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Marine Salmon Interactions Program

Predation Impact on Juvenile Salmon by Sablefish in Southeast Alaska

Sablefish (Anoplopoma fimbria) and Pacific salmon (Oncorhynchus spp.) are important components of marine ecosystems throughout the eastern Pacific Rim and comprise valuable socioeconomic resources to coastal communities. In Southeast Alaska, for example, exvessel commercial values of these species over the past 5 years averaged $27.0 million for sablefish, and $75.7 million for salmon. Because interannual abundances of sablefish and salmon are both highly variable, understanding mechanisms that govern the production of these species, including trophic interactions, may help to explain fluctuations in year-class strength and improve management of these fishery resources. For example, sablefish and immature salmon may compete for food on the continental shelf, age-1+ sablefish moving inshore may prey on juvenile salmon or compete with them for plankton prey as they migrate to sea in summer, and juvenile sablefish may in turn be preyed on by returning adult salmon as they move onshore in summer and fall.

During routine sampling of the coastal epipelagic habitats of Southeast Alaska in 1999, high abundances of age-1+ sablefish were encountered with juvenile salmon. Many of the sablefish sampled for stomach analyses had eaten juvenile salmon. These events were not observed in prior or subsequent years of monitoring during the Southeast Coastal Monitoring (SECM) project. Consequently, we collected sablefish for laboratory studies to determine gastric evacuation rates, and then combined these data with our field observations to estimate predation impact. We also examined juvenile salmon and sablefish abundance data from the SECM 10-year time-series and adult pink salmon (O. gorbuscha) harvest data for the northern region of Southeast Alaska to infer relationships with sablefish.

Predation on juvenile pink, chum (O. keta), and sockeye (O. nerka) salmon was observed for 43%-60% of the estimated 95,328 sablefish in a 500 km2 area encompassing Icy Strait in June and July 1999. Age-1+ sablefish had each consumed an average of 1.8 salmon; they also preyed on planktonic invertebrates common in juvenile salmon diets, including pteropods, hyperiid amphipods, euphausiids and some gelatinous taxa. Pink and chum salmon constituted 33% and 60%, respectively, of the juvenile salmon eaten, in proportion to their abundance in the surface trawls. Examination of juvenile salmon size frequencies in the catches and in sablefish guts indicated that prey length was typically about one-third of predator length; in July, the biggest month for predation, sablefish tended to select the smaller juvenile chum salmon available, were not size selective of juvenile pink salmon, and could consume sockeye salmon that were up to 53% of their body length.

In the laboratory, we fed individual sablefish a whole, thawed, previously-frozen juvenile chum salmon in flow-through experimental tanks that simulated summer temperatures of 7 and 12C. We sacrificed predators at predetermined time intervals (0.5-96 hours) to model the percent decline in prey biomass over time. Instantaneous evacuation rates, r, from the exponential model were -0.027 at 7C and -0.049 at 12C. Using these evacuation rates to derive meal frequency, we estimated that the age-1+ sablefish population in the study area consumed 1.13 - 2.77 million juvenile salmon in the summer of 1999. Further, our trawl catch data for the same area indicated that 1999 was an anomalously high year of age 1+ sablefish abundance, an unusually low year for juvenile salmon abundance, and was followed by an extremely low adult pink salmon harvest in the year 2000. These results suggest that sablefish predation on juvenile salmon can be an important ecological interaction, and that strong sablefish year classes may impact salmon survival.

By Molly Sturdevant, Joe Orsi, and Mike Sigler


Auke Creek Weir Installation Delayed by Weather

Auke Creek weir and hatchery
Figure 1.  Auke Creek weir and hatchery 7 March 2007 before weir installation.  Stream flow was low, air and stream temperatures were near record lows, and there was 5 feet of snow on the ground.  Photo by Jerry Taylor.
 
 

The key to all fisheries research projects at Auke Creek is the fish counting weir. Auke Creek weir is made operational in early March each year for counting seaward migrating salmonids leaving the Auke Creek -Auke Lake drainage. The 45-year biological data set provides important information about the freshwater life history stages and marine survivals of seven species of anadromous species that migrate in and out of this drainage system, as well as time series data for analysis of climate change and indices for salmon harvest management programs of the Alaska Department of Fish and Game (ADF&G).

The weir is operated on a cooperative basis with the University of Alaska School of Fisheries and Ocean Sciences and the ADF&G. Both of these agencies conduct specific research projects on one or more species that migrate through Auke Creek weir. The weir is a permanent structure with the capability of capturing all downstream and upstream migrants, and it is designed to operate even during extreme water flows.

Auke Creek weir was installed in the downstream capture mode on 9 March 2007 during the worst weather in the history of the March weir installations at Auke Creek. Installation was delayed about a week because of low air and water temperatures, but nonetheless occurred during a blizzard. Five feet of snow covered the ground when the weir was installed (Fig. 1). Weir traps were buried and were lifted out of the snow with a hydraulic crane. Stream flow was low; however, the weir was operating within a few hours of installing the last stop logs. Stream flow remained low throughout the month, and only three of five fry traps were operated during the month. Decreasing air temperatures during the last week of March resulted in the accumulation of ice on the weir traps, collection trough, and holding tank. The weir pool was frozen over for several days, but not to the extent seen in 2006. Auke Lake remained ice covered throughout March, and water temperatures were between 0.4 and 0.9C.

Pink and chum salmon fry usually dominate the number of emigrants captured at Auke Creek weir during March, with an occasional Dolly Varden char or cutthroat trout being captured. The March 2007 migration of pink salmon fry at Auke Creek is one of the lowest on record. The daily counts were less than average for the entire month of March, and a total of only 805 pink salmon fry were counted at the weir through March. For comparison, the average number of pink salmon fry leaving Auke Creek during the last 3 weeks in March is 10,000 with a range in March pink salmon emigrations of 135 (1979) to 45,000 (1984). The highest daily count for March 2007, 121, (30 March) is an order of magnitude lower than the historical average daily count of 1,000 pink salmon fry. Chum salmon fry, 308 total, were the only other species caught during March 2007.

Low numbers of emigrants in March do not necessarily portend a small 2007 emigration, as the pink salmon fry emigration this year is expected to be later than in recent years. Based on average stream temperature from August 2006-March 2007 the predicted midpoint of pink salmon emigration is 28 April. The average midpoint of emigration for pink salmon at Auke Creek is 20 April, and latest midpoint of emigration is 7 May 1982.

By Jerry Taylor
 

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