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: 64 3 545 7020 Fax: 64 3 545 7021 Email: clement@fishinf.c.nz
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 450-1500 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) 70-250 20-70 Headline Height (m) 20-100 2-9 Mesh size/number Netting Head 1.8 40+m 30, 25, 15 cm Netting Tunnel (cm) 20-80 15, 11 Netting Bagend (cm) 12 11 Lengthener (cm) 11 11 Mesh rund (meshes) 100-150 60-150 Cdend (cm) 11 11 Mesh rund (meshes) 100-150 60-150 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 20-100m. 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 (3.5 5.5 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
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 150-700metres. 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
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
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
References B Page, J McKenzie, S Gldwrthy (2005) Inter-sexual differences in New Zealand fur seal diving behaviur, Vl 304:249-264 Marine Eclgy Prgress Series RH Mattlin, N Gales, D Csta (1998) Seasnal dive behaviur f lactating New Zealand furseals (Arctcephalus frsteri) Vl 76:350-360 Canadian Jurnal f Zlgy FISHERIES ADVISORS & ANALYSTS 6