Auke Bay Laboratory  (cont.)

Habitat Use and Early Marine Ecology of Juvenile Salmon Studied

The Marine Salmon Investigations Program completed five research cruises during 2001 in continuation of the Southeast Alaska Coastal Monitoring (SECM) Project.  The SECM project was initiated in 1997 to study the habitat use and early marine ecology of juvenile Pacific salmon (Oncorhynchus spp.).  In five sampling intervals from May through October, biophysical parameters were sampled in inshore, strait, and coastal habitats along a primary seaward migration corridor used by juvenile salmon.  These habitats spanned the geographic area extending 250 km from near Juneau westward through Icy Strait to 64 km offshore in the Gulf of Alaska.  Data were collected on intra- and interannual variation of juvenile salmon size, distribution, and abundance; predation by species such as spiny dogfish, walleye pollock, sablefish, and immature and adult salmon; and biophysical characteristics of the habitats influencing salmon growth and survival.  Zooplankton samples were collected to assess prey abundance, and fish samples were collected to determine stock composition and whole-body energy content of juvenile salmon.

In 2001, 13 sampling stations were located in Auke Bay and along three transects with four stations each in upper Chatham Strait, Icy Strait, and Icy Point.  Stations sampled in prior years at Cross Sound, Taku Inlet, Lower Favorite Channel, and False Point Retreat were omitted to focus on key stations and to free up time for additional studies (nearshore sampling, gastric evacuation and diel feeding periodicity).  The standard fishing gear used was a NORDIC 264 surface rope trawl 24-m wide x 20-m deep fished for 20 minutes per station from the NOAA ship John N. Cobb to within about 1 km of shore.  Oceanographic samples taken included a 20-m vertical haul with a 0.5-m diameter, 243-µm mesh NORPAC net; a double oblique bongo net system sample (0.6- m diameter opening, 333-µm and 505-µm mesh nets) to 200-m depth or within 20 m of bottom; a 0.6-m diameter opening, 202-µm mesh WP-2 net sample to 200-m depth at selected stations; surface water samples for chlorophyll and nutrient determinations; and CTD casts for temperature and salinity profiles (Table 1 below).  To target the prey resource potentially most important to juvenile salmon in the near-surface waters they inhabit, a 20-m bongo sample series was collected with diel and systematic samples in Icy Strait.  When time allowed, replicate trawl samples were collected in straits to increase samples of juvenile salmon for thermal-mark analysis and catch-per-unit-effort (CPUE) estimates.

Process studies on juvenile salmon feeding periodicity and gastric evacuation rates were added to the suite of SECM objectives in 2001.  Diel samples were collected over a 24-hour period every 3 hours at a single station in Icy Strait in each month to examine feeding periodicity of juvenile chum, pink and coho salmon. Shipboard gastric evacuation experiments were conducted in May and in July by holding fish from single-haul catches in live tanks at ambient seawater temperatures; we then sacrificed lots of approximately 10 individuals per species every 2-3 hours up to 32 hours after capture to monitor the rate of passage of food from the gut.  In May, beach seine-caught pink and chum salmon fry about 40-50 mm in length were studied; in July, trawl-caught juvenile pink and chum salmon averaging 135 mm were studied. Information from the feeding periodicity and gastric evacuation studies will be combined with size information to develop bioenergetic models of juvenile salmon growth.

Over the 2001 season, 360 plankton samples, 56 water samples, and 120 temperature-salinity profiles were collected, and 110 trawl hauls were made (Table 1 above).  More than 52,000 fish were caught in the trawl between May and September (Table 2 below). Most nonsalmonids caught were juvenile walleye pollock, eulachon, and capelin from night trawls during the diel series. Sampling was not conducted at Icy Point in August or September due to weather and time constraints. 

By Molly Sturdevant

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AFSC Quarterly
Research Reports
July-Sept 2001