For decades, stomach content analysis has been the go-to method for studying the feeding habits of sharks and rays. This technique, which involves dissecting the stomachs of euthanized animals, provides a snapshot of their recent meals. While highly effective in identifying specific prey species, SCA has historically required large sample sizes, sometimes exceeding a thousand individuals in a single study. But many sharks and rays face a slew of conservation challenges, and researchers are now reconsidering the ethical and ecological implications of this lethal sampling method. The question is no longer just about collecting data but about how we can do so without further threatening already vulnerable populations.
Lethal sampling has long been justified by the valuable biological insights it provides, from diet composition to growth and reproductive parameters. Understanding what sharks eat remains crucial for managing fisheries, assessing ecosystem health, and identifying critical feeding habitats. Ethical considerations aside, the removal of even a few individuals from an at-risk population can have ripple effects, potentially disrupting local ecosystems. And so, the idea of “killing for conservation” has become increasingly difficult to defend.
Sharks and rays have diverse and opportunistic feeding habits, meaning their diets can vary widely based on location, season, and prey availability. For species like the broadnose sevengill shark (Notorynchus cepedianus) or tiger shark (Galeocerdo cuvier), which consume a broad range of prey, a single stomach sample may not be representative of their typical diet. Thus, when fisheries already harvest large numbers of sharks, collecting stomachs from individuals that are already being killed can still be a viable research option. But for species not commonly caught for consumption… well, alternative approaches are necessary.
One such alternative is stable isotope analysis, which has been used in marine ecology since the late 1980s. Unlike stomach content analysis, which identifies prey at the species level, SIA provides a broader view of an animal’s diet over weeks or months by analyzing chemical signatures in tissues like muscle or blood. These signatures can reveal where a shark has been feeding and its position in the food chain. However, SIA comes with its own limitations. It can be difficult to pinpoint specific species within a reading, especially if their isotopic compositions overlap. Not to mention that without prior knowledge of what a shark eats, interpreting stable isotope data can be challenging. For this reason, SIA is most effective when paired with SCA, allowing researchers to combine the precision of direct stomach content analysis with the long-term insights of biochemical markers.
Gastric lavage is another promising non-lethal method that has gained traction in recent years. This technique involves flushing a shark’s stomach with water to retrieve its contents without harming the animal. First introduced for sharks in the 1980s, gastric lavage has been successfully used on various species, demonstrating high efficiency in recovering stomach contents. It is relatively simple to perform, requiring minimal equipment and training; the process involves inserting a soft tube into the shark’s stomach and using water pulses to induce regurgitation. Once the stomach contents are collected, the shark is released back into the wild. But despite its advantages, gastric lavage has not been widely adopted. This is partly due to concerns about its effectiveness and potential impacts on the animals.
In a recent study, a team of scientists summarized gastric lavage studies on elasmobranchs to identify which species/groups it has been effective for, the difficulties encountered and if post-release survival has been evaluated. “[We also]
used a field study to (1) demonstrate how to perform gastriclavage on juvenile rays, (2) assess its effectiveness and (3) verify post-release survivalusing mark-recapture techniques,” the researchers explained. They found that across 23 published studies between 1985 and 2023, gastric lavage was performed on a variety of shark and ray species with success rates ranging from 29% to 100% for sharks and 60% to 95% for rays. The technique has proven particularly effective in juveniles and smaller-bodied species, though larger individuals can also be sampled with proper handling. The most important part of this literature review? No studies reported mortality or lasting harm as a direct result of gastric lavage, and some even confirmed survival through short-term monitoring and recapture studies.
The need to study shark diets has not diminished, but the ways in which we gather this information must adapt to modern conservation challenges. By refining and expanding non-lethal techniques, scientists can continue to unlock the secrets of the feeding habits of these predators without compromising the very populations they seek to protect.
