Milton et al.: Reproductive biology and egg production of three species of Clupeidae 



103 



may be increased if they spawned multiple batches 

 of eggs. 



Egg production of multiple spawning species de- 

 pends on reproductive life span, the time between 

 spawnings, and the age structure of the population 

 (Parrish et al., 1986). Batch fecundity of S. delica- 

 tulus varies widely between sites, both within and 

 between countries (Milton et al., 1990). In a short- 

 lived species such as S. delicatulus (<5 months; 

 Milton et al., 1991), reproductive life span may have 

 an important influence on potential lifetime egg 

 production. 



Batch fecundity of H. quadrimaculatus does not 

 appear to vary throughout its distribution, and 

 ranges from 4,000 to 10,000 eggs (Marichamy, 1971; 

 Hida and Uchiyama, 1977; Williams and Clarke, 

 1983; Moussac and Poupon, 1986; Conand, 1988). 

 Fish mature at about 90 mm in length at six months 

 of age (Williams and Clarke, 1983), and they sur- 

 vive for at least one year (Milton et al., 1993). Little 

 is known of fecundity and egg production of A. sirm. 

 Fecundity of the species is related to length and 

 weight, with a mean of 20,000 eggs per batch, and 

 individuals probably spawn more than one batch of 

 eggs (Conand, 1988). 



Temperate clupeids vary widely in life-history 

 parameters (e.g., Clupea spp., Jennings and 

 Beverton, 1991). Food availability and environmen- 

 tal conditions affect the size and number of eggs of 

 Pacific herring (Clupea pallasi) (Hay and Brett, 

 1988). Results of studies of temperate clupeoids 

 suggest that they do not spawn during periods of 

 high food abundance, but store energy as fat for 

 later reproductive activity (Hunter and Leong, 1981; 

 lies, 1984). There are no similar studies of tropical 

 clupeids. Encrasicholina heterolobus, a tropical 

 engraulid, does not deplete energy reserves in the 

 liver or soma during spawning (Wright, 1990). Fish 

 with higher condition factor (K) also had higher 

 fecundity. 



Stored energy or fish condition that may influence 

 both spawning frequency and batch fecundity have 

 a marked influence on egg production and, hence, 

 affect subsequent recruitment (Ricker, 1954; 

 Beverton and Holt, 1957). Adult reproductive varia- 

 tion should strongly influence recruitment in short- 

 lived tropical species that have short larval phases 

 and rapid growth. An example is S. delicatulus 

 which, in the Solomon Islands, live a maximum of 

 five months and mature at about two months of age 

 (Milton and Blaber, 1991; Milton et al., 1991). 

 Amblygaster sirm and H. quadrimaculatus live less 

 than two years (Milton et al., 1993) and mature in 

 6-12 months (Williams and Clarke, 1983; Conand, 

 1988). 



In this study, we examined the variability in re- 

 productive biology of the three major baitfishes in 

 Kiribati to determine the influence of adult repro- 

 ductive variability on subsequent recruitment. Our 

 objective was to test the hypothesis that reproduc- 

 tive biology of short-lived clupeids is adapted to 

 maintaining relatively stable population sizes. We 

 determined potential life-time egg production and 

 whether estimated egg production is related to the 

 frequency distribution of back-calculated birthdates. 



Methods and materials 



Study areas 



The Republic of Kiribati covers an area of 3 x 10 6 

 km 2 in the central Pacific ocean and comprises three 

 main island groups (Gilbert, Phoenix, and Line Is- 

 lands) (see Inset Fig. 1). The Gilbert Island group 

 is the most populated, consisting of 16 coral reef 

 islands. All islands in the group have a typical ocean 

 platform coral reef structure and have been built up 

 by scleractinian corals and coralline algae on a sub- 

 merged mountain (Gilmour and Colman, 1990 3 ). 

 Most atolls consist of small islets lying on the east- 

 ern side of a lagoon with an open western side due 

 to the prevailing easterly winds. Most typically have 

 passages between the islets through which water is 

 exchanged. 



The four study sites (Abaiang, Butaritari, Tarawa, 

 and Abemema) were typical of islands in the Gilbert 

 Island group; all had narrow islets on their south- 

 ern and eastern sides, except Abaiang (Fig. 1). La- 

 goons were mainly shallow (20-30 m deep), often 

 with large areas of intertidal seagrass or sand on 

 their eastern sides. Bottom topography of the deeper 

 parts of the lagoon was generally smooth, with some 

 coral outcrops. Our study sites were similar to those 

 described by Hobson and Chess (1978) in the 

 Marshall Islands. 



Environmental parameters 



On each sampling occasion, we measured the time 

 of collection, sea surface temperature (°C), cloud 

 cover (okters), wind direction and speed, and moon 

 phase because these factors may be related to 

 spawning or recruitment (Dalzell, 1985, 1987; 

 Peterman and Bradford, 1987; Milton and Blaber, 

 1991). For each site, monthly rainfall data for 1989 



Gilmour, A. J., and R. Colman. 1990. Report on a consultancy 

 on a pilot environmental study of the outer island development 

 program. Republic of Kiribati. Graduate School of the Environ- 

 ment, Macquarie Univ., Australia, 151 p. 



