Invertebrate Forage 



Most taxa of invertebrate forage belonged to two 

 groups, Mollusca and Crustacea (Appendix Table 1). 

 Salps and scyphozoan medusae were also present, but 

 were not identifiable. 



Thunnus. — Cephalopods were the most frequently oc- 

 curring invertebrates (80-90%) in the stomachs of all 

 species of Thunnus (Appendix Tables 6-8). Crustaceans 

 were present in 30-80% of the stomachs, depending on 

 the species of tuna. In stomachs where the two groups 

 co-occurred, the percent-volume ratios of cephalopods: 

 crustaceans were about 75:25 (range 67-79:21-33). 

 Cephalopods were, furthermore, more often the exclusive 

 content of a stomach than were crustaceans (Appendix 

 Table 10). Heteropods and pteropods were also abun- 

 dant, more so than indicated in our tables because they 

 were removed from many samples by Russell (1960). 



Cephalopods were the largest invertebrates eaten 

 (mantle lengths up to 200 mm). The most voluminous 

 and abundant were species of the family Om- 

 mastrephidae. Gonatus fabricii (Gonatidae) was also 

 common. The most common octopods were the species of 

 Argonauta (including A. argo) and Alloposus mollis. 

 Surface-dwelling Argonauta occurred far more often in 

 the stomachs of T. albacares (36%) and T. thynnus (25%) 

 than in T. alalunga (8%), and was not found in T. obesus 

 (Appendix Table 11). The gelatinous octopod, Alloposus 

 mollis, was frequently consumed by T. thynnus (48% 

 frequency), but rare or absent in the diet of other species 

 of tunas. 



Even though abundant, pteropods and heteropods are 

 not as important a component of the forage as are 

 cephalopods and crustaceans because of the small size of 

 these mollusks. They occurred in 52% of the T. thynnus 

 stomachs examined (Appendix Table 8). Russell (1960) 

 found the heteropod Carinaria lamarcki in T. albacares 

 stomachs at 14 localities but gave no indication of either 

 the number of heteropods found or the number of 

 stomachs examined. He reported the pteropod Cavolina 

 tridentata from T. albacares stomachs at three localities. 



Crustaceans were generally smaller than cephalopods 

 and were usually consumed in larger numbers, par- 

 ticularly amphipods and larval forms. The number of the 

 hyperiid amphipod Phrosina semilunata (lengths of 20- 

 30 mm) per single stomach sometimes exceeded 100 and 

 constituted a considerable portion of the forage volume. 

 Phrosina semilunata was present in the stomachs of 65% 

 of the T. alalunga, and in 20-30% of the other tuna 

 species. Although not in large numbers, various species 

 of the hyperiid genus Phronima were also frequently 

 utilized. They were found in 15% of T. albacares and T. 

 thynnus, and in 9% of T. alalunga. Several other species 

 of hyperiid amphipods collectively showed a high fre- 

 quency of occurrence (10-20% depending on species of 

 tuna). 



Decapods were the largest crustaceans consumed. Por- 

 tunids, represented by the sargassum crab (Portunus 

 sayi), when identifiable, were eaten by 11% of T. 



albacares. Brachyuran and anomuran larvae were found 

 in about 10% of T. albacares and T. alalunga, though 

 rarely present in T. thynnus and absent from T. obesus. 

 The large penaeid larva Cerataspis petiti comprised a 

 significant portion of the crustacean volume in 5% of T. 

 albacares stomachs. Scyllarid larvae and juveniles were 

 also consumed by T. albacares and T. alalunga, although 

 less frequently. Other decapods, mostly larvae, occurred 

 in low frequency when considered separately, but were 

 present in a sizeable percentage of tuna stomachs (about 

 10-30%) when considered as a group. 



Isopods and copepods were not significant in either 

 volume or frequency (1-2%). In one sample, it appeared 

 that copepods had been eaten by another forage 

 organism and were in the tuna stomach only secondarily. 

 Also, the isopod, Ceratothoa sp., a parasite of exocoetids, 

 was probably ingested only incidentally, in the capture of 

 its host. 



Alepisaurus. — Most of the invertebrates eaten by 

 tunas were also consumed by Alepisaurus (Appendix 

 Table 1). Forage composition differed, however, in the 

 relative importance of different taxonomic groups. The 

 percent-volume ratio of cephalopods: crustaceans was 

 roughly 50:50 (Appendix Table 10). Crustaceans out- 

 ranked cephalopods in frequency, however, in a ratio of 

 75:50. Deep-dwelling soft-bodied squids 

 (Histioteuthidae, Bathyteuthidae, Cranchiidae; Roper 

 and Young 1975) occurred more often in Alepisaurus 

 (15%) than in any of the tunas. Other squids, although 

 eaten less than by tunas, were still common (42%). Oc- 

 topods were often present in Alepisaurus forage, 

 Argonauta in 25%, Alloposus in 32%, and all others in 

 32% (Appendix Table 11). 



Crustaceans were more often the exclusive content of 

 an Alepisaurus stomach than were cephalopods. 

 Phronima spp. (51%), other hyperiids (47%) (including 

 two not found in the tunas), and anomuran larvae (22%) 

 occurred in generally higher numbers and with higher 

 frequency than in any of the tunas. No portunids were 

 found in Alepisaurus, although two or three unidentified 

 crab larvae were present. 



Heteropods and pteropods were present in 14% and 

 12% of the Alepisaurus stomachs that we examined 

 (Appendix Table 9) but the actual occurrence was much 

 higher because Russell (1960) removed these mollusks 

 from many of the Alepisaurus stomachs. Russell reported 

 seven species of heteropods from three families and five 

 species of pteropods from two families in his sample of 

 Alepisaurus stomachs (Appendix Table 1). The 

 heteropod Carinaria lamarcki and four species of the 

 pteropod genus Cavolina were found at most stations, 

 but Russell gave no numbers of forage organisms or of 

 stomachs examined. 



An important constituent of the Alepisaurus diet was 

 the family of pelagic polychaetes, Alciopidae, present in 

 38% of the stomachs and forming much of the total food 

 bulk. Gelatinous organisms (salps, pyrosomas, medusae, 

 and siphonophores) were also more common in 

 Alepisaurus stomachs than in tunas. 



