Milton et al.: Reproductive biology and egg production of three species of Clupeidae 105 



tively estimated. Post-ovulatory follicles were aged 

 according to stages found in other multiple-spawning 

 clupeoids (Hunter and Goldberg, 1980; Goldberg et al., 

 1984; Isaac-Nahum et al., 1988). Gonosomatic indices 

 (GSI) were calculated as the ratio of wet gonad weight 

 to somatic weight (total weight minus gonad weight), 

 expressed as a percentage. Similarly, we calculated a 

 hepatosomatic index (HSI) as the ratio of liver dry 

 weight to somatic dry weight (total weight minus en- 

 tire viscera), expressed as a percentage. 



Length and age at sexual maturity were defined 

 as the minimum size and age at which fish had ripe 

 oocytes (Stage 4), determined by histological exami- 

 nation. Fish that had running-ripe oocytes (Stage 5) 

 were recorded as in spawning condition. We defined 

 the length and age at first spawning as the small- 

 est size where the proportion of running-ripe oocytes 

 in the section exceeded 85% for more than 50% of 

 the fish of that length or age. We chose this crite- 

 rion after examining large numbers of histological 

 sections with running-ripe oocytes. In these sections 

 they always represented more than 85% of the sec- 

 tion area. Our results were similar to that found in 

 other tropical clupeoids (Milton and Blaber, 1991). 

 The reproductive life span of the population of each 

 species at each site each month was determined 

 from the oldest fish (Milton et al., 1993) in each 

 sample minus the age at first spawning. 



We estimated batch fecundity for each species 

 from fish that had been examined histologically and 

 had oocytes that were starting to hydrate ( ripe-early 

 running ripe; Stages 4-5; Table 1), but we did not 



examine the fecundity of fish with any empty fol- 

 licles. An advanced modal size group of oocytes could 

 be distinguished in ripe fish. We separated a 

 subsample of between half (A. sirm) and all (S. 

 delicatulus) of the ovary and weighed it. The num- 

 ber of eggs in the advanced mode was counted and 

 the fecundity was estimated by multiplying the 

 number of eggs in the subsample by the ratio of total 

 gonad weight to subsample weight. Fecundity esti- 

 mates were made within three to four days after the 

 ovary was removed from the fish to minimize the 

 potential bias of differential absorption of fixative 

 by oocytes and surrounding somatic tissue. 



We used hydrated oocytes from fish caught be- 

 tween 2000 and 2330 hours to estimate egg weight. 

 Oocyte weights were estimated from hydrated oo- 

 cytes in ovaries that were almost ready to spawn 

 (late Stage 5; Table 1). We measured oocyte dry 

 weight by counting 10 samples of 10 oocytes from 

 each ovary, drying the oocytes at 50° C to a constant 

 mass and weighing each subsample separately. 



We scored visceral fat on a five-point scale. If a 

 fish had less than 25% of the intestine covered in 

 fat deposits, it was scored as (1); 25-50%, (2); 50- 

 75%, (3); and 75-100%, (4). A fish scored (5) when 

 all intestine was covered with fat and deposits were 

 also present around the stomach (Nikolsky, 1963). 



The proportion of females examined histologically 

 each month that had post-ovulatory follicles (POF; 

 Stage 6) was used to evaluate reproductive season- 

 ality. We determined that these fish had spawned 

 within the previous 15-48 hours, because these 

 structures decompose and cannot be recognised af- 

 ter that time (Hunter and Goldberg, 1980; Clarke, 

 1989). In samples where no fish had POF's, we used 

 the proportion of fish in the histological subsample 

 whose sections had greater than 85% running-ripe 

 oocytes (Milton and Blaber, 1991). We used this 

 proportion to calculate monthly estimates of mean 

 daily oocyte production and the number of batches of 

 oocytes spawned each month (Parrish et al., 1986). 



We estimated daily oocyte production (n/kg of 

 adults; egg production index) for samples collected 

 from commercial baitfishing, because these samples 

 were assumed to be most representative of the popu- 

 lation. Our methods were similar to those of Parker 

 (1980, 1985), which have been used to estimate the 

 spawning biomass of a number of multiple spawn- 

 ers (Armstrong et al., 1988; Pauly and Palomeres, 

 1989; Somerton, 1990). However, our methods dif- 

 fered because we used commercial catch per unit of 

 effort (CPUE) as an index of adult abundance. 



Egg production 

 index 



^(fiPF.SR^^WiYcPUE (1) 



