MFR PAPER 999 Both Parker S. Trefethen and Wesley J. Ebel are members of the staff of the NMFS Northwest Fisheries Center, 2725 Montlake A new method has been devised Boulevard East, Seattle, WA 98112. by NMFS to increase the survival of juvenile salmon. per~odof yeais could severely deplete the runs of salnlon in a river. To offset this. the NMFS Northwest Fisheries Center's Division of Coastal Zone and Collection and Transportation System Estuarine Studies designed a collection system at Little Goose, the uppermost of Juvenile Salmonids dam in the lower . The Evaluated at Little Goose Dam collected fish are transported nearly 350 rniles downstream around hazar- dous reaches of the river and released PARKER S. TREFETHEN and WESLEY J. EBEL in the below Bonne- ville Dam (Figure 2). Development BACKGROUND vides the circulation necessary for and tests of the collection and trans- AND OBJECTIVES rapid release of dissolved gases. Vir- portation system were endorsed by tually all of the Columbia River below State fishery agencies and financed by Survival of juvenile salnion-as well the U.S.-Canadian border and many the U.S. Army Corps of Engineers. The as steelhead trout-in the Snake River miles of the lower Snake River are system is now in the third year of a has declined significantly in recent seasonally supersaturated to varying 6-year program. years owing mainly to gas bubble dis- degrees, depending on river flows and ease, a condition caused by supersatur- amount of spill discharges at the dams. FISH PROTECTIVE SYSTEM ation of the river with nitrogen and "Gas bubble disease" in fish is sinlilar AND FACILITIES other dissolved gases from spilling of to the "bends" in dcepsea divers. Fish. water at dams. Before Lower Monu- however, can contract the disease At Little Goose Dam the young fish mental and 1,ittle Goose Dams were merely by remaining in supersaturated are diverted from turbine intakes into built, over 90 percent of the young surface waters. An example of gas intake gatewells by means of traveling salmon managed to survive their sea- bubbles in a young fish is shown in screens (Figure 3). From the gatewells, ward migration down the lower Snake Figure I. the fish pass through a submerged ori- River. Reservoirs, or pools, delay the mi- fice into a bypass pipe (Figure 4) and The tern1 "nitrogen supersaturation" gration of juveniles by prolonging their are routed around the dam to a collec- has fallen into common use (largely exposure to high concentrations of dis- tion area (Figure 5) below the power- through convenience) to describe a solved gases, predation, and other house. At this point the fish are either condition known as excessive total dis- adverse conditions. Such losses over a released into the tailrace or transferred solved gas pressure. Recause air is near- ly four-fifths nitrogen, references to the gas supersaturation problem have in- variably highlighted nitrogen. even though the sum of all gas pressures is actually involved. Most of the Colum- bia River (and a major portion of the lower Snake River) is now a series of impoundments, or dams. The long succession of pools no longer pro-

Figure 1. - Gas bubbles beneath skin on head of young chinook salmon. When bubbles burst, infections may set in and kill the fish. Gas absorbed into blood stream will cause emboli, which can block circulatory system and cause death. survival of truck-transported fish was about twice that of those migrating naturally downstream. Survival of fish transported during periods of heavy spills and high gas supersatura- lion was about three times that of non- transported fish.

EVALUATION OF THE SYSTEM

Returns of marked adult fish provide data on the effectiveness of the system. Juveniles are marked at the collection area (Figurc 6) with a coded magnetic OREGON wire tag, a cryo-brand, and an adipose fin clip. Control groups are releas