In the Pacific Northwest, juvenile hatchery-reared salmonids (Oncorhynchus sp.) attract predators or are eaten by northern pikeminnows (Ptychocheilus oregonensis)(Thompson 1959:50, 56; Thompson and Tufts 1967, Maxfield et al. 1970:1, Uremovich et al. 1980:31, Buchanan et al. 1981:362, Schroder and Fresh 1992:281-287, Ledgerwood et al. 1993, Collis et al. 1995, Shiveley et al. 1996), other predatory fish (Beamish et al. 1992, Kruzynski and Birtwell 1994:1786, National Research Council 1996:40), and birds (Mace 1983:33, Macdonald et al. 1988:1370-1371, Jones et al. 1996:12, Scheel and Hough 1997, Roby et al. 1998, Bayer 2000:16, Collis et al. 2000, Stahl et al. 2000). Most (79% or more) juvenile salmonids eaten by Caspian terns (Sterna caspia) and double-crested cormorants (Phalacrocorax auritus) at the Columbia River Estuary were from hatcheries, and terns took hatchery fish significantly more often than their relative in-river abundance (Roby et al. 1998:42-43, 47-48; Collis et al. 2000:25-26, 61-62).
Hatchery-reared juvenile salmonids can be impaired after release for several reasons, and thus they may be more vulnerable to predation (Mesa et al. 1994, Roby et al. 1998:64, Collis et al. 2000:36-37). First, they have been stressed, disoriented, and forced into an unfamiliar environment; these stressors can be compounded by the additional stress of trying to adapt to seawater (Wedemeyer et al. 1980, Schreck 1990:31- 32, Brauner et al. 1994, Shrimpton et al. 1994). Second, hatchery juvenile salmonids are easily detectable to predators because some behave inappropriately in their new environment. Since they are fed on pellets spread on the water surface, many hatchery fish come to the surface to feed shortly after release and in so doing are easily seen by potential predators; they also often jump out of the water or roll, exposing their highly conspicuous silver sides, which makes it very easy for predators to find them (Bayer 1986). Their presence near the surface makes them particularly vulnerable to aerial predators such as Caspian terns (Roby et al. 1998:64, Collis et al. 2000:37). Third, hatchery fish are vulnerable to predation because they are not wary of predators (e.g., Bayer 1989:61-63; Suboski and Templeton 1989, Olla et al. 1998). Fourth, hatchery fish have sometimes been released that are nearly blind (Mace 1983), diseased (Bley and Moring 1988:15, Morley et al. 1988:80, Raymond 1988:18, Johnson et al. 1990:431, Cross et al. 1991:24, Witty et al. 1995:23-26, Maule et al. 1996, National Research Council 1996:311-312), or of poor quality (Dickhoff et al. 1995), so it is not surprising that they may attract predators. Because it has been recognized that hatchery fish are vulnerable to predators, changing rearing practices to reduce post-release predation has been proposed (Olla et al. 1994, Maynard et al. 1995, ODFW 1998:27-28, Hansen 2000).
The purpose of this paper is to examine the increased abundance of bird predators after releases of hatchery salmon smolts at Yaquina and Coos Bays in Oregon.
Almost all juvenile salmon released into Yaquina Bay during 1975- 1983 were by Oregon Aqua-Foods, Inc. (OAF), a private aquaculture business (ODFW 1991:22-23, 36-37). Most were coho (Oncorhynchus kisutch) smolts, although some chinook salmon (O. tshawytscha) smolts were also released (Table 1). The western tip of the jetties (Mile 0.0) was 2.0 mi west of the OAF release site.
Many coho smolts were very conspicuous after release because they seemed disoriented and milled around the surface and jumped or rolled on their sides, exposing their silver undersides (Bayer 1986:282).
After hatchery releases of coho in 1982 and 1983, common murres, gulls (Larus sp.), Heermann's gulls (L. heermanni), brown pelicans (Pelecanus occidentalis), Brandt's cormorants (Phalacrocorax penicillatus), pelagic cormorants (P. pelagicus), double-crested cormorants, pigeon guillemots (Cepphus columba), and Caspian terns were observed eating smolts in the estuary (Bayer 1986:281-282). Murres were the most numerous bird that appeared to be feeding on smolts, but there were also peak counts of 1,311 gulls, 286 cormorants, and 84 brown pelicans in 1983 (Bayer 1986:284). The numbers of common murres, gulls, and brown pelicans averaged greatest the first day after a release than on later days, but the average number of cormorants peaked on the second day after a release (Bayer 1986:283-284).
Within Yaquina Bay, counts of over 100 murres were only recorded in years when four million or more coho smolts were released (Table 1). The increase in murre numbers during years with large releases is even greater than indicated in Table 1 because peak counts of murres in 1971 and 1993 (when no smolts were released) included the "jaws" of the jetties (Mi 0-0.8), but results for 1982 and 1983 (when many smolts were released) did not include the "jaws." In 1982 and 1983, peak counts just for the "jaws" were 2,886 and 2,703 non-flying murres, respectively (unpubl. data used in Bayer 1986).
Murre predation was not limited to within Yaquina Bay as peak counts of 5,011 non-flying murres in 1982 and 8,155 murres in 1983 were seen between the jetties and about 0.6 mi (1.0 km) to the west and to the south of the jetties in the Pacific Ocean (unpubl. data for Bayer 1986:280). Uncounted murres were probably also to the north or further than 0.6 mi from the jetties in the ocean. Total counts for areas within the Estuary and for part of the Pacific Ocean near the jetties peaked at 5,988 non-flying murres in 1982 and 9,638 murres in 1983 (Bayer 1986:Table 4). Since these are counts only of non-flying murres at one time, they underestimate the total number of murres present during a day because murres were often flying between the estuary and their nesting colony at Yaquina Head, so probably not all that fed at the estuary were present during a count. Further, these counts are underestimates because they did not include all the area in the ocean near the jetties.
In the ocean near Yaquina Bay during May-August in 1969-1971, 120 common murres were collected, but none were found with salmon smolts (Scott 1973:58); in these years, no coho or chinook smolts were released into the Yaquina (ODFW 1991:22-23, 36-37). In 1982, when 21 million coho were released (ODFW 1991:36-37), 28% of the diet of murres collected near Yaquina Bay (Newport) in June was coho smolts (Matthews 1983:98), and coho smolts were 13% of the diet of murres within 2 km of the Yaquina jetties compared to 2% farther away (Matthews 1983:32, 49). Matthews (1983) did not distinguish between hatchery and wild smolts, but the change in murre diet with increased hatchery releases is probably not coincidental.
--------------------------------------------------------------------------TABLE 1. Hatchery releases by Oregon Aqua-Foods or public hatcheries of coho or chinook smolts into Yaquina Bay and the number of common murres inside of Yaquina Bay. 1971-1983 release data are from ODFW (1991:22-23, 36-37); 1993 and 1997-1998 release data are from Bob Buckman (ODFW Biologist, pers. com.). There were also 0-0.2 million coho fry or presmolts released by state hatcheries into the Yaquina Basin during 1971-1983 (ODFW 1991:36). MAX=maximum number of murres recorded during a single count. -=no data. Count Area=portion of Yaquina Estuary counted for murres; the boundaries are estimated in miles from the western tip of the jetties (Mi 0), Mi 0.8 is where the narrow channel between the jetties widens, Mi 2.5 is the Oregon State University Hatfield Marine Science Center Small Boat Dock, Mi 3.5 mi is at the east edge of Idaho Flats, and Mi 6.9 is at the east edge of Poole's Slough.
--------------------------------------------------------------------------
Common
Murres_____
Millions of Hatchery Count
Smolts Released____ Area
Year(s) Coho Chinook MAX (mi) Reference
----------------------------------------------------------------------
1971-72* 0 0 32* 0-2.5 Scott (1973:13, 40)
1975 ** ** 37 0.8-6.9 Bayer (1983:79-80)
1979 4 1 198 0.8-6.9 Bayer (1983:79-80)
1980 8 ** 1,053 0.8-6.9 Bayer (1983:79-80)***
1981 12 ** - - -
1982 21 1 1,992 0.8-3.5 Bayer (1986) ****
1983 15 1 1,884 0.8-3.5 Bayer (1986) ****
1993 0 0 51 0.5-6.9 Merrifield (1998:4,
157)
1997 ** ** 23 0.5-6.9 Merrifield (in prep.)
1998 0 ** 22 0.5-6.9 Merrifield (in prep.)
* Scott's counts were during May 1971-May 1972; his "MAX" count of 32 is
the average in August 1971, which was the month when he found
murres to be most numerous on average and is estimated from his
graph.
** Less than 0.5 million.
*** The 1,053 murres in Bayer (1983:79-80) includes area 6.9-11.0 mi;
examination of count data reveals that no murres were found
upstream of 6.9 mi.
**** MAX murre counts given in Bayer (1986:Table 4) include murres in the
ocean and in Mi 0-0.8, so only unpubl. data used in Bayer (1986)
for 0.8-3.5 mi are given here to be comparable to 1975, 1979, and
1980.
--------------------------------------------------------------------------
The attraction of birds to releases of coho smolts was also apparent at Coos Bay, where they were released into the estuary, mostly by private aquaculture.
In the ocean near the mouth of Coos Bay (see maps in Matthews 1983:55-71), Dan Varoujean (in Ward 1983) and Dan Matthews (1983) found that common murres became more abundant or dense and fed relatively more on coho salmon smolts during years of big hatchery releases of coho smolts into Coos Bay (Table 2).
During 1979-1982, Matthews (1983:87-90) examined 4,977 prey items of common murres collected off Coos Bay and only found one other salmonid besides coho smolts--a chinook salmon smolt. The low incidence of chinook smolts may be because Paul Reimers (ODFW Biologist, pers. com.) reported that only 0.04-0.24 million fall chinook were released per year into Coos Estuary during 1979-1982.
--------------------------------------------------------------------------TABLE 2. Hatchery releases by Oregon Aqua-Foods and Anadromous, Inc.of coho and chinook salmon smolts into Coos Bay and Dan Matthews' (1983:28, 35-36) and Dan Varoujean's (in Ward 1983) common murre data in the Pacific Ocean near the mouth of Coos Bay. Release data are from Paul Reimers (ODFW Biologist, pers. com.), Matthews (1983:36), and Ward (1983). % of diet=the percentage of all prey items by number that were coho salmon smolts in common murre diets in Matthews (1983:28). ND=no data.
--------------------------------------------------------------------------
Common Murres in Ocean near Coos Bay__
Matthews' (1983)______
Murre % of Coho Varoujean's
Millions of Hatchery Density Smolts in Murre
Smolts Released______ (Murres/ Murres' Abundance
Coho Chinook sq km) Diet (numbers)
-------------------------------------------------------------------------
1979 1 * ND 0.1 ND
1980 7 * ND 0.3 4,400
1981 12 * 123 6.5 13,600
1982 2 * 41 0.1 3,000
* Less than 0.5 million.
--------------------------------------------------------------------------
Birds, particularly, common murres, became abundant at Yaquina and Coos Bays after many hatchery coho smolts were released. This attraction was conspicuous to observers, and hatchery managers tried to reduce predation by releases at night (Bayer 1986:282) or by barging hatchery fish offshore (Gowan 1988, Bayer 1989:47-48, McNeil et al. 1991).
This attraction also occurred at Alsea Bay, where local residents reported seeing large numbers of great blue herons (Ardea herodias) and cormorants after hatchery releases. In May and June 1997, Bob Buckman (Oregon Dept. of Fish and Wildlife Biologist, pers. com.) also observed concentrations of birds and harbor seals (Phoca vitulina) that appeared to be feeding on juvenile salmon after a hatchery release into the Alsea. In the spring of 1998, when there were no June releases, he did not see any concentrations of predators like in June 1997, so the concentrations of bird predators appear correlated with hatchery releases at the Alsea.
The increased abundance of predators after releases of hatchery juvenile salmonids has also been observed elsewhere in the Pacific Northwest (e.g., see Introduction). However, Caspian terns and double- crested cormorants generally did not become more abundant after a release at the Columbia River Estuary (Roby et al. 1998:55, Collis et al. 2000:29). This lack of a response may be because the Columbia is so large that hatchery fish can disperse more; alternatively, there were so many hatchery juvenile salmonids released into the Columbia Basin (e.g., about 200 million in 1990 and 1992 [Kaczynski and Palmisano 1993:165, 295; Beamish et al. 1997:1211]) that they were more continuously available to predators than in other stream basins, so a single release into the Columbia Estuary was not as attractive.
The greater vulnerability of hatchery fish to predators appears to continue into the ocean because Holtby et al. (1992:2174) found that juvenile hatchery chinook and coho were more apt to be caught in saltwater sports fisheries than their relative abundance compared to wild fish.
Given the significantly selective predation of Caspian terns on hatchery salmonids at the Columbia River Estuary (Roby et al. 1998:42-43, 47-48; Collis et al. 2000:25-26, 61-62) and the observed attraction of predators to hatchery fish releases, one may wonder if hatchery fish increase predation on wild juvenile salmon, many of which are listed under the Endangered Species Act in Oregon and Columbia River Basin, or if the abundance of hatchery fish decreases predation on wild fish by "swamping" predators or "masking" the passage of wild fish (see Witty et al. 1995:29-30, Columbia Basin Fish and Wildlife Authority 1996:83).
Having seen the attraction of predators after hatchery releases at the Yaquina and noting the great change in murre abundance in years with large hatchery releases, I suspect that wild fish that happen to be passing after a hatchery release are much more vulnerable to predation. This may be particularly true in Oregon coastal estuaries because they are relatively narrow and shallow, so that wild fish have little room to avoid predators that have been attracted by the hatchery fish.
I am grateful to Bob Buckman and Paul Reimers of the Oregon Department of Fish and Wildlife and to Kathy Merrifield for providing unpublished data. I also thank Janet Webster, Susan Gilmont, and Judy Mullen of the Guin Library at the Oregon State University Hatfiled Marine Science Center for providing library assistance.
Bayer, R. D. 1983. Seasonal occurrences of ten waterbird species at
Yaquina Estuary, Oregon. Murrelet 64:78-86.
Bayer, R. D. 1986. Seabirds near an Oregon estuarine salmon hatchery in
1982 and during the 1983 El Nino. Fish. Bull. 84:279-286.
Bayer, R. D. 1989. The cormorant/fisherman conflict in Tillamook
County, Oregon. Studies in Oregon Ornithology No. 6. (This is at
http://www.orednet.org/salmon/salmon.htm#cormorant)
Bayer, R. D. 2000. Cormorant harassment to protect juvenile salmonids
in Tillamook County, Oregon. Studies in Oregon Ornithology No. 9.
(This is at http://www.orednet.org/salmon/salmon.htm#hazing)
Beamish, R. J., B. L. Thomson, and G. A. McFarlane. 1992. Spiny dogfish
predation on chinook and coho salmon and the potential effects on
hatchery-produced salmon. Trans. Am. Fish. Soc. 121:444-455.
Beamish, R. J., C. Mahnken, and C. M. Neville. 1997. Hatchery and wild
production of Pacific salmon in relation to large-scale, natural
shifts in the productivity of the marine environment. ICES Journal
of Marine Science 54:1200-1215.
Bley, P. W. and J. R. Moring. 1988. Freshwater and ocean survival of
Atlantic salmon and steelhead: a synopsis. Biological Report
88(9).
Brauner, C. J., G. K. Iwama, and D. J. Randall. 1994. The effect of
short-duration seawater exposure on the swimming performance of
wild and hatchery-reared juvenile coho salmon during
smoltification. Can. J. Fish. Aquat. Sci. 51:2188-2194.
Buchanan, D. V., R. M. Hooton, and J. R. Moring. 1981. Northern
squawfish (Ptychocheilus oregonensis) predation on juvenile
salmonids in sections of the Willamette River Basin, Oregon. Can.
J. Fish. Aquat. Sci. 38:360-364.
Collis, K., R. E. Beaty, and B. R. Crain. 1995. Changes in catch rate
and diet of northern squawfish associated with the release of
hatchery-reared juvenile salmonids in a Columbia River reservoir.
No. Am. J. Fish. Mgmt. 15:346-357.
Collis, K., S. Adamany, D. D. Roby, D. P. Craig, and D. E. Lyons. 2000.
Avian predation on juvenile salmonids in the lower Columbia River.
1998 annual report to the Bonneville Power Administration,
Portland, OR, Contract No. 97BI33475, Project No. 97-024-00, 101
electronic pages (BPA Report DOE/BP-33475-2). (This is available
at
http://www.efw.bpa.gov/Environment/EW/EWP/DOCS/REPORTS/DOWNSTRM/D33475-2.pdf)
Columbia Basin Fish and Wildlife Authority. 1996. Impacts of artificial
salmon and steelhead production strategies in the Columbia River
Basin. Draft Programmatic Environmental Impact Statement for the
U. S. Fish and Wildlife Service, National Marine Fisheries Service,
and Bonneville Power Administration.
Cross, C. L., L. Lapi, and E. A. Perry. 1991. Production of chinook and
coho salmon from British Columbia hatcheries, 1971 through 1989.
Can. Tech. Rep. Fish. Aquat. Sci. No. 1816.
Dickhoff, W. W., B. R. Beckman, D. A. Larsen, C. V. W. Mahnken, C. B.
Schreck, C. Sharpe, and W. S. Zaugg. 1995. Quality assessment of
hatchery-reared spring chinook salmon smolts in the Columbia River
Basin. Am. Fish. Soc. Symp. 15:292-302.
Gowan, R. 1988. Release strategies for coho and chinook salmon released
into Coos Bay, Oregon. P. 75-80 in W. J. McNeil (ed.), Salmon
production, management, and allocation. Oregon State Univ. Press,
Corvallis.
Hansen, D. 2000. Hatchery attempts to boost salmon's IQ: Yakama Nation
facility might be fish's best hope for survival. In March 27
Spokesman-Review (Spokane, Washington newspaper). (This is at
http://www.nwefish.com/psga/news/media_2k/hatchery_iq_032700.html)
Holtby, L. B., R. K. Kadowaki, and K. S. Simpson. 1992. Factors
affecting the vulnerability of juvenile coho (Oncorhynchus kisutch)
and chinook salmon (O. tshawytscha) in a saltwater sport fishery.
Can. J. Fish. Aquat. Sci. 49:2164-2178.
Johnson, S. L., M. F. Solazzi, and T. E. Nickelson. 1990. Effects on
survival and homing of trucking hatchery yearling coho salmon to
release sites. No. Am. J. Fish. Mgmt. 10:427-433.
Jones, S. T. , G. M. Starke, and R. J. Stansell. 1996. Predation by
birds and effectiveness of predation control measures at
Bonneville, The Dalles, and John Day dams in 1995. U.S. Army Corps
of Engineers, CENPP-CO-SRF, Bonneville Dam, Cascade Locks, OR
97014.
Kaczynski, V. W. and J. F. Palmisano. 1993. Oregon's wild salmon and
steelhead trout: a review of the impact of management and
environmental factors. Oregon Forest Industries Council. Salem,
Oregon.
Kruzynski, G. M. and I. K. Birtwell. 1994. A predation bioassay to
quantify the ecological significance of sublethal responses of
juvenile chinook salmon (Oncorhynchus tshawytscha) to the
antisaptstain fungicide TCMTB. Can. J. Fish. Aquat. Sci. 51:1780-
1790.
Ledgerwood, R. D., E. M. Dawley, P. J. Bentley, L. G. Gilbreath, T. P.
Poe, and H. L. Hansen. 1993. Effectiveness of predator removal
for protecting juvenile fall chinook salmon released from
Bonneville Hatchery, 1991. U.S. Dept. of Commerce, National Oceanic
and Atmospheric Administration, National Marine Fisheries Service,
NOAA Tech. Memo. NMFS-NWFSC-9. (This is at
http://www.nwfsc.noaa.gov/pubs/tm/tm9/index.html)
Macdonald, J. S., C. D. Levings, C. D. McAllister, U. H. M. Fagerlund,
and J. R. McBride. 1988. A field experiment to test the
importance of estuaries for chinook salmon (Oncorhynchus
tshawytscha) survival: short-term results. Can. J. Fish. Aquat.
Sci. 45:1366-1377.
Mace, P. M. 1983. Bird predation on juvenile salmonids in the Big
Qualicum Estuary, Vancouver Island. Can. Tech. Report Fish. Aquat.
Sci. No. 1176.
Matthews, D. R. 1983. Feeding ecology of the Common Murre, Uria aalge,
off the Oregon Coast. M. S. Thesis, Univ. Oregon, Eugene.
Maule, A. G., D. W. Rondorf, J. Beeman, and P. Haner. 1996. Incidence
of Renibacterium salmoninarum infections in juvenile hatchery
spring chinook salmon in the Columbia and Snake Rivers. J. Aquat.
Anim. Health 8:37-46.
Maxfield, G. H., R. H. Lander, and C. D. Volz. 1970. Laboratory tests
of an electrical barrier for controlling predation by northern
squawfish. U.S. Fish and Wildlife Service, Special Scientific
Report--Fisheries No. 611.
Maynard, D. J., T. A. Flagg, and C. V. W. Mahnken. 1995. A review of
seminatural culture strategies for enhancing the postrelease
survival of anadromous salmonids. Am. Fish. Soc. Symp. 15:307-314.
McNeil, W. J., R. Gowan, and R. Severson. 1991. Offshore release of
salmon smolts. Am. Fish. Soc. Symp. 10:548-553.
Merrifield, K. 1998. Waterbird censuses of Yaquina Bay, Oregon: March
1993-February 1994. Oregon Dept. Fish and Wildlife, Wildlife
Diversity Program Technical Report 98-1-01.
Merrifield, K. In Prep. Larid, alcid, and crow censuses of Yaquina Bay,
Oregon: June 1997-June 1999. Oregon Dept. Fish and Wildlife,
Wildlife Diversity Program Technical Report.
Mesa, M. G., T. P. Poe, D. M. Gadomski, and J. H. Petersen. 1994. Are
all prey created equal? A review and synthesis of differential
predation on prey in substandard condition. J. Fish. Biol. 45
(Supplement A):81-96.
Morley, R. B., H. T. Bilton, A. S. Coburn, D. Brouwer, J. Van Tine, and
W. C. Clarke. 1988. The influence of time and size at release of
juvenile coho (Oncorhynchus kisutch) on returns at maturity:
results of studies on three brood years at Quinsam Hatchery, B.C.
Can. Tech. Rep. Fish. Aquat. Sci. No. 1620.
National Research Council. 1996. Upstream: salmon and society in the
Pacific Northwest. National Academy of Sciences Committee on
Protection and Management of Pacific Northwest Anadromous
Salmonids. National Academy Press, Washington, D.C. (This is at
http://www.nap.edu/books/0309053250/html/index.html)
ODFW (Oregon Department of Fish and Wildlife). 1991. Yaquina River Basin
fish management plan. Oregon Dept. of Fish and Wildlife.
ODFW (Oregon Department of Fish and Wildlife). 1998. Draft predation
action plan: avian species. Oregon Dept. of Fish and Wildlife.
(This is at
http://www.dfw.state.or.us/ODFWhtml/Research%26Reports/PredatorPlans/DearRev.html)
Olla, B. L., M. W. Davis, and C. H. Ryer. 1994. Behavioural deficits in
hatchery-reared fish: potential effects on survival following
release. Aquaculture and Fish. Mgmt. 25 (Suppl. 1):19-34.
Olla, B. L., M. W. Davis, and C. H. Ryer. 1998. Understanding how the
hatchery environment represses or promotes the development of
behavioral survival skills. Bull. Marine Science 62:531-550.
Raymond, H. L. 1988. Effects of hydroelectric development and fisheries
enhancement on spring and summer chinook salmon and steelhead in
the Columbia River Basin. No. Am. J. Fisheries Mgmt. 8:1-24.
Roby, D. D., D. P. Craig, K. Collis, and S. L. Adamany. 1998. Avian
predation on juvenile salmonids in the lower Columbia River. 1997
annual report to the Bonneville Power Administration and U.S. Army
Corps of Engineers. Revised September 1998. (This is at
http://www.efw.bpa.gov/Environment/EW/EWP/DOCS/REPORTS/DOWNSTRM/D33475-1.pdf)
Scheel, D. and K. R. Hough. 1997. Salmon fry predation by seabirds near
an Alaskan hatchery. Marine Ecology Progress Series 150:35-48.
Schreck, C. B. 1990. Physiological, behavioral, and performance
indicators of stress. Am. Fish. Soc. Symp. 8:29-37.
Schroder, S. and K. Fresh (eds.). 1992. Results of the Grays Harbor
coho survival investigations, 1987-1990. Washington Dept. of
Fisheries, Tech. Report No. 118.
Scott, J. M. 1973. Resource allocation in four syntopic species of
marine diving birds. Ph.D. Thesis, Oregon State Univ., Corvallis.
Shiveley, R. S., T. P. Poe, and S. T. Sauter. 1996. Feeding response by
northern squawfish to a hatchery release of juvenile salmonids in
the Clearwater River, Idaho. Trans. Am. Fish. Soc. 125:230-236.
Shrimpton, J. M., N. J. Bernier, G. K. Iwama, and D. J. Randall. 1994.
Differences in measurements of smolt development between wild and
hatchery-reared juvenile coho salmon (Oncorhynchus kisutch) before
and after saltwater exposure. Can. J. Fish. Aquat. Sci. 51:2170-
2178.
Stahl, T. P., C. B. Schreck, and D. D. Roby. 2000. Avian predation in
Oregon estuaries and juvenile salmonid migration. Final Draft,
Annual Report. Oregon Cooperative Fish and Wildlife Research Unit,
Dept. of Fisheries and Wildlife, Oregon State University.
Suboski, M. D. and J. J. Templeton. 1989. Life skills training for
hatchery fish: social learning and survival. Fisheries Research
7:343-352.
Thompson, R. B. 1959. Food of the squawfish Ptychocheilus oregonensis
of the lower Columbia River. Fish. Bull. 158.
Thompson, R. B. and D. F. Tufts. 1967. Predation by dolly varden and
northern squawfish on hatchery-reared sockeye salmon in Lake
Wenatchee, Washington. Trans. Am. Fish. Soc. 96:426-427.
Uremovich, B. L., S. P. Cramer, C. F. Willis, and C. O. Junge. 1980.
Passage of juvenile salmonids through the ice-trash sluiceway and
squawfish predation at Bonneville Dam, 1980. Oregon Dept. of Fish
Wildlife, Fish Division, Annual Progress Report.
Ward, D. E. 1983. Fish runs suffer loss to murres. P. B2 in July 12
Oregonian (newspaper). Portland, Oregon.
Wedemeyer, G. A., R. L. Saunders, and W. C. Clarke. 1980. Environmental
factors affecting smoltification and early marine survival of
anadromous salmonids. Marine Fish. Review 42(6):1-14.
Witty, K., C. Willis, and S. Cramer. 1995. A review of potential
impacts of hatchery fish on naturally produced salmonids in the
migration corridor of the Snake and Columbia Rivers. S. P. Cramer
and Associates Final Report to the Columbia Basin Fish and Wildlife
Foundation. Given as Appendix A in Columbia Basin Fish and
Wildlife Authority (1996).