FISHERY BITLETIN: VOL. 69, NO. 2 



Table 3. — The average size of individual crustaceans, squids, and fishes in the stomachs of scombrids from the 



central Pacific. 



Species 



Volume of individual organisms (ml) 



Squids 



Fishe 



Volume 



of food 



analyzed 



(mi) 



Source 



Thunnut albacareS 

 Thunnus obesus 

 Tkunnus albacares 

 Katsuwonus pelamxs 

 Mean (unweighted) 



1.0, 1.7, a.nd 4.5 mm, respectively. Selectivity 

 of the gill raker appai'atus would vary with gill 

 raker gap, a function of both species and length 

 of the fish. Thus, a small T. albacares and a 

 large K. pelamis should have more similar diets 

 than a small and a large T. albacares. Any 

 number of such predictions can be generated 

 from Figure 3. A fish with smaller mean gap, 

 regardless of its species or length, would be 

 expected to be more planktivorous. 



RELATION BETWEEN GILL RAKER 

 GAP AND DIET 



Stomach-content data from published liter- 

 ature from the central Pacific were compared 

 with the mean gill raker gaps reported here to 

 test the hypothesis that fish with a finer gill 

 raker gap have a greater proportion of smaller 

 organisms (crustaceans) in their diet. 



Crustaceans in the diet of scombrids from 

 the central Pacific were smaller than were the 

 other major food organisms (squids and fishes) 

 (Table 3). The volume of individual, partially 

 digested crustaceans in the stomachs of five spe- 

 cies averaged 0.3 ml whereas individual, partially 

 digested squids and fishes averaged 5.6 and 5.7 

 ml, respectively. The much smaller body size 

 of the crustaceans was not likely the result of 

 diflferential digestion, especially since the exo- 

 skeleton of crustaceans, if anything, might be 

 expected to slow, rather than accelerate, diges- 

 tion (Pandian, 1967). 



For comparison with gill raker data, the per- 

 cent volumes of the stomach content comprised 

 by crustaceans, squids, or fishes are presented 

 in Table 4 for five scombrids and one coryphae- 

 nid. Only stomach data from the central Pa- 

 cific were used because difl'erences in typical 

 body size of crustaceans in scombrid stomachs 

 from other regions would have invalidated these 



Table 4. — Food of scombrid and coryphaenid fishes from the central Pacific divided into the percentages of the 

 stomach volume that were crustaceans, squids, or fishes. The median fork length of fishes in the sample is also 

 given along with the literature source for the data. 



Species 



Stomach content 

 {percent volume) 



Crusta- 

 ceans 



Squids 



Fork length 



Median 

 ("n) 



Range 

 (cm) 



Number 



of 

 stomachs 



Literature source 



Acanthoeybium solanderi 

 Aeanthocybium iolanderi 

 Euthynnus affinis 

 Katsuwonus pelamxs 

 Katsuwonus petamis 

 Katsuwonus pelamis 

 Katsuwonus pelamxs 

 Katsuwonus pelamis 

 Katsuwonus pelamis 

 Thunnut albacares 

 Thunnus albacares 

 Thunnus albacares 

 Thunnus albacares 

 Thunnus albacares 

 Thunnus obesus 

 Thunnus obesus 

 Coryphaena hippurus 



366 



