Fishing gears that minimise the damage incurred by discarded spanner crabs (Ranina ranina): Laboratory and field experiments

Abstract

Large numbers of undersized spanner crabs (Ranina ranina) are damaged and killed during capture and subsequent discarding from tangle nets. Fishing methods that reduce this mortality were examined in this study. Males and females of all sizes readily negotiated inclined ramps made from many materials, suggesting that they could be caught in non-entangling traps with entrances. A variety of traps with top or side entrances were compared with tangle nets in laboratory trials but the best catch rates of non-entangling traps were less than half those achieved using traditional tangle nets. This was due largely to the greater time required by crabs to find entrances of traps. Field trials supported the laboratory observations with traps catching significantly fewer crabs than tangle nets. When fishing times of non-entangling traps were increased, catches increased, but the logistics and cost-effectiveness of their commercial use were considered prohibitive. Field trials using conventional tangle nets with different combinations of mesh sizes, numbers of netting layers, twine thicknesses and hanging methods identified the most efficient net configuration that minimised damage while maintaining catch rates. Significantly more time was required to disentangle crabs from small (25 mm) and large (85 mm) mesh than from intermediate mesh sizes. Likewise, loss of dactyls (a major cause of post-release mortality) was greater for these mesh sizes, particularly the 25 mm mesh. Use of loosely hung nets (hanging coefficient of 0.5) resulted in over twice the loss of dactyls than tightly hung nets (hanging coefficient of 0.7) and required more time to disentangle crabs. The catch rates of undersized crabs (less than 10 cm carapace length) was significantly greater in loosely hung nets but this effect was not observed in legal sized crabs (over 10 cm carapace length). Tightly hung (hanging coefficient of 0.7), single netting layer nets with a mesh size greater than 25 mm and less than 85 mm proved to be most effective for minimising damage whilst maintaining catch rates.