FISHERY BULLETIN: VOL. 84, NO. 2 



posed 13% of 287 prey items of common murres col- 

 lected within 2 km of the Yaquina's jetties during 

 the summer of 1982. 



Salmon smolts appeared to be most vulnerable to 

 predation soon after a release. When they first 

 entered the estuary after exiting a pond through a 

 large tube, smolts seemed disoriented and milled 

 around the surface where they could easily be caught 

 by birds. Night releases allowed smolts several hours 

 to become adjusted before becoming vulnerable to 

 predators at daylight. (The only somewhat signifi- 

 cant nocturnal bird predator were heerman's gulls, 

 L. heermanni, but they usually numbered <50 birds, 

 were not present for every release, and were pres- 

 ent mainly in late July and August.) 



Within about 4 h after daylight after a release, 

 some smolts were observed jumping at the mouth 

 of the jetties in regions A and B, where birds also 

 concentrated. For censuses of regions A-D within 

 2 h of low tides and within 2 d of a release in 1982, 

 an average of 97.9% (SD - 6.3, N - 17 d) of the 

 common murres, 91.5% (SD = 16.2, N = 17 d) of 

 the "active" gulls, and 90.5% (SD = 26.8, N = S 

 d) of the "active" cormorants censused were at 

 regions A and B. But regions A and B accounted 

 for only about 27% of the area of regions A-D. 

 Evidently, the turbulent action of the estuarine 

 water entering the ocean and/or the funneling ef- 

 fect of the jetties (Fig. 1) caused the smolts to be 

 particularly vulnerable to predators there. 



During the first 12 h of daylight after a release, 

 some smolts within 0.5 km of the release site were 

 still vulnerable to bird predation as many smolts 

 were near the water surface. Many jumped out of 

 the water, and some rolled on their sides exposing 

 their silver undersides, which were highly conspic- 

 uous against the dark water background. Gulls often 

 sat on the water and grasped a fish as it jumped into 

 the air. Schools of smolts also milled near the sur- 

 face where they were clearly visible to humans (and 

 presumably birds). 



Within-Day Variation in Bird Numbers 



Bird abundance was clearly not constant within 

 a day, and taxa did not reach maxima synchronously 

 (Fig. 2). Censuses within 2 h of early low tides (i.e., 

 low tides before 1500 PDT) averaged closer to the 

 maximum number censused daily for all taxa, and 

 censuses near high tide were usually closer to the 

 daily maximum than counts within 2 h of evening 

 low tides (i.e., after 1800 PDT) for all taxa except 

 brown pelicans (Table 3). But differences in censuses 

 among tidal conditions within a day were only sig- 



IOO 



COMMON MURRES 

 ( MAX=442 I) 



LO A 0.8 



0600 0800 1000 1200 1400 1600 1800 2000 

 PACIFIC DAY L IGHT TIME 



Figure 2.— Percentage of daily maximum number of common 

 murres, "active" gulls, "active" cormorants, and pigeon guillemots 

 (PCs) observed on 5 August 1982 (which was two days postrelease) 

 at regions A-D. Times and heights of measured low (LO) and high 

 (HI) tides are indicated by open and closed triangles, respective- 

 ly. MAX - maximum number of birds seen on 5 August. 



nificant for common murres and "active" gulls 

 (Table 3). 



A single census at any time of day is unlikely to 

 estimate accurately the maximum number of birds 

 of any taxon present that day (Table 3). The average 

 census only ranged from 10.8% to 63.7% of the daily 

 maximum (Table 3). The best censuses to use for 

 estimates would be those within 2 h of a morning 

 or afternoon low tide because their averages 

 (44-64% of daily maxima) were greater than for 

 high and evening low tides, and their CV's (41-82%) 

 were generally lower than for other tides (Table 

 3). 



Daily Variation in Bird Numbers 



On a day to day basis, bird numbers could often 

 be seen to increase in the first day postrelease and 

 then to decline (Fig. 3). However, the degree of in- 

 crease was variable. Overall, murres, "active" gulls 

 in 1983, and brown pelicans exhibited the same pat- 



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