The Ocean InfluenceIdaho Fish & Game Report to the Director 5/1/98
Why would the ocean be selectively bad for Snake Rive salmon and steelhead?
Conclusion:Ocean conditions influence smolt-to-adult survival of salmon and steelhead. However, there have been no hypotheses advanced or evidence presented to explain why the estuary or ocean would be selectively bad for Snake River fish.
The following material is adapted from Schaller et al. (1996)
The conclusion by Schaller et al. (1996) that hydropower was a primary cause of productivity and survival rate declines of Snake River and upper Columbia river stocks was conditioned on evidence that the estuary and early ocean conditions do not have a systematically different effect on survival for interior Columbia River basin stream-type chinook stocks. This is reasonable in view of the similarity of these stocks, the overlap in time and space of these stocks during their early ocean residence (and beyond), and the broad-scale nature of climatic influences described in the literature.
There are several lines of evidence suggesting that the interior Columbia Basin stocks are exposed to similar estuary and ocean conditions, particularly during the critical first year. Beamish and Bouillon (1993) and others provided evidence that indices of climate over the north Pacific Ocean may play an important role in production of different species of salmon originating over a wide geographic range. Deriso et al. (1996) found evidence of a common year effect for these index stocks of stream-type chinook. Smolts of these lower Columbia River, Snake River and upper Columbia River stocks migrate through the mainstem to the estuary primarily in late April and May. Current hypotheses regarding ocean survival of Pacific salmon generally focus on the juveniles' critical first months at sea, where juveniles of these index stocks are most likely to overlap in time and space. Year class strength for these spring and summer chinook is established, for the most part, within the first year in the ocean, as evidenced by the ability of fishery managers to predict subsequent adult escapements from jack counts.
Since it appears that Columbia Basin stream-type chinook share a common estuary and nearshore ocean environment and a more common ocean distribution than stocks evaluated by Beamish and Bouillon (1993), it is very unlikely that differential estuary and ocean conditions could explain systematic differences in stock survival. The ocean recoveries of coded wire tagged (CWT) stream-type chinook were infrequent. The few recaptures (62 recoveries from 8 release years) from hatchery stocks in both the Snake river (21 recoveries) and lower Columbia river (41 recoveries) were widely scattered from California to Alaska ocean fisheries (Pacific States Marine Fisheries Commission unpublished data). The average annual proportion of CWT recoveries from ocean fisheries north and south of the Columbia River mouth appears similar between the Snake and lower Columbia hatcheries.
It is noteworthy that even the lower Columbia stocks have been affected by hydropower, and that a portion of their recent decline may be hydropower related, rather than environmental. John Day River stocks, in particular, were likely subjected to a higher smolt passage mortality at John Day Dam relative to other stocks passing the dam between 1968 and 1984. Development of the Canadian storage projects in the upper Columbia river in the mid-1970s, and hydrosystem regulation have reduced flows during the spring smolt migration for all stream-type chinook. Since Columbia Basin stream-type chinook share a common lower river migratory corridor and estuary, changes that may have occurred due to the development of storage projects in the mid-1970s are unlikely to account for the differential decline in productivity and survival rates between the upriver and downriver index stocks.
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