FOOD HABITS OF BAIT-CAUGHT 



SKIPJACK TUNA, KATSUWONUS PELAMIS, FROM 



THE SOUTHWESTERN ATLANTIC OCEAN 



Lisa Ankenbrandt' 



ABSTRACT 



Stomach cdntents ofskipjai'k tuna captured in 1981-82 by live jiole-aml-lino vessels off the southern coast of 

 Brazil were analyzed for the presence of larval and juvenile skipjack tuna. The percentage frequency of 

 occurrence, perc'ent number, and percent volume were evaluated. Of the 1,041 stomachs that were exam- 

 ined for food. 48B were empty. The mean volume of focxi in all stomachs analyzed was 3ti.9 niL. of which 

 18.9 mL was bait and 18.0 mL was prey. 



The g-onostomatid Moiirolicii.'i muelleri and the euphausiid Euphauxin simHi^ were the principal foods. 

 Other important food.s were the chub mackerel, Sramberjaponiem; the fripjate tuna, Auj-is thuziird; ^jem- 

 pylids: trichiurids; and carangids. In the study area, adult skipjack tuna were not found to feed on their 

 young. 



Kruskall-Wallis nonparametric one-way analysis of variance was used to test for differences in the mean 

 volumetric ratios of food items in relation to skipjack size. The percentage oiE. xiwUv^ in the diet was found 

 to decrease, while the proportion of M. muelleri was found to increase with increasing skipjack size. 

 Seasonal variations in the diet were also examined and discussed. 



Apparently the anatomy of their gill raker apparatus allows skipjack to ingest a wide variety of prey 

 ts'pes above a minimum size. These variations in the food can be attributed to the number and size of the 

 prey species in an area. 



A Brazilian skipjack pole-and-line fishery has been 

 developing in the Rio de Janeiro area since 1979 

 (Fig. 1). Because skipjack tuna, Katswonus pelamis, 

 is one of the major tuna species harvested at maxi- 

 mum sustainable yield in the tropical and subtropical 

 oceans (Kearny 1976; Evans et al. 1981), estimation 

 of the fishery potential requires information on the 

 distribution and concentration of its spawning stock. 

 One technique used to determine the existence of a 

 spawning stock is to quantify the distribution of its 

 larvae. Obviously, the presence of large numbers of 

 larvae would indicate a spawning stock occupies an 

 area. 



Knowledge of the distribution and abundance of 

 juvenile skipjack tuna is limited. Occasionally, speci- 

 mens have been found in experimental plankton 

 hauls or in the stomachs of apex predators (Kearny 

 1976). From ichthyoplankton surveys, Matsuura 

 (1982) and Nishikawa et al. (1978) reported larvae in 

 warm tropical waters north of the study area (Fig. 

 1), and juvenile skipjack tuna have been found in the 

 stomachs of adult skipjack tuna captured off west 

 Africa and in the Caribbean (Suarez-Caabro and 



■Southwest Fisheries Center La JoUa Laboratory, National 

 Marine Fisheries Service, NOAA, P.O. Box 271. La Jolla, CA 

 92038; present address: School of Fisheries, University of 

 Washington, Seattle, WA 98195. 



Duarte-Bello 1961; Klawe 1961; Dragovich 1970; 

 Dragovich and Potthoff 1972). Their occurrence in 

 the diet of central and south Pacific skipjack tuna 

 caught by pole-and-line has been used to deduce their 

 distribution and abundance (Waldron and King 1963; 

 Nakamura 1965; Argue et al. 1983). 



20°S 



30°S 



60°W 



500W 



40°W 



30°W 



40°S 



Manuscript accepted October 1984. 



FISHERY BULLETIN: VOL. 83, NO. 3, 1985. 



Figure 1.- Solid area indicates fishing localities from where skip- 

 jack tuna stomachs were obtained. Hatched area shows larval occur- 

 rence (Matsuura 1982). 



379 



