ON THE ROLE OF FOOD-SEEKING IN THE SUPRABENTHIC HABIT 

 OF LARVAL WHITE CROAKER, GENYONEMUS LINEATUS 



(PISCES: SCIAENIDAE) 



A. E. Jahn, D. M Gadomski, and M. L Sowby' 



ABSTRACT 



Fish larvae and their prey were sampled from discrete depths within the bottom meter and at 

 middepth near the 15 m depth contour off southern California. The smallest white croaker larvae 

 (<2.7 mm NL) occurred mostly at middepth. Mid-sized larvae (2.7 mm to the beginning of flexion) 

 were almost all collected at the two depths nearest the bottom. All preflexion-stage larvae ate small 

 (50-300 (im in length) prey, chiefly rotifers, copepod nauplii, tintinnids, and invertebrate eggs. 

 Although small and mid-size larvae ate these items in different proportions, this difference could not 

 be ascribed to vertical distribution. Diet of the largest larvae, flexion and postflexion (roughly 5-15 

 mm), consisted mainly of copepods and differed by >90% from diets of smaller larvae. Though largest 

 larvae were only captured 50 cm above the bottom, their prey, with one exception (amphipods), were 

 more abundant at or above 1 m. It was concluded that the observed suprabenthic concentration of 

 older white croaker larvae was probably not motivated by food-seeking. 



Disparity between concentrations of food re- 

 quired for survival and growth of laboratory- 

 reared fish larvae and observations of average 

 concentrations of food organisms in the ocean has 

 led to the widely accepted idea that aggregations 

 offish larvae and their food must frequently over- 

 lap in nature (see reviews by Theilacker and 

 Dorsey [1980] and Hunter [1981]). Direct and in- 

 direct evidence for the importance of overlapping 

 concentrations of larvae and their prey (Lasker 

 1975, 1978; Govoni et al. 1985; Buckley and 

 Lough 1987) comes from sampling at fronts and 

 discontinuities in the pelagic environment. One 

 interface that attracts many zooplankters is the 

 seabed itself (Hamner and Carleton 1979; Wish- 

 ner 1980; Sainte-Marie and Brunei 1985). On the 

 southern California continental shelf, the seabed 

 serves as a surface of aggregation for larvae of 

 numerous fish species (Brewer et al. 1981; Schlot- 

 terbeck and Connally 1982; Barnett et al. 1984; 

 Jahn and Lavenberg 1986) and other zooplankton 

 (Clutter 1969; Barnett and Jahn 1987) and of 

 large-zooplankton biomass (Jahn and Lavenberg 

 1986). While it is tempting to suggest a trophic 

 advantage to the suprabenthic habit of the fish 

 larvae, near-bottom concentrations of organisms 

 actually eaten by larval fishes have yet to be 

 demonstrated along the open coast. 



iNatural History Museum of Los Angeles County, 900 Expo- 

 sition Boulevard, Los Angeles, CA 90007. 



In all cases reported, concentration in the near- 

 bottom zone was greater in older larvae and, 

 when observations permitted, greater during the 

 day than at night (Brewer and Kleppel 1986; 

 Jahn and Lavenberg 1986). The phenomenon is 

 therefore thought to be behavioral. Possible ad- 

 vantages of such behavior, including avoidance of 

 midwater predators, maintenance of position on 

 the shelf, and increased encounters with high 

 concentrations of food, have been discussed else- 

 where (Barnett et al. 1984; Brewer et al. 1984; 

 Brewer and Kleppel 1986; Jahn and Lavenberg 

 1986). In discussing the near-bottom schooling 

 behavior of a larval clupeoid in Japan, Leis (1986) 

 stated, "knowledge of the biology of epibenthic 

 fish larvae is too rudimentary to allow a clear 

 assessment of the advantages and disadvan- 

 tages. . . ." Whatever the advantages, a seemingly 

 more answerable question about the near-bottom 

 habit is what causes the larvae to behave as they 

 do? In another study from Japan, Tanaka (1985) 

 showed that juvenile red sea bream, Pagrus 

 major, exploited suprabenthic copepod popula- 

 tions, and he speculated that the distribution of 

 prey was a template for the descent of the fish 

 from midwaters and its subsequent migration 

 into estuaries. The question addressed in the 

 present study was whether the fine-scale layering 

 of larval fishes was a direct response to that of 

 their prey field. 



Because of the immediate behavioral aspect of 



Manuscript accepted Februar>' 1988. 

 FISHERY BULLETIN: VOL. 86, NO. 2, 1988. 



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