Figure 1  Screwtrap at Lake Eyak outflow

In Spring 2004, a two year investigation began into the early life histories of coho and sockeye salmon outmigrating from the Copper River Delta.  The project, funded by the North Pacific Research Board, is a cooperative effort between the Prince William Sound Science Center, Dauphin Island Sea Lab of the University of South Alabama, and the Pacific Northwest Research Station, USDA Forest Service.

As coho and sockeye emerge from freshwater sites and begin their migration to the ocean, estuarine habitats provide rich foraging opportunities and a salinity gradient over which physiological adjustments to an oceanic environment can occur.  This estuarine phase of the life cycle is the primary focus of the study because of its importance in affecting growth and survival of juvenile salmon.

The study objectives are to quantify the ages at freshwater outmigration and the subsequent time spent in the estuarine environment.  Knowledge of these parameters and how estuarine habitats are utilized by juvenile coho and sockeye are critical for stock assessments and forecasting how manmade and natural disturbances along the Copper River Delta might affect salmon production. 

Our field program consists of four sampling techniques that allow us to sample fish as they outmigrate through salinity regimes in the Eyak River and Alaganik Slough systems.  Screwtraps are deployed for continuous freshwater sampling at the Lake Eyak outflow and at Salmon Creek and 18 Mile Creek, both tributaries to Alaganik Slough.  A screwtrap is a stationary sampling device with a 1.5 meter diameter, chambered funnel designed to spin in the current and trap fish in a live box at the base of the funnel (fig.1).

Exploring Habitat Utilization by Juvenile Salmon on the Copper River Delta

Figure 2  Fyke net set at Alaganik site with view of Scott Glacier

Figure 3 Mid-water trawl at lower estuary site

Upper estuary sites located near the mouths of Eyak River and Alaganik Slough are sampled with beach seines and fyke net deployments.  Because juvenile salmon are thought to migrate down the estuary on the ebb tide as a means of energy conservation, fyke nets are deployed on the intertidal substrate with the wings and net openings oriented up-river to catch fish migrating on the ebb flow.

Surface and mid-water trawls are conducted at the middle and lower estuary sites (fig.3).  Trawls are conducted with two 6’x8’ trawling nets towed parallel during mid to low tides when water levels are lower and sampling is more efficient because outmigrating fish are constricted to a narrower corridor.

 The lengths, weights and juvenile stage (fry, parr, or smolt) of coho and sockeye captured at each site are recorded and a subsample is retained for laboratory analysis of stomach contents and otoliths.  Identifying stomach content yields data on fish diet and foraging variety for the different sampling sites. 

We are experimenting with otolith microchemistry analysis.  The otolith, or earbone, is an inert calcified structure that absorbs and imprints elements from the fish’s environment.  As the fish grows, the otolith grows annuli that correspond with the fish’s age, and within the annuli an elemental record of the fish’s environment for that particular age is retained.  Strontium is an element found in saline waters, and by analyzing an otolith for the strontium to calcium ratio, residence times in various salinity regimes could be determined.

Field seasons May-November 2004, and March-November 2005.

Journal publication in review.

Project PI: Dr. Mary Anne Bishop

Article by: Brad Reynolds

Field biologists: Brad Reynolds, Marguerite Leeds. Ben Heermans, Sarah Ecolano