USGS - science for a changing world

Alaska Science Center

white dothome: white dotscience: white dothighlights: white dotmaps, products & publications: white dotpartners & education: white dotcontact us:   white dotinternal:

Methods

Study Area


Map showing some seabird and sea lion colonies in the Gulf of Alaska and Aleutian Islands
This map shows the study area and includes major seabird colonies (red dots) and Steller Sea Lion rookeries (yellow dots). (click on image for larger view)
The Aleutian Archipelago is formed by a volcanic mountain range that spans nearly 2000 km from the Alaska Peninsula to the Kamchatka Peninsula. There is a general northward flow of waters originating in the Gulf of Alaska or North Pacific into the Bering Sea through several major passes between the islands.  Recent studies have shown that an abrupt change in oceanographic conditions, zooplankton and benthic fish community composition occurs at Samalga Pass, and this has a major influence on patterns of  predator abundance (Hunt and Stabeno 2005). Areas of focus for our study included seabird colonies and Steller sea lion rookeries within the Alaska Maritime National Wildlife Refuge.

 

Shipboard Ocean Sampling

Acoustic signal that represents capelin biomass in the water column
This is an image of the acoustic signal that represents capelin biomass in the water column. This particular echogram shows a school of capelin we documented in the Shumagin Islands. The color bar to the left indicates the target strength and the bold numbers indicate depth on the Y axis and time on the X axis. (click on image for larger view)

During transit between Homer and Adak, Alaska (a distance of about 1700 km), we continuously monitored changes in biomass using hull mounted split beam echosounders (120 and 38 kHz). This hydroacoustic equipment can detect small and large organisms including zooplankton and Walleye Pollock, although net samples are required to determine which species contribute to the observed acoustic signal. We also measured sea surface temperature and salinity using the ship’s thermosalinograph. These data were useful in detecting changes over large spatial scales.

Coastal Fish Sampling

We used beach seines to capture nearshore fishes to document species occurrence and relative abundance. The small-meshed beach seine was deployed from a small skiff and pulled into shore by two or three people.
The beach seine is hauled in by the 30 m long ropes attached to the wings of the net

The beach seine is hauled in by the 30 m long ropes attached to the wings of the net. The top of the net is floated by corks and the bottom of the net is weighted with leads to keep the net hanging vertically in the water column
Modified Herring Trawl (red triangles), Beach Seine (yellow dots) and Isaacs-Kidd Trawls (blue squares) gear types were used to sample different habitats in the Gulf of Alaska and Aleutian Islands
Modified Herring Trawl (red triangles), Beach Seine (yellow dots) and Isaacs-Kidd Trawls (blue squares) gear types were used to sample different habitats in the Gulf of Alaska and Aleutian Islands. (click on image for larger view)

Open Ocean (Pelagic) Sampling

Small meshed research trawls were used to sample forage fish within the watercolumn. The modified herring trawl is deployed from a reel on the stern of the ship. This cone-shaped net is designed to herd fish into the back of the net. The net mouth is held open by hydrodynamic steel doors attached to both sides of the net mouth. An acoustic transducer is also attached to the mouth of the net to manage the depth of the net. The Isaacs-Kidd Midwater trawl is a smaller net that uses a single v-shaped depressor bar to hold the net mouth open.

At each trawl station we collected oceanographic data using a conductivity-temperature-depth (CTD, SeaBird® SBE 19) water profiler equipped with a fluorometer.   We also sampled zooplankton using a 0.6 m diameter ring net with 211μm mesh on a 50 m vertical tow. We collected water samples for chlorophyll a (chl a) and nutrients analysis using a 1.2 liter Niskin bottle fired with a messenger at 10 m depth.

Mid-water trawl attached to the stern of the Tiglax
This is where the modified herring trawl is attached to the stern of the Tiglax. (click on the image for a larger view)
Modified herring trawl with floats
The modified herring trawl uses floats on the headrope (top) and lead chain on the footrope (bottom) to help keep the net open while fishing. (click on the image for a larger view)
Zooplankton are sampled using a ring net
Zooplankton are sampled using a ring net. (click on the image for a larger view)
A conductivity-temperature-depth (CTD) profiler
A conductivity-temperature-depth (CTD) profiler measures oceanography and phytoplankton by taking about two readings per meter as it is lowered through the water column. (click on the image for a larger view)

Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: http://alaska.usgs.gov/science/biology/seabirds_foragefish/foragefish/Aleutian/methods.php
Page Contact Information: ascweb@usgs.gov
Page Last Modified: December 27 2010 18:11:15.