Determining the impact of environmental variability on the sustainability, fishery dynamics and economic performance of the West Coast Prawn Trawl Fishery
Spencer Gulf and West Coast Prawn Fishermens Association
Neil A. Carrick
The program addresses the three strategic challenges outlined in FRDC’s Research & Development Plan, 2005-10 namely: • Natural resources sustainability-development of spatially explicit management models for fisheries sustainability and will include temporal (cycles) effects driven by environment. • People Development-greater understanding of the processes affecting stocks and better management through industry involvement in decision making. • Community and Consumer support-through education about factors affecting stocks. The WCPF production has largely declined over the last 4 years and remains at a depressed state. Industry is faced with paying high interest rates on loans and licence fees for research and management. Moreover, Industry pay for costs (additional to licence fees) associated with fishery independent trawl surveys. There is a need to analyse data and demonstrate that the sustainability and profitability of the fishery is undermined by catastrophic downturns in recruitment attributable to environmental variation linked to El Nino Southern Oscillation (ENSO) events. The fishery is a unique case and will be used as a model to demonstrate the catastrophic impact of environmental disturbance on a fishery for application to the Australian government for funding support through the ‘Exceptional Circumstances” programme.
1. To assemble andpdate analyses of fishery dependent catch-effort and fishery independent trawl survey data and environmental information relating to the West Coast prawn fishery (WCPF).
2. To determine the impact of environmental variation (ENSO and upwelling events) on the sustainability, fishery dynamics and economic performance of the WCPF.
3. To develop a case model for an application for support provided under the Australian governments Exceptional Circumstances program (EC).
The project has provided an understanding of how environmental variation has impacted on WCPF production. Chapter 1 contains the objectives of the study, background and need for the work. Chapter 2 provides a context for understanding of: a) oceanic and climatological processes which are associated with El Nino episodes and cold water upwelling in WCPF waters and b) the potential impact of a cold water environment in reducing larval survival, advective dispersal (larval supply) to nurseries and reduced recruitment to grounds. Chapter 3 details commercial logbook catch, effort and CPUE on temporal and spatial trends in the WCPF. Catch-effort data were reported by financial and calendar year with calendar year being more informative of the dynamic changes in the stock. Historical catch-effort data show that the WCPF fishery has strong cycles in production and CPUE. CPUE fell to the lowest levels in 1978/79, 1992/93 and in 2003/04. However, the recent decline in WCPF production and CPUE was more prolonged than in the past with zero catch in 2006/07 due to the closure of the fishery. The prolonged decline has association with more frequent El Nino episodes.
The WCPF is considered unique for the following reasons: 1) It is based exclusively on the capture of the Western King prawn (Melicertus latisulcatus) for income; 2) Is an oceanic penaeid fishery situated in a region, the Great Australian Bight (GAB), where El Niño events and cold water upwelling have strong effect on local oceanography; and 3) no other established fishery in Australia has shown such catastrophic stock collapse over an extended period (some 6 years from 2002) which has link to El Nino episodes. Chapter 4 uses catch-effort data to derive Leslie depletion estimates which are integrated with fishery independent recruitment estimates in an evaluation of the effects of El Nino and exploitation on recruitment in Chapter 8. A significant project outcome was the development of a prawn size grade database where grade data was joined to catch-effort data for parameter estimates using SQL procedures. Chapter 5 reports on the size composition in catches from 1996-2005 and results show an exponential decline in the abundance of large spawners over this period. The analysis of commercial prawn grade and depletion data does not support the claim that overfishing is the cause of the recent collapse of the fishery.
Chapter 6 uses fishery-independent sampling surveys to show that the most productive ground, Venus Bay, is a key spawning area (highest egg production) and that changes in abundance (and mortality) in 1991/92 were linked to an El Nino event culminating in stock collapse in 1992/93. Chapters 7 and 8 integrate information from previous chapters and provide estimates of fishery-independent biomass density, annual recruitment trends and spawner abundance trends which: 1) demonstrate that the fishery was in a depressed state in 2006; 2) show that recruit abundance decreases with El Nino indices.
Keywords: Melicertus latisulcatus, stock collapse, GAB, environmental variation, El Nino, upwelling, recruitment, fishery collapse and Exceptional Circumstances Scheme.