Effects of twine material on the marine debris and relative ghost fishing of portunid hoop (tangle) nets

Abstract

Portunids are important to recreational fisheries in Australia and typically are harvested by baited gears. Most of these baited gears are considered environmentally benign, although small conical hoop (tangle) nets fished in southeastern Australia sometimes are damaged and lose twine, which becomes marine debris. Substituting conventional multifilament polyamide (PA) twine with alternative multi-monofilament and monofilament PA twines in hoop nets can dramatically reduce marine debris, but there remain unanswered questions concerning the relative fishing power of alternative twines among derelict hoop nets. This study addresses the shortfall in data by comparing the efficiencies and damage to replicate hoop nets made from the three twines and left to fish for short (one and two days), medium (three to five days) and long (ten and fourteen days) soaks. Irrespective of twine material or soak time, all hoop nets entangled the same numbers of giant mud crabs, Scylla serrata implying catch saturation within two days. Only four other animals were caught (all yellowfin bream, Acanthopagrus australis) and all died. At least some replicates of all hoop nets were damaged and lost twine, but this was greatest among the multifilament (up to a maximum of 9.3 m hoop-net deployment−1), followed by the multi-monofilament (up to 7.4 m hoop-net deployment−1) and monofilament hoop nets (1.4 m hoop-net deployment−1). Replacing the existing multifilament twine in hoop nets with monofilament when targeting S. serrata would maintain catches and minimise marine debris during conventional soaks, without increasing ghost fishing over the short term. Keywords: Bycatch, Decapods, Discard mortality, Unaccounted fishing mortality