324 



Fishery Bulletin 105(3) 



Table 5 



Correlation coefficients for the depth-stratified sample 

 means (n=74) of eleven variables sampled during the 

 day and night from a single station (HH-05) off the 

 Oregon coast in August 2000 and 2002: water tempera- 

 ture (°C), salinity (kg/m''), and log^.-transformed densi- 

 ties (number/1000 m'^) of Lyopsetta exilis, Sebastes spp., 

 Stenobrachius leucpsarus, Tarletonbeania crenularis, and 

 total fish larvae, and Chauliodus macouni, Icichthys lock- 

 ingtoni, Sardinops sagax, and total fish eggs. *=P<0.05, 

 *=P<0.001. * '*=P<0.0001. 



Salinity 

 Lyopsetta exilis 

 Sebastes spp. 



Stenobrachius leucopsarus 

 Tarletonbeania crenularis 

 Total larvae 

 Chauliodus macouni 

 Icichthys lockingtoni 

 Sardinops sagax 

 Total eggs 



In addition, we found evidence that S. sagax spawn in 

 this region, as documented since the 1990s (Emmett et 

 al., 2005; Auth and Brodeur, 2006). Similarly, we col- 

 lected T. symmetriciis eggs, which are normally found 

 off southern California but which have been reported off 

 southern Oregon by Kendall and Clark', off Washington 

 by Ahlstrom (1956), and most recently in the Columbia 

 River plume by Parnel et al.^ 



Our finding that the vast majority (96^^) offish larvae 

 were present in the upper 100 m of the water column 

 has been supported by several other studies (Brodeur 

 and Rugen, 1994; Suntsov, 2002; Sabates, 2004; Auth 

 and Brodeur, 2006). This finding has significant im- 

 plications for strategies to sample ichthyoplankton off 

 Oregon and potentially throughout the California Cur- 

 rent. Sebastes spp. primarily occurred in the 10-50 m 

 depth range, as previously reported (Ahlstrom, 1961; 

 Boehlert et al., 1985; Sakuma et al., 1999; Auth and 

 Brodeur, 2006), whereas L. exilis larvae were found 

 between 20 and 100 m as observed by Auth and Brodeur 

 (2006) and during daytime collections made by Boehlert 

 et al. (1985). Also, myctophid larvae within the subfam- 

 ily Myctophinae (T. crenularis) were found at greater 



1 Kendall, A. W., Jr., and J. Clark. 1982. Ichthyoplankton 

 off Washington, Oregon, and northern California April-May 

 1980. AFSC Proc. Rep. 82-11, 44 p. National Oceanic 

 and Atmospheric Administration, Alaska Fisheries Science 

 Center, 7600 Sand Point Way NE, Seattle, WA 98115. 



2 Parnel, M. M., R. L. Emmett, and R. D. Brodeur. 2006. In- 

 terannual and seasonal variation in ichthyoplankton collected 

 off the Columbia River. Unpubl. nianuscr., 20 p. Oregon 

 State Univ., Hatfield Marine Science Center, 2030 SE Marine 

 Sci. Dr., Newport, OR 97365. 



depths than those within the subfamily Lampanyctinae 

 (S. leucopsarus) as reported by Sassa et al. (20021, al- 

 though the vast majority was still found above 100 m. 



One implication from these findings is that sampling 

 in the upper 100 m of the water column should be suf- 

 ficient to characterize pelagic summer ichthyoplankton 

 abundances and distributions of the majority of fish 

 taxa along the northeastern Pacific coast (Brodeur and 

 Rugen, 1994; Auth and Brodeur, 2006). A further im- 

 plication is that sampling strategies without discrete 

 vertical strata, which expend additional effort collect- 

 ing ichthyoplankton below 100 m, may underestimate 

 larval fish abundances by using samples from largely 

 uninhabited portions of the water column. 



Differences in the abundance and size of fish larvae 

 collected in diel samples have often led researchers to 

 suspect bias due to net avoidance, resulting in under- 

 estimation of larvae (especially larger larvae) collected 

 during the day (Richardson and Pearcy, 1977; Boehlert 

 et al., 1985). These differences can vary depending on 

 such factors as gear type, size, and tow speed. The pres- 

 ent study supports this contention, particularly for the 

 upper 20 m of the water column, where light penetra- 

 tion is of greatest concern. Sakuma et al. (1999) found 

 that total adjusted catches of Sebastes spp. larvae off 

 the central California coast were significantly greater 

 at night than during the day. Richardson and Pearcy 

 (1977) observed that large Sebastes spp. larvae (9-11 

 mm) were collected exclusively during the night, while 

 smaller larvae (3-4 mm) were collected during both day 

 and night at the 0-50 m depth off the central Oregon 

 coast. However, they did not find evidence of daytime 

 net avoidance for S. leucopsarus (5-11 mm) or Isopsetta 

 isolepis (butter sole) (14-23 mm) larvae collected during 

 the same study. In addition, Laroche and Richardson 

 (1979) found no evidence of increased net avoidance by 

 larger Parophrys vetulus (English sole) larvae collected 

 during the day versus night off the Oregon coast. How- 

 ever, considering results from the present and similar 

 studies on differences in diel ichthyoplankton abun- 

 dance (Ahlstrom, 1959), we believe that ichthyoplankton 

 sampling should be conducted primarily at night if at 

 all possible to eliminate any potential bias due to net 

 avoidance. Net avoidance should at least be factored 

 into any model estimating abundances and depth distri- 

 butions of larvae collected during both day and night. 



As noted earlier, diel vertical migration (DVM) has 

 been well documented for larvae of many marine fish 

 species. A variety of theories have been put forth to 

 explain DVM in larval fish, including predator avoid- 

 ance (Hunter and Sanchez, 1976; Yamashita et al., 

 1985), the pursuit of zooplankton prey (Fortier and 

 Leggett, 1983; Munk et al., 1989), facilitated larval 

 transport in varying tidal currents (Norcross and Shaw, 

 1984; Hare and Govoni, 2005), optimization of the en- 

 ergetic advantage gained by larvae at certain depths 

 in thermally stratified water (Wurtsbaugh and Never- 

 man, 1988; Neilson and Perry, 1990), and the pursuit 

 of optimum light conditions for larval survival (Heath 

 et al., 1988J. Although this study was not designed to 



