Connecting post-release mortality to the physiological stress response of large coastal sharks in a commercial longline fishery

Abstract

Bycatch mortality is a major factor contributing to shark population declines. Post-release mortality (PRM) is particularly difficult to quantify, limiting the accuracy of stock assessments. We paired blood-stress physiology with animal-borne accelerometers to quantify PRM rates of sharks caught in a commercial bottom longline fishery. Blood was sampled from the same individuals that were tagged, providing direct correlation between stress physiology and animal fate for sandbar (Carcharhinus plumbeus, N = 130), blacktip (C. limbatus, N = 105), tiger (Galeocerdo cuvier, N = 52), spinner (C. brevipinna, N = 14), and bull sharks (C. leucas, N = 14). PRM rates ranged from 2% and 3% PRM in tiger and sandbar sharks to 42% and 71% PRM in blacktip and spinner sharks, respectively. Decision trees based on blood values predicted mortality with >67% accuracy in blacktip and spinner sharks, and >99% accuracy in sandbar sharks. Ninety percent of PRM occurred within 5 h after release and 59% within 2 h. Blood physiology indicated that PRM was primarily associated with acidosis and increases in plasma potassium levels. Total fishing mortality reached 62% for blacktip and 89% for spinner sharks, which may be under-estimates given that some soak times were shortened to focus on PRM. Our findings suggest that no-take regulations may be beneficial for sandbar, tiger, and bull sharks, but less effective for more susceptible species such as blacktip and spinner sharks.

Stress related blood values in Scyliorhinus canicula as live-indicators of physiological status after bottom trawling capture activity

“…Previous work by Cliff and Thurman (Cliff and Thurman, 1984) investigating the physiology consequences of stress in in Carcharhinus obscurus during hand net captures, reported that potassium plasma values above 7 mmol/ l could generate myocardial disruption. Although, the potential levels of K + that may impact cardiac function in sharks remain unknown (Schwieterman et al, 2021) the dogfish captured in deeper waters in our study had elevated levels of K + an may have experienced a larger osmotic disruption than those captured in shallower waters as reported by (Skomal and Mandelman, 2012) (Whitney et al, 2021).…”

Diving into the vertical dimension of elasmobranch movement ecology

“…When considering management and conservation, data on vertical distributions can inform us of a species’ vulnerability to anthropogenic threats, which is particularly relevant for elasmobranchs given that they can be subject to both targeted fisheries and bycatch worldwide ( 8 , 9 ). For example, capture probabilities and levels of mortality are influenced by depth overlap between elasmobranchs and fishing gears ( 10 ), as well as gear selectivity ( 11 ) and species physiology ( 12 ). Data on vertical distributions can consequently inform encounter rates and catchability with various fishing gears and inform potential mitigation measures to reduce bycatch of vulnerable species while maintaining capture of fishery targets ( 10 ).…”

Are we any closer to understanding why fish can die after severe exercise?

“…The mechanism of increased circulating potassium has not been clearly defined, though some researchers identify concurrent increased circulating potassium and intracellular acidosis in sharks that perish after being released (Whitney et al, 2021). Additionally, it is also suspected that hyperkalemia plays a role in pathological cardiac function during stress in elasmobranchs, suggesting a potential connection between potassium concentrations and dysfunctional myocardial function .…”

“…Several studies have observed marked increases in plasma potassium concentrations following the induction of stress (Brooks et al, 2012; Mandelman & Farrington, 2007; Manire et al, 2001; Moyes et al, 2006), indicating a potential loss of potassium ions from intracellular compartments. The mechanism of increased circulating potassium has not been clearly defined, though some researchers identify concurrent increased circulating potassium and intracellular acidosis in sharks that perish after being released (Whitney et al, 2021). Additionally, it is also suspected that hyperkalemia plays a role in pathological cardiac function during stress in elasmobranchs, suggesting a potential connection between potassium concentrations and dysfunctional myocardial function (Skomal & Mandelman, 2012).…”