Trophic Interactions Among Wild and Hatchery Juvenile Chum Salmon in Taku Inlet
The Early Ocean Salmon (EOS) task of the Marine Salmon Interactions Program at ABL continued work on a collaborative investigation of the ecological interactions of hatchery and wild juvenile chum salmon in Taku Inlet near Juneau. This research is funded by the Southeast Sustainable Salmon Fund to address concern about the recent decline of wild chum salmon in the Taku River, which coincided with increased production of enhanced stocks of chum salmon in nearby waters.
Cooperating agencies and institutions include the University of Alaska Juneau Center for Fisheries and Ocean Sciences, ADF&G, and Douglas Island Pink and Chum Hatchery (DIPAC). The DIPAC hatchery releases chum salmon juveniles at several locations near Taku Inlet in May during the wild fry out-migration; all of these hatchery releases are marked with thermally-induced patterns on their otoliths. Collaborating researchers sampled wild chum and DIPAC hatchery chum salmon juveniles that utilize Taku Inlet, using beach seines in littoral habitat and tow nets in nearshore habitat at sampling stations in inner, middle, and outer Taku Inlet from late April to late June in 2004 and 2005.
The EOS component of the Taku project is assessing trophic interactions of juveniles as a potential cause for declining wild adult chum salmon returning to the Taku River. Mortality of Pacific salmon during their marine life history is highest during this early period and may be related to competition for food. The university component of the Taku project is examining juvenile chum salmon abundance and distribution and found that hatchery fish comprised greater than 90% of catches in the outer inlet, but only 10% of catches in the inner inlet. Hatchery fish were also larger than wild fish, but smaller hatchery fish spent more time in the inlet than larger individuals.
We are examining diet and energy content of these wild and hatchery chum salmon juveniles, which were preserved for stomach analyses or frozen for calorimetry, to determine energy content. Sample processing is complete and analysis is under way for 2004, the first of the two project years; sample processing for the 2005 collections is on-going. Otoliths were extracted and examined for hatchery thermal marks, then subsamples representing wild and hatchery stocks were processed for diets (n = 486) and calorimetry (n = 571). We also determined a baseline energy content for hatchery chum salmon (n = 63) upon their release in May and diet and energy content for both stocks from summer trawl catches in Icy Strait (n = 114) to compare condition of the fish as they approach the Gulf of Alaska (GOA).
Hatchery chum salmon were initially larger and had greater energy content (cal/g wet weight) than wild fish; however, energy values converged by mid-June in outer Taku Inlet, and wild fish in Icy Strait were in similar condition as hatchery fish. Multivariate analysis of 54 prey measures indicated that diets of the two groups were distinct throughout the season in all inlet locations and converged in Icy Strait. Further analysis is needed to determine if the diet partitioning and energetic differences in Taku Inlet are related to high densities of hatchery fish residing short term in outer Taku Inlet. If density-dependent interactions are affecting wild chum salmon in the inlet, the negative effects must occur very rapidly because juvenile survivors enter the GOA with no apparent disadvantage.
By Molly Sturdevant and Alex Wertheimer
Little Port Walter 2006 Chinook Salmon Return
Chinook salmon produced as part of the ABL research at the Little Port Walter (LPW) Marine Research Station contributed strongly to the commercial and sport fisheries during 2006 in Southeast Alaska. Commercial and sport catches totaled nearly 3,600 LPW Chinook salmon. Numerical expansions for fishery sampling fractions for many of the individual tag recoveries are not yet available, so this number is likely a minimum estimate. Of particular note is the large number of LPW Chinook salmon caught in the Juneau sport fishery during the Golden North Salmon Derby. Of the 514 Chinook salmon landed in the derby, 63, or roughly 12%, of the Chinook salmon caught were produced at LPW. This is likely the highest single source contribution for Chinook salmon to the local fishery.
In addition, LPW Chinook salmon represented a large percentage of all tagged Chinook salmon from Southeast Alaska facilities that were recovered in Southeast Alaska commercial fisheries. The catch rate for these fish was as high as 25% during particular troll and net openings in Southeast Alaska. The high rate of recoveries not only provides data for ABL research on distribution of hatchery and wild stocks, but also gives fishery managers important information regarding the catch and distribution of Alaska origin fish. These data help to optimize harvest strategies in order to maximize exploitation of hatchery stocks, and thereby increase the number of Chinook salmon that may be caught and retained under the Pacific Salmon Treaty between the United States and Canada.
The preliminary total escapement of Chinook salmon to LPW in 2006 is estimated to be 2,700-2,800 fish. Capture for returning adult Chinook salmon began on 29 June with the installation and deployment of the LPW floating fish aggregating device (FAD). Fish capture and processing began on 30 June with a catch of nearly 300 Chinook salmon. During the first week of capture we were able to capture and either hold or process almost 1,000 returning Chinook salmon.
As part of the Chinook salmon research on hatchery-produced and wild fish, Miles Johnson, an intern with the Hollings Scholarship Program, carried out a project to increase understanding of the relationship between the arrival timing and maturation timing in the LPW hatchery stocks and their donor wild stocks. With the assistance of LPW biologists, he tagged 1,541 large, returning adult Chinook salmon with Floy Tags over the time period from 30 June to 10 August. The holding pens, which used freshwater lens technology, reached maximum holding capacity at 1,600 fish on 17 July. The FAD accounted for almost 2,200 Chinook salmon captured in 2006.
The FAD effectiveness at capturing fish diminishes by the end of July, due to water flow gradients between the outfall of Sashin Creek and the culture water used as an attractant near the FAD location and other factors such as fish behavior. After July, fish aggregate at the creek mouth and swim closer to the surface and thus may be less susceptible to the FAD. Fouling of the FAD netting may also decrease its effectiveness. The presence of seals actively hunting in the FAD and weir areas may also have affected our ability to capture adult salmon. The FAD catches were supplemented with fish recovered from the creek mouth using a drop seine. A few Chinook salmon were also captured with sport and other gear.
More precise estimates of the LPW escapement will come from the removal and decoding of all coded wire tags recovered from Chinook salmon during October and November 2006. Returning Chinook salmon were processed to provide fertilized eggs for a project funded by the U.S. section of the Pacific Salmon Commission (PSC) studying the effects of ventral fin marking of juvenile Chinook salmon. Additional information from these fish on distribution, exploitation rate, and age and size at maturation will be used for further analysis as part of another U.S. PSC-funded project comparing these same parameters between wild Alaska Chinook salmon stocks and the LPW hatchery stocks derived from these wild stocks.
By John Joyce
Auke Creek 2006 Adult Pink Salmon Migration
Pink salmon spawn throughout the Auke Lake system in the tributaries to Auke Lake, Auke Creek, and in the intertidal area where the creek enters seawater. Pink salmon were counted at Auke Creek in 1967 and 1968, and each year since 1971. Annual counts of pink salmon at Auke Creek show the interannual variability common in this fish. Pink salmon escapements at Auke Creek ranged from 650 to 28,000 fish over the last 40 years, and it was not unusual to see a 5- to 10-fold increase or decrease in consecutive even- and odd-year brood lines.
A total of 13,198 wild pink salmon returned to Auke Creek in 2006, the largest even-year run since 1994. These fish were produced by the 2004 brood-year spawners, the progeny of which emigrated to the ocean as juveniles in 2005. In 2006, pink salmon were counted daily at the fish-counting weir at Auke Creek from the fourth week of July through mid-September, which is within the normal range of timing at Auke Creek. The run was characterized by several peaks of high daily counts at approximate weekly intervals beginning the first week of August. This was a departure from the historic pattern of pink salmon migrations at Auke Creek, with migrations that usually peaked in mid-August and mid-September.
The overall midpoint of the 2006 run was 18 August, which along with 1983 and 1992 was the earliest on record for Auke Creek. The average midpoint of adult migration for 1967-2005 was on 30 August, and during the 1960s and 1970s the midpoint of the run was in September. Prespawning mortality of adult pink salmon in Auke Creek was high, and estimates from daily recovery of fish at the fish counting weir showed that approximately 50% of the females died before spawning.
In even- and odd-numbered years there are two distinct runs of pink salmon at Auke Creek, referred to as the early and late runs. Because all returning fish are individually handled and counted by sex at the fish counting weir, it is possible to estimate the number of fish in both runs. Based on the increase in the proportion of silver-colored females with deciduous scales, 31 August was considered the start of the late run in 2006. At that time, early-run females were ready to spawn while late-run females were not, and fish could be easily distinguished when handled during counting. The early and late runs for 2006 were 10,685 and 2,513 fish.
The larger proportion of fish in the early run continued the pattern of greater abundance of that component, which became apparent in the mid-1980s. Before then the late even- and odd-year runs dominated the total returns of pink salmon at Auke Creek and averaged 73% of the total return. Before the mid-1980s there were often large numbers of fish into the third or fourth weeks of September. Since the mid 1980s, in both even- and odd-year runs, the average late component of the run averaged about 30%, and the run often dwindled to low numbers and ended during the second week of the month. It appears that the late September component of the Auke Creek pink salmon run continues to decrease.
