BARNETT ET AL.: DISTRIBUTION OF ICHTHYOPLANKTON OFF SAN ONOFRE, CALIF. 



top of the range of the epibenthic gear, could make 

 large differences (by a factor of 2) in the abundance 

 estimates of some taxa. 



Other studies from the Southern California Bight 

 have shown cross-shelf patterns similar to those 

 which we describe. For example, Gruber et al. (1982; 

 sampling neuston and midwater) and Brewer et al. 

 (1981; sampling the entire water column) both 

 showed vertical and cross-shelf changes in species 

 composition. In both studies, atherinid larvae were 

 principally neustonic. Brewer et al. (1982) took 69% 

 of all larvae on their surveys from the epibenthic 

 stratum. Both studies showed that clinids, most 

 gobiids, sciaenids, and atherinids were most pre- 

 valent nearer shore. Such inshore-offshore patterns 

 have also been shown further north along the west 

 coast (Pearcy and Meyers 1974; Richardson and 

 Pearcy 1977). 



Icanberry et al. (1978) conducted a distributional 

 study of ichthyoplankton above the epibenthic 

 stratum at two nearshore stations off Diablo Canyon, 

 about 100 km northwest of the Southern California 

 Bight. Though there is taxonomic overlap between 

 their study and ours, their sampling was too 

 nearshore to delimit the offshore extent of any 

 species in our study. Published data on widely 

 (offshore) ranging species are contained in the 

 CalCOFI atlas series (Kramer and Ahlstrom 1968; 

 Ahlstrom 1969, 1972; Ahlstrom and Moser 1975) and 

 complement some of the offshore patterns report- 

 ed here. 



Engraulis mordax, one of these widely ranging 

 species, spawns principally offshore (Richardson 

 1981; Brewer and Smith 1982). The number of 

 excess E. mordax larvae (over those which can be 

 accounted for by eggs) in the nearshore zone must 

 come from outside the sampling area, and these lar- 

 vae must begin moving shoreward at an early age. 

 Richardson (1981) suggested that currents might be 

 a mechanism through which larvae of the northern 

 subpopulation of E. mordax are redistributed. We 

 presently cannot identify a mechanism for the redis- 

 tribution off San Onofre. However, if one assumes it 

 involves some behavioral response to environmental 

 cues, it is worth considering just how far a larval 

 anchovy might swim. Hunter (1972) estimated cruis- 

 ing speed on the order of one-half body length/s. At 

 this speed, a 6 mm larva would swim about 250 m/d, 

 far enough to move several kilometers along an 

 environmental gradient during the larval period. Any 

 behavior allowing larvae to remain in the nearshore 

 zone (e.g., orientation toward the bottom), once 

 encountered, could help explain their observed 

 concentration. 



The increased concentration of older larvae of E. 

 mordax, Genyonemus lineatus, and Seriphus politus 

 nearshore and near the bottom is reminiscent of the 

 invasion and retention of larval and postlarval fishes 

 in estuaries and tidal creeks of the Atlantic coast (cf. 

 Chao and Musick 1977; Weinstein et al. 1980). Older 

 larvae of Paralichthys californicus, although too rare 

 for statistical analysis, also appeared more concen- 

 trated nearshore than did the younger larvae. 

 Whatever the mechanisms for such ontogenetic 

 redistribution, they must be at least partly 

 behavioral. Weinstein et al. (1980) found vertical 

 movements in response to tides, whereby postlarvae 

 became more concentrated near the bottom during 

 ebb flows, thus taking advantage of the slower 

 seaward current in the boundary layer. In the 

 Southern California Bight the mean nearshore flow is 

 alongshore, with relatively weak cross-shelf com- 

 ponents (Hendricks 1977; Reitzel 1979 11 ; Parrish et 

 al. 1981; Winant and Bratkovich 1981). The major 

 source of cross-shelf water motion is internal waves 

 of tidal frequency (Winant and Olson 1976) which 

 propagate toward shore. For these waves to pro- 

 pagate, the water column must be stratified. It is not- 

 able that larval S. politus, which displayed the most 

 intense ontogenetic redistribution, is most abundant 

 during late summer-early fall (Walker et al. foot- 

 note 11), the season of maximum thermal 

 stratification in the Bight (Cairns and Nelson 1970). 

 Thus it may be that S. politus and other semi- 

 planktonic organisms of the shallow shelf waters take 

 advantage of internal tides in somewhat the same way 

 that the estuarine fauna use the surface tide to regu- 

 late position. It is conceivable that due to dissipation 

 of energy, seaward motions in the boundary layer are 

 slower than shoreward motions. 



A similar internal wave mechanism for shoreward 

 migration has been suggested by Norris (1963). He 

 hypothesized that postlarval Girella nigricans might 

 swim ahead of the cold waters of the incoming inter- 

 nal wave fronts, thus producing the observed early 

 shoreward migration of that species. 



Brewer and Smith (1982) estimated that the num- 

 bers of E. mordax larvae spawned in the nearshore 

 waters were approximately proportional to the area 

 the nearshore waters represented in the total waters 

 inhabited by the central subpopulation. They con- 

 cluded that the nearshore region off southern 



"Reitzel, J. 1979. Physical/chemical oceanography. In Interim 

 report of the Marine Review Committee to the California Coastal 

 Commission. Part II: Appendix of technical evidence in support of 

 the general summary. MRC Document 79-02(11), p. 6-23. Marine 

 Review Committee of the California Coastal Commission, 631 

 Howard Street, San Francisco, CA 94105. 



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