Return of the giants: conservation works when you give it time
- Eloise
- Jun 13
- 5 min read
Masters of the open ocean, apex predators, top speeds of 45mph, weighing over 600kg, and up to 10ft long. Which species could I be talking about?
Scientifically named Thunnus thynnus, the Atlantic bluefin tuna originates from the North Atlantic Ocean where they spawn. The word Thunnus derives from the Greek “thynō”, meaning “to rush; to dart”. True to their name, tuna spend most of their time cruising at around 2-5mph, between short bursts of high-speed swimming to hunt. With in-built adaptations such as torpedo-shaped bodies, specialised swimming muscles along their backs, and most propulsion coming from the tail, tunas use a form of movement aptly named “thunniform” swimming!
In other words, they’re quite literally built for speed.
Migratory marvels
The Atlantic Ocean is home to two distinct populations of bluefin tuna: one breeding in the Gulf of Mexico (West Atlantic), and one in the Mediterranean (East Atlantic). Tuna are experts in the field of migration, crossing entire oceans to breed and feed in record-breaking times. Atlantic bluefin tuna are unsurprisingly famous for crossing the Atlantic Ocean, some 4,800 miles, in <60 days! That’s an average of 80 miles a day, which would be tiring to drive let alone swim!
Deep divers
Atlantic bluefin tuna are typically found in the uppermost layers of the Ocean where sunlight penetrates, known as the photic* zone. But they don’t just stay here! Individuals tagged by scientists were found to dive to well over 1000 meters! That’s over 3 times as deep as military submarines, and covers the same range as well-known deep-sea creatures like the giant squid. This species continues to astound me – would you ever expect your tinned tuna to be such a talented diver?
*The photic zone extends to around 200m and allows phytoplankton and other species to harness the sun’s energy through photosynthesis.
Fishing fever
Historically British waters were described as “boiling” with marine life, full of large predatory species like Thunnus thynnus. In the late 20th and early 21st centuries, these magnificent hunters experienced population collapse on several occasions due to overfishing. Numbers got so low that the species almost disappeared by the 1990s!
The problem with overfishing
Ever heard the phrase “fishing down the line”? This is when commercial fisheries progressively target smaller and smaller species, as a result of unsustainably depleting wild populations. As larger, higher-trophic level* species like tuna are depleted, they shift to medium-sized species like cod (Gadus morhua). The pattern continues, impacting smaller species like anchovy (Engraulis encrasticolus).
*Trophic levels allude to the position and role a species holds in an ecological food web. Whilst many species occupy several regions of a food web, larger predators tend to occupy higher trophic levels.
Return of the giants
In recent years, several of these larger, predatory species have resurfaced in Cornish waters. This includes Thunnus thynnus, with Falmouth holding the first ever catch-and-release competition for this species in September 2024. Met with joy from both fishers and researchers alike, let’s explore what the return of these giants means for British wildlife, and ocean-users like us.
Balancing act
As top predators, Thunnus thynnus maintain balance in the ecosystem and are responsible for regulating population sizes of other species in the marine food web. Marked as a keystone species*, Atlantic bluefin tuna influence important oceanic processes like nutrient and carbon cycling. With juveniles feeding on squid and crustaceans, and adults mainly on baitfish such as herring and mackerel, tuna’s return could see the restoration of balance to Cornish populations of other well-fished species. Not only this, but bluefin tuna are prey too! Other predators such as Orca (Orcinus orca) benefit from tuna’s presence in the marine foodweb, hunting both adults and juveniles.
*Keystone species play a crucial role in their ecosystem, upholding biodiversity and balance. With a disproportionately larger responsibility than their abundance, removal of one keystone species could result in ecosystem collapse.
Protected, but still landing on our plates
Atlantic bluefin tuna have been firmly under protective measures after drastic exploitation and population decline. Recently, they’ve made their return to British waters and can now be sighted for 5 months of the year. Restaurants responded by flocking to serve tuna to eager customers, and it can now be seen on menus for the first time in what feels like forever. This begs the question, are we fishing them again too soon?
I spoke to marine biologist and diver Ryan Hunnisett (otherwise known as Scuba-Ry) to get his perspective on the issue. Ryan is the owner of a small dive shop in Helston, and has been diving in Falmouth for over 10 years with 1000+ dives in Cornwall. He has noticed some drastic changes to our marine wildlife during this time. As a business-owner, he says he understands why seafood restaurants and the fishing industry need to spice things up and create opportunities for growth. But when it comes to nature, Ryan explained that these “new opportunities” usually result in a negative impact for the target species. Giving tuna a longer chance to repopulate and establish themselves in the ecosystem before fishing them, would bring about more opportunities for businesses in the long run.
Biological clocks
Why is it so important to give tuna time? Let’s talk biology. Historically, we’ve underestimated the amount of time it takes for tuna to reproduce. It takes between 3-12 years for an individual to reach sexual maturity, by which point they can weigh up to 250kg and reach 8ft in length. Imagine that on your dinner table!
Setting and maintaining conservation measures like minimum size, science-based catch quotas, and individual boat limits, helped to bring Atlantic bluefin tuna back from the brink of extinction. However, the current minimum size for Atlantic Bluefin Tuna to be landed in the UK is 115cm fork length* or 30kg, meaning that caught individuals could be a long way from reaching sexual maturity. Natural variation in large predatory species like tuna means we can’t always rely on size and weight classes to define their catchability.
* Fork length is a measurement of a fish's length, not how much you can fit on your fork. It’s used for species with forked tails, from the tip of the snout to the centre of the tail fork.
One to watch
After decades of Atlantic bluefin tuna being absent from UK waters, there is a steep learning curve for the fishing industry now that these ocean-roaming giants are back. Upon their return, tuna were subject to a trial commercial quota of 39 tonnes annually, and any recreationally caught individuals must be caught under an active permit. As of 2025, the quota has increased to 66.15 tonnes with 45 of those reserved for commercial vessels. How might this impact our recently recovered tuna populations?
For now, it’s watch-this-space. There will almost certainly be delayed effects from current fishing efforts on Atlantic bluefin tuna populations, and marine scientists are increasingly interested in the behaviour of this fascinating species. With fishing regulations on the brink of yet more changes, how will we manage our fish stocks in order to avoid future population crashes? Follow the FMC blog to find out more about Cornish marine wildlife, and receive updates on local events so you can join our growing ocean literacy movement.
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