Neil Loneragan

The Northern Prawn Fishery (NPF) is Australia’s most valuable Commonwealth fishery, with an average annual catch of about 8,000 tonnes, worth between AUS$100 and $175 million and now taken by 104 modern trawlers. The fishery survived the early history of overcapitalization/overfishing common to most prawn trawl fisheries during the 1970s and early 1980s, when up to 302 trawlers were operating. Since the mid 1980s, fishing effort has been greatly reduced through industry-funded buybacks, spatial closures to protect small prawns and their nursery habitats and severe reductions in the fishing season from the entire year to just over 4½ months. Fishers, managers, researchers and environmentalists now share the responsibility for managing the NPF through their positions on the Northern Prawn Management Advisory Committee (NORMAC). A common vision has evolved of pursuing ecologically sustainable development through ecosystem-based management. The fishery has been highly innovative in addressing bycatch issues and also has established a large system of “fishery closure areas” to protect juvenile prawn stocks, comprising about 8.7% of the NPF-managed zone. The NPF is working with government agencies and other stakeholders to develop a system of “no-take” marine protected areas in northern Australian waters that will both ensure biodiversity conservation and protect nursery and other habitats important to the sustainability of the prawn fishery. The research program to support ecologically sustainable development in the NPF includes research on assessing the status of the target stocks, bycatch and the impacts of trawling on animals in the soft sediments. The potential benefits to the fishery from marine protected areas are summarized.

Effective management of estuarine and coastal fisheries resources requires detailed information on the relationships between the habitats being protected and the fisheries dependent on them. Past research of nekton has focused on comparisons of abundance and species composition between single habitats (e.g. mangroves versus seagrass or vegetated versus unvegetated habitats). These studies have provided valuable insights into the role of coastal habitats for sustaining fisheries and biodiversity but have not considered the importance of adjacent habitats to the overall value of an area. For example, fish are only able to occupy mangrove forests for a restricted amount of any high tide period. The nature of the habitats lower down the shore may be crucial to the overall value of any patch of mangrove for supporting fisheries. We are taking a new approach to assessing the value of estuarine habitats for fisheries and biodiversity that considers the spatial arrangement of different habitats within an area - or the “mosaic” of habitats within the area. The scale of the area for study is defined by the life history and biology of the species of interest. In addition to estimating the abundance, biomass and community structure of nekton (e.g. fish, crustaceans, molluscs), the functioning of mosaics will be studied by estimating growth rates and describing the food webs in different mosaics and the characteristics of the mosaics will be measured. This approach has the potential to be extended to allow much better criteria to be developed for the selection of marine reserves by managers.

Recently fisheries management in Australia has shifted to emphasise management of resources within the principles of ecologically sustainable development. This has resulted in management to sustain fish stocks, maximise economic efficiency when harvesting those stocks, and a trend towards granting property rights to the fishers. To achieve the goal of management to sustain fish stocks, a major focus of fisheries agencies has been to preserve the critical habitats upon which the long-term productivity of the fisheries depends. For penaeid prawns this has meant that seagrass (tiger prawns), and mangroves (banana prawns) have achieved special status to fishers, fisheries biologists, managers and legislators. Is this justified? Is this the appropriate management strategy to preserve critical fisheries habitat? We examine these questions using two case studies: cyclones, seagrasses and tiger prawns in the Gulf of Carpentaria and king prawns in the Peel-Harvey estuarine system in Western Australia. It is clear that a greater understanding of the key processes operating in the coastal zone is a critical requirement for fisheries management. It is not enough to just map, monitor and maintain subsets of these systems based on coarse distribution and abundance studies of prawn populations. With increasing pressure on the coastal zone from competing interest groups, fisheries managers need a greater understanding of the factors which determine the carrying capacity of nursery habitats for juvenile penaeid prawns, and the factors which limit the distribution of key fisheries habitats within coastal ecosystems. Fisheries scientists and managers need to develop the knowledge base and management procedures for the implementation of ecosystem management.

Some of the recent literature is reviewed and results of detailed studies of fish and crustacean populations in temperate estuaries of south-western Australia and New South Wales are synthesised and the approaches to these studies discussed. Studies on the west coast of Australia have concentrated on defining seasonal, annual and spatial patterns of change in the fish fauna of the Swan and Peel-Harvey estuaries. The emphasis has been on obtaining detailed knowledge of the life history strategies of fish in estuaries and interpreting the main factors affecting the fish populations and community structure in light of this information. On the east coast, more effort has been directed towards evaluating the importance of various habitats to fish in estuaries, particularly seagrass habitats in several different estuarine and inshore coastal systems. Conventional sampling techniques (i.e. beach seines, gill nets, otter and beam trawls) have been used to study fish populations in estuaries of both regions. In addition, in Western Australia, commercial catch data in the Peel-Harvey and Swan estuaries have been used to assess how fish populations have responded to the marked eutrophication in the former system. Artificial seagrass has also been used in NSW to test hypotheses about the importance of seagrass to larval and juvenile fish.

A checklist is provided for the fish that have been caught in the Swan estuary. These species each fall into one of the following categories: 1. marine stragglers, 2. Marine species which use estuaries extensively at some stage of their life cycle but spawn at sea, 3. estuarine species, i.e. those species which can pass through the whole of their life cycle in the estuary, 4. anadromous species, i.e. those species which migrate from the sea through the estuary to breeding grounds in reduced salinities or fresh water. Examples of different life cycles are provided by reference to mullets (Mugil cephatus and Aldrichetta forsteri), gobbleguts (Apogon ruepellii), five species of hardyhead (Atherinidae), Perth herring (Nematalosa vlaminghi) and cobbler (Cnidoglanis macrocephalus). Details are provided (or the commercial fish catches in the Swan estuary and for the recreational fishery for teleosts and prawns.

The initial work on the fish fauna and the blue manna crab in the Peel-Harvey estuarine system, which commenced in April 1979, was aimed at elucidating the patterns of distribution, abundance and growth of the different species. At the same time, the data on the commercial fish catches for the last 40 years were subjected to detailed analysis to ascertain whether any changes in catch during the 1970s could be related to the increased growth of macroalgae. More recent work has focussed on determining whether Nodularia has an effect on the fish fauna either through inducing changes in behaviour or by causing mortality.