Evaluating time-depth recorders as a tool to measure the behaviour of sharks captured on longlines

Quantifying the behavioural response of chondrichthyans to capture in longline fisheries can assist in understanding the physiological changes resulting from capture stress and ultimately aid in developing fishing practices that increase the survival of released bycatch species. Here, we evaluated the use of time-depth recorders (TDR) as a tool to quantify the amount of movement during capture across 42 animals from seven species of shark and one species of ray caught on hooks with TDRs attached in either a demersal or surface longline. Depth changes over time were analysed using three methods to estimate the percentage of time sharks and rays struggled on the line. Methods used were; 1) a Visual Assessment Method (VAM) of the TDR trace conducted by two investigators quantifying movement by summing the duration of movement bouts visually identified by erratic changes of depth; 2) the Gangion Extension Method (GEM) which quantifies movement by summing periods when captured animals altered their depth by > 50% of the gangion length; and 3) the Vertical Excursion Method (VEM) which quantifies movement by summing periods when the absolute depth change between successive data points exceeded a threshold determined from the maximum depth change in the TDR data prior to capture of the animal. We found that the VAM was consistent across investigators and produced significantly higher estimates of movement than GEM and VEM. Estimates of movement from GEM and VEM were not significantly different to each other, but unlike GEM, VEM could be applied to TDRs used in both surface and demersal longlines. The amount of movement observed was different between species and such differences were consistent across all methods, indicating that species-specific behavioural responses to capture can be identified. The ability to assess capture behaviour using VEM allows inter-species comparisons, which may be used as a metric for rapid, generalised assessment of species' responses to longline capture where physiological data may be limited or lacking. Such assessments are important in the design of species-specific management for bycaught animals.