Productivity and survival rate trends in the freshwater spawning and rearing stage of Snake River chinook salmon (Oncorhynchus tshawytscha) C.E. Petrosky, H.A. Schaller, and P. Budy Abstract: Stream-type chinook salmon (Oncorhynchus tshawytscha) populations in the Snake River (northwest United States) have declined dramatically since completion of the federal hydrosystem. Identifying the life stage that is limit- ing the survival of these stocks is crucial for evaluating the potential of management actions to recover these stocks. We tested the hypothesis that a decrease in productivity and survival rate in the freshwater spawning and rearing (FSR) life stage since completion of the hydropower system could explain the decline observed over the life cycle. The de- cline of chinook populations following completion of the hydrosystem was not accompanied by major survival changes in the FSR life stage. FSR productivity showed no significant decline, and the FSR survival rate decline was small rel- ative to the overall decline. However, significant survival declines did occur in the smolt-to-adult stage coincident pri- marily with hydrosystem completion, combined with poorer climate conditions and possibly hatchery effects. Potential improvements in survival that occur only at the FSR life stage are unlikely to offset these impacts and increase survival to a level that ensures the recovery of Snake River stream-type chinook. Résumé : Les populations du Saumon quinnat (Oncorhynchus tshawytscha) de type riverain de la rivière Snake du nord-ouest des États-Unis ont décliné de façon spectaculaire depuis l’achèvement de l’hydrosystème fédéral. L’identification du stade du cycle biologique qui limite la survie de ces stocks est une étape cruciale dans l’évaluation du potentiel des aménagements nécessaires pour la récupération de ces populations. Nous posons en hypothèse que la diminution de la productivité et du taux de survie dans la partie du cycle relié à la fraye et au développement en eau douce (stade FSR), depuis l’achèvement du système hydroélectrique, peuvent expliquer le déclin observé dans tout le cycle biologique. Le déclin des populations de Saumons quinnat après le parachèvement de l’hydrosystème n’a pas été marqué par des changements majeurs de la survie durant le stade FSR; la productivité durant ce stade n’a pas baissé de façon significative et la réduction des taux de survie a été faible par comparaison au déclin total. Cependant, des réductions significatives de la survie se sont manifestées durant le passage du stage saumonneau au stade adulte qui ont coïncidé particulièrement avec l’achèvement de l’hydrosystème, mais aussi avec des conditions climatiques défavorables et peut-être aussi avec des effets de pisciculture. Des améliorations potentielles de la survie au seul stade FSR sont donc peu susceptibles de compenser les impacts et d’augmenter la survie à un niveau qui assure la récupération des stocks de Saumons quinnat de type riverain de la rivière Snake. [Traduit par la Rédaction] Petrosky et al. 1207 Introduction The Columbia River Basin once contained extremely abun- dant runs of chinook salmon (Oncorhynchus tshawytscha) that supported a substantial freshwater harvest (Chapman 1986). Over the last century, however, these stocks, in addition to sockeye (Oncorhynchus nerka) and steelhead (Oncorhynchus mykiss), have declined dramatically in response to a combina- tion of factors including extensive hydroelectric development, harvest, habitat degradation, and emphasis on mitigating anthropogenic losses with hatchery releases. Snake River spring and summer chinook populations, once the predominant run of stream-type chinook in the Columbia River Basin, have declined dramatically since completion of the Federal Colum- bia River Power System (hereafter referred to as the hydro- system). They have been listed as threatened under the Endangered Species Act since 1992. To address this population decline, the National Marine Fisheries Service’s 1995–1998 biological opinion on operation of the hydrosystem created a process called PATH (Plan for Analyzing and Testing Hypoth- eses) (Marmorek et al. 1998). The PATH process evaluated hy- potheses about the distribution of mortality over the life cycle and strengths and weaknesses of supporting evidence and tested those alternative hypotheses that had significant manage- ment implications (Marmorek et al. 1998). The PATH modeling forum was a critical component of the evaluation of the effects of breaching four large hydro- electric dams on the lower Snake River for salmon recovery, Can. J. Fish. Aquat. Sci. 58: 1196–1207 (2001) © 2001 NRC Canada 1196 DOI: 10.1139/cjfas-58-6-1196 Received August 9, 2000. Accepted February 28, 2001. Published on the NRC Research Press Web site on May 7, 2001. J15911 C.E. Petrosky. 1 Idaho Department of Fish and Game, 600 S. Walnut Avenue, P.O. Box 25, Boise, ID 83707, U.S.A. H.A. Schaller. Columbia River Fisheries Program Office, U.S. Fish and Wildlife Service, 9317 Hwy 99, Suite I, Vancouver, WA 98665, U.S.A. P. Budy. Utah Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife, Utah State University, Logan, UT 84322-5210, U.S.A. 1 Corresponding author (e-mail: cpetrosk@idfg.state.id.us).