Guts and Glory: why I’m rummaging around in shark stomachs

It’s been a few weeks since I returned, unexpectedly and abruptly (thanks pandemic), from Galveston, Texas. I’d been there to study the stomach contents of blacktip sharks at Texas A&M University. When people ask me what I was doing out there, I usually say “rummaging around in shark guts”, but today I’d like to dive a little deeper into what that actually involved and why I was doing it. After all, subjecting yourself to the smell of years old shark stomachs, in various states of preservation, isn’t usually something one does for fun. (For those wondering about the smell, think alcohol, fish, and the sickeningly sweet scent of decay. Also sometimes the acidic smell of tomato soup?).

What is stomach contents analysis?

As the phrase suggests, stomach contents analysis involves sorting through the material in stomachs from your species of interest, and trying to identify what’s inside. It gives a snapshot of what the animal was eating in the minutes to days before it was caught. It’s often the first method we ecologists will use to find out what an animal is eating.

How do you do some stomach contents analysis?

The process begins with a stomach, usually zip-tied at both ends to keep that all important contents in place. The stomach can be preserved in fluid (usually ethanol or formalin), or it can be frozen. I’d by carefully removing the zip-ties, and weighing the whole stomach. Then I’d grab a scalpel and made a cut from the top of the stomach, where the oesophagus (food pipe) is, down to where the spiral valve begins (the shark version of intestines). I’d then flip the stomach inside out, depositing the contents in a series of sieves. At this point I’d also be washing water over the inside-out stomach to dislodge all the contents into the sieve. Once that was done I’d weigh the empty stomach, and have a look in the sieve. I picked out and set aside any material that looked identifiable, as well as any parasites. Most of the stomachs I’d looked at have at least a few small worms in them.

Selection of stomachs preserved in ethanol

Identification was my favourite part, but is was also the most tricky. Most ID guides work on the assumption that you have a whole, fresh animal to look at, so with SCA you have to play detective. I’d start by taking a photo of the object in question, take notes of any body measurements I could make, and then start noting down any features which stood out to me. I might make a sketch of anything notable too. Next, I’d hit the ID guides. My usual method here was to have an initial flick through of the colour photos and note down any possible species. I’d then go through each of the possibles in more details, noting down matching characteristics. Once I had a positive ID, I’d note down all the reasons that my specimen matches the species, which was very useful if I was having doubts later on. I’d also note down the weight of the specimen. This, along with the number in each stomach, is what I’m now using to calculate the index of relative importance (IRI) of each species to the diet of blacktip sharks.

A relatively fresh Atlantic bumper (Chloroscombrus chrysurus)

Where did you get the stomachs from?

All of the sharks used in this study were collected opportunistically from recreational anglers. Angling is big in Galveston, and people fish from the jetties and beaches as well as out on chartered boats. The lab here work closely with these anglers to make sure that they’re able to generate scientific knowledge from what would otherwise be discarded material. Also, by talking to the anglers they can ask them what bait they used, which is important to know if that bait has ended up in the stomach! Unfortunately, due to the virus-which-will-not-be-named, I had to return home early and so wasn’t able to collect any samples myself. Instead, I worked on processing the samples that had been stored between 2016 and 2018.

A selection of otoliths or fish ear stones. Very useful for ID as they tend to stay intact.

Why is it important to know what blacktip sharks eat?

Blacktip sharks are the second most landed shark in the USA, second only to sandbar sharks. They’re popular with anglers, but are also caught in the commercial Atlantic shark fishery. The IUCN currently classifies them as “Near Threatened”, meaning the species may become at risk of extinction in the near future. Species don’t exist in isolation; when we’re considering conservation measures it’s important to consider the whole ecosystem.  Each species is always interacting with other species, primarily through eating each other and being eaten. It’s important that we understand these interactions so we can build food web models, and envisage the ecosystem as a whole, and understand our role in it.

Blacktip sharks are a good example of this. We know they eat fish, and that their diet varies from region to region. What we don’t know is what fish they are eating in the northern Gulf of Mexico. Consider a fish species, call it fish A. If we want to create catch limits for fish A, we might study them in isolation to work out how many we can catch while leaving enough for their population to sustain itself. But blacktip sharks are also eating fish A and the combined effects of us and the sharks eating them might cause their population to be unsustainable. On the other hand, if we know that blacktips are eating fish A, we can incorporate that into the models, and make more informed catch limits. This kind of holistic approach is called “ecosystem based management” and is pretty much the gold standard for conservation and management.

The best preserved thing I found in a stomach, a very fresh Gulf menhaden (Brevoortia patronus).

How come you got to go on such a cool placement?

This placement was funded by the SPITFIRE Doctoral Training Partnership, with whom I’m doing my PhD. It was a great opportunity to travel to another university and do research that was different to what I normally do. I learned so much, not just the obvious things like species identification and diet analysis, but also how research is done across the world. Turns out that “grad school” in the USA has some differences from “PhD life” in the UK. I’ve also met some wonderful people, and I hope this will be the start of many a productive collaboration between Texas A&M University and the University of Southampton.

A very intact squid.
Sarah Alewijnse is a PhD student at the University of Southampton and the Natural History Museum London, studying the field metabolic rates of fishes using otolith carbon stable isotopes. This placement was funded by the SPITFIRE DTP (grant number NE/L002531/1). Thanks to Dr David Wells and the Shark Biology and Fisheries Science Laboratory for hosting this placement. For more information please contact Sarah at


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