Size-Specific Vulnerability to Predation and Sensory 



System Development of White Seabass, 



Atractoscion nobilis, Larvae 



Daniel Margulies 



ABSTRACT: The size-specific vulnerability of 

 white seabass, Atractoscion nobilis, larvae (2.5-15.0 

 mm SL) to two types of fish predators— adult 

 northern anchovy, Engraulis mordax, and juvenile 

 white seabass — was examined in laboratory preda- 

 tion trials. Concurrent analyses were made of the 

 developmental ontogeny of larval visual and mech- 

 anoreceptive systems. The proportion of larvae re- 

 sponding to or escaping attacks by either predator 

 increased with larval size. There were no signifi- 

 cant differences in proportions of lar\ ae responding 

 to or escaping attacks of either predator until 

 larvae were 6.0-7.5 mm SL (early postflexion 

 stage). At this size, larvae were better able to evade 

 the slower, more discontinuous attacks of juvenile 

 white seabass compared with the faster attacks of 

 adult anchovy. Major developmental events occur 

 during this larval stage, including rapid improve- 

 ment in visual acuity, visual accommodation to dis- 

 tant objects, growth and stratification of the optic 

 tectum, and large increases in the numbers of free 

 neuromasts on the head and trunk. It is likely that 

 at larval sizes >7 mm SL, the slower attacks of 

 juvenile white seabass allow more time for larvae to 

 process and integrate visual and mechanoreceptive 

 sensory input from several modalities. White sea- 

 bass larvae (and sciaenids in general) become more 

 demersal in distribution during the late larval 

 stages in nearshore southern California waters. 

 This ontogenetic shift deeper into the water col- 

 umn may be related to predator-avoidance capabil- 

 ities of the larvae, since most demersal planktivores 

 exhibit some type of hovering, ambush, or discon- 

 tinuous mode of predation similar to that of juve- 

 nile white seabass and attack at much slower 

 speeds than pelagic, shoaling fish predators. As 

 they shift downward, older white seabass larvae 

 may maximize their predator-detection capabilities 

 when encountering demersal fish predators. 



For most marine and estuarine fishes, preda- 

 tion-induced mortality during early life stages 

 may be crucial in determining year-class 



Daniel Margulies, Soutliwest Fisheries Center La JoUa 

 Laboratory, National Marine Fisheries Service, NOAA, 

 P.O. Box 271, La JoUa, CA 92038; present address; Inter- 

 American Tropical Tuna Commission, c/o Scripps Institution 

 of Oceanography, La JoUa, CA 92093. 



Manuscript Accepted April 1989. 

 Fishery Bulletin, U.S. 87: 537-552. 



strength. Predation has long been recognized as 

 an important potential regulatory mechanism in 

 prerecruit hfe stages (Houde 1978, 1987; Hunter 

 1981, 1984), but many of the specific factors that 

 influence predation rates on early life stages are 

 not well understood (Bailey and Houde 1989). 



One of the crucial aspects of predator/prey 

 dynamics involves the developmental or physio- 

 logical basis for capture and escape responses. 

 Fish larvae possess several sensory systems pre- 

 sumed to be important in predator detection; 

 these include visual, mechanoreceptive, and 

 auditory systems. During early ontogeny, these 

 sensory systems undergo rapid development, 

 and the improvement in these systems is 

 thought to be crucial in controlling vulnerabihty 

 to predation (Hunter 1984; Blaxter 1986). How- 

 ever, experimental investigations combining the 

 study of larval sensory system development and 

 vulnerability to predation are limited. Larvae of 

 several species, including the northern anchovy, 

 Engaulis mordax, (O'Connell 1981; Webb 1981; 

 Folkvord and Hunter 1986), Atlantic herring, 

 Clupea harengus liarengus, (Blaxter et al. 1983; 

 Blaxter and Batty 1985; Fuiman 1989), Cape 

 anchovy, Engraulis capensis, (Brownell 1985), 

 bloater, Coregonus hoyi, (Rice et al. 1987), and 

 white perch, Morone americana, (Margulies in 

 press) have been studied to examine either sen- 

 sory system development or predator-avoidance 

 behaviors. Similar studies have been conducted 

 on larvae of flatfish (Neave 1984, 1986) and sev- 

 eral gadoids (Fridgeirsson 1978; Bailey 1984). 

 These types of investigations, however, have not 

 been combined during one study using a single 

 cohort of experimental fishes. 



The purpose of this study was to examine, 

 experimentally, the size-specific vulnerability of 

 white seabass, Atractoscion nobilis, larvae to 

 different types of fish predators, and to deter- 

 mine if there was a developmental or neuro- 

 logical basis for any observed changes in larval 

 vulnerability or behaviors. The developmental 

 studies of larval white seabass centered on the 

 ontogeny of two sensory systems, vision and 



537 



