Release of unwanted flat-bodied fish from a horizontal-bar grid system as revealed through comparative fishing trials and underwater video observations

摘要

Grid systems separate fish species primarily through physical means: fish size and body shape. On Georges Bank off the northeast USA, many species of flounders are overfished, and their catch needs to be reduced. Flat-bodied skates are also often discarded. We tested a European style horizontal-bar grid system to reduce these flat-bodied low quota species in a trawl targeting the haddock ( Melanogrammus aeglefinus ), whose population is robust. The grid system consisted of 4 grid sections, two on each side, with horizontal bars 70?mm apart. The alternating tow method was used to compare the catch characteristics between a trawl with a grid section and the same trawl without a grid section. A video camera was used to observe fish escape in the grid section. The results indicate that the grid system reduced the flounder catch rate (mainly winter flounder, Pseudopleuronectes americanus ) by 51.3%, and skates (mainly winter skate, Leucoraja ocellata and little skate, Leucoraja erinacea ) by 29.4%, while there were no differences in the catch of Atlantic cod ( Gadus morhua ). While haddock was reduced by 37% by weight, the reduction was primarily small undersized individuals. There was no reduction in large haddock greater than 50?cm. Video observations indicates that flounders and cod mainly escaped from the bottom half of the grid while haddock were from the top half. These finding suggest that this system reduced low quota flounders, as well as discarded skates and small haddock, while retaining Atlantic cod and large size haddock. Underwater observations indicate that differential spacing (narrower on the top and wider on the bottom) may improve the system performance by releasing more flat-bodied fish, that were observed to escape from the bottom part of the grid, while retaining more haddock, which typically escape from the top part of the grid.