I. Keay

Breathing life into fisheries stock assessments with citizen science

Citizen science offers a potentially cost-effective way for researchers to obtain large data sets over large spatial scales. However, it is not used widely to support biological data collection for fisheries stock assessments. Overfishing of demersal fishes along 1,000 km of the west Australian coast led to restrictive management to recover stocks. This diminished opportunities for scientists to cost-effectively monitor stock recovery via fishery-dependent sampling, particularly of the recreational fishing sector. As fishery-independent methods would be too expensive and logistically-challenging to implement, a citizen science program, Send us your skeletons (SUYS), was developed. SUYS asks recreational fishers to voluntarily donate fish skeletons of important species from their catch to allow biological data extraction by scientists to produce age structures and conduct stock assessment analyses. During SUYS, recreational fisher involvement, sample sizes and spatial and temporal coverage of samples have dramatically increased, while the collection cost per skeleton has declined substantially. SUYS is ensuring sampling objectives for stock assessments are achieved via fishery-dependent collection and reliable and timely scientific advice can be provided to managers. The program is also encouraging public ownership through involvement in the monitoring process, which can lead to greater acceptance of management decisions.

Broad-scale coastal movements of white sharks off Western Australia described by passive acoustic telemetry data

Movements of 89 acoustically tagged subadult and adult white sharks (Carcharodon carcharias) were monitored off the south and west coasts of Western Australia (WA) between December 2008 and May 2016 by a network of up to 343 passive acoustic receivers. In all, 290 inter-regional movements, totalling 185092km were recorded for 73 of these sharks. Estimated rates of movement in excess of 3kmh–1 (mean 1.7kmh–1; maximum 5.6kmh–1) were common, even over distances of thousands of kilometres. Detections indicated that white sharks may be present off most of the south and lower west coasts of WA throughout the year, although they are more likely to be encountered during spring and early summer and are least likely to be present during late summer and autumn. There was limited evidence of predictable return behaviour, seasonal movement patterns or coordination of the direction and timing of individual shark’s movements. Nevertheless, the data suggest that further analyses of movements in relation to ecological factors may be useful predictors of shark activity at local scales. It is hoped that these data may be useful for informing public safety initiatives aimed at mitigating the risks associated with human encounters with white sharks off the WA coast.

Accepting final counts from repeat readings of otoliths: should a common criterion apply to the age estimation of fish?

Multiple readings of otoliths are often carried out to assess the repeatability and reliability of increment counts for estimating fish age. Various criteria have been used to assign or discard age estimates from repeated counts when discrepancies occur although the reasons for doing so are usually not stated or justified. Trends in relative frequencies (percentage disagreement, PD) and magnitudes (inter-read discrepancy, IRD) of otolith-count discrepancies were explored for 15 species of fish collected from a range of locations around Australia to explore generality in the best explanatory model(s) for otolith-count discrepancies and, hence, the most appropriate criterion for accepting or rejecting age estimates from multiple-count data. Increasing discrepancies with increasing age, according to a constant per-increment probability of error, was the best-approximating model for 9 of the 15 species for PD data but for only two species for IRD data. Our results indicated disproportionately higher rates of rejection of estimates from older age groups if exact agreement between repeated counts was required for age acceptance. Results varied with the reader, region and the method of otolith reading, indicating that multiple criteria for accepting or rejecting counts from multiple readings may be required among or even within species.