Stoner et al : Behavior of Pseudopleuronectes americanus during spawning, feeding, and locomotion 



101 1 



verged suddenly on a spawning pair or 

 group. A variety of sensory mechanisms 

 could play a role in detecting opportunities 

 for egg fertilization. Male fish may respond 

 to the obvious visual signal of light reflected 

 from the white underside of spawning fish. 

 It is also possible that hydroacoustic cues 

 were emitted from vigorous bouts of spawn- 

 ing that occurred just above the bottom. It 

 is less likely that spawning was detected 

 by an olfactory cue because there was al- 

 most instantaneous response to spawning 

 activity and there was little horizontal cur- 

 rent in the aquarium. However, olfactory 

 cues may be important in certain circum- 

 stances. Occasionally, males converged on 

 locations where spawning had occurred al- 

 though the spawners had departed, and 

 these spawning locations were sometimes 

 visited by additional males for several min- 

 utes after the initial spawning event. Fu- 

 ture research will be needed to determine 

 the precise mechanisms of spawning group 

 formation and detection. 



In female winter flounder just one batch of oocytes 

 matures per year (Burton and Idler, 1984); never- 

 theless, spawning in the aquarium population of win- 

 ter flounder had a duration of 60 days. It is clear 

 that individual females spawn many times over a 

 period of at least one week, and perhaps longer. Cir- 

 cumstantial evidence indicates that individual males 

 were capable of spawning over most of the reproduc- 

 tive season in the research aquarium. Large num- 

 bers of males were often engaged in single spawning 

 events; the total number of spawnings yielded a high 

 average spawning frequency by males that would 

 necessitate several weeks of spawning by each; and 

 feeding by males did not begin until most females 

 stopped spawning. It is unknown whether male win- 

 ter flounder produce more than one batch of sperm 

 or simply release one batch over time (Bert et al., 

 1988). 



Spawning in the aquarium occurred over a broad 

 range of times, but always during hours of darkness 

 or just before. Nocturnal spawning in winter floun- 

 der is probably a function of endogenous diurnal 

 rhythms rather than light level because Breder 

 (1922) noted that spawning occurred between 2200 

 and 0330 h even under artificial light. Spawning 

 times vary with flatfish species and are probably 

 species-specific. For example, plaice (Nichols, 1989) 

 and other pleuronectids (Woodhead, 1966) are also 

 known to spawn primarily at night, but Bothus 

 ocellatus spawns only around the time of sunset 

 (Konstantinou and Shen, 1995). 



The role of multiple releases of eggs and sperm is 

 poorly understood but may be important in the re- 

 productive biology and recruitment of winter floun- 

 der. Although release of gametes could not be re- 

 corded or verified beyond the fact that larvae were 

 produced in the aquarium, it is evident that numer- 

 ous males spawned in response to one female. It is 

 likely, therefore, that eggs released in a single spawn- 

 ing event were often fertilized by more than one male. 

 Given that the average female fish spawned 40 times 

 during the reproductive season in the aquarium and 

 that most of the males appeared to remain ripe 

 throughout the observation period, it is likely that 

 many, if not all, of the males could have contributed 

 genetic material to the offspring of each female. Ge- 

 netic diversity of embryos produced in the field could 

 be high for similar reasons. 



Although male winter flounder appeared to be in 

 a constant state of readiness for spawning, future 

 experiments will be needed to determine whether 

 frequency of spawning and fertilization success is 

 uniform among individuals and how this might be 

 influenced by size structure, sex ratio, or density of 

 fish on the spawning ground. For example, small ( 10— 

 15 cm ) but reproductively ripe males were abundant 

 in the Navesink River estuary. It is possible that 

 these small individuals have a lower frequency of 

 success than large males in inducing females to 

 spawn, given the order-of-magnitude difference in 

 body weight between 10 and 22 cm fish. However, 

 Baynes et al. (1994) observed female Dover sole 

 spawning with males smaller than themselves, and 



