2nd Meeting of the Scientific Committee Honolulu, Hawaii, USA 1-7 October 2014 SC-02-10 An assessment of the potential for near-seabed midwater trawling to contact the seabed and to impact benthic habitat and Vulnerable Marine Ecosystems (VMEs) Geoff Tingley 1 3 Sep 2014 SC-02-10 An assessment of the potential for near-seabed midwater trawling to contact the seabed and to impact benthic habitat and vulnerable marine ecosystems (VMEs) SPRFMO number SC-02-10 MPI Technical Paper No: 2014/30 ISBN No: 978-0-478-43746-1 (online) ISSN No 2253-3923 (online) September 2014 3 Sep 2014 SC-02-10 Disclaimer While every effort has been made to ensure the information in this publication is accurate, the Ministry for Primary Industries does not accept any responsibility or liability for error of fact, omission, interpretation or opinion that may be present, nor for the consequences of any decisions based on this information. Requests for further copies should be directed to: Publications Logistics Officer Ministry for Primary Industries PO Box 2526 WELLINGTON 6140 Email: [email protected] Telephone: 0800 00 83 33 Facsimile: 04-894 0300 This publication is also available on the Ministry for Primary Industries website at http://www.mpi.govt.nz/news-resources/publications.aspx © Crown Copyright - Ministry for Primary Industries 3 Sep 2014 SC-02-10 Contents Page 1 Introduction 1 2 Data and methods 2 3 Discussion 8 4 References 11 i 3 Sep 2014 SC-02-10 3 Sep 2014 SC-02-10 An assessment of the potential for near-seabed midwater trawling to contact the seabed and to impact benthic habitat and vulnerable marine ecosystems (VMEs) Geoff Tingley, Ministry for Primary Industries September 2014 1 Introduction In general pelagic (midwater) trawling is perceived as having a much lower potential for causing adverse environmental impacts than demersal (bottom) trawling, especially with respect to benthic environments (SPRFMO, 2012; Ministry of Fisheries, 2008). This view is largely driven by the way pelagic trawls are mostly used, being fished in the upper layers of the sea to catch pelagic schooling fish. In some circumstances, it is not uncommon for pelagic trawl gear to be used to catch fish near the seabed and is seen in New Zealand in the fisheries for southern blue whiting (Micromesistius australis) (MPI, 2013; MPI, 2014). Typically, this latter type of use occurs in areas of relatively flat and/or soft seabed where the lighter, more easily damaged midwater gear, is at lower risk of damage but this approach can be extended into rougher terrain if the fishers are prepared to accept the risk of damaging the gear. Within a SPFRMO context, the level of potential benthic impact of midwater trawl gear has been considered both in the SPRFMO Benthic Impact Assessment Standard (SPRFMO, 2012) and in individual member fishing impact assessments (Ministry of Fisheries, 2008). However, the New Zealand Bottom Fishing Impact Assessment (op. cit.), which drew on accepted understandings that different gear types carry different benthic risk profiles (see Chuenpagdee et al, 2003), was principally focussed on assessing demersal fishing activity and gears and the consideration of the use of pelagic trawl gear was not considered in depth. Defining the potential benthic impact of trawl gear is inherently difficult, as it is not just the type of gear but the manner in which it is employed that determines the risk and potential impacts. The SPRFMO Benthic Impact Assessment Standard (SPRFMO, 2012) recognises this, and defines bottom trawl gear as “any trawl net fished in such a way that it has a likelihood of coming into contact with the seabed at some time during the trawling operation”. What constitutes ‘a likelihood’ in this context is not defined. The 2nd SPRFMO Commission Meeting in January 2014 adopted a Conservation and Management Measure for the Management of Bottom Fishing in the SPRFMO Convention Area (CMM 2.03) that defines the term ‘bottom fishing’ as “fishing using any gear type likely to come in contact with the seafloor or benthic organisms during the normal course of operations”. As its name suggests, in normal use, midwater trawl gear would not necessarily be expected to contact the seabed. However, as noted above, midwater trawl gear can be, and is, effectively operated as demersal trawl gear, designed to fish on the seabed in some fisheries. The demersal trawl fisheries in the western SPRFMO Convention Area have a single target species, orange roughy (Hoplostethus atlanticus), which tend to aggregate in areas of steep slopes and rough terrain, often on seamounts, hills and knolls. Due to a historic lack of success in using midwater trawls to target orange roughy, believed to be due to orange roughy Ministry for Primary Industries Assessment of the potential for near-seabed midwater trawling to contact the seabed • 1 3 Sep 2014 SC-02-10 having a strong tendency to dive with the approach of fishing gear (Koslow et al., 1995; Kloser et al., 1997, 2002; O’Driscoll et al., 2012), and because of the nature of the ground where orange roughy tend to aggregate, midwater nets have not been used to target orange roughy in the SPRFMO Convention Area. New Zealand vessels started targeting alfonsino (Beryx spp.) in the SPFRMO Convention Area using midwater trawls in about 2011. This activity came under scrutiny following the adoption of SPRFMO CMM 2.03 as it was unclear whether this activity should be covered by a ‘bottom fishing’ CMM or not. All use of midwater trawls to target alfonsino has been undertaken under a New Zealand High Seas Fishing Permit for bottom fishing and each vessel carried a Ministry observer who recorded benthic interactions. The analyses of data collected on benthic bycatch and other indicators of benthic contact from the observer programme form the basis of this paper. The analyses presented in this paper are to inform the discussion of whether midwater trawling for bentho-pelagic species such as alfonsino should continue to be included in the approach to manage bottom fishing or not and the likely impact on benthic habitats and vulnerable marine ecosystems (VMEs). 2 Data and methods Ministry observers on New Zealand vessels operating in the SPRFMO Convention Area record standardised information. Specifically, observers record two types of information that can inform on whether there is any benthic contact by fishing gear. These two types of information are, (i) the presence or absence of benthic material brought up in the gear for each set or tow and (ii) the recording of a gear event code. The frequency with which benthic material is brought up when using midwater trawl gear is relatively low, with six of 238 (3%) midwater tows during the period 2011–2013 with a recorded presence of benthic material. Over the same time period, the rate for demersal trawl tows with benthic material was 37%. To consider the evidence of contact with the seabed in the gear event codes recorded by observers for each tow, the description gear event codes must first be explored. There are thirteen gear event codes in use (Table 1), of which a number are absolute indicators of benthic contact and others can be interpreted as indicators of benthic contact of varying strength. More than one gear event code can be recorded for each tow, thus a tow that gets caught fast and tears the net would be coded as ‘AB’ or ’BA’. If more than three codes are required for a single tow, code ‘Y’ is used and the details recorded in a comments field. 2 • Assessment of the potential for near-seabed midwater trawling to contact the seabed Ministry for Primary Industries 3 Sep 2014 SC-02-10 Table 1: Gear event codes used by NZ observers and the interpretation of whether they indicate bottom contact when reported for tows using midwater trawl gear. Code Description Indicator of bottom contact A Net torn Yes, in some instances B Net caught/net fast Yes C Winch failure during setting No D Winch failure during hauling No E Net deliberately towed for a long period at non-fishing depth No F Haul gear to doors up, turn and then shoot away again No G Twisted warps, crossed doors Possibly H Gear lost Yes I Broken or snapped warp Possibly O Other (detail in comments) Possibly U Unknown, unobserved No Yes, if at least one code is indicative Y More than three gear event codes, see comments section of bottom contact. Z No gear events No The observer recorded gear event codes ‘B’ and ‘H’, which indicate respectively the net caught on the seabed and gear lost, are categorical indicators of gear contact with the seabed. Codes that may indicate seabed contact are ‘A’, ‘G’, ‘I’, ‘O’ and ‘Y’. The argument for these latter codes in indicating bottom contact varies from strong to weak. Logically, those codes indicating significant gear damage (e.g. ‘A’ and ‘I’) are more likely to indicate bottom contact than codes that do not reflect actual damage to the gear. An examination of each gear event code and other evidence in the observer record are presented in Table 2, which shows an interpretation of the definitive and imputed evidence for bottom contact for each gear event code type from the 238 midwater tows by New Zealand vessels in the SPFRMO Convention Area observed during the period 2011–2013. Gear codes ‘D’, ‘H’, ‘U’ and ‘Y’ were not recorded for midwater tows over the period 2011– 2013 (Table 2). Codes ‘C’, ‘E’ were rarely recorded and are assumed to not be indicators of bottom contact. These code groups will not be considered further in these analyses, other than to note that they have a non-zero probability of bottom contact.