Shark behaviors, particularly those post-capture and release, have long intrigued researchers. One such phenomenon that has puzzled academics is stomach retraction in sharks, particularly in response to capture stress and release efforts. For years, scientists have observed stomach eversion in elasmobranchs and pelagic fishes, especially when they are captured in hook and line fisheries. This fascinating behavior, where the stomach is inverted, is believed to serve as a mechanism for expelling noxious or indigestible items from the gastrointestinal tract. This behavior is often triggered by the stress of capture, especially in hook and line fisheries, where the hook itself may act as a stimulus for eversion. “Circle hooks, which are often used in catch and release fishing, have been reported to reduce the chances of gut hooking when compared to J hooks, and therefore may reduce the likelihood of eversion,” explains Dr. Samantha Andrzejaczek, a Postdoctoral Research Fellow at the Hopkins Marine Station of Stanford University.
But the repercussions of capture and release on shark physiology have remained relatively elusive. Concerns about mortality following release, especially in the face of declining shark populations, have underscored the urgency of understanding the recovery process of released sharks after being worked on by scientists. This is especially true for individuals with everted stomachs, where stomach retraction and its associated energetic costs play a vital role in post-capture recovery.
“Stomach eversion is relatively common in fish that are caught and brought up from depth in a phenomenon known as ‘barotrauma’. In this case, the quick release of pressure causes the gasses within a fishes body to expand, often causing the stomach to be pushed out through the mouth – a different mechanism to that observed in sharks (which don’t have a swim bladder). Similar to sharks, marlin caught in sports fisheries have often been observed to evert their stomachs upon capture, though their ability to successfully retract the stomach upon release is unknown,” says Andrzejaczek. “Across different shark species, this behaviour has not been widely studied. It has been noted in sharks that are susceptible to capture by fishing, but observations in a natural setting are very rare. Ultimately, the prevalence of this pattern suggests its utility as a natural strategy for ridding the stomach of harmful substances. Nevertheless, the capacity to retract after a prolonged period of eversion, particularly in a fisheries context, remains largely unexplored across most of these species.”
In a groundbreaking study published in a recent issue of Fisheries Research, scientists delved into the intricate process of stomach retraction in sharks, particularly focusing on a tiger shark captured and released off the coast of Western Australia. Led by Andrzejaczek, a team used state-of-the-art animal-borne video and tri-axial accelerometers to document the retraction of the stomach and the behavior of the shark post-release. Their findings revealed a remarkable sequence of events. Upon capture, the tiger shark’s stomach was everted and even after release, the stomach remained as such. However, within minutes of release, the shark exhibited vigorous movements, including bursts of tailbeats, leading to the retraction of the stomach back to its normal position.
“While stomach eversion had been documented in line-caught tiger sharks previously, the subsequent retraction of the stomach upon release had not been observed, thereby restricting our understanding of how these sharks might recover or even endure such an event,” says Andrzejaczek. “Our video and accelerometer tags showed that not only could a tiger shark retract its stomach shortly after release, but it also resumed normal swimming behavior, implying that stomach eversion and retraction did not create any further adverse effects beyond those of capture and restraint.”
Despite the energetic costs associated with stomach retraction, the tiger shark exhibited a swift return to normal swimming behavior. Nevertheless, it remains uncertain to what degree other shark species, particularly those more prone to stress and mortality during capture, can successfully retract an everted stomach: “Based on previous observations of stomach retraction in a Caribbean reef shark and a shortfin mako shark, it seems probable that the capacity for stomach retraction is widespread among sharks in their natural habitats. However, in capture scenarios where sharks endure stress and incur high energetic costs, the ability to retract the stomach upon release may vary among species.”
The study also underscored the potential risks associated with prolonged stomach eversion, including increased vulnerability to predation and competition. While Andrzejaczek is not currently further investigating these behaviors, she believes the increasing utilization of biologging technologies, like camera tags, among various shark species increases the likelihood of documenting these events in the future. “Overall, it’s an exciting time in shark science where novel behaviors such as these are being remotely recorded for the first time.”