The Murray-Darling Basin (MDB) Fish Survey is currently reliant on electrofishing and un-baited bait traps to sample fish populations. Water clarity can impact on the efficiency of these methods. Highly turbid waters are prevalent across much of the Codamine, Warrego and Paroo systems. Therefore, it was considered some key species or life stages could be missed by electrofishing and bait traps in these catchments. In response to this, a comparison of fish catches using the current MDB sampling techniques with fish catches in fine meshed fyke nets (a method perhaps less affected by turbidity) was carried out to evaluate the extent to which fyke nets could add value to survey techniques in highly turbid northern Basin rivers.
Reliance on electrofishing and bait traps alone in turbid waters provides a biased estimate of the fish assemblage and population structure. Addition of overnight set fine meshed fykes to MDB fish surveys in turbid sites may substantially improve estimates of relative abundance of native fish, native fish biomass and native fish recruitment.
During the 2015/16 MDB Fish Survey use of day set fykes and overnight set fykes was trialled at MDB Fish Survey sites in Queensland sections of the Condamine, Warrego and Paroo River Valleys. The fixed sites in these river valleys are characterised by highly turbid water. Fyke netting was run in conjunction with the standard MDB Fish Survey techniques of electrofishing and unbaited bait traps. At each site fyke nets were set in three back-t0-back pairs in water less than 1m deep.
Fyke net catches detecting additional species
The fine-meshed fykes used in this survey captured species of fish not captured by electrofishing, whereas bait traps did not capture any additional species to those captured by electrofishing. Additional species detected by fykes included Freshwater catfish, Silver perch, Australian smelt and Hyrtl’s tandan.
Fyke net catches providing evidence of recruitment
Overnight set fykes were also very effective at detecting juvenile Golden perch and large numbers of Carp gudgeons at sites where few or none were detected by electrofishing or bait traps. Juvenile Golden perch detected by overnight set fykes were too small to have been stocked fish, and therefore are evidence of natural recruitment. Most size classes of Golden perch detected by electrofishing would require genetic fingerprinting or calcein tagging of stocked individuals for reliable conclusions of natural recruitment. Overnight set fykes also detected significantly more Un-specked hardyhead than electrofishing. Overnight set fykes indicated that Hyrtl’s tandan was a common species at some sites, yet electrofishing and bait traps failed to detect any.
Day set fykes vs Overnight set fykes
Day time fyke nets were set prior to the onset of electrofishing and cleared at the end of all electrofishing activity; a set period of approximately three hours. Overnight fyke nets were set late in the afternoon and cleared the next morning.
While day set fykes detected additional species to electrofishing alone, this was at fewer sites than overnight set fykes. With the exception of Carp gudgeons, total captures of fish were usually higher in overnight set fykes than in day-set fykes, especially in the case of Hyrtl’s tandan and juvenile Golden perch.
The perfect pair: electrofishing + fyke nets
Electrofishing was better than overnight set fyke nets at capturing large Golden perch, Bony bream, Carp, Murray cod and Eastern gambusia. Therefore, overnight fyke netting and electrofishing complement each other well. Continued reliance on just electrofishing and bait traps in northern turbid rivers will produce misleading information on fish assemblages and fish population structure. Use of overnight fyke nets will increase sampling time and slightly increase operational costs,but will also significantly improve the detection of key fish species and life history stages that are essential for evaluating the success of environmental watering strategies.
To find out more, download the pdf of the full report here.
Sophie Van Dijk
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