THE SPAWNING FREQUENCY OF SKIPJACK TUNA, 

 KATSUWONUS PEL AMIS, FROM THE SOUTH PACIFIC 



J. Roe Hunter, 1 Beverly J. Macewicz, 1 

 and John R. Sibert 2 



ABSTRACT 



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 number 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, Euthyn- 

 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 



Southwest Fisheries Center La Jolla Laboratory, National 

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

 92038. 



2 South Pacific Commission, P.O. Box D5, Noumea, CEDEX, 

 New Caledonia. 



oocytes and the other is based on the frequency with 

 which they contain postovulatory follicles of known 

 age (Hunter and Macewicz 1985a). These methods 

 have been used to measure the rate of spawning in 

 a number of marine fishes: Engraulis mordax 

 (Hunter and Goldberg 1980; Hunter and Macewicz 

 1980); Engraulis ringens (Alheit et al. 1984); Hypso- 

 blennius jenkinsi (Present 1985); Sardinella brasi- 

 liensis (Isaac-Nahum et al. 1985); Seriphus politus 

 (DeMartini and Fountain 1981); and Euthynnus 

 lineatus (Schaefer 1986). Postovulatory follicles 

 were used in most studies, but DeMartini and Foun- 

 tain (1981) and Schaefer (1986) used the incidence 

 of females with hydrated oocytes to estimate spawn- 

 ing frequency. The hydrated oocyte method may 

 produce a biased estimate in some species because 

 of increased vulnerability of hydrated females to net- 

 ting gear (Alheit et al. 1984). 



The objective of this paper was to estimate the 

 spawning rate of South Pacific skipjack tuna by 

 applying some of these techniques. It was not possi- 

 ble to use the hydrated ovary method in our study 

 because fish were not caught during the period of 

 the day when the ovary was hydrated. Instead, we 

 used the incidence of females having ovaries con- 

 taining postovulatory follicles to estimate the fre- 

 quency of spawning of skipjack tuna. This method 

 requires ovaries to be preserved immediately in for- 

 maldehyde solution when the fish is caught, a 

 histological examination of the ovary, and the devel- 

 opment of a staging system for estimating the age 

 of the postovulatory follicle. Our histological classi- 

 fication included not only an assessment of spawn- 

 ing frequency but also an assessment of the extent 

 of ovarian atresia. The atretic condition of the ovary 

 is a sensitive index of the reproductive state of 



Manuscript accepted June 1986. 



FISHERY BULLETIN: VOL. 84, NO. 4, 1986. 



895 



