Abstract.- Length-at-sexual- 

 maturity and spawning periodicity 

 of the tuna baitfish Encrasicholina 

 devisi, E. heterolobus, SprateUoides 

 delicatulus, S. gracilis, S. lewisi, and 

 Archamia zosterophora were studied 

 at two exploited fishing grounds and 

 one unexploited site in the Solomon 

 Islands. All species became sexually 

 mature and capable of spawning at 

 70% of the largest size, except the 

 apogonid A zosterophora which ma- 

 tured at a larger size (80%). There 

 was little site-related variability in 

 length-at-flrst-spawning, although S. 

 lewisi from Tulagi grew to a larger 

 size and was larger than S. lewisi 

 from other sites when it spawned for 

 the first time. There was no evidence 

 that length-at-first-spawning was af- 

 fected by commercial baitfishing. 



The timing and intensity of spawn- 

 ing of each species were extreme- 

 ly variable. All species spawned 

 throughout the year, with one or two 

 periods of more intense activity. The 

 spawning peaks of the same species 

 at different sites did not coincide, 

 and no proximate stimuli correlated 

 with spawning by any species at all 

 sites. The timing of major spawning 

 events was not random, nor did fish 

 spawn as soon as they reached ma- 

 turity. Spawning events at the three 

 sites correlate with particular en- 

 vironmental conditions, especially 

 moon phase and, less importantly, 

 rainfall and temperature. These re- 

 sults are not consistent with the 

 hypothesis that spawning is timed to 

 maximize either local dispersal or the 

 potential for larvae to find suitable 

 food. Lack of clear proximate stimuli 

 for spawning among the six species 

 examined makes it difficult to predict 

 the timing of major spawning events 

 by these species. 



Maturation, Spawning Seasonality, 

 and Proximate Spawning Stimuli 

 of Six Species of Tuna Baitfish 

 in the Solomon Islands 



David A. Milton 

 Stephen J.M. Blaber 



CSIRO Division of Fisheries, Marine Laboratories 



PO Box 120, Cleveland. Queensland 4163. Australia 



Manuscript accepted 17 December 1990. 

 Fishery Bulletin, U.S. 89:221-237 (1991). 



The pole-and-line fisheries for skip- 

 jack tuna Katsuwonus pelamis in the 

 Pacific are dependent on adequate 

 supplies of suitable bait. Engraulids 

 (genus Encrasicholina) and dussumi- 

 erids (genus SprateUoides) are the 

 basis of the Solomon Islands tuna 

 baitfishery, the largest in the region 

 with catches of over 2000 1 annually 

 (Anon. 1988). 



Knowledge of the reproductive biol- 

 ogy of the main bait species may be 

 important in developing management 

 regimes to minimize the impact of the 

 baitfishery on these species. Some 

 aspects have been studied in the Sol- 

 omon Islands (Evans and Nichols 

 1984) and elsewhere in the Pacific 

 (Tester 1955; Tiews et al. 1971; Leary 

 et al. 1975; Dalzell and Wankowski 

 1980; Conand 1985; Dalzell 1985, 

 1986, 1987ab; Lewis et al. 1983; 

 McCarthy 1985; Clarke 1987) and in 

 southeast Asia (Dharmamba 1960; 

 Tham 1965; Luther 1979; Chen 1984, 

 1986). These studies suggest both 

 genera spawn year-round, with peri- 

 ods of increased spawning during 

 spring and summer (Leary et al. 

 1975, Tiews et al. 1971, Luther 1979) 

 or with the change of monsoon (Dal- 

 zell and Wankowski 1980, Dalzell 

 1987b) or periods of high zooplankton 

 production (Sitthichockpan 1972). How- 

 ever, timing of peak spawning is vari- 

 able, both temporally (e.g., Dalzell 

 1987b) and between regions. 



There have been several reviews 

 (e.g., Scott 1979, Lam 1983, Bye 



1984) of the importance of various 

 cues which stimulate gonadal devel- 

 opment and cause fish to spawn 

 (proximate factors). Among temper- 

 ate species, temperature and light 

 are the most common cues (Scott 

 1979, Bye 1984). Other cues, such as 

 food supply, moon phase, and rain- 

 fall, have also been suggested as 

 important for spawning by tropical 

 marine fishes (Johannes 1978, Lam 

 1983, Walsh 1987). However, the 

 proximate stimuli that arouse in- 

 creased spawning activity among 

 baitfish remain obscure. Tester 

 (1955) found that variations in egg 

 production by Encrasicholina pur- 

 pureas in Hawaii could not be ade- 

 quately explained by temperature, 

 salinity, or moon phase. Similarly, 

 Muller (1976) showed that fluctua- 

 tions in salinity and zooplankton 

 biomass accounted for only 30% of 

 the variation in egg production of E. 

 heterolobus at Palau. Such poor cor- 

 relations suggest that spawning may 

 be random, or fish may begin to 

 spawn as soon as they are physio- 

 logically capable of doing so. 



The length at which Encrasicho- 

 lina and SprateUoides become sex- 

 ually mature appears to be variable 

 both between and within countries of 

 the southwestern Pacific (Dalzell and 

 Wankowski 1980, Conand 1985, Dal- 

 zell 1985, McCarthy 1985, Dalzell 

 1987ab, Wright 1989), and these fish 

 may adjust their life-history param- 

 eters to changes in their demography 



221 



