Operating Parameters of Trawl Gear in Relation to Fur Seal Mitigation

Transcription

1 Operating Parameters f Trawl Gear in Relatin t Fur Seal Mitigatin JUNE 2008 Reprt Prepared by: FISHERIES ADVISORS & ANALYSTS 1 98 Vickerman St PO Bx 1460 Nelsn, New Zealand Phne: Fax: clement@fishinf.c.nz

2 Fishery Characteristics The bulk fisheries that have the majrity f fur seal interactins (hki, suthern blue whiting, squid and assciated fish bycatch species) have the fllwing relevant characteristics: The majrity f the catch is taken by large (>28m) vessels which are ften the same vessels in the squid fishery that utilise Sea Lin Exclusin Devices, r SLEDs). The inshre fleet invlved in the abve fisheries are less than 28metres in length and generally h.p. Fishing gear cnsists f tw key types bttm trawl and mid water trawl. Sme dimensinal infrmatin is presented belw: Overall net dimensins Midwater Trawl Bttm Trawl Overall Length (m) Headline Height (m) Mesh size/number Netting Head m 30, 25, 15 cm Netting Tunnel (cm) , 11 Netting Bagend (cm) Lengthener (cm) Mesh rund (meshes) Cdend (cm) Mesh rund (meshes) The peratin f mid water trawls can be pelagic (ff bttm), semi-pelagic (n and ff), r in full and cntinuus bttm cntact. Head line heights when in pelagic mde are frm m. This value may be halved when perated n bttm (net clsed up ). Due t the bulk nature f the relevant fisheries mst vessels are perating significant lengtheners (e.g. 2 x 6.5metres f extensin between net and cdend) as well as multiple cdend sectins (e.g. 2 x 7metres). This means there is a large distance (25-30 metres) between the bdy f the net and the end f the cdend. Netmakers invlved in the develpment and building f SLEDs suggest that placement f exclusin devices is best at the intersectin f net bdy and lengthener. This allws fr excess catch t fill the lengthener if required and als mre space fr animals in frnt f the grid t make sensible escape decisins, rather than being cnstrained within a 13metre lng tunnel f relatively limited diameter. Tw speeds are generally cnsistent and reasnably high ( knts) acrss the relevant trawl fisheries (i.e. thse with frequent fur seal interactins). SLEDs are perable at these speeds as demnstrated in the squid trawl fishery. Cntinued n next page FISHERIES ADVISORS & ANALYSTS 2

3 Fishery Characteristics, Cntinued Fishery perating parameters that may influence r increase interactin risk have been lng discussed and attempts t mitigate made n the basis f: Offal discharge attractant Deck lights attractant Winch nise attractant Stickers in net attractant Drs up turns higher entanglement risk Gear n surface (breakdwns etc) higher entanglement risk Vessels may perate at high density and clse prximity in fisheries such as the Snares Shelf and Auckland Islands Shelf squid, West Cast hki r Ck Strait hki. Trawling can be characterised in sme fisheries by vessels turning 180 t tw back alng their riginal tw path t re-target a fish aggregatin. This peratin is variable in the time taken depending n gear size and vessel pwer. Turns with trawl nets at r near the surface have been implicated in captures. Trawling depths fr mst relevant target species are in the range f metres. Squid and suthern blue whiting fisheries are predminantly in less than 300m. NZ fur seals are certainly capable f diving t this latter depth but have seasnally varying average dive depths f between 30-75metres (Mattlin, Gales and Csta, 1997; Page, McKenzie and Gldswrthy, 2005). These depths are fr fur seals targeting natural prey. This suggests that fur seals may be at mst risk during shting and hauling r when the gear is held at r near the surface, nt while it is in active fishing mde. Anecdtal evidence shws fur seals actively feeding frm either stickers (meshed fish) r catch in the cdend directly frm the gear n the surface. The hki fishery in particular has at times a wide range f ther fish species such as spiny dgfish, ling, hake and warehu present in the catch. Prcedures prpsed t help prevent fur seal captures have been dcumented and used fr many years. Relevant parts are presented belw. FISHERIES ADVISORS & ANALYSTS 3

4 Mitigatin Measures Used in Vessel CP All vessels shuld adpt the fllwing practices t minimise accidental catches f marine mammals. Shting and hauling Befre shting gear, all stickers must be remved frm the trawl s marine mammals are nt attracted t the net. Shting and hauling fishing gear must be undertaken as quickly as pssible in rder t lessen the risk f capture f marine mammals at r near the sea surface. Gear failures, particularly when shting r hauling can create high risk situatins fr marine mammal captures. Evidence suggests that fur seal captures ccur when there are gear failures and the gear is left n the surface f the water with the net muth pen. In the event f a gear failure which may delay the shting r hauling f the gear, the fllwing shuld ccur: Keep the gear deep in the water even if this means re-shting the gear if the gear is t remain in the water the gear headline height shuld be at least belw 50m and preferably belw 100 m. Bring the gear n bard r at least the grund rpe and net headline t ensure the net muth remains clsed. It is als critical that the net shape is maintained when trying t fix any gear failures. If shape is nt maintained, this can make it difficult fr marine mammals particularly dlphins t escape the net. FISHERIES ADVISORS & ANALYSTS 4

5 Mitigatin f Risks Fr well ver a decade there has been a prgram t mitigate peratinal risks by an industry Cde f Practice. T an extent there is uncertainty as t level f risk engendered by varius aspects f peratins; the imprvement that this mitigatin has delivered r which f the risks are currently unable t be mitigated well by changes in peratinal practice is uncertain Prbably the majr risk is fishing gear n r very near the surface fr prlnged perids due t slw deplyment r retrieval (which can ccur fr a variety f reasns); this appears t lead t multiple capture events. FISHERIES ADVISORS & ANALYSTS 5

6 References B Page, J McKenzie, S Gldwrthy (2005) Inter-sexual differences in New Zealand fur seal diving behaviur, Vl 304: Marine Eclgy Prgress Series RH Mattlin, N Gales, D Csta (1998) Seasnal dive behaviur f lactating New Zealand furseals (Arctcephalus frsteri) Vl 76: Canadian Jurnal f Zlgy FISHERIES ADVISORS & ANALYSTS 6