Efficacy of an egress window to reduce the capture of undersize waved whelk (Buccinum undatum) in conical traps: laboratory experiments on the effect of vertical surge and trap angle during haul back.

Abstract

The objective of this study was to develop a means to reduce the capture of undersize waved whelk (Buccinum undatum) in small mesh (51–64 mm) conical traps. To achieve this, egress windows were incorporated above the bottom ring of a trap to produce openings of sufficient width to allow undersize whelk to be released as a string of traps is being hauled to the surface. Analysis of the passage of undersize whelk through three egress window sizes revealed that a 30 mm wide window was most suitable. Simulations of two vertical surge distances and two trap angles were carried out in a controlled environment to provide a better understanding of trap performance. Experiments revealed that when a trap is oriented at an angle of 40–45o a substantial percentage (29–36%) of undersize whelk can be removed at vertical surge distances of 0.3–1 m. Few (3–4%) legal size whelk were emitted at a trap angle of 40–45o and all whelk emitted from a trap were observed to pass through an egress window. Increasing the angle of the trap to 70–75o resulted in loss of legal size whelk through the entrance located on top of the trap. The results of these simple experiments are promising but require verification through fishery trials before they can serve management decisions. Trap modifications and a trap line configuration that will maximize the egress of undersize whelk during fishing trials are discussed.