(Hunter, 1980; Walsh, et al., 1981). This distribution will accentuate aggregation. 

 On the other hand, passive dispersion will continue longer than 24 hours, at least 

 among pelagic spawners. The actual duration will depend on the time of hatching, 

 the onset of larval swimming, and perhaps on the overall length of the pelagic 

 period. In cold-water fishes, the egg stage may endure from 2-4 days to 2 years 

 (Hempel , 1979), but the precise time is species- and temperature-dependent. In 

 coral reef fishes, where incubation temperatures are high, pelagic eggs hatch 

 generally after 15-36 hours (Thresher, 1980). Demersal eggs usually require longer 

 to hatch, up to 12 days in coral reef fish, but, since dispersion does not begin in 

 these species until emergent larvae enter the water column, the duration of the egg 

 stage does not influence spread. 



Once young larvae begin actively swimming, the possibility exists for behavioral 

 mechanisms to influence dispersion. Swimming begins between 0.5-3 days post- 

 hatching in coral reef fishes (Thresher, 1980) and 1-3 days after hatching in 

 anchovies and Pacific mackerel (e.g., Hunter & Kimbrell , 1981). Larvae hatch from 

 demersal eggs at a more advanced stage of development than larvae from pelagic eggs. 

 Sensory and motor systems are more fully functional in the former and new larvae 

 swim and catch prey very soon after hatching (Iwai, 1980). The short time interval, 

 for these larvae, between hatching and onset of swimming provides short purchase 

 for turbulent diffusion to act. Hence, we should expect dispersion to be far less 

 in larvae from demersal than from pelagic eggs. 



Finally, very little is known about the total length of the pelagic period. 

 Estimates vary from 3 days to 10 weeks in some coral reef species (Thresher, 1980; 

 Brothers & McFarland, 1981). The density of eggs and larvae, whether aggregated or 

 not, will clearly vary with mortality. During the pelagic egg stage, mortality 

 varies between 2-95% per day, with larval mortality running 2-15% per day (Jones & 

 Hall, 1975; Hempel , 1979). 



In spite of the large area covered by eggs in the model, eggs did aggregate 

 within some 1 m? units simply from random processes. The probability of finding 

 eggs or larvae in aggregates would be substantially increased if selective forces 

 favored their survival over that of isolated eggs or larvae (Hewitt, 1981). On 

 theoretical grounds, the probability that a predator will detect aggregated eggs 

 may be less than the corresponding probability for dispersed eggs (Rubinstein, 

 1978). The same phenomenon will favor aggregated larvae as well. Larval aggrega- 

 tion may also increase the difficulty of prey capture for a predator (Milinski, 

 1977). The larger the larval aggregation, the less likely it is that any particu- 

 lar larva will be eaten, provided the predator is incapable of eating the entire 

 patch. It is not known whether more active anti-predator behavioral adaptations 

 occur in fish larvae or not, e.g., increased total vigilance as aggregate size 

 increases, but such processes are well documented for adult social groups of 

 terrestrial animals and for some marine insects (Treherne & Foster, 1980). 



Food densities necessary for larval survival and growth tend to be substantially 

 higher than mean densities of food in the open sea. Consequently, there should be 

 strong selective pressures for larvae to locate and remain in high-density food 

 clumps (Hunter, 1980). One way to achieve this is through local enhancement: 

 larvae in clumps should find food patches faster than isolated individuals because 

 each larva could watch its neighbor's behavior as well as look for food. When a 

 larva begins feeding, the observant neighbor could swim to the feeding site and 

 itself begin feeding. Since (1) young larvae feed largely through visual mechanisms 

 of perceiving prey (Iwai, 1980), (2) perceptive distance increases with the size of 

 the perceived object (Hunter, 1980), and (3) neighbors are much larger than food 

 items, a larva should see neighbors at greater distances than food. The effect will 

 be a substantial increase in the search volume for food by each aggregated larva. 

 Search volume, currently estimated at 0.1-1.0 liter/hour for individual larvae 

 (Hunter, 1980), will be determined by larval swimming speed. In general, early 



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