Maturity and Spawning of Skipjack Tuna 

 ( Katsuwonus pelamis ) in the Atlantic Ocean, 

 with Comments on Nematode 

 Infestation of the Ovaries 



By 

 DAVID C. SIMMONS, Fishery Biologist 



Tropical Atlantic Biological Laboratory 



Bureau of Commercial Fisheries 



Miami, Florida 33149 



ABSTRACT 



Ovaries were examined from 537 fish collected in the eastern tropical Atlantic, 

 western tropical Atlantic, and off New York. The reported incidence of larval and 

 juvenile K.. pelamis was also reviewed. The minimum fork length of skipjack tuna at 

 maturity was 435 mm. in the eastern tropical Atlantic, and 410 mm. in the western 

 tropical Atlantic. All ovaries collected off New York were in an early stage of de- 

 velopment. The percentage of skipjack tuna near spawning or recently spawned was 

 greater in the western tropical Atlantic than in the eastern tropical Atlantic. Skip, 

 jack tuna spawn throughout the year in the areas studied in the tropical Atlantic. The 

 number of eggs per spawning for fish 465 mm. to 809 mm. long was 262,000 to 

 1,331,000. 



Nematodes identified as Philometra sp. and Spiruroidea infested the ovaries of 

 about 90 percent of the mature skipjack tuna. Both taxa were found in the ovaries of 

 tuna collected in all three areas. 



INTRODUCTION 



Until recently skipjack tuna (Katsuwonus 

 pelamis) have not been of great importance 

 to the Atlantic tuna fishery. Although the 

 northwest Atlantic purse -seine fishery in- 

 creased its catch of skipjack tuna from 12 

 percent of the total in 1962 (Wilson, 1965) to 

 55 percent in 1964 (Shomura, 1966), the fishery 

 has not contributed much to the total Atlantic 

 catch. Most of the catch is taken by longline, 

 a type of fishing that is selective against 

 smaller scombroids such as skipjack tuna, 

 which made up less than 2 percent of the total 

 Atlantic tuna catch in 1964. The surface 

 fishery in the eastern Atlantic is concen- 

 trated on yellowfin tuna, but skipjack tuna are 

 becoming more important. In 1967, skipjack 

 tuna formed 12 percent of the total catch of 21 

 live -bait boats and 18 purse -seiners that fished 

 along the West African coast in and south of 

 the Gulf of Guinea (LeGuen, Poinsard, and 

 Gayde, 1968). About 40 percent of the tuna 

 caught in the same general area by American 

 purse-seiners in 1967 were skipjack. 



Gonad development and the distribution of 

 identifiable larvae and juveniles have been 

 used as evidence for spawning of skipjack tuna 

 (Rothschild, 1965). Klawe (1963) said: "Knowl- 

 edge of spawning habits is useful in the eluci- 

 dation of the life history, ecology, and popu- 

 lation structure of tropical tunas and is 

 essential to the sound management of these 

 resources." 



I report here the results of examination of 

 ovaries from 537 skipjack tuna, present de- 

 terminations of fecundity, and review the oc- 

 currences of larvae and juveniles. Nematode 

 infestation of the ovaries is also dis- 

 cussed. 



Previous work on the biology of skipjack 

 tuna in the Atlantic Ocean is limited. Howell - 

 Rivero and Juarez F. (1954), Postel (1955), 

 Frade and Postel (1955), and Gorbunova and 

 Salabarria (1967) examined gonads of skip- 

 jack tuna; Klawe and Shimada (1959), Klawe 

 (I960; 1961), Marchal (1963), Gorbunova and 

 Salabarrai (1967), and Richards (in press) re- 

 ported on the occurrences of larvae and juve- 

 niles. 



