The brilliant world of the blue shark
Show notes
This week on World of Sharks, we are diving into the world of an iconic species: the blue shark, Prionace glauca. Known for their big eyes, pointed snouts, mesmerising metallic blue colour and inquisitive nature, blue sharks are a species of requiem shark that occur in temperate and tropical oceans around the world. We are joined by Dr Jenny Bortoluzzi, a shark scientist and SOSF project leader, who studied this species off the coast of Ireland for her PhD and tried to understand why they came to the area in such large numbers and discovered some quite surprising things about their eating habits…
Two years ago, Jenny appeared on our first-ever episode to talk about shark reproduction – and she’s had a lot of exciting experiences since! One that stands out for her is a particularly good day in the field, looking for blue sharks [6.30]. Not only did she have a successful day with her study species, but Jenny also saw multiple basking sharks breaching and had a very special experience with dozens of sunfish who gathered around their boat. Jenny and her team couldn’t quite believe this had all happened in less than 24 hours – but in the North Atlantic, almost anything can happen.
Jenny’s passion for sharks started in school, where she watched the 2007 film Sharkwater [10.20]. It follows filmmaker Rob Stewart as he investigates the vital role that sharks play in marine ecosystems and the devastating effects of the shark fin trade. This was the first time that Jenny learned of the dire situation sharks were in. She couldn’t understand why there was such fear and hatred of these animals, which to her, were beautiful and fascinating. From that moment, she decided her goal was to save sharks which led her to study marine science at the University of Southampton [12.00]. While at university, she also learned how complicated conservation was – it wouldn’t be as simple as just banning the practices that threatened them. This was cemented by her experiences after university, which included an internship at the Environmental Justice Foundation [13.40]. Jenny was tasked with monitoring fishing vessels and looking for ‘suspicious’ activities, but what really stood out for her was just the sheer scale of commercial fisheries. Another internship, a few years later, took Jenny to the Philippines, where she was assisting with mobula ray research [17.20]. The research team were working in a small, artisanal fishing village, and at the time, the Philippines were banning the capture and trade of any CITES-listed species. Jenny saw the impact of this on the fishers, as well as that of the commercial vessels operating in the same area. She felt conflicted: happy that the rays were protected but concerned that the artisanal fishers had lost their livelihood. This experience motivated her to find solutions that not only protected sharks but also considered people: a theme that continued throughout her PhD.
Before getting into Jenny’s research, we first get to know her study species [20.20]. Blue sharks are an oceanic species spending most of their time in the open ocean. They have long, pointy snouts, dotted with numerous Ampullae of Lorenzini, big eyes, and get their name from their beautiful blue colour. Blue sharks are also notoriously curious. They have even been known to ‘boop’ scuba divers right on their face masks!
Despite being considered the most abundant oceanic shark species, blue sharks are also sadly the most frequently fished [22.20]. They have a high reproductive rate relative to most other sharks; females can give birth to up to 135 pups. This, coupled with their tendency to hang out in the high seas (where little fisheries management is enforced), makes them vulnerable to intense fishing pressure. Blue sharks are fished for their fins and meat, and their population worldwide is in decline. They are classed as Vulnerable by the IUCN Red List of Endangered Species and Critically Endangered in the Mediterranean. In the Atlantic, where Jenny works, they have declined by around 70%.
This is why it is so important to know which areas are important for blue sharks and why. They are found in abundance in the Irish Sea and begin to appear in large numbers around May [23.50]. Here, they are very important for recreational angling, and in Jenny’s study site, a catch-and-release fishery operates, where blue sharks are caught for sport and then released back into the wild. Jenny’s PhD aimed to find out why the area was so significant for blue sharks, and there were two main questions that she was interested in [25.55]. The first regarded reproduction. She knew from talking to anglers that they only seemed to catch females closer inshore – males were found much farther offshore. Jenny wanted to know the age of these females and their reproductive stage (e.g. were they too young to reproduce, pregnant, or had just given birth?). Information like this could indicate that the area was important for a certain reproductive stage, like birthing or nursery grounds.
Jenny also wanted to know what the sharks were eating while they were in the area. Blue sharks are opportunistic feeders, and there was also anecdotal evidence of something very unusual: blue sharks eating jellyfish! Jenny’s supervisor, Nick Payne, had previously tagged blue sharks with speed sensors and had discovered blues swimming much slower than they should be [28.00]. They wondered if this could be an indication that they were feeding on superabundant, slow-moving prey, like jellyfish, which would back up the stories from the local community. They could even target specific jellies [29.50]. Jellyfish sometimes trap other animals in their bell – like other fish, or tiny, lipid-rich crustaceans called amphipods. Jenny theorised that they could be saving energy by eating jellyfish that had already done the work for them and captured more nutritious food. But she needed proof…
A ‘typical’ day in the field for Jenny would start with a good weather window (which in Ireland is hard to find!) [33.20]. She and her colleagues would set out with an angler who had valuable knowledge of where to find the sharks and the necessary equipment. They would bait and wait – sometimes for hours at a time – until finally getting a bite and reeling in the shark. Once the shark was on the boat, they had to work quickly to ensure minimal stress to the animal. The whole team worked collaboratively to get all the data they needed in ten minutes, which Jenny says involved “a lot of yelling”! After everything was done, they would release the shark back into the sea and watch to make sure it was happy and well.
But all the stress was worth it. Jenny came away with blood samples, which she could use to analyse the levels of different hormones and assess reproductive state [32.40]. Her findings indicated that the anglers were right – during late spring and summer, the majority of sharks were juvenile or subadult (essentially teenage) females who were not yet mature enough to reproduce. But around November, sexually mature adult females moved inshore. Why this is requires further research – but it could still have conservation implications.
And as for the jellyfish theory? Well, Jenny also obtained muscle samples, from which she did something called a fatty acid analysis [40.00]. ‘Fatty acid’ is the scientific term for fats the body can’t make on its own, and we get the majority of them from our diet (such as Omega-3’s). They are essential for survival, particularly for active, predatory shark species, as they provide energy and insulate the body and vital organs. Each animal will have a ‘fatty acid profile’ that reflects their diet, and so these can be compared to examine what different animals are eating. As Jenny sampled throughout the year, she could also look at seasonal changes in the blue shark’s diet. Her results indicate that blue sharks are shifting their diet to jellyfish later in the year – pretty unusual for a shark! She was also able to attach a camera to some individuals, in the hope of gaining visual evidence for this [43.15]. Although she didn’t capture a blue shark actually chowing down on a jellyfish, she did capture an individual circling back to check out a jellyfish, which suggests interest. It could be that the shark was investigating to see if the jelly contained any of those tasty amphipods, and moved on when it discovered none were present. Pretty cool stuff!
So, what does this all mean for conservation of the blue shark? Well, put simply, the more we know, the better we can protect them [50.00]. In identifying important areas, and why they are important, we can also better understand where overlaps with human activities occur, and which threats to manage. This is where Jenny’s work with the angling community comes in. Her aim is to work with them, learning from their knowledge and diverse perspectives, building respectful relationships, and collaboratively working on better practices to limit pressure on an already threatened species.
About our guest
DR JENNY BORTOLUZZI
Jenny is currently a Postdoctoral researcher with the IUCN SSC Shark Specialist Group on the Important Shark and Ray Areas project. She recently completed her PhD at Trinity College Dublin where she studied the ecology and physiology of Irish elasmobranchs. Her broad interests are in elasmobranch feeding, movement and reproductive ecology and collaborative conservation research.
She also became an SOSF project leader in 2022, to support her research on the reproductive status of female of blue sharks in Irish waters. The information from this project will help guide responsible handling practices for local fishers, but also scale up to inform regional and global policies. You can read more about it here: https://saveourseas.com/project/baby-blues-female-blue-sharks-in-irish-waters/.
Website: https://jennybortoluzzi.wordpress.com/
Instagram: @drsharkyjenny
Twitter: @DrSharkyJenny
