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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.

Estimated declines in shark and ray populations worldwide have raised major, widespread concern about the impacts of global fisheries on elasmobranchs. The mechanisms causing elasmobranch mortality during fisheries' capture are not fully understood, but we must gain greater clarity on this topic for fisheries managers to develop effective conservation plans to mitigate further population declines. To evaluate how two important factors, respiratory mode and fishing gear type, impact elasmobranch survival, we compiled publicly available data sources on the immediate mortality percentages of 83 species and post-release mortality percentages of 40 species. Using Bayesian models, we found that sharks and rays captured in longlines had significantly lower immediate mortality than those caught in trawls or gillnets. Our models also predicted the mean total discard mortality (combined immediate and post-release mortality) percentages of obligate ram-ventilating elasmobranchs caught in longline, gillnet and trawl gear types to be 49.8, 79.0 and 84.2%, respectively. In contrast, total discard mortality percentages of stationary-respiring species were significantly lower (longline capture mean = 7.2%, gillnet capture mean = 25.3%, trawl capture mean = 41.9%). Our global meta-analysis provides the first quantified demonstration of how mortality is affected by these two factors across a broad range of species. Our results and approach can be applied to data-deficient elasmobranchs and fisheries to identify species that are likely to experience high rates of mortality due to respiratory mode and/or fishing methods used, so that appropriate mitigation measures can be prioritized and investigated.

Incidental fisheries capture has been identified as having a major effect on shark populations throughout the world. However, factors that contribute to the mortality of shark bycatch during fisheries capture are not fully understood. Here, we investigated the effects of capture duration, sea surface temperature, and shark total length (snout to the tip of the upper caudal lobe) on the physiology and condition of longline-caught bronze whalers, Carcharhinus brachyurus. Plasma lactate and potassium concentration had a positive linear relationship with capture duration, indicating that this species experiences increasing physiological challenges while on fishing gear. Additionally, we used stereotype logistic regression models to determine variables that could predict the capture condition of sharks (categorized as “healthy”, “sluggish”, or “moribund or dead”). In these models, elevated plasma lactate concentration, plasma potassium concentration, and capture duration increased the likelihood of C. brachyurus being captured in a “sluggish” condition or in a “moribund or dead” condition. After plasma lactate concentration exceeded 27.4 mmol/L, plasma potassium concentration exceeded 8.3 mmol/L, or capture durations exceeded 293 minutes, the majority of captured sharks (>50%) were predicted to be “moribund or dead.” We recommend that a reduction in the amount of time longlines are left fishing (soak time) will reduce immediate and post-release mortality in C. brachyurus bycatch and that our methods could be applied to identify causes of fisheries-induced mortality in future studies. The identification of operational, environmental, and biological variables contributing to poor condition will be necessary to implement conservation strategies that reduce mortality during capture.

To investigate factors contributing to immediate mortality in gill-net-caught elasmobranchs, we caught and blood sampled a total of 64 sharks and rays. Blacktip reef sharks (Carcharhinus melanopterus) were the most commonly caught species and had significantly elevated plasma lactate (mean 23.3 mM) and potassium (mean 6.2 mM) concentrations following capture, suggesting physiological disturbance. The overall immediate mortality rate of C. melanopterus was 38% and mortality was significantly influenced by body size, but not location of entanglement or sex. Of the other species caught, nervous sharks (Carcharhinus cautus) were of particular concern, because this species is often caught as bycatch by gill-net fisheries throughout its range and experienced high immediate mortality rates when captured (66%; n = 12) in the present study. The results suggest that juvenile C. melanopterus are particularly susceptible to gill-net-related mortality and that fisheries management strategies for C. melanopterus bycatch should focus on reducing the likelihood of juvenile sharks encountering this gear. Given that this species is not currently considered to be threatened with extinction, population monitoring should be performed such that the suggested regulations can be implemented if populations decline to levels threatening the conservation status of the species.