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Bottom Trawl Survey Objectives and Standard Methods


The main objective of Groundfish trawl surveys is to collect fishery-independent data. The data are used to describe the following about groundfish:

  • temporal distribution and abundance of the commercially and ecologically important groundfish species.
  • changes in the species composition and size and age compositions of species over time and space.
  • reproductive biology and food habits of the groundfish community.
  • the physical environment of the groundfish habitat.

Dataflow from catch processing to fishery management.

The Groundfish trawl survey data are used in mathematical models that researchers use to analyze fish biomass and mortality dynamics.
Information derived from the mathematical models is then used by fishery management scientists to help predict appropriate harvest guidelines and regulatory measures for commercial groundfish species in upcoming seasons.

Standard Methods

Trawl and Mensuration Gear

fishing netThe Groundfish Assessment Program uses a range of standard and specialized trawl gear and equipment during its assessment surveys. The RACE Division's Survey Gear and Support Program builds and maintains a large inventory of standard fishing gear, including nets, footropes, and longlines. The Poly Nor'eastern trawl (shown in the photo at right), one of the standard trawl nets used in groundfish surveys, can be outfitted with one of three different types of footropes, depending on the condition of the bottom to be surveyed. The main trawl gear components include the net and footrope combination, the trawl doors, and the ship's winch and cable system.

groundfish sensor As indicated in the picture at left, trawl nets are also outfitted with various electronic sensors, such as net spread and height sensors, fishing dimension (area) sensors, bottom contact sensors, and dataloggers that collect depth, temperature, and global position measurements. This equipment is attached to specific parts of the trawl prior to each tow, and collectively records the various data types while allowing scientists to monitor gear performance during each tow event. The sensors are removed at the end of each tow, and recorded data is downloaded to shipboard computers for processing.

In addition to standard gear, RACE scientists also use a variety of specialized equipment for special purpose studies. Camera sleds, towed video systems, and manned submersibles are all used to observe trawl performance, fish habitat, and fish and crab behavior during capture by fishing gear.

Trawl Catch Processing and Sampling

A small catch of mixed fish and invertebrate species emptied on sorting table. Groundfish catches are hauled onboard, dumped into a sorting table, sorted by species, and weighed. Catches range in size from 1 kg to more than 5,000 kg. No two catches are exactly the same. Some catches consist mostly of a single species, while other catches can be a mix of both fish and invetebrate species. Small hauls may be sampled entirely (every fish accounted for), or subsampled if the catch quite large. Each sampled species of fish, depending on the partucular survey objectives, is further sorted by gender (done by examining the gonads of each fish via a cut in the stomach), placing females into orange baskets and males into gray baskets.

A Polycorder: An electronic fish length recorder. After the catch is sorted, sampled fish are lengthed using a bar-coded length strip and an electronic polycorder with an infrared light pen (the picture at the left shows a length measurement of a shortspine thornyhead). Length data collected during groundfish surveys are analyzed to generate information about species-specific size composition.

A fish otolith Otoliths (fish ear bones) provide important information about the various ages of fish. Otoliths are extracted from priority fish species in each catch. Biologists from the Age and Growth Laboratory in the AFSC's Resource Ecology and Fisheries Management Division estimate the age of fish by counting rings in the annular pattern contained in the otoliths, similar to the process for estimating the ages of trees.

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            | Home | Site Map | Contact Us | FOIA | Privacy | Disclaimer | | Accessibility | Print |           doc logo
            | Home | Site Map | Contact Us | FOIA | Privacy | Disclaimer | | Accessibility | Print |           doc logo