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

Mud crabsFisheries Queensland did not observe any increase in the level of shell lesions in mud crabsin Gladstone Harbour compared with the reports in 1998 to 2001 (Andersen and Norton2001). Fisheries Queensland observed 2253 mud crabs in Gladstone in 2011/12 (Phase 1and 2 combined).SummaryIt is obvious from all the reports that something happened to aquatic life in GladstoneHarbour in 2011. The findings provided in this study indicate that the fish health in GladstoneHarbour had returned to a more normal situation in 2012. Other than the physical damage orrecovery from physical damage, this study found the fish in a health status similar to thatobserved in the Bundaberg and Fitzroy reference sites.All industrial activities in Gladstone that were operating in Gladstone in 2011 have continuedto operate, including dredging. The only factor different in 2011 compared to previous yearswas the significant rainfall, the consequent flooding and the introduction of a significantbiomass of fish from Lake Awoonga. Flooding has subsequently occurred in 2012 and 2013.However, other than continuing reports of barramundi showing physical damage as a resultof being washed over the Awoonga spillway in these flooding events, widespread reports ofsick fish have not been received by DAFF.The introduction of the large number of barramundi into the Gladstone ecosystem wouldhave caused the system to become imbalanced. There are a number of studies that showthe impact of introduced and invasive species on ecosystem, which in this situationbarramundi could be classified. Eby et al. (2006) found that stocking introduced fish resultedin predation, competition for food, and declines in animal abundance at different trophiclevels, which in turn drastically altered these ecosystems.The change in biomass of stocked fish is a gradual process commencing after the firstintroduction of hatchery reared fingerlings into new ecosystems. The ecosystem and speciescomposition change slowly over time. This was not the situation in the Boyne River in 2011where the biomass of barramundi was increased by 300 t in less than three months, havingstarted from a very low base. The changes identified by Eby et al. (2006) were compressedinto a short time frame, which did not allow the ecosystem to gradually change to a newequilibrium.The stocking of Lake Awoonga with 4.5 million fingerlings since 1996 meant that prior to the2010/11 flood, the Boyne River (including Lake Awoonga) was populated with more fish thanthe Boyne River would have naturally contained if Awoonga Dam was not present.Consequently, when these stocked fish moved downstream of Awoonga Dam, the biomassof barramundi was far in excess of the carrying capacity of the Boyne River. The populationof barramundi in Gladstone Harbour also increased to higher levels than normal, as indicatedby seven barramundi being caught while trawling for prawns. This population increasecaused stress to the whole ecosystem.The results in this study support that it was flooding combined with the introduction of a largenumber of barramundi that stressed the ecosystem in Gladstone. This study cannot rule outthe possibility that the activity of dredging and associated turbidity provided additional stressto the ecosystem, but it was not the primary stressor.76