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



I / 



to food supply in other fish species (Townshend and 

 Wootton, 1984). Fish at Butaritari may experience 

 a more predictable environment that enables them 

 to produce more eggs of smaller size than fish in 

 more variable environments. 



In contrast, A. sirm delayed spawning beyond the 

 size and age at sexual maturity and did not spawn 

 until one year old. As fecundity was related to 

 weight, delayed spawning enabled A. sirm to grow 

 faster than the other species (Milton et al., 1993) 

 and have a higher batch fecundity when spawning 

 started. Murphy (1968) hypothesized that delayed 

 spawning and longer reproductive life span would 

 evolve in response to variable reproductive success. 

 However, Armstrong and Shelton (1990) demon- 

 strated that, even with a short reproductive life 

 span, multiple spawners had a high probability of 

 successful reproduction when subject to random 

 environmental fluctuations over time. Thus, delay- 

 ing spawning would be of adaptive advantage if 

 mortality was low (Roff, 1984) because batch fecun- 

 dity and lifetime egg production would be increased. 



Our estimates of the reproductive lifespan of A. 

 sirm indicate that this species spawns fewer times 

 in their lifetime than other species and thus would 

 also have less chance of successful spawning than 

 other species. Given that this is the longest-lived of 

 the species examined, our estimate of overall mean 

 lifespan may be biased by the small number of 

 months sampled. Large fish may be under-repre- 

 sented in small catches and may contribute to un- 

 derestimating the reproductive potential of A. sirm. 



Herklotsichthys quadrimaculatus had a longer 

 reproductive life span and spawned more frequently 

 than did the other species. Reproductive life span 

 varied little among sites (except Abemama) and 

 there was no significant temporal variation, which 

 suggests that survival rates of large adult H. 

 quadrimaculatus are fairly constant in Kiribati. 

 This is reflected in their life-history parameters, 

 which varied little among sites or over time. In con- 

 trast, the frequency distribution of back-calculated 

 birthdates indicated that overall survival was vari- 

 able both between and within years, and was not 

 related to monthly egg production. We have no es- 

 timates of adult abundance during the study period, 

 and so population egg production could not be as- 

 sessed. However, the annual CPUE and abundance 

 of H. quadrimaculatus in the baitfishery were simi- 

 lar in the three years for which both data sets were 

 available (Rawlinson et al., 1992). This suggests that 

 population size was relatively constant during this 

 period. If so, then variation in post-hatching survival 

 probably has an important influence on recruitment 

 in this species (Smith, 1985). 



The reproductive life span of the smallest species, 

 S. delicatulus, was intermediate between the other 

 species and varied little among sites during 1989 

 and 1990. Unlike H. quadrimaculatus, the reproduc- 

 tive life span of S. delicatulus varied between years, 

 which suggests that survival rates are not as con- 

 stant or as predictable as those of H. 

 quadrimaculatus. Potential lifetime egg production 

 of each female was only one tenth that of other spe- 

 cies, but, because of the larger number of females, 

 monthly estimates of daily egg production were 

 higher. The distribution of back-calculated 

 birthdates varied between years, but a greater pro- 

 portion of births fell in May-August, irrespective of 

 che pattern of egg production. Annual CPUE of S. 

 delicatulus (Rawlinson et al., 1992) was similar in 



1989 and 1990, which suggests that fishing mortal- 

 ity had not contributed to the increased mortality 

 that reduced the reproductive life span in 1990. 



The reproduction and abundance of S. delicatulus 

 may be more directly influenced by its environment 

 than are the other species. Adult survival is vari- 

 able and low (Tiroba et al., 1990); egg production 

 varies, probably in response to food supply, and sur- 

 vival to recruitment is unpredictable. Yet the poten- 

 tial for successful reproduction with this strategy 

 may still be relatively high (Armstrong and Shelton, 

 1990). In contrast, H. quadrimaculatus appears to 

 be able to offset environmental variability to produce 

 a relatively constant supply of eggs. 



The distribution pattern of back-calculated 

 birthdates of each species was not consistent among 

 species. Months when a higher proportion survived 

 differed for each species during all years; months 

 with highest mean survival were not the same for 

 any species. This suggests that the effects of envi- 

 ronmental conditions such as seasonal food avail- 

 ability or favorable physical conditions are not the 

 same for each species. Alternatively, other factors 

 such as predation (Rawlinson et al., 1992) may have 

 greater influence on survival to recruitment. Egg 

 production by S. delicatulus was positively corre- 

 lated to survival rates in 1989 and negatively cor- 

 related in 1990. This seems unrelated to fish abun- 

 dance as catch rates were higher in 1989 than in 



1990 (Rawlinson et al., 1992). 



Large variations in recruitment, reflected in catch 

 rates of the main baitfishes do not appear to be di- 

 rectly linked with variations in egg production. All 

 spawn in the lagoon for most of the year, and dis- 

 tribution of birthdates indicated recruitment in most 

 months. Although the absolute level of recruitment 

 varied throughout the year, multiple spawning re- 

 duces fluctuations in population size due to environ- 

 mental variability and should ensure that relatively 



