Stable Isotopes Reveal Diet of Irish Small Sharks

Here is a press release on my recently published paper from the work I did together with Dr. Anne Marie Power and Dr. Conor Graham during my Bachelors.

Dogfish (Scyliorhinus canicula), which are an abundant small shark species in Irish coastal waters, have been shown to have a very varied diet, preferring soft-bodied animals over crabs and fish, according to a new study by marine scientists at the Ryan Institute in NUI Galway. The study was published in the Journal of Fish Biology.

Previous studies which were purely based on examining what is found inside the stomach of a dogfish had described these sharks as mainly feeding on crabs and fish. In this new study, the researchers extracted muscle tissues from these sharks and analysed them using chemical tracers, called stable isotopes, to determine whether these ecologically important predators actually feed on other prey types.

Using a combined approach of novel chemical tracers and analysis of the stomach contents of the dogfish, the researchers were able to show that these small sharks feed mainly on soft-bodied animals such as sea squirts and other soft-bodied organisms. The reason crab and fish prey might have been over-estimated in the past is because these hard-bodied prey types tend to be over-represented in the dogfish’s stomach contents due to their slower rates of digestion.

Dogfish are one of the most abundant sharks and can be found from Norway down to western Africa and are common along all Irish coasts. In contrast, other species of predatory fish such as cod, are described to be on the decline due to an increase in heavily-overfished areas worldwide.

Alina Wieczorek, lead author of the study from the Ryan Institute at NUI Galway, says: “It is important to know what dogfish feed on because with the decrease in commercially important fish, these small sharks are likely to become the top predators of our future coastal ecosystems.”

The researchers were able to infer from a muscle sample of the shark what it primarily had been feeding on in the preceding 200 days. This is due to the fact that some chemical tracers of elements such as carbon accumulate in different ratios in the animal’s tissue, depending on what they have been feeding on. The higher up the foodchain an organism feeds, the more carbon stable isotopes can be found its tissues.

Alina Wieczorek, added: “A vegetarian, for instance, would have less of the heavier carbon in their muscle than someone who prefers a meaty diet. So the sentence, ‘You are what you eat’ isn’t that far-fetched.”

The research article resulted from Ms Wieczorek’s undergraduate project which was supervised by Dr Anne Marie Power, and was carried out during the final year of her B.Sc. in Marine Science at

NUI Galway.

Professor Mark Johnson, the coordinator of the B.Sc. Marine Science course at NUI Galway, explains: “We feel it is important that teaching should be research-led and this is a great example of how undergraduate students can produce high quality research during their final year projects.”

This research was funded as part of the NEPHROPS project (www.nephrops.eu) by the European Community’s Seventh Framework Programme.

To read the full study in the Journal of Fish Biology, visit: https://onlinelibrary.wiley.com/doi/10.1111/jfb.13770

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