Easy Winters Hard On Fish Survival

By Ed Buettner and Scott Putnam - Idaho Dept. of Fish and Game

While Idaho's big game animals have enjoyed an extremely mild winter, record low snow pack and eventual spring runoff mean trouble for Idaho's migrating salmon and steelhead smolts.

This is basic science. As springtime river discharge decreases, downstream juvenile fish migration rates also decrease. Drought and water storage for hydroelectric generation severely reduce flows necessary for the downstream movement of young fish.

Migration rate is a critical factor in smolt survival and is affected not only by discharge but also by the level of smoltification of the fish. Smoltification is a physiological change that prepares young fish that have been living in freshwater to live in saltwater. If a smolt does not find saltwater in the spring, it will revert back to its freshwater state. That means if a fish is delayed for too long on its journey to the ocean, it will die when it enters the estuary because it can no longer survive in saltwater.

Another reason why migration rate is so important is because in low-flow years, as 2005 appears to be, fish may be caught in a river system that is warming up because of the lack of flow. Decreased summer flows have the potential to expose delayed fish to lethal water temperatures. Water temperatures around 72 degrees are sub lethal, while those greater than 75 degrees will kill juvenile salmonids.

A monitoring program has been established throughout the Columbia River basin to collect biological and environmental data necessary for water management decisions that are beneficial to maximizing smolt survival. Idaho Department of Fish and Game's Smolt Monitoring Project (SMP) provides information on the migration characteristics of the various stocks of steelhead and salmon within the Snake River basin. Since 1984, the Lewiston-based SMP has operated large fish traps on the Clearwater, Snake and Salmon rivers.

At these traps, ocean-bound juvenile salmon and steelhead trout, or smolts, are captured, examined, counted, and released back into the river daily from early March through May.
   
Passive Integrated Transponder (PIT) tags are injected into some of these fish. PIT tags are the size of a grain of rice and contain a computer chip that provides each smolt with a unique identifier that is recorded by strategically placed interrogation equipment throughout the Columbia River Basin. Date and time of passage are also recorded for each tag encountered. Fish marked at SMP facilities provide information about smolt movement and survival estimates.

The Salmon River trap provides information about smolt migration rates in a free-flowing river. Among the juvenile salmonids collected each day at the trap is a small proportion that were previously PIT-tagged upstream of the trap. Equipment on the trap records the date and time the fish was observed. These data are compared to previous interrogation events upriver to calculate migration rates. For example, 2004 data shows that a hatchery steelhead trout released 287 miles upstream of the trap arrived at the trap in 5.6 days. That means the steelhead migrated 51 miles per day.

The Salmon River trap is 576 miles from the Pacific Ocean. Therefore, before man altered the lower Snake and Columbia River systems it would take that steelhead smolt about 10-14 days to reach the Pacific estuary. Today it might take between 30 to 60 days.

The Snake River trap, located 466 miles from the Pacific Ocean, evaluates smolt migration from the head of Lower Granite Reservoir to Lower Granite Dam, the first of eight such complexes encountered by migrating smolts. During the low discharge of 40,000 cubic feet per second in 2004, steelhead trout migrated seven miles per day from the Snake River trap to Lower Granite Dam located 32 miles downstream. In contrast, steelhead trout traveled more than three times faster, 23 miles per day, when the discharge was 130,000 cubic feet per second.

With 2005 shaping up to be a low-flow year, smolt survival will be compromised. How successfully managers are able to accommodate the fish's needs is reflected in future fishing opportunities. Lower juvenile survival ultimately means fewer returning adults for anglers to harvest in Idaho's rivers.

Ed Buettner, Sr. Fisheries Research Biologist, is IDFG's smolt monitoring program leader. Scott Putnam, Sr. Fisheries Technician, has worked for IDFG since 1998.

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