Characteristics of catch losses in stow nets with finless porpoise excluder devices and a strategy to reduce them

Abstract

Finless porpoise excluder devices have been developed to prevent bycatch of finless porpoise by stow net in Korea; however, fishermen are hesitant to use them owing to concerns of associated catch losses. To promote their use, we aimed to identify the characteristics of catch losses associated with finless porpoise excluder devices used along the Korean west coast and suggest a strategy to reduce these losses. Therefore, we analyzed the selectivity of the separator net in excluder devices, the catch loss rates in stow net with upward- or downward-facing exit holes, and the size distribution between the lost and caught fish for significant species of stow net. As a result, it was confirmed that the selectivity of the separator was best expressed by the bi-normal function for Japanese anchovies (L50 = 8.1 to 10.9 cm) and golden cuttlefish (L50 = 6.7 to 8.7 cm) and by the constant function for hair tails and swimming crabs. In addition, the size distribution between lost and caught fish species was not significantly different except for Japanese anchovies. Analysis results of the catch loss rate by the position of exit of the excluder devices were as follows. Upward-facing exit holes showed relatively high losses of Japanese anchovies (upward exit 14.40 vs. downward exit 1.11%) and hair tails (3.38 vs. 0.26%). In contrast, downward-facing exit holes showed high losses of golden cuttlefish (3.58 vs. 7.55%) and swimming crabs (0% vs. 15.72%). However, these losses significantly decreased when the exit hole direction was reversed, suggesting that changing the exit hole direction depending on the species could reduce catch losses, increasing fishermen’s willingness to use finless porpoise excluder devices and reducing bycatch.