Milton and Blaber: Sexual maturity and spawning of tuna baitfish in the Solomon Islands 



233 



unlike others, used histological examination of gonads 

 to verify macroscopic stages. 



In the Solomon Islands, Encrasicholina devisi 

 reached sexual maturity at 45 mm, and some fish at this 

 size were in spawning condition. This is consistent with 

 results from Papua New Guinea (Dalzell and Wankow- 

 ski 1980), New Caledonia (Conand 1985), and southern 

 India (Luther 1979). Spratelloides delicatulus also 

 showed little difference throughout the region in 

 length-at-sexual-maturity (Lewis et al. 1983, Conand 

 1985, McCarthy 1985). 



Spratelloides lewisi showed local variation in length- 

 at-sexual-maturity. Fish from Tulagi were not sexual- 

 ly mature at less than 40 mm (compared with 35 mm 

 at other sites) and they also grew to a greater size than 

 fish from the other sites (unpubl. data). 



Length-at-first-spawning, however, was similar for 

 both Encrasicholina species and S. delicatulus 

 throughout the region (Dalzell and Wankowski 1980, 

 Lewis et al. 1983, Conand 1985), except for E. hetero- 

 lobus at Munda, where fish were in spawning condi- 

 tion at a smaller length than at the other sites in the 

 Solomon Islands. 



Both Spratelloides lewisi and S. gracilis showed 

 variation in length-at-first-spawning between sites. At 

 Tulagi, S. lewisi did not spawn until a much greater 

 length was attained. These results are consistent with 

 those of Dalzell (1987), who found a difference of 9 mm 

 in the length-at-first-spawning in two populations of 

 S. lewisi in Papua New Guinea waters. Dalzell (1985) 

 also found that Papua New Guinea S. gracilis did not 

 develop ripe eggs until 44 mm, which was much larger 

 than the length-at-first-spawning of fish from the 

 Solomon Islands (35 mm). Unfavorable conditions for 

 reproduction may delay the onset of gonadal develop- 

 ment in these species at some sites to help offset 

 reproductive uncertainty (Mann and Mills 1979). 



No comparative data on length-at-sexual-maturity of 

 A. zosterophora are available, and there are few data 

 for other similar-sized apogonids. However, this species 

 matures at almost 80% of maximum size, which is 

 larger than in the clupeoids (70%). The subtropical 

 Australian species Apogon fasciatus also matures at 

 about 70% of maximum size (90 mm) near Brisbane (K. 

 Warburton, Zool. Dep., Univ. Queensland, Brisbane, 

 Australia, unpubl. data). 



The relationship between length-at-first-spawning 

 and maximum length of the Solomon Islands baitfish 

 closely fits that found for other clupeoids (Beverton 

 1963). Beverton (1963) showed that length-at-first- 

 spawning among clupeoids is closely proportional to 

 maximum length, with smaller species spawning at a 

 smaller size (relative to their maximum) than larger 

 ones (Blaxter and Hunter 1982). Longhurst and Pauly 

 (1987) hypothesize that length-at-sexual-maturity and 



maximum length are determined by the interactions 

 of oxygen supply and demand. Any species of fish living 

 in cold water should grow to a greater size and mature 

 at a larger size than the same species in warm water, 

 given similar food supplies. Water temperature at 

 Tulagi was consistently 1-2°C colder than at the other 

 sites, and this may account for differences in these 

 parameters in S. lewisi at this site. The regional dif- 

 ferences seen in E. heterolobus and S. gracilis also 

 support this idea, with fish from higher latitudes 

 maturing at greater lengths. However, similar patterns 

 were not found in the other species. Availability of food 

 also must play an important role by affecting growth 

 rates. 



Many clupeoids, including most engraulids, are multi- 

 ple spawners (Blaxter and Hunter 1982), and studies 

 of other engraulids suggest they spawn batches of eggs 

 every 2-10 days (Hunter and Goldberg 1980, Alheit et 

 al. 1984, Clarke 1987). Smaller species (e.g., Encrasi- 

 cholina purpureus) spawn more frequently during the 

 peak spawning period (as often as every 2 days; Clarke 

 1987). The spawning frequency of multiple spawning 

 fish can be determined by the presence or absence of 

 postvitellogenic follicles in the ovaries; their presence 

 indicates that a fish has spawned within the previous 

 24-48 hours (Hunter and Goldberg 1980). Clarke (1987) 

 reported that postvitellogenic follicles were distinguish- 

 able in Encrasicholina purpureus up to 16 hours after 

 spawning. In our study, we did not find either post- 

 vitellogenic follicles or a continuous egg-size distribu- 

 tion indicative of multiple spawning (Blaxter and 

 Hunter 1982). However, the similarity in reproductive 

 behavior of Encrasicholina species and the presence 

 of eggs in the plankton throughout most of the year 

 suggest that E. devisi and E. heterolobus are also multi- 

 ple spawners. Our data on Encrasicholina devisi and 

 E. heterolobus were consistent with that of Leary et 

 al. (1975) on Encrasicholina purpureus. We found, as 

 had Leary et al., only two egg sizes: one advanced and 

 one with all eggs at the yolk-precursor stage of develop- 

 ment. Leary et al. (1975) interpreted these results to 

 indicate that E. purpureus spawned once in its lifetime. 

 However, Clarke (1987) found that E. purpureus eggs 

 could mature very rapidly, and hence timing of sam- 

 pling was critical. Running- ripe eggs were only present 

 shortly before spawning, which occurred one or two 

 hours after sunset. If E. devisi and E. heterolobus are 

 batch spawners similar to E. purpureus, then we would 

 expect to collect only fish about to spawn in our 

 samples, as the postvitellogenic follicles of fish that 

 spawned the previous night would have degraded 

 (Clarke 1987). However, spawning is probably less fre- 

 quent than in the smaller E. purpureus because greater 

 energy is required by larger fish to maintain continuous 

 spawning (Hunter and Leong 1981). 



