Development of non-damaging trapping apparatus and methods of limiting damage caused by traditional tangle nets in the spanner crab (Ranina ranina) fishery

Abstract

Both male and female spanner crabs (Ranina ranina) readily moved up inclined ramps made from numerous materials and there were no significant size related behavioral differences. This suggested that Ranina ranina could be trapped in "entrance type" non­-entangling apparatus. A range of top and side entrance traps as well as conventional entanglement nets were trialed in the laboratory with entanglement nets entrapping over double the number of crabs caught in the most efficient non-entanglement trap tested. This was largely due to the greater time required by spanner crabs to find the entrance of a trap. Field trials supported the laboratory behavioural observations with no trap obtaining comparable catches to the conventional entangling apparatus. When fishing times of non-entangling traps were increased, catch rates likewise increased however the logistics and cost-effectiveness of their commercial use proved prohibitive.

Field trails using different configurations of mesh size, numbers of mesh layers, ply and net tension for conventional entangling apparatus were also conducted to identify the most efficient net configuration for minimising damage whilst maintaining catch rates. Both small (25 mm) and large (85 mm) mesh size required more time to clear than intermediate sizes. Likewise dactyl loss was higher for these meshes, particularly the 25 mm mesh. Loosely hung nets induced over double the dactyl loss of tightly hung nets, with longer clearance time. In addition the catch of undersize crabs was also significantly greater in loosely hung nets whilst there was no significant difference in the catch of legal sized crabs. Tightly hung single mesh nets of a mesh size greater than 25 mm and less than 85 mm proved to be the most effective net for minimizing damage whilst maintaining catch rates.