Using acoustic telemetry to inform standardisation of catch rates from passive fishing gear: A case study using dusky flathead (Platycephalus fuscus)

Estuaries are dynamic ecosystems and the species which inhabit them often support important commercial, recreational and indigenous fisheries. Catch rate standardisations are an essential component of many contemporary stock assessments, yet these standardisation processes often do not incorporate environmental variability and its influence on catchability. Here, we use acoustic tagging to quantify the relationship between environmental variation and small-scale, within-estuary, movement rates of dusky flathead ( Platycephalus fuscus ), a key fishery species in New South Wales (NSW), Australia. These relationships were then included as environmentally-mediated proxies for catchability in the standardisation of commercial mesh (gill) net catch rates. The case study showed that the probability of movement increased with freshwater inflow and varied across the lunar cycle. The effect of freshwater inflow was reflected in the commercial catch data of five estuaries, with greater catches of dusky flathead correlated with higher freshwater inflow, including one of the estuaries in which fish were acoustically tagged. Incorporating environmental effects (such as freshwater inflow) into catch standardisations of the five estuaries did not alter the overall trajectory of the time series but there were differences of up to 50.9 % (mean 12.3 %, SD 10.2 %) in the standardised index values in some years. Additionally, the study also found that large-scale weather drivers correlate with small biases in abundance indices when environmental conditions were excluded from standardisation processes. These results highlight the value of biological information and ecological experiments (including acoustic tracking studies) for validating assumptions in the broader stock assessment process such as how environmental variables may be influencing catchability or availability/density. • Environmental variation influences catchability or availability in passive gear fisheries. • Telemetry data showed the probability of movement increased with freshwater flow. • Catchability of estuarine fish likely increased with freshwater flow. • Catch indices can vary up to 50.9 % if environmental variables are included. • Results support the use of environmental variables in catch rate standardisations.