BOEHLERT ET AL: VERTICAL DISTRIBUTION OF ICHTHYOPLANKTON 



Table 8. — Ranges and mean standard lengths (mm) for dominant fish larvae. N = number of larvae; Min. = 



minimum; Max. = maximum. 



spp., Cyclopteridae spp. 1, and Isopsetta isolepis. 

 These differences indicate the need for more exten- 

 sive sampling before the variability of vertical 

 distributions off Oregon can be understood, par- 

 ticularly as they relate to hydrographic condi- 

 tions. 



We found Engraulis mordax larvae entirely at 0-10 

 m. Brewer et al. (1981) found greater concentrations 

 of Engraulis below 10 m, while Ahlstrom (1959) 

 found Engraulis to be concentrated in the upper 23 

 m with some specimens occurring to 105 m. Off 

 Oregon, Engraulis larvae are found concentrated at 

 0-20 m (Richardson 1973), in association with the 

 Columbia River plume, a lens of warm, low salinity 

 water usually 20-40 m deep (Richardson 1980). Our 

 limited data suggest that Engraulis mordax larvae 

 occur at depths that would place them within the 

 plume, rather than beneath it or at its boundary. The 

 vertical distribution suggests restriction to the 

 warmest part of the water column (Fig. 2d); north- 

 ern anchovies rarely spawn in waters with surface 

 temperatures below 14°C (Lasker et al. 1981). 



The seasonal differences in species composition be- 

 tween the April-May and July samples were those 

 that would be expected in samples from winter and 

 summer hydrographic regimes, except that Artedius 

 fenestralis and A. meanyi have been taken in April 

 and May of other years (Mundy 1984). The presence 

 of Clupea harengus, Radulinus asprellus, myctophid, 

 and bathylagid larvae only in April-May, during a 



winter hydrographic regime, is expected from 

 previous studies (Richardson and Pearcy 1977; 

 Mundy 1984). 



Studies of day/night differences in the distribution 

 of fish larvae are confounded by daytime avoidance 

 of nets by larvae (Ahlstrom 1959). Daytime avoidance 

 of nets is suggested in our study by the greater 

 numbers of larvae taken during the night than day 

 at all but two depth strata. The lack of length dif- 

 ferences between larvae caught in day and night, 

 however, and the fact that no taxa were taken only 

 in night samples during July suggests that diurnal 

 net avoidance was not related to taxon or size The 

 same comparisons with 70 cm bongo net samples 

 (Richardson and Pearcy 1977) suggest that diurnal 

 avoidance by large larvae was greater for bongo nets 

 than for the Tlicker trawl. 



Evidence for vertical migration exists for several 

 species in this study (Tkbles 3, 4, 6, 7). Psettichthys 

 melanostictus abundance in surface waters (0-10 m) 

 increased greatly at night (Fig. 4). Engraulis mor- 

 dax were most abundant at 5-10 m than 0-5 m dur- 

 ing the day, but more evenly distributed at night. 

 This could be due either to vertical migration or net 

 avoidance in the shallowest stratum during the day. 

 Ahlstrom (1959), however, presented evidence for 

 negative phototaxis by anchovy larvae, and Hunter 

 and Sanchez (1976) demonstrated nighttime migra- 

 tion to the surface in larvae larger than 10 mm SL. 

 Thus larvae migrate upwards at night, but are con- 



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