Blue shark sighting informs conservation research

Dr Steph Smith reflects on a rare encounter with a blue shark in Weymouth Harbour and explores what its behaviour might tell experts about this near-threatened species. Images and video by Dr Scott Denholm, with expert comment from Dr Lauren Smith and Hayley Roberts.

Weymouth is a picturesque seaside town in Dorset¹ on the south coast of England, positioned along the Jurassic coast. Its busy harbour entrance lies between two stone piers² and is traversed by a historic lifting bridge to allow larger vessels into the marina. Walking alongside it on a warming July morning, the water was calm and the tide was out. Suddenly I was made aware of something less common gliding through the water, towards the harbour entrance. It was approx. 150cm and an exceptional blue colour with an elongated tail like a thresher shark. Almost entirely due to its colour, but with the added evidence of a useful guide from the Dorset Wildlife Trust³, I suspected a blue shark.

This sighting was picked up by an independent observer and reported in the local paper⁴, as well as the Sun⁵ and the Independent⁶. These confirmed the individual was a young blue shark.

However, where we observed it, we got an insight into shark behaviour in the shallows, captured by Dr Scott Denholm, with independent but complementary footage to that reported in the news. Understanding how marine wildlife respond to environmental change is an important part of protecting biodiversity and building sustainable food systems.

The shark seemed to glide slowly and unflustered in parallel with the harbour wall. However, as it approached shallow stone steps leading out of the harbour it appeared to be ‘scratching'. It breached the water several times as it rubbed itself on the stone steps, beyond that captured by the footage. Finally, it left towards the middle of the harbour, leading out to the sea.

Due to its size, it is likely a juvenile, with adults maturing around five to six years old at c.180cm⁷ and a British record at almost 9ft (2.7m)⁸. Juvenile blue sharks tend to be found in shallower and more inshore waters than adultswhich can dive down to over 1000m^{9, 10, 11}. This usually open-ocean species was breaching the water and harbour steps in the shallows of the harbour.

Blue sharks are found across the world's oceans. They usually appear in UK waters during the summer, mainly around South West England and the western English Channel¹⁰, where sea conditions suit them best¹². This is part of their trans-Atlantic migratory route which is driven by habitat use, their reproduction cycle requirements and distribution of their prey^11,13. As such, this blue shark sighting is not unique, but certainly rare given the shallow, inshore water and further east along the south coast.

There was another potential blue shark sighting in Weymouth Bay recently. However, the species was unconfirmed¹⁴. Marine engagement assistant for the Dorset Wildlife Trust, Hayley Roberts, explains, “In recent years there has been an increase in blue shark sightings, alongside other shark species such as porbeagle and thresher sharks, off the south coast.” The images captured in a recent article¹⁵ cites some historic sightings in the local area.

blue shark pictured close to shore — Young blue shark (Prionace glauca). Image credit: Dr Scott Denholm.

Behaviour

Regarding our observation and footage captured of the blue shark in Weymouth harbour. As this was of a single individual and lasted only a few minutes, we cannot be certain why it entered such shallow water or displayed this behaviour.

It may be that this usually open-ocean, or pelagic, species was tempted into the harbour foraging for prey. Hayley Roberts elaborates, “Blue sharks are opportunistic predators with an excellent sense of smell, allowing them to detect food over long distances. As a result, they may be attracted to fish offal and other discards from fisheries in the harbour. There have also been reports of blue sharks approaching fishing boats as soon as the engines are switched off, even before bait is deployed, suggesting that they may associate the presence or sounds of fishing vessels with feeding opportunities through learned behaviour.” As to its behaviour on the harbour steps – this may have been curiosity; investigating the surroundings in which it found itself, having come into the confines of the harbour. This interpretation could be borne out by its unhurried, deliberate movements.

However, the repeated nature of the ‘scratching’ and ‘rubbing’ against the harbour wall and steps, even breaching the water several times, made me suspect it was trying to satisfy an itch. Sharks can be plagued with ectoparasites and scratching (or ‘chafing’) behaviour has been observed elsewhere; a blue shark scratching their underbelly against a floating fish box was observed in the North Celtic sea¹² and by salmon sharks using barnacle - encrusted floating oceanic debris and floating fibreglass off the coast of Canada¹⁶. One study even captured smaller sharks rubbing against their larger shark predators¹⁷ to try and dislodge parasites and address skin irritation. With pervasive plastic and anthropogenic pollution in the open ocean, sharks seeking to scratch risk entanglement and death^{16, 18}.

Dr Lauren Smith, shark expert and founder of Saltwater Life¹⁹, a marine research and conservation organisation based in Scotland, summarised; “The footage provides a fascinating glimpse into the behaviour of a blue shark exploring the harbour. Blue sharks are highly migratory pelagic predators and are known to carry external parasites. It's possible the animal was attempting to dislodge these by rubbing against the harbour steps, although it's equally plausible that the abrupt change in swimming behaviour after contact with the structure was simply an instinctive response to avoid becoming stranded in shallow water. While we can't say for certain why it behaved this way, both are plausible explanations”.

Conservation

The blue shark is listed as ‘near threatened’ on the IUCN Redlist²⁰ - which assesses species’ risk of extinction - indicating its global population is declining⁹. Protection is crucial to recovery efforts; but strict global enforcement of this is needed²⁰. As Dr Lauren Smith explains; ‘Alongside the growth of responsible wildlife tourism, blue sharks remain one of the most heavily exploited shark species in the world due to commercial fishing pressure. Whilst there have been an increase in sightings in UK waters and even blue shark strandings in parts of the North East of Scotland, encounters like this are incredibly special and serve as a reminder of the remarkable marine wildlife found in our waters’.

It is unclear how the record-level marine heatwave^{21, 22}will impact various marine populations, but evidence elsewhere shows that movement and feeding patterns of open-water fish is influenced by water temperature profiles across depths, to maintain their bodily functions and those of their prey and their predators^{10, 23}.

Warming waters may also increase the potential for waterborne pathogens²⁴, increased mortality particularly in seagrass and kelp²⁵ and exacerbate harmful algal blooms²⁶; all of which may further disrupt the intricate food web systems maintained by so called ‘keystone’ species, including many sharks. Whilst the change species face is largely too fast to allow adaptation through the slow, incremental process of evolution, there is evidence species are responding to these climate-induced pressures²⁷.

By liaising with the Dorset Wildlife Trust and the Shark Trust²⁸, I logged the blue shark sighting in Weymouth harbour to be included in relevant marine sightings databases. This so called ‘citizen science’²⁹ (whereby the general public can help collect data and raise awareness of species of conservation concern) can help researchers track how wildlife responds to environmental change and can provide valuable evidence for future conservation efforts.

Addressing the implications of climate change for marine wildlife is captured in the holistic and ambitious United Nations Sustainable Development Goals³⁰, in this case particularly SDG14 and the goal to conserve and sustainably use the oceans. By capturing observations from the public like this, we can help understand and protect global biodiversity and its role in the health and wellbeing of people and our planet.

Behaviour

Conservation

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