Many fish form spawning aggregations. Targeted fishing of these increases short-term catch rates but can lead to serious declines in aggregation biomass, reproductive output and stock sustainability. Traditional monitoring and stock assessment methods can also be compromised by aggregation fishing (Die and Ellis 1999; McAllister and Kirchner 2000). Globally, the deleterious impacts of aggregation fishing are becoming more obvious and the issue is receiving increasing attention. This is highlighted by recent temporal/spatial closures to protect spawning shallow water aggregations of snapper in Western Australia. However, there is little knowledge about the prevalence of aggregating species or their aggregations in WA waters, or of the affects that aggregation fishing has on stock sustainability. Management of aggregation fishing is thus a significant consideration for the Ecological Sustainable Development initiative and the ‘wetline’ review process that is currently underway within the West Coast Bioregion. There is a pressing need to obtain biological and ecological information about aggregations and to factor their exploitation into management plans.
This project has been developed with, and has the strong support of, recreational and charter fishers and their peak bodies, who have expressed considerable concerned over growing fishing pressure on key recreational species. Commercial interests will similarly benefit from long-term conservation of these species and the project has received support from the WA Fishing Industry Council. The project relates to the following Department of Fisheries Strategic Plan Objectives: 1) Improved sustainability of fish stocks and production. 2) Increased economic benefits to the community from fish (eg charter fishers will benefit from a sustainable sportsfishery and the positive image resulting from research and management). 3) Increased industry and community involvement in the development and implementation of management strategies (eg the inception of this project has and will require considerable liaison with fishers, universities and CALM scientists. Completion of the project will require open and ongoing communication between groups). This project will provide advice to the Integrated Fisheries Management Advisory Committee that will be established to deal with issues associated with the proposed Integrated Fisheries Management Plan (A. Cribb, pers. comm.).
Die DJ, Ellis N (1999) Aggregation dynamics in penaeid fisheries: banana prawns (Penaeus merguiensis) in the Australian Northern Prawn Fishery. Mar.Freshwater Res. 50:667-675
McAllister MK, Kirchner CH (2000) Development of bayesian stock assessment methods for namibian orange roughy Hoplostethus atlanticus. S.Afr.Mar.Sci. 23:241-264
Many strategies have evolved among fishes to maximise spawning success. One of the most striking of these is aggregation spawning, in which individuals group together, often at predictable times and locations in order to reproduce (cf. a school, which refers to a group of non-spawning fish). Aggregation spawning may enhance the capacity of individuals to select mates, synchronise spawning and optimise survival of offspring. It can also make these fish more vulnerable to capture by fishers. Nevertheless, despite the fact that numerous species of fish aggregate to spawn, it is only in recent years that the ecological and fisheries importance of spawning aggregations has become widely recognised by management agencies.
In Western Australian (WA) waters various species are known to form spawning aggregations and hence may be particularly vulnerable to overfishing. This is the case within the West Coast Bioregion (WCB), where fishing pressure is high and stocks of key aggregating species such as pink snapper and dhufish are depleted. As such, an objective of the current project was to describe the spawning strategies of these and other aggregating species of fish within the WCB, using a range of data sources including formal interviews with fishers and acoustic and video surveys. A preliminary list of 22 aggregating species of fisheries importance was compiled. The review of dhufish (Glaucosoma hebraicum) ecology showed that this species has a complex reproductive strategy in which size-based dominance hierarchies determine spawning success amongst individuals. In post-war years aggregations of this species containing hundreds of individuals were observed fairly regularly by fishers. These days dhufish more typically form small aggregations numbering from a few to tens of individuals. Dhufish are typically quite sedentary and individuals may not move far in order to aggregate. Nevertheless those found in southern parts of the WCB may migrate tens of miles to spawn, with the area west of Cape Naturaliste considered to be particularly favoured for spawning by this species. Dhufish are also common within the mid-west zone of the bioregion although individuals are generally smaller in size and do not appear to move as far as those living further south, possibly because of spatial differences in habitat. Whilst this review provided important information about dhufish ecology it also highlighted the poor understanding of this species.
Keywords: Spawning aggregation, West Coast Bioregion, Western Australia, acoustics, underwater video, dhufish, pink snapper, Samson fish, mulloway, fisheries management, recreational fishing.