Resource Assessment Report No. 19 Statewide Large Pelagic Resource in Western Australia P. Lewis May 2020 Executive Summary The large pelagic resource is distributed throughout Western Australia (WA) in offshore pelagic and nearshore waters and includes a range of tropical and temperate species, such as mackerels, barracuda, billfishes, cobia, large trevallies, mahi mahi and tunas. The three indicator species for the resource are Spanish mackerel (Scomberomorus commerson) and Grey mackerel (Scomberomorus semifasciatus) for the tropical species and Samson fish (Seriola hippos) for the temperate species. Status of Australian Fish Stocks (SAFS) reporting for the large pelagic resource requires the additional assessment and reporting of Cobia (Rachycentron canadum), Yellowtail kingfish (Seriola hippos), Mahi mahi (Coryphaena hippurus), School mackerel (Scomberomorus queenslandicus) and Spotted mackerel (Scomberomorus munroi), although the catch levels in WA for the later four are low (<5 t) or negligible. SAFS reporting also includes large pelagic species caught in the Commonwealth managed Western Tuna and Billfish Fishery (WTBF) such as, Swordfish (Xiaphus gladius), Bigeye tuna (Thunnus obesus), Southern bluefin tuna (Thunnus maccoyii), and Yellowfin tuna (Thunnus albacares), for which WA does not currently contribute to the reporting or assessment as state based commercial fishery catch levels are negligible but there is some minor state based recreational and charter catch of the latter two tuna species of up to 15 t. Commercially the resource is predominantly accessed by the Mackerel Managed Fishery (MMF) in the North Coast (NCB) and Gascoyne Coast Bioregions (GCB) using trolled lines targeting Spanish mackerel which constitutes up to 80% of the WA commercial large pelagic catch. The recreational and charter fishery accesses the large pelagic resource throughout the state mainly by line fishing and some spearfishing with high release rates. Annual catch data for each indicator species and annual commercial catch rate for Spanish mackerel are monitored in each management area of the MMF. From 2008-2017 the annual WA commercial catch range for Spanish mackerel of 272- 320 t has been within the catch tolerance range of 246-430 t but the 2018 catch of 214 t was below this, for various reasons. The nominal catch rates (kg/day) for the two management areas in the NCB (Kimberley and Pilbara) of the MMF have been relatively stable but declined over the past 4 years. The annual commercial catches of Spanish mackerel in the Pilbara and Gascoyne/West Coast management areas of the MMF have been at or below catch tolerance levels for most years since they were set due to a reduction in fishing effort with the introduction of quotas in 2006 and require review. ii No reference levels are set for Grey mackerel or Samson fish and annual commercial catches for the later are at historically low levels, due primarily to low targeting and recent management changes, particularly in the West Coast Bioregion (WCB). The MMF has reported catches in a daily logbook for over 12 years and these finer scale data have been assessed to conduct a catch rate standardisation for the fishery, although 90% of catch is taken by four vessels so confidentiality is an issue. The estimated statewide retained harvest by boat-based recreational fishers of the three large pelagic indicator species over the past four surveys from 2011/12 to 2017/18 is 42-107 t. Over the past 10 years (2009-18) the charter boats combined retained catch for these species ranged 15-25 t. Spanish mackerel dominate the retained recreational and charter catches, at over 75% of the resource, and by weight are the 5th and 2nd highest retained finfish species, respectively. For most large pelagic species the majority of the recreational and charter catch is released/discarded, typically 70%, but up to 100% for some species with possible high mortality rates. Ongoing unknown, but likely high degree of depredation and discard mortality are common among the species in the large pelagic resource. This is particularly important with the high release rates reported by recreational and charter fishers for many species. Large pelagic species are generally highly resilient to fishing pressure with low risk PSA scores from 2.02-2.5, with the added resilience of size limits, bag limits for most species and commercial quotas for Spanish mackerel and Grey mackerel. Weight of evidence assessments have deemed that all large pelagic species have an acceptable/adequate level of fishing mortality and estimates of relative spawning biomass, with current management settings maintaining risk at acceptable (medium) level for Spanish mackerel, Samson fish, and Yellowtail kingfish plus acceptable (low) levels for Cobia, Grey mackerel, School mackerel, Spotted mackerel and Mahi mahi. Future monitoring includes the industry based collection of Spanish mackerel samples for an age-based assessment in 2021 of the main exploited commercial species and development of standardised catch rates from the daily logbook data. Important disclaimer The Chief Executive Officer of the Department of Primary Industries and Regional Development and the State of Western Australia accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it. Copyright © Department of Primary Industries and Regional Development, 2020. iii Table of Contents Contents Executive Summary .............................................................................................................. ii Table of Contents ................................................................................................................. iv List of Abbreviations ............................................................................................................. x 1.0 Scope .............................................................................................................................. 1 2.0 How the Department Operates ......................................................................................... 1 3.0 Aquatic Environment ...................................................................................................... 2 4.0 Resource Description ................................................................................................... 8 4.1 Large Pelagic Resource ........................................................................................... 8 4.2 Selection of Indicator Species for Resource ........................................................... 8 5.0 Species Description .................................................................................................... 11 5.1 Spanish Mackerel (Scomberomorus commerson) .................................................... 11 5.1.1 Taxonomy and Distribution ............................................................................... 11 5.1.2 Stock Structure ......................................................................................... 11 5.1.3 Life History .............................................................................................. 12 5.1.3.1 Habitats and Movements .......................................................................... 13 5.1.3.2 Age and Growth ....................................................................................... 14 5.1.3.3 Natural Mortality ..................................................................................... 15 5.1.3.4 Reproduction ............................................................................................ 15 5.1.3.5 Factors Affecting Year Class Strength and Other Biological Parameters .. 16 5.1.3.6 Diet and Predators .................................................................................... 16 5.1.3.7 Parasites and Diseases .............................................................................. 16 5.1.4 Inherent Vulnerability .............................................................................. 16 5.2 Grey mackerel (Scomberomorus semifasciatus) ................................................... 18 5.2.1 Taxonomy and Distribution ...................................................................... 18 5.2.2 Stock Structure ......................................................................................... 18 5.2.3 Life History .............................................................................................. 18 5.2.3.1 Habitats and Movements .......................................................................... 19 5.2.3.2 Age and Growth ....................................................................................... 20 5.2.3.3 Natural Mortality ..................................................................................... 20 5.2.3.4 Reproduction ............................................................................................ 20 5.2.3.5 Factors Affecting Year Class Strength and Other Biological Parameters .. 20 5.2.3.6 Diet and Predators .................................................................................... 20 5.2.3.7 Parasites and Diseases .............................................................................. 21 5.2.4 Inherent Vulnerability .............................................................................. 21 5.3 Samson fish (Seriola hippos) ............................................................................... 22 5.3.1 Taxonomy and Distribution