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



235 



suggests that either local conditions exert a strong in- 

 fluence on the timing of reproduction in these species 

 or that at these sites reproduction is not strongly linked 

 to particular environmental events. Wright (1989) also 

 found no relationship between spawning by E. hetero- 

 lobus in Indonesia and temperature, rainfall, or tidal 

 phase, and suggested that these factors were not in- 

 fluencing spawning. 



However, among the other species, temperature and 

 reduced tidal range appear to be more important at 

 Tulagi. Higher temperatures should allow increased 

 growth under favorable conditions, and reduced tidal 

 exchange would reduce egg movement away from 

 favorable habitats. 



While rainfall has been suggested as a proximate fac- 

 tor influencing spawning by Encrasicholina species in 

 Papua New Guinea (Dalzell and Wankowski 1980; 

 Dalzell 1984, 1987b), in our study rainfall was only 

 weakly correlated with spawning of Encrasicholina at 

 Munda (Tables 4, 5). Spawning appeared to be greater 

 during periods of lower rainfall. Positive relationships 

 between spawning and time since rain or less rain may 

 be indirectly linked to phytoplankton production, which 

 is determined by light or the depth of mixing (Wyatt 

 1980). Variations in wind strength and cloudiness will 

 cause major variations in the onset and end of the 

 phytoplankton production cycle (Blaxter and Hunter 

 1982). If rainfall at Munda is linked to the phytoplank- 

 ton cycle, spawning during periods of low rainfall would 

 be consistent with Cushing's (1967) hypothesis. Some 

 previous studies in southeast Asia and Papua New 

 Guinea (Tiews et al. 1971, Sitthichockpan 1972, Dalzell 

 1987b) have suggested that Encrasicholina spawned 

 more intensively during the months of peak zooplank- 

 ton production. 



The variation in spawning frequency observed among 

 the species in this study suggests that each responds 

 differently to local conditions, reacting to those vari- 

 ables most appropriate to maximize reproductive suc- 

 cess in the immediate environment (Bye 1984). No ob- 

 vious differences were detected in baitfish spawning 

 patterns between the exploited fishing grounds and the 

 unexploited site. If baitfish numbers are higher at the 

 unexploited site, this suggests, in turn, that differences 

 in the observed spawning patterns were not density- 

 dependent. Lack of clear proximate stimuli for spawn- 

 ing among the six species examined makes it difficult 

 to predict the timing of major spawning events by these 

 species. 



However, protracted spawning and the baitfish 

 population's adaptation to local conditions suggests 

 that these species should be resilient to increased 

 fishing mortality. Factors affecting larval survival and 

 growth may be more important in determining recruit- 

 ment to the fishery. 



Acknowledgments 



We thank N. Rawlinson, G. Tiroba, and J. Leqata of 

 the Fisheries Division, Solomon Islands, for assistance 

 in the field and laboratory, and J. Kerr for statistical 

 advice. Drs. R. Thresher, R. Johannes, and R. Harden 

 Jones provided useful comments on the manuscript. 

 This work is part of a CSIRO/Solomon Islands govern- 

 ment collaborative research project funded by the 

 Australian Centre of Agricultural Research (ACIAR 

 project 8543). 



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