THE SPAWNING FREQUENCY OF SKIPJACK TUNA, KATSUWONUS PELAMIS, FROM THE SOUTH PACIFIC

Abstract

ABSTRACf Histological criteria to age postovulatory follicles were developed from examination of laboratory-spawned skipjack tuna; the criteria were used to estimate the frequency of spawning of skipjack tuna from the South Pacific. Examination of 87 skipjack tuna from field cOllections taken in October-November indicated that spawning occurred nearly every day. The fraction of mature females with postovulatory follicles, <24 hours old, was 0.85 (standard deviation = 0.071) indicating that the mean interval between spawn­ ings was only 1.18 days. Estimates of the frequency of spawning of multi­ ple spawning fishes are essential for understanding their reproductive biology. To estimate annual reproductive effort or fecundity, and how these variables are related to size or age structure of a population requires knowledge of the frequency of spawning and the number of eggs produced per spawning. Batch fecundity, the number of eggs pro­ duced per spawning, has been estimated for skipjack tuna a nwnber of times (see review by Matsumoto et al. 1984) but the spawning rate of the skipjack is unknown. Thus spawning frequency is one of the missing links in an assessment of the reproduction of skipjack populations. It has long been recognized that skipjack tuna spawn more than once in a season because more than one mode of advanced oocytes are found in active ovaries (Brock 1954; Bunag 1956; Joseph 1963; Raju 1964; Simmons 1969; Batts 1972; Cayre 1981; Goldberg and Au 1986). The frequency of occurrence of female black skipjack tuna, Eutkyn­ nus lineatus, throughout the spawning season with ovaries containing hydrated oocytes led Schaefer (1986) to conclude that the average interval be­ tween spawnings of black skipjack in the eastern tropical Pacific was 2.1-5.7 d depending on the region. Over the last 6 years, two methods have been developed for measuring the spawning rate of multi­ ple spawning marine fishes: One method is based on the frequency of ovaries containing hydrated