VERTICAL DISTRIBUTION OF ICHTHYOPLANKTON OFF 

 THE OREGON COAST IN SPRING AND SUMMER MONTHS 



George W. Boehlert,' Dena M. Gadomski,^ and Bruce C. Mundy^ 



ABSTRACT 



Day and night discrete-depth tows were taken off the Oregon coast in spring and summer months to 

 assess the vertical distribution of ichthyoplankton in nearshore waters. Over 1,000 larvae representing 

 33 taxa of both coastal and offshore ichthyoplankton assemblages were taken; Psettichthys melanostictus 

 was the most abundant coastal species and Lyopsetta exilis the most abundant offshore species. Larvae 

 were generally most abundant at 10-30 m, near the seasonal thermocline in both day and night collec- 

 tions. Larval abundance in July was much higher than in April-May collections. Limited evidence for diel 

 vertical migration suggests that Psettichthys melanostictiis moves to surface waters at night and Gadus 

 macrocephalus moves to deeper water at night. No trends of changes in depth distribution were observed 

 with increasing size 



Knowledge of the vertical distributions of larval 

 fishes is crucial to full understanding of their biology 

 and to understanding the results of ichthyoplankton 

 surveys (Ahlstrom 1959; Kendall and Naplin 1981). 

 The interaction between vertical distributions and 

 physical processes can have important effects on 

 onshore-offshore distributions of planktonic 

 organisms in upwelling regions such as the coastal 

 northeastern Pacific (Peterson et al. 1979; Parrish 

 etal. 1981; Wroblewski 1982; Rothlisberg et al. 1983). 

 Near-surface distribution, for example, may result 

 in shoreward transport in slicks associated with in- 

 ternal waves (Shanks 1983). In the coastal region off 

 Oregon, the only information on larval fish vertical 

 distribution is a comparison between abundances of 

 Paro'phrys vetultcs and Isopsetta isolepis larvae from 

 neuston and oblique bongo net tows (Laroche and 

 Richardson 1979) and one 24-h study with stratified 

 samples taken by bongo nets without opening-closing 

 devices (Richardson and Pearcy 1977). With the ex- 

 ceptions of the classic study by Ahlstrom (1959) and 

 recent studies by Brewer et al. (1981) and Schlotter- 

 beck and Connally (1982), little else is known about 

 the vertical distribution of larval fishes in north- 

 eastern Pacific coastal waters. In this paper, we pre- 

 sent information on vertical distributions of larval 

 fishes off Oregon. 



'College of Oceanography and Marine Science Center, Oregon 

 State University, Newport, OR; present address: Southwest 

 Fisheries Center Honolulu Laboratory, National Marine Fisheries 

 Service, NOAA, P.O. Box 3830, Honolulu, HI 96812. 



^College of Oceanography and Marine Science Center, Oregon 

 State University, Newport, OR; present address; Section of Fishes, 

 Los Angeles County Museum of Natural History, 900 Exposition 

 Boulevard, Los Angeles, CA 90007. 



METHODS 



Six series of samples were collected in 1982, four 

 during daylight (30 April, 14 May, 2 and 13 July) and 

 two during night (2 and 6-7 July). The first two series 

 (30 April, 14 May) were taken at station NHIO, 10 

 nmi (18.5 km) off Newport, OR, on the Newport 

 hydroline (lat. 44°40'N; Fig. 1). All others were col- 

 lected at NHS (9.2 km offshore). Each sample series 

 consisted of a variable number of tows at discrete 

 depth strata from the surface to within about 4 m 

 of the bottom (Tkble 1). 



Tows were stepped oblique in five intervals of 3 

 min each, resulting in a total sampling time of 15 

 min in each 5 or 10 m depth stratum. The sampler 

 was an opening-closing lUcker trawl (Clarke 1969) 

 with three nets and a double-release mechanism 

 operated by messengers. The nets were 0.505 mm 

 mesh (Nitex^) with a 1 m^ mouth; all tows were at 

 a wire angle of 45° at approximate tow speeds from 

 0.9 to 1.1 m/s. At this angle, effective mouth area 

 of the net is 0.71 m^. An uncontaminated, discrete 

 depth sample was collected in the second net by 

 lowering the trawl with the first net open, opening 

 the second net for the desired sampling time, and 

 retrieving the trawl with the third net open. Water 

 volumes filtered were estimated with General 

 Oceanics flowmeters mounted in the center of each 

 net. Volumes of water filtered usually ranged be- 

 tween 250 and 450 m^/sampla Temperature and 

 salinity data were collected throughout the water col- 

 umn on each cruise using Niskin bottles to collect 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



Manuscript accepted January 1985. 

 FISHERY BULLETIN: VOL. 83, NO. 4, 1985. 



611-9l| 



