Gladstone Fish Health Investigation 2011 - 2012 - Western Basin ...

from advice from a panel of experts. Next, the likelihood that they were affecting (positively ornegatively) the health of fish and crabs was considered. Development of the modelconsidered the impact on fish health of changes to the ecosystem, such as changes to thefood web and competition among predators, as well as the introduction of toxicants thatcould directly affect fish health.The model allows for different hypotheses (e.g. competition for food versus effects ofturbidity) to be visualised and their likelihood to be compared. It can also be used to identifywhich steps in the pathway can be monitored to determine whether or not the particularpathway is actually affecting fish health. For example, to test the ’competition for food’pathway, the abundance of prey fish should be assessed and compared to other systems.The model was never intended to be a quantitative tool with the sophistication to predict thehealth of fish given various levels of driving factors. It can, however, be utilised to explain thethinking and justification for the likely pathways affecting fish health and help improve theunderstanding of the ecological processes that potentially result in a reduction in the healthof fish in Gladstone waterways.The conceptual model indicated that water quality, including turbidity, was unlikely to be theprimary contributing cause for the fish health issues observed in Gladstone Harbour andadjacent waterways.Study areaThe study area during Phase 2 was the same as the study area described for Phase 1, withsampling sites illustrated in Figure 2.Candidate speciesThe monitoring program focused on seven species of finfish, one species of shark, onespecies of prawn, and one species of crab (species listed below). These species represent arange of different life cycles (e.g. catadromous and estuarine) and trophic levels (e.g.predatory, omnivorous detritivores and scavengers). They are species that were encounteredduring Phase 1 sampling in the principal study area and reference areas, and had displayeda variety of abnormalities.Barramundi (Lates calcarifer): Barramundi is a predatory species and a principle target inthe region for recreational line and commercial net fishers. It is catadromous (live in freshand salt water), but must migrate to salt waters to spawn. Barramundi fingerlings are stockedinto freshwater impoundments throughout the Port Curtis and Fitzroy River catchments,including Awoonga Dam.Mullet (Mugil sp.): Mullet is an omnivorous detritivore with a catadromous lifecycle. Thisspecies is caught mainly by commercial net fishers, although smaller numbers are alsocaught by recreational fishers (mainly for bait) using cast nets. Sea mullet have been stockedinto Lake Awoonga.Banana prawn (Fenneropenaeus merguiensis): Banana prawns are omnivorousdetritivores and a principle target for the commercial trawl fishery in the region, as well as forrecreational fishers using cast nets. Banana prawns use the numerous intertidal mangrovelinedcreeks as nursery habitats, then move into waters including Gladstone Harbour as theygrow.17