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Resource Assessment and Conservation Engineering

Field Videos

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PROCESSING AND ENUMERATING TRAWL CATCH

 

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Sorting Catch

Video clips:   1 |  2 |  3 |  4 |  5 |  6 |  7

 

Every trawl is an interesting experience, with no two trawls exactly the same. A wide-range of variables influence what is captured in a single haul, such as type of gear used, substratum, season, water depth, etc. Sometimes a catch will be quite large, weighing several tons, and other times consisting no more than a few baskets of fish. Also, a trawl catch may consist primarily of one type of species, such as the "clean" Atka mackeral catch shown in video clip 1, while other times a wide variety of fish and invertebrate species may be present. Such factors as catch size and animal consistency will influence which type of quantitative sampling method will be used when processing the catch.

 

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Determining Fish Gender

Video clips:   1

 

Once fish are sorted by species, counted and weighed, their gender is then determined. This process is done by making a cut -- as clean as possible -- along the abdomen of each fish, enabling scientists to look at the reproductive organs (gonads). Ovaries and testes will vary considerably according to species, age of the fish, and time of year. A clean cut is strived for for several reasons. It is often necessary to collect fish stomachs for predator/prey studies, so the stomachs must not be accidently cut open, which can be a tricky process if the fish has recently eaten and is quite full. Similarly, the gonads of certain target species may be analyzed for "ripeness" (a measure of fertility and development) during spawning seasons, and cutting the gonads can make classification more difficult. Also, whether or not organs are being collected or analyzed, cutting internal tissues is often a messy situation and can interfere with the gender identification process, especially if the fish is young and underdeveloped. Nevertheless, making clean cuts is not as easy as a scientist would often like. Rough seas, heavely-gloved fingers, and a slippery, flopping fish does nothing to help the process.

 

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Measuring Fish Length

Video clips:   1

 

After catch has been sexed and sorted, sampled fish are lengthed using a bar-coded length strip and an electronic polycorder with an infrared light pen. Length data collected during groundfish surveys are analyzed to generate information about species-specific size composition. The length board is measured off in units of centimeters and each unit has a specific bar code associated with the length measure. Fish are placed on the board, gender is entered into the polycorder as a code, and a length measure is recorded by scanning the bar code at the tail end of the fish with the infrared pen. When all fish are measured, polycorder data is downloaded into ships computers for safe storage and processing.

 

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Collecting Fish Otoliths

Video clips:   1

 

Otoliths (think of them as fish ear bones) provide important information about the various ages of fish. Otoliths are extracted from priority fish species in each catch. Researchers at the AFSC estimate fish age by counting the annular rings in the contained in the otoliths, similar to the process of estimating the ages of trees.

 

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Data Entry

Video clips:   1

 

Data entry is probably one of the most critical activities at sea. Time is a precious resource during field operations and is usually slim. Scientists can spend many laborious hours of sorting, hauling, weighing, and slicing fish in rough, cold weather, sometimes only working-up one large haul in a day due to weather limitations. Data gathered from this labor is hard-earned, and getting it recorded and archived correctly is very important, as "repeat" tows are often hard to achieve.


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