samples were taken from captured squids for stable isotope analyses, 143 O. bartramii and 160 S. oualaniensis. SIA was conducted on the mantle muscle of O. bartramii that were divided into five categories based on mantle length, (1-7 mm) was 6.4 , (75-100 mm) was 6.9 , (200-300 mm) was 11.1 , (300-400) was 13.3%, (400-570 mm) was 12.8 . The S15N values for all O. bartramii mantle muscle samples showed a logistic increase with mantle length. The mean S15N value for S. oualaniensis sub-adult and adult mantle muscle (128 to 324 mm) was 8.2 The mean S15N value for paralarvae was 6.2 . The S15N values for all S. oualaniensis mantle muscle samples showed an exponential increase with mantle length. Eye lenses, and blood samples were also taken from each squid species and showed similar patterns of S15N increase with mantle length respectively, blood was unavailable in the smaller size ranges of O. bartramii.