BAYER: SEABIRDS NEAR OREGON ESTUARINE SALMON HATCHERY 



pigeon guillemots because the numbers of these 

 birds during this period did not show any signs of 

 seasonal variation. But for common murres, the 

 periods of comparison were 1 June-31 July and 1-30 

 August, and the periods for Caspian terns were 1 

 June-10 July, 11 July-5 August, and 6-30 August. 

 The periods for common murres and Caspian terns 

 were chosen because in one or both years there were 

 marked seasonal changes in bird numbers between 

 or among these periods. 



The number of days postrelease refers to the num- 

 ber of daylight periods after a smolt release (Myers 

 1980). For example, if smolts were released on Mon- 

 day night or early Tuesday morning, then Tuesday 

 after dawn would be considered as 1-d postrelease 

 (i.e., the first day, or first daylight period, after a 

 release). 



If variances were not significantly different, then 

 the student's £-test for two means or the analysis 

 of variance (ANOVA) for three or more means were 

 calculated to determine statistical differences be- 

 tween or among means. If variances were signifi- 

 cantly different, the Mann-Whitney U test or nor- 

 malized Mann-Whitney z test (Zar 1974, p. 109-113) 

 for two samples or the Kruskal-Wallis rank H or H c 

 (if ranks were tied) test (Zar 1974, p. 139-142) for 

 three or more samples was used. All tests were 

 two-tailed. 



RESULTS AND DISCUSSION 



Smolt Releases 



Oregon Aqua-Foods, Inc. (OAF) has released 2 

 million or more salmon smolts (almost all coho 

 salmon, Oncorhynchus kisutch) each year since 1977 

 into Yaquina Estuary between June and August. In 

 1982 and 1983, the proportion that were coho 

 salmon was 98% and 94%, respectively; the re- 

 mainder were chinook salmon, 0. tshawytscha. Un- 



til 1983, these releases were under variable tidal con- 

 ditions in the evening just after dark to minimize 

 bird predation of smolts as they were released. In 

 1983, salmon smolts were released either in the 

 evening or early morning on the ebbing tide while 

 it was still dark. 



Salmon smolts do not immediately swim to the 

 ocean after they are released. Myers (1980) found 

 that the number of OAF smolts in the Yaquina 

 Estuary declined exponentially after a release. Dur- 

 ing June- August releases in 1978, half the smolts 

 left the estuary within an average of 3.9 d (SE = 

 0.7 d, range 1.7-9.0 d, N = 9 releases) with a few 

 smolts remaining in the estuary several months 

 (calculated from Myers 1980). There are no data to 

 determine if the smolt residency time in the estuary 

 differed between 1982 and 1983. 



In 1982 and 1983 from June through August, the 

 interval between releases averaged <2.5 d, and an 

 average of 0.2-0.3 million fish were released each 

 time (Table 2). Although the average release inter- 

 val was longer and the number of fish per release 

 usually greater in 1983, these differences were not 

 significant (Table 2). But the biomass of fish per 

 release was significantly greater in 1983 than in 

 1982 in the June- July and June- August periods 

 (Table 2). Overall, 1.6 million fewer fish were re- 

 leased in 1983 than 1982, but the total biomass 

 released was almost 38 metric tons (t) greater (Table 

 2); this resulted from smolts weighing more on the 

 average in 1983 (32.9 g/smolt) than in 1982 (26.7 

 g/smolt) (calculated from Table 2). 



Bird Predation of Salmon Smolts 



Although all birds in this study except marbled 

 murrelets were observed with salmon smolts in their 

 bills, the importance of smolts in these birds' diets 

 was not documented in this study. However, Mat- 

 thews (1983) found that coho salmon smolts com- 



Table 2.— Releases of salmon smolts in 1982 and 1983 at Yaquina Estuary. Total 

 = total number or biomass of fish released during a period. Differences between years 

 tested with student's f, Mann-Whitney U, or normalized Mann-Whitney z test. NS = 

 not significant. 



Re- 



Release 



interval 



(d) 



No. fish/release 

 (millions) 



Fish biomass/release 

 (t) 



281 



