SEABIRDS NEAR AN OREGON ESTUARINE SALMON HATCHERY IN 



1982 AND DURING THE 1983 EL NINO 



Range D. Bayer 1 



ABSTRACT 



In the summer of 1982, 14.4 million salmon, Oncorhynchus sp., smolts were released at the Yaquina 

 Estuary, OR; and in the summer of 1983, 12.8 million salmon smolts were released. Within hours after 

 release, fish-eating seabirds aggregated at the estuary mouth. In 1982, the number of no seabirds was 

 significantly correlated with the number of days since a release. In 1983, however, numbers of common 

 murres, Uria aalge; gulls, Larus sp.; and brown pelicans, Pelecanus occidentalis, were significantly in- 

 versely correlated with the date of a release, and the number of cormorants, Phalacrocorax sp., was 

 significantly more abundant the second day after a release. In contrast, numbers of Caspian terns, Sterna 

 caspia, and pigeon guillemots, Cepphus columba, showed no relationship with releases in 1983. 



There were significantly more cormorants and marbled murrelets, Brachyramphus marmoratus, 

 in 1983 than in 1982. There were also significantly more murres in 1983 than in 1982 before 1 August, 

 but fewer afterwards. Gull and brown pelican numbers were about the same between years, but significant- 

 ly fewer pigeon guillemots were present in 1983 than in 1982. 



Seabirds have been estimated to consume 29% of 

 the pelagic fish production within 45 km of a British 

 seabird colony (Furness 1984b), and several simula- 

 tion models for various geographical areas indicate 

 that 20-30% of the annual pelagic fish production 

 may be preyed upon by seabirds (Furness 1984a). 

 Since the mortality of salmon, Oncorhynchus sp., 

 smolts as a result of predation and environmental 

 factors is greater shortly after they first enter the 

 ocean than after they move offshore (Parker 1962, 

 1968), the impact of seabird predation on salmon 

 smolts just released along a coast could also be 

 significant. 



El Nino is the intrusion of anomalously warm 

 water off the coast of Peru and Ecuador (Barber and 

 Chavez 1983); an El Nino of varying intensity oc- 

 curs on the average of every 3-5 yr (Quinn et al. 

 1978; Duffy 1983a). Along the Oregon coast, warm- 

 water conditions concurrent with an El Nino appear 

 much more rarely, and in the last century have oc- 

 curred only in 1983, 1957-1958, and perhaps in 1941 

 (Huyer 1983; Reed 1983). The impact of seabirds on 

 hatchery-released salmon smolts would be expected 

 to be greater in years of anomalously warm water 

 associated with El Nino, when natural prey for sea- 

 birds become rare and seabirds starve or have low 

 nesting success (Boersma 1978; Duffy 1983a, b; 

 Ainley 1983; Schreiber and Schreiber 1984). 



'Oregon Aqua-Foods, Inc., 2000 Marine Science Drive, New- 

 port, OR 97365; present address: P.O. Box 1467, Newport, OR 

 97365. 



Here, I correlate bird numbers with salmon smolt 

 releases at Yaquina Estuary, OR, and examine 

 variation in bird numbers between the summer of 

 1982 and the summer of 1983, when warm water 

 associated with an El Nino was present. 



STUDY AREA AND METHODS 



Yaquina Estuary (Fig. 1) is located on the mid- 

 Oregon coast and is a drowned river valley. It has 

 an intertidal and submerged area of 15.8 km 2 

 (Oregon State Land Board 1973). During this study, 

 all releases were from the site designated as OAF 

 in Figure 1. 



The most abundant seabird nesting nearby was 

 the common murre, Uria aalge, but western gulls, 

 Larus occidentalis; Brandt's cormorants, Phalacro- 

 corax penicillatus; pelagic cormorants, P. pelagicus; 

 and pigeon guillemots, Cepphus columba, also nested 

 there (Table 1; Pitman et al. in press). Within Ya- 



Table 1.— Distance of nesting birds from the mouth of Yaquina 

 Estuary in 1979 (calculated from Pitman et al. in press). 



Cumulative number of nesting birds 

 <7 km <20 km <25 km <45 km <50 km 



common murres 1 2 2,800 2 6,800 2 6,800 



western gulls 398 528 536 



cormorants 418 653 1,581 



pigeon guillemots 45 191 201 



26,800 322,000 



541 1,231 



1,727 3,041 



206 220 



'Includes all breeding and nonbreeding adults at colony. 

 ^Estimated for 1983 (USFWS, aerial survey; pers. obs.). 

 includes 1983 as well as 1979 estimates. 



Manuscript accepted July 1985. 



FISHERY BULLETIN: VOL. 84, NO. 2, 1986. 



279 



aS'T-f- 2-*^ 



