MARGULIES: VULNERABILITY AND SENSORY DEVELOPMENT OF WHITE SEABASS 



Table 1 . — Modes of predation exhibited by adult Engraulis mordax and juvenile Atractosclon nobilis. 



dow on one side for observation. Flourescent 

 lamps produced 2,000-3,000 mc at the surface of 

 each tank. A black plastic tent enclosed the win- 

 dow, providing a darkened compartment for 

 observations. Metric rules placed on the tank 

 window allowed estimation of predator attack 

 distances. The tanks were supplied with flow- 

 through ambient seawater (17°-19°C), except 

 during an experiment when the water was 

 static. Since adult northern anchovy often occur 

 in large schools and juvenile white seabass are 

 solitary or found in loose aggregations, it was 

 decided that northern anchovy predators in 

 groups of five individuals and white seabass in 

 groups of three should be used. This simulated 

 the predators' natural condition yet prevented 

 predator "swamping" of larval prey. 



Feeding motivation of predators was stan- 

 dardized by presenting constant numbers of 

 adult Artemia to predators prior to predation 

 trials with larvae. (Artemia are a good standard 

 prey because they do not avoid predatory at- 

 tacks by fishes.) Preliminary experiments had 

 indicated that feeding behavior of adult northern 

 anchovy was more variable than that of juvenile 

 white seabass; thus, five gi'oups of five Ariernia 

 each were presented to each anchovy predator 

 group and three groups of five Aiiejiua each 

 were presented to white seabass. After the 

 Artemia additions, white seabass larvae were 

 introduced into the predation tanks in groups of 

 five (each larval group added = trial). Trial dura- 

 tion was 10 minutes or until all prey were eaten. 

 Larvae and Artemia were added to the tanks in 

 clear beakers that were gently submerged at the 

 water surface. After the initial Artemia trials, 

 each predator group was tested with 6-8 larval 

 trials, followed by a final Artemia trial to test for 

 predator satiation. 



On the same day as predation experiments, 

 subsets of 10-12 larvae were removed from cul- 



ture tanks and fixed in 5% formalin for calcula- 

 tion of mean larval sizes; additional subsets of 

 larvae were removed for examination of sensory 

 system ontogeny (discussed below). The total 

 number of predator-prey interactions observed 

 for each larval size class and predator type is 

 presented in Table 2. 



Table 2. — The total number of predator-prey interactions ob- 

 served for each larval size and age class and predator type. 



Classification of behaviors followed Folkvord 

 and Hunter (1986). Four measures of predator- 

 prey interactions were calculated: mean and 

 maximum attack distances; percentage of larval 

 avoidance responses; percentage of larval 

 escapes; and predation rate (percentage of 

 larvae captured during each 10 min trial). Ap- 

 proximate predator attack speeds also were 

 estimated. A predator attack was a directed 

 movement toward a prey with the mouth open. 

 Predator attack distance was the distance in 

 decimeters (dm) from a prey to the point of 

 origin of attack. An avoidance response was a 

 change in speed or direction of a larva occurring 

 before a predator could complete an attack. An 

 escape occurred when a predator failed to cap- 

 ture a larva during a single attack. Repeated 

 attacks were scored as separate events. 



Two potential biases to the predator-prey 



539 



