FISHERY BULLETIN: VOL. 86, NO 2 



The only unequivocal instance in which a prey 

 item of larval white croaker was vertically dis- 

 tributed similarly to the larvae was the trace of 

 amphipods found in the guts of competent (flexion 

 and postflexion) larvae. At the lengths of larvae 

 sampled (<12 mm) the prey were all planktonic 

 and nearly all about equally abundant in mid- 

 waters as near the bottom. The small numbers of 

 amphipods eaten may indicate an incipient tran- 

 sition to larger, suprabenthic crustacean prey. 

 The size gap between the large prey of these com- 

 petent larvae and the smaller prey of preflexion 

 larvae is probably an artifact of the bimodal size 

 distribution of sampled larvae. Though all of the 

 prey eaten by size-1 (<2.7 mm) larvae were <300 

 |xm in length, the more varied diet of larger pre- 

 flexion larvae contained some copepods as big as 

 500 |xm. There is therefore nothing in these data 

 to suggest that the switch from microplanktonic 

 to macroplanktonic prey is anything but a grad- 

 ual transition as the larvae grow. 



Brewer and Kleppel (1986) also reported a 

 change to copepod prey in white croaker larvae 

 >6 mm. Our findings are further similar to those 

 of Brewer and Kleppel in that there was no indi- 

 cation that food-seeking had anything to do with 

 the descent of larval white croaker from mid- 

 waters to the near-bottom zone. The other defin- 

 able dietary trend in this study (besides ontoge- 

 netic change) was the high percentage of rotifers 

 eaten by midwater preflexion larvae. This was 

 apparently related to subtle but important differ- 

 ences in the available planktonic prey — signifi- 

 cantly, to a greater abundance of suitable-size ro- 

 tifers — at the time the midwater stratum was 

 sampled. 



It seems safest to conclude that white croaker 

 larvae descend toward the bottom for reasons 

 quite apart from seeking food (see discussions in 

 Barnett et al. [1984], Brewer and Kleppel [1986], 

 Jahn and Lavenberg [1986]) and simply eat what- 

 ever they find there that suits them. Many poten- 

 tial macroplanktonic prey also favor the near- 

 bottom layer (Jahn and Lavenberg 1986; Barnett 

 and Jahn 1987). Older larvae and their prey may 

 occupy the near-bottom layer for different rea- 

 sons, or it may be that a single advantage, or set 

 of pressures, underlies the behavior of these di- 

 verse planktonic and semi-planktonic taxa. Some 

 species need to remain near shore, and living in 

 the bottom boundary layer helps assure this. The 

 boundary layer also tends to be more turbid than 

 overlying waters and so may lessen an animal's 

 jeopardy to visual (biting) planktivores. (The gen- 



erality of the latter explanation only holds if 

 suprabenthic fish larvae are less important 

 planktivores than other water-column inhabi- 

 tants — see Gushing 1983.) 



Rotifers have never to our knowledge been re- 

 ported as an important food of ocean-caught fish 

 larvae, even though the genus Brachionus is com- 

 monly cultured for feeding larval fish in the labo- 

 ratory. Schmitt (1986) reported that small, 

 laboratory-reared larval northern anchovy read- 

 ily fed upon (unidentified) wild-caught rotifers. 

 Rotifers are only occasionally abundant in neritic 

 waters, and never in oceanic waters (J. Beers^). 

 Their rarity notwithstanding, rotifers have the 

 ability very rapidly to dominate marine mi- 

 croplanktonic assemblages (Hernroth 1983), and 

 their good food quality (Theilacker 1987) and 

 high secondary productivity for a period of weeks 

 might constitute a significant enhancement to 

 growth and survival of a larval fish cohort. 



Our previous experience in handling larval 

 white croaker specimens agrees with the findings 

 of Brewer and Kleppel (1986) in that lamelli- 

 branch larvae, easily seen through the body wall, 

 are a common food for small white croaker larvae. 

 In our study, this taxon was a minor constituent 

 of the plankton and of the larval fish diet. We 

 cannot say how unusual were the circumstances 

 we encountered, but we know that in terms of 

 diatom numbers and larval fish diversity these 

 conditions were not typical of March on the south- 

 ern California continental shelf That white 

 croaker larvae appeared to find these conditions 

 salubrious may be one reason this species is so 

 successful in southern California (Love et al. 

 1984). 



ACKNOWLEDGMENTS 



Thanks go to S. Caddell, R. Feeney, T. Garrett, 

 R. Lavenberg, J. McGowen, J. Petersen, 

 J. Rounds, and Captain L. Nufer for able partici- 

 pation in the field work. D. Carlson-Oda, 

 J. Rounds, and S. Shiba helped process larval fish 

 samples, and H. Schwarz helped prepare the 

 manuscript. K. Zabloudil generously loaned the 

 current meters, and R. Erdman assisted in proc- 

 essing data therefrom. We also thank J. Beers 

 and R. Brusca for help in accessing literature on 

 rotifers. D. Cohen, R. Lavenberg, and J. Petersen 



5J. Beers, Scripps Institution of Oceanography, La Jolla, CA 

 92093, pers. commun. November 1986. 



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