FISHERY BULLETIN: VOL. 83, NO. 4 



in April and May were about 9°-ll°C (Fig. 2b). No 

 thermocline was present. Thus, hydrographic 

 regimes support grouping April and May samples 

 together. Temperature and salinity profiles in April 

 and May were typical for the winter Oregon 

 hydrographic regime prior to the onset of late spring- 

 early summer upwelling (Huyer 1977). 



In July the salinity and temperature profiles dif- 

 fered from those in April and May (Figs. 2c, d). 

 Salinities were more uniform in summer than spring 

 throughout the water column, ranging from 32.07oo 

 at the surface to 33.4"/u„ below about 20 m (Fig. 2c). 

 The temperature gradient in July was greater than 

 in April and May due to warmer surface waters (Fig. 

 2d). Surface temperatures ranged from 12.8° to 

 14.6°C, decreasing with depth to about 8.2°-9.6°C at 

 40 m. A thermocline was present at about 10-20 m. 

 Temperature and salinity profiles in July were typical 

 of the summer Oregon hydrographic regime (Huyer 

 1977). Surface temperatures suggest that samples 

 were not taken during active upwelling. 



RESULTS 



In this study, a total volume of 29,145.5 m^ was 

 filtered and 1,007 larvae, representing 33 taxa, were 

 enumerated from 75 discrete depth tows. Larvae 

 were most abundant in summer, with an abundance 

 peak 10-30 m deep during daytime and 20-30 m deep 

 during nighttime (Fig. 3). In spring, larvae were 

 distributed relatively uniformly throughout the 

 water column below 5 m with small abundance peaks 

 at 10-20 and 40-50 m. During daytime in both spring 

 and summer, larvae were least abundant at the sur- 

 face (0-5 m), although abundance at the surface in- 

 creased at night. The depth distribution at night also 

 differed in having a secondary abundance peak near 

 the bottom (50-60 m) and greater overall larval abun- 

 dance than during the day. 



The larval fish species were categorized as coastal 

 (most abundance 2-28 km from the coast, see Tkbles 

 2, 3, and 4), or offshore (most abundant 37-111 km 

 from the coast, see Ikbles 5, 6, and 7), according to 

 larval assemblages described by Richardson and 

 Pearcy (1977). Most larvae in this study were of the 

 coastal assemblage because samples were collected 

 at NH5 and NHIO (9.2 and 18.5 km from the coast, 

 respectively). The spawning seasons of the dominant 

 species off Oregon are discussed in Mundy (1984); 

 most of the fall-winter spawning species were not 

 represented in this study. Since many species were 

 not abundant enough to demonstrate trends, only 

 the dominant species will be discussed below. 



Coastal Assemblage 



Gadus macrocephalus, Microgadus proximtis, 

 Isopsetta isolepis, and Psettichthys melanostictiis lar- 

 vae were abundant in all three sampling periods 

 (Tkbles 2, 3, 4, Fig. 4). Gadus macrocephalus larvae 

 were most abundant during the day at 20-30 m in 

 both spring and summer, but were very abundant 

 in the deepest stratum (below 50 m) in night samples 

 (Fig. 4). Microgadus proximvs larvae do not show 

 as clear a trend, but were most abundant in deeper 

 water during summer, particularly at nighttime In 

 spring they were distributed throughout the water 

 column. Isopsetta isolepis and P. melanostictus were 

 also most abundant in nighttime samples. More /. 

 isolepis larvae were found at 10-20 m in spring, 

 whereas in summer they were collected throughout 

 the water column, with abundance peaks near the 

 bottom. Psettichthys melanostictus larvae were more 

 abundant in summer than spring samples. During 

 daytime in summer, P. melanostictus were most 

 abundant below 10 m, whereas at nighttime, 

 although found throughtout the water column, they 

 were most abundant in waters shallower than 10 m 

 (Fig. 4). 



Seasonal abundance changes were observed for 



-I 

 5 

 10 



20 



-^ 30 



E 



a. 



O 40 



50 



4.4 



4>--., 



3.3,3 



• .4 Day, Spring 



• • Day, Summer 



•— — • Nig hi, Summer 



H 



I — I — I — I — I — I — I — I — I — 



10 20 30 40 50 60 70 80 90 



Mean Abundance (No./IOOOm^) 



Figure 3.— Overall larval abundance (larvae per 1,000 m^) for all 

 collections. Numbers adjacent to data points indicate the number 

 of samples taken. 



614 



