An underwater drone has highlighted that catch and release fishing may have significant damaging effects on sharks, increasing their risk of predation.
The study, led by University of Newcastle marine ecologist Dr Vincent Raoult, also showed that a shark’s ability to swim immediately after being released does not relate to good recovery.
“The footage captured in our study showed that sharks captured and exposed to a boat deck for 15 minutes swam for a maximum of 30 seconds after release, before becoming largely immobile for five minutes,” Dr Raoult said.
“A possible explanation for the lengthy period of immobility is that deck-exposed sharks use the remaining oxygen contained in their blood to try to escape the stress event, leaving their tissues starved of oxygen or ‘hypoxic’,” Dr Raoult said.
“The short burst of rapid swimming immediately after release could be an attempt to reverse these hypoxic conditions, an escape reaction to high stress environments or the result of thermal shock when the shark re-enters the water.
Dr Raoult said the footage from the underwater drone, also known as a Remote Operated Vehicle (ROV), provided a new window into the behaviour of a shark after release.
“Anglers needed to consider the exposure of sharks. Measuring and weighing sharks or posing for social media photos prolongs their exposure and is detrimental to the animal and potentially fatal in the long run,” Dr Raoult said.
“People should enjoy their sport fishing but should try to limit how long they handle sharks to minimise injury to the animal. Best practice is to release them immediately.”
Dr Raoult said bycatch of sharks formed a substantial component of both commercial and recreational fishing.
“Improved bycatch handling practices would reduce negative effects to sharks and help stop the decline in their numbers.”
Just ‘beat’ it
Shark ‘tail beats’ helped researchers undertake the study – like a human heart beat, a shark’s tail beat is easily measured and provides useful health insights. A shark’s tail beat frequency is also linked to their metabolic rates – the rates at which their bodies use oxygen and chemicals to undertake processes necessary for survival.
The study involved the capture of two species of shark – the draughtboard shark (Cephaloscyllium laticeps) and piked spurdog shark (Squalus megalops).
All sharks spent 24 hours in a holding pool flushed with fresh seawater to return them to a relaxed, pre-catch state. Sharks then experienced one of two different scenarios – brief handling (to catch them from within the holding pool) and immediate release into the ocean, or 15 minutes of boat deck exposure (to mimic the scenario of catch and release) before being released into the ocean and tracked by an ROV.
The research team assessed the behaviour of all sharks by measuring tail beats at 15-second intervals for a period of five minutes immediately after release.
“Preliminary analysis of the ROV video footage from the deck-exposed sharks showed they had low tail beat frequencies, were in distress and usually had rapid, strained breathing,” Dr Raoult said.
The study showed that sharks that were handled but immediately released displayed rapid recovery.
“This indicates that even sharks previously exposed to high capture stress, such as trawling, can rapidly recover if exposure times are limited,” Dr Raoult said.
The team
The research team comprises Dr Vincent Raoult, Dr Troy Gaston and Dr Tim Smith of the University of Newcastle’s School of Environmental and Life Sciences and Associate Professor Jane Williamson of Macquarie University’s Department of Biological Sciences.
The full findings are published in Journal of Experimental Marine Biology and Ecology.
