(Table 1). A total of 2,152 larvae and postlarvae 

 were taken in 84 tows with the 0.5-m net and 784 

 postlarvae and juveniles were captured in 10 tows 

 with the 0.9-m trawl. 



Early stages of 22 species were taken with the 

 0.5-m net. The catch was dominated numerically 

 by the Osmeridae which accounted for 89% of the 

 total. Spirinchus thaleichthys were the most 

 numerous — composing 67% of the total catch. 

 Thaleichthys pacificus represented 19% of the 

 total. Cottus asper made up 7% of the total and 

 each of the remaining individual species ac- 

 counted for less than 1%. 



Twelve species were captured with the trawl at 

 Station 2. Spirinchus thaleichthys, 22-64 mm, 

 composed 92% of the catch. Post-larval Hypomesus 

 pretiosus, Allosmerus elongatus, and juvenile 

 Engraulis mordax represented the majority of the 

 remaining total. The trawl captured three species 

 not taken with the 0.5-m net: Ophiodon elongatus, 

 Hemilepidotus hemilepidotus, and Alosa 

 sapidissima. 



Species composition of ichthyoplankton in the 

 Columbia River estuary differed from that found 

 in other northwest estuaries. Waldron (1972) and 

 Blackburn (1973) found larval Gadidae dominated 

 catches in Puget Sound. In Humboldt Bay, El- 

 dridge and Bryan (1972) reported 82% of the total 

 catch was Clupea harengus pallasi and 

 Lepidogobius lepidus. In Yaquina Bay, Pearcy and 

 Myers (1974) reported this combination of species 

 was 90% of the catch. Clupea h. pallasi in the 

 Columbia River estuary composed less than 1% of 

 the total and no L. lepidus were captured. 



Seasonal Abundance 



Larval and post-larval fishes were most 

 abundant January through May. During the 

 summer no larval or post-larval stages were taken 

 at any of the seven stations. Similar findings were 

 reported in Humboldt Bay (Eldridge and Bryan 

 1972) and in Yaquina Bay (Pearcy and Myers 

 1974). 



Abundance estimates are based on average 

 monthly catches at all stations with the 0.5-m net 

 (Figure 2). A peak of 1.1/m 3 occurred in March, 

 primarily the result of an influx of newly hatched 

 Spirinchus thaleichthys. A maximum average 

 catch of 1.5/m 3 occurred in May, the result of an 

 increased number of Thaleichthys pacificus and 

 Cottus asper. Maximum catch during the year was 

 4.0/m 3 and occurred at station 2 in May. The 



1.5 



< 



5 1.0 



5 0.5 

 til 



2 



FIGURE 2. — Seasonal density of ichthyoplankton at seven loca- 

 tions in the Columbia River estuary during 1973. These results 

 show average catch at seven stations with the 0.5-m plankton 

 net. 



composition was entirely S. thaleichthys, T. 

 pacificus, and C. asper. 



Juveniles were the only stage captured with the 

 trawl from summer through fall. Those captured 

 were: Microgadus proximus (60-61 mm), Sebastes 

 melanops (55-67 mm), Alosa sapidissima (44 mm), 

 Leptocottus armatus (11-13 mm), Allosmerus 

 elongatus (49-58 mm), Engraulis mordax (45-68 

 mm), and Spirinchus thaleichthys (45-64 mm). 



Horizontal Variation 



The greatest variety of species was captured at 

 stations nearer the mouth where salinities were 

 higher. Large variations in tides and river flow 

 combine to create a fluctuating horizontal saline 

 intrusion; salinity is dissipated upstream and 

 station 7, except during reduced river flow in the 

 fall, is essentially fresh water (Haertel and Os- 

 terberg 1967 and Misitano 1974). The reduction in 

 salinity upstream was reflected by a correspond- 

 ing decrease in the variety of species (Figure 3). At 

 station 1 there were 22 identifiable species and at 

 stations 5 and 7 three species: S. thaleichthys, T. 

 pacificus, and C. asper. Stations 5, 6, and 7, which 

 exhibited similarly reduced salinities, accounted 

 for 47.8% of the total larvae captured in the es- 

 tuary with the 0.5-m net. This high percentage is 

 due to the influx of the two species of osmerid 

 larvae entering the estuary during the first part of 

 the year. 



220 



