Trophic variation with length in two ommastrephid squids, Ommastrephes bartramii and Sthenoteuthis oualaniensis

Abstract

From 1998 to 2001 a total of 200 Ommastrephes bartramii (27 paralarvae) and 170 Sthenoteuthis oualaniensis (14 paralarvae) were sampled from the Central North Pacific. One group of non-paralarval O. bartramii (n = 30) was sampled from farther northwest in 1996. The δ15N of mantle muscle of non-paralarval O. bartramii ( \( \overline{x} \) = 12.4‰) was significantly greater than that of non-parlarval S. oualaniensis (\( \overline{x} \) = 8.1‰) (P < 0.001). The δ15N of whole paralarvae of O. bartramii (\( \overline{x} \) = 6.4‰) was not significantly different than parlarvalae of S. oualaniensis (\( \overline{x} \) = 6.1‰) (P = 0.528). There was no significant difference between the mantle muscle δ15N values of male (n = 95, \( \overline{x} \) = 13.3‰) and female (n = 18, \( \overline{x} \) = 12.9 ‰) O. bartramii greater than 300 mm mantle length (ML) (P = 0.15). There was also no significant difference between the mantle muscle δ15N values of male (n = 15, \( \overline{x} \) = 7.2‰) and female (n = 26, \( \overline{x} \) = 7.3 ‰) S. oualaniensis in the same size range (P = 0.41). Overall there was a distinct logistic increase in δ15N with mantle length for O. bartramii, whereas S. oualaniensis showed an exponential increase in δ15N with mantle length that was stronger within individual years than with all samples combined. In general, adult O. bartramii are more than a trophic level above S. oualaniensis (4.3‰, 1.3 TLs). Because of the nature of the sampling protocol, this study could not separate spatial and temporal effects on the δ15N signals from each squid species. This study demonstrates the ability of stable isotope analyses to differentiate trophic levels between squid species as well as track trophic changes across size ranges from paralarvae to adults. Additional research is needed to validate these trophic changes across size within individuals.