Fisheries biology and habitat ecology of the southern sea garfish (Hyporhamphus melanochir) in southern Australia
In 1992, following a detailed review of the South Australian Marine scalefish fishery, South Australian garfish stocks were assessed as fully-exploited and a range of measures suggested to prevent any future increase in overall catch and to better utilise the available resource (SA Dept of Fisheries White Paper, 1992). Since that time no dedicated garfish research has been undertaken nor have any of the management options been acted upon, apart from the introduction of a recreational bag-limit and some areas closed to netting. There has been an increase in the targeting of garfish by the dab-net sector and the recent development of an experimental pair-trawl fishery in Upper Spencer Gulf has the potential to further exert pressure on the resource. Increasing interest in the species is not restricted to South Australia, with similar moves afoot in Western Australia and Tasmania.
A management plan is currently being developed for the South Australian marine scalefish fishery which will require the development of biological reference points to facilitate sustainable resource management into the next century. The management plan requires information on the genetic stock structure of garfish to enable the appropriate spatial management unit to be decided. The paucity of information available for garfish on stock structure and other fisheries biology parameters will impede this process. As a result, southern sea garfish has been identified as a research priority by the South Australian Marine Scalefish Fishery Management Committee (SMC).
There are also developments towards management of marine resources at the system level and there will be a clear need to identify areas and/or habitats of particular importance for fishery production, and to be able to assess the impact of environmental loss and degradation on species productivity, including those critical to garfish. Anecdotal information exists suggesting the importance of particular spawning habitats to garfish. A closely related species attaches its eggs to seagrass blades and the eggs of southern sea garfish are known to be adhesive (SA Dept of Fisheries Green Paper 1990). However, the degree of selectivity or reliance on seagrass or other benthic structure is unknown. Substantial seagrass loss has occured over recent decades particularly in SA, Tasmania and Victoria. The impact of such habitat degradation on species closely associated with such habitats, such as garfish, remains to be assessed. The loss of seagrass, and possible effects on garfish spawning success, early life-history and adult productivity is of concern in all states where such losses/reduction in habitat quality have been identified.
A yield per recruit model developed in SA to assess the effects of different fishing strategies, and based upon growth and mortality parameters from earlier studies, assumed constant recruitment, the validity of which is unknown. The model has been used to establish current size limits in the SA fishery. Spatial differences in age and growth require investigation allowing the development of a population model, building on the earlier research. More detailed catch sampling from all sectors would obtain data for such a model and allow comparison of growth and age structure between areas and with existing data obtained 10-15 years ago.
As a result of increasing development of the fishery in terms of the introduction of new techniques and the opening up of new areas to harvesting, information regarding the seasonal movements of adult garfish from their inshore summer habitats to deeper waters in the South Australian gulfs during the cooler months would allow the extent of potential inter-sectorial conflict to be established. In Gulf St Vincent, fishing effort in the winter “deep water” fishery does appear to have adversely affected catch rates in the summer shallow water fishery (Green Paper 1990) and this report highlighted the need to determine the seasonal, inshore-offshore movements of these fish. Such movements may be a feature in the populations of the other states. Better understanding may help in determining gear-sector interactions and allow some prediction of the effects of one sector (in one season) on others in subsequent seasons. Information quantifying the impacts of the different gear sectors and the level of interactions (between gear-types and seasons) in South Australia have been specifically requested by the SA Marine Scalefish Management Committee. There appears to be general agreement on the need to make better use of the available resource by catching larger fish (White Paper, 1992) and therefore the potential for more specific targeting of larger adult garfish (preferred by the fresh fish buyers) with methods such as the experimental pair-trawl being currently developed in upper Spencer Gulf, SA, needs to be assessed.
