AREA-1 



10 10 



60 70 



AREA-2 



Z 



10 30 40 }0 60 n 



f-^ o 



AREA-3 



10 10 30 40 



60 70 io 



ALL AREAS 

 COMBINED 



10 20 30 



FORK LENGTH (cmj 



FORK LENGTH (cm) 



Figure 2. — Length-frequency distribution of skipjack tuna from which stomachs were collected. 



Table 1. — Tuna stomachs examined from, three areas 

 of the Atlantic Ocean 



IDENTIFICATION OF TUNA FORAGE ORGANISMS 



The identification of tuna forage was com- 

 plicated by the poor condition of many of the 

 organisms and by the predominance of juvenile 

 or larval forms, for which taxonomic literature 

 is practically nonexistent. Partially digested 

 organisms also were difficult to identify. The 

 organisms that lacked external protective de- 

 vices (such as the scales of fish or the exo- 

 skeletons of crustaceans) were digested most 

 rapidly and were, therefore, the most difficult to 

 identify. Cephalopods, for instance, presented 

 difliculties because they are identified primarily 

 on the basis of skin, suckers, and fins. Squids 

 were separated from octopuses on the basis of 



number of arms (10 in squid and 8 in octopus), 

 lack of a gladius ("pens," or internal skeleton) 

 in the octopus, and the shape of the beaks. 

 Ommastrephid squids were identified by the in- 

 verted T-shaped funnel (known as the locking 

 apparatus), which distinguishes this family 

 from all others, even in the youngest stages. 



The shelled mollusks, heteropods, pteropods, 

 and nautiloid cephalopods were identified from 

 their shell remains. Exoskeletons provided 

 diagnostic characters for identification of ar- 

 thropods. 



The crustaceans were identified mostly on 

 the basis of their remaining exoskeletons. 

 Larvae of crustaceans were sorted into zoea 

 and megalopae or their equivalent stages, and 

 then identified as far as was possible. 



Identification of fish was troublesome because 

 their best diagnostic features (scales, fins) are 

 external and are the first to be digested. Partly 

 digested teleost fishes that lacked external 

 characters were identified from skeletal re- 

 mains. Other characters used were scutes, 

 dorsal spines, gill rakers, photophores, teeth, 

 and body shape. 



SKIPJACK AND YELLOWFIN TUNA FOOD IN ATLANTIC OCEAN 



447 



