In 2000 and 2001, scientists from the Resource Assessment and Conservation Ecology (RACE) and Resource Ecology and Fisheries Management (REFM) Divisions designed a multi-year investigation of the effects of fishing on sea lion prey abundance and distribution in a commercial fishing ground located on in the Gulf of Alaska off the east side of Kodiak Island.  In 2001, investigators from Pacific Marine Environmental Laboratory joined the project to provide enhanced bio-physical sampling to characterize the marine habitat.

The sampling design utilizes control (unfished) and treatment (fished) areas.  When control and treatment sites are reasonably similar, the control allows the analyst to differentiate responses to the treatment from factors due to natural variability.  Barnabas and Chiniak troughs share many biological and physical characteristics making them good candidates for the control and treatment sampling sites.  Feasibility studies conducted in 2000 in the absence of commercial fishing revealed that the distribution and abundance of fish in each trough was stable during the study period (two passes in each trough).   In 2001, the survey design was expanded to include periods before and during the commercial fishing season beginning in August (C season).  Regulations were established to close Chiniak trough to fishing.  Two passes were made in both troughs before the start of the fishing season and one full pass of each trough was made after fishing commenced.  This type of before-after-control-impact study was conducted off Kodiak in 2001, 2002, 2004 and 2006.

Fish distributions in each trough are assessed using Echo Integration Trawl (EIT) survey techniques during daylight hours aboard the NOAA Research Vessel, Miller Freeman.   The survey methods are similar to those used during other routine acoustic surveys.  Trawl hauls are conducted where significant acoustic sign is encountered to determine the species composition and to collect other biological information of the acoustic layer. 

A variety of oceanographic data are collected to describe the influence of bio-physical factors on the distribution and abundance of sea lion prey.  Moorings designed to measure temperature, salinity, fluorescence and currents are deployed.  Temperature and salinity measurements are also collected while the ship is underway and by water column profiles taken at selected sites.  Satellite track drifters are deployed to measure the spatial characteristics of the Alaska Coastal Current.   Finally, satellite images provide information on the spatial distribution of chlorophyll - a in the study region. 

In 2001 and 2004, substantial amounts of adult pollock were removed from our study area during the C season.  Results from the 2001 experiment show high temporal variability in adult pollock biomass in the treatment area, but not in response to fishing.  In contrast, results from 2004 show a statistically significant decrease in pollock biomass in the treatment area following the start of commercial fishing.   No concurrent decrease in adult pollock biomass in the control area was observed.  Results from 2000 and 2002 are not shown because the region was closed to pollock fishing in 2000, and fishery removals were very small in the study area in 2002.   In 2006 an intermediate amount of adult pollock was removed during the commercial fishery.  Results show a decline in pollock abundance after the start of the fishery, but the decline occurred in both the treatment and control area, so this response is not likely due to fishing. 

Bio-physical data from all years show that adult pollock and age-1 pollock had an apparent preference for warmer nearshore water where production was likely to be concentrated.  During years when age-1 pollock are abundant in the study area, capelin were associated with cool slope water offshore.  In contrast, capelin were distributed in warmer nearshore waters in years when age-1 pollock were absent from the study area, suggesting that competition with age-1 pollock may influence capelin distribution.  This bio-physical research is being conducting in collaboration with the Recruitment Processes Program (RACE Division) and is funded in part by the North Pacific Research Board (NPRB project 524). 

Contact

For more details on the results summarized above, see the documents below, or contact Anne Hollowed (anne.hollowed@noaa.gov or 1-206-526-4223) or Chris Wilson (chris.wilson@noaa.gov or 1-206-526-6435)