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Simple Closing Midwater Trawl for Small Boats
Author(s) -
Enzenhofer Hermann J.,
Hume Jeremy M. B.
Publication year - 1989
Publication title -
north american journal of fisheries management
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.587
H-Index - 72
eISSN - 1548-8675
pISSN - 0275-5947
DOI - 10.1577/1548-8675(1989)009<0372:scmtfs>2.3.co;2
Subject(s) - limnetic zone , fishery , closing (real estate) , fish <actinopterygii> , population , environmental science , towing , sampling (signal processing) , oncorhynchus , marine engineering , oceanography , geography , geology , biology , computer science , engineering , littoral zone , telecommunications , demography , sociology , political science , detector , law
A closing midwater‐trawl system for a small boat (7 m) was developed to apportion hydroacoustic population estimates among fish species by collecting vertically stratified limnetic fish samples. These stratified samples could also be used for providing biological data on the fish or for feeding and behavior studies. The 3 × 7‐m rectangular trawl used a double‐line, hydraulically operated cable system that sampled down to a depth of 80 m. Launching and retrieval required minimum time and only one person plus the driver; cumbersome messengers, planer boards, or fixed frames were not needed. Effectiveness of the closing mechanism was tested by sonar observation and by towing the net in a closed position through dense layers of juvenile sockeye salmon Oncorhynchus nerka . No fish were caught when the net was closed, whereas 3.8‐56 sockeye salmon/min were caught when the net was open. When the closing trawl was used to apportion hydroacoustic population estimates among species in a lake with a mixed limnetic fish population, the estimate of sockeye salmon was more than double the estimates made with the continuously open trawl.