A study of feeding relationships in tuna and porpoise through the application of cephalopod beak analysis

Abstract

A method of identifying the beaks and estimating body weight and mantle length of eighteen species from the eastern tropical and central Pacific is presented and used in a stomach content analysis of 35 Thunnus albacares and 32 Stenella attenuata. Twenty specimens were selected from each of the following cephalopod species: Symplectoteuthis oualaniensis, Dosidicus gigas, Ommastrephes bartramii, Symplectoteuthis luminosa, Nototodarus hawaiiensis, Todarodes pacificus, Hyaloteuthis pelagica, Ornithoteuthis volatilis, Liocranchia reinhardti, Leachia danae, Abraliopsis affinis, Abraliopsis felis, Pterygioteuthis giardi, Histioteuthis heteropsis, Histioteuthis dofleini, Gonatus onyx and Loligo opalescens. Dimensions measured on the upper and lower beak are converted to ratios and compared individually among the species using an analysis of variance procedure with Tukey's omega and Duncan's multiple range test. Significant differences (p = .05) observed among the species' beak ratio means, and structural characteristics, are used to construct artificial keys for the upper and lower beaks of the 18 species. Upper and lower beak dimensions are used as independent variables in a linear regression model with mantle length and body weight (log transformed). Species and sizes of cephalopods are determined from the predators' stomach contents using the beak characterization procedure. Three species of ommastrephid squid, Ommastrephes bartramii, Dosidicus gigas and Symplectoteuthis oualaniensis, dominate the stomach contents of both predators in terms of number, frequency of occurrence and weight. The total biomass of prey in the stomachs is found to represent between less than one to as many as three feedings. A hierarchical agglomerative clustering procedure based on species abundances separated the stomachs into groups of large dolphin, small dolphin, tuna and mixed small dolphin and tuna. Using these groups in a multiple discriminant analysis procedure, two cephalopod species are identified as the primary source of group separation; D. gigas and O. banksii. Prey size distribution and behavioral characteristics of the predator and prey, suggests dolphin, in feeding dives, cause cephalopods to be driven toward the vertically limited tunas in the surface layers. The potential of cephalopod beak characterization in expanding information on areal and size distribution and taxonomic relationships is discussed.