CONOVER: PATTERNS IN FECUNDITY OF ATLANTIC SILVERSIDE 



lower in fish nearly ready to spawn than in fish 

 where batch eggs are still maturing). Linear correla- 

 tions of batch size and mean diameter of the 

 maturing egg batch for each of the four dates on 

 which fecundity was measured were nonsignificant 

 (P > 0.05), suggesting that the number of eggs in a 

 batch does not decline much as the oocytes grow to 

 maturity. 



Assuming that the recruitment pool of immature 

 eggs is fully formed prior to the breeding season, an 

 estimate of the actual number of eggs produced an- 

 nually can be derived from the above data. Because 

 recruitment eggs remain at the season's end, actual 

 egg production is best represented by the total 

 number of eggs present just prior to the beginning of 

 the spawning season minus the number of recruit- 

 ment eggs retained when the spawning season ends. 

 For the above data, this provides a value of (1,609 ± 

 126) - (716 ± 164) = 893 ± 197 eggs/g ovary-free 

 body weight (±95% C.L.). The mean body weight of 

 females during the breeding season was 5.6 g so that 

 the average female would have spawned about 5,001 

 eggs in a season. The mean batch fecundity over the 

 spawning season was 223 eggs/g ovary-free body 

 weight or about 1,249 eggs/ female. Hence, if the ini- 

 tial assumption is correct, the average female must 

 spawn about four times during the breeding season. 



phic variation in M. menidia also support this conten- 

 tion (Johnson 1975). 



Daily observations of the number of spawning ag- 

 gregations sighted during high tide at Salem Harbor 

 in 1979 showed that populations of Atlantic silver- 

 sides breed on a semilunar periodicity coinciding 

 with new and full moons (see figure 2 in Conover and 

 Kynard 1984). Middaugh (1981) has reported similar 

 observations based on a 3-yr study of populations in 

 South Carolina. Within each semilunar spawning 

 period of 1979 in Salem Harbor, the majority of 

 spawning (40-90%) occurred on a single day, sug- 

 gesting that females spawn, at most, once per semi- 

 lunar cycle. This conclusion is further supported by 

 the observation that sex ratios in spawning aggrega- 

 tions were highly male biased, whereas during non- 

 spawning intervals, the sex ratio was near unity 

 (Conover 1984). The sex ratio data is explained if 

 females spawn, at most, once per cycle, while males 

 spawn each day of a spawning period. 



During 1979 in Salem Harbor, there were five 

 semilunar spawning periods, although the first in 

 late April was very light. In Essex Bay during 1977, 

 there were four semilunar phases during the period 

 defined as the breeding season. Hence, estimated 

 spawning frequency based on gonadal analysis and 

 direct observations of spawning fish agreed closely. 



Frequency of Spawning in Nature 



To test the prediction of spawning frequency 

 derived above, spawning periodicity was determined 

 from direct, visual observation of breeding events in 

 the field. In 1978, 1 discovered a large spawning site 

 in Salem Harbor, MA, where numerous eggs of M. 

 menidia were deposited amongst mats of filamen- 

 tous algae in the upper intertidal zone. Salem Harbor 

 is located 20 km southwest of Essex Bay; however, 

 individuals in each area are likely members of the 

 same population because extensive mixing occurs 

 during the offshore winter migration (Conover and 

 Murawski 1982). Electrophoretic studies of geogra- 



Spawning Frequency and Egg Production 

 in the Laboratory 



Females held in laboratory tanks, whether housed 

 individually indoors or outdoors in groups where 

 natural day and night illumination was present, did 

 not maintain a fortnightly spawning periodicity. In- 

 stead, spawning occurred much more frequently: the 

 interval between spawnings averaged about 4 

 d/female (Table 1; see also figure 4 in Conover and 

 Kynard 1984). Batch fecundity averaged 99-187 

 eggs/g ovary-free body weight among different 

 tanks. Total egg production averaged 1,425-3,375 

 eggs/g ovary-free body weight. Hence, the experi- 



Table 1. — Egg production by Menidia menidia on unlimited food rations in the laboratory. Field data are 



also provided for connparison. 



'Low values reflect the fact that 9 C died before the conclusion of the experiment. 



337 



