Basking Sharks

Photo by David Mark of Pixabay

Basking Sharks are so named because they’re often seen feeding at the surface of the water where they look like they’re basking in the sun!  They are enormous and often spotted in UK waters during summer months.
APPEARANCE:
Apart from their large size, Basking Sharks have:

  • a very large mouth – this can be well over 1m wide!
  • 5 huge gills which almost encircle the head
  • and a powerful crescent-shaped tail

COLOUR:
They tend to be greyish-brown with a lighter underbelly. Often they have irregular patches, patterns and streaks on their flanks and fins. Using photo-ID we can use these distinct markings to identify individual Basking Sharks.

SIZE:
The largest reported Basking Shark was 12m long. But most don’t get bigger than 9.8m.

WEIGHT:
The average Basking Shark weighs 4.5 tonnes. Yet, they can weigh up to 7 tonnes!

DIET:
Basking Sharks eat zooplankton. This includes small copepods, barnacles, decapod larvae, fish eggs and shrimp.  They’re one of 3 filter-feeding sharks but are the only species that feeds entirely passively. They swim through the water with their mouth wide open, rather than actively sucking water in. Only closing their mouths to swallow their food. Long comb-like structures on their gills (known as gill-rakers) trap and filter zooplankton. These can strain up to 2000 tonnes of water per hour!

REPRODUCTION:
It’s thought that Basking Sharks live for at least 50 years. Males reach maturity at 12–16 years. And females at 20 years (around 4.6-6.1m in length).
Females produce eggs, which develop and hatch inside their body. They then give birth to fully developed young, which are around 1–1.7m long. This makes Basking Shark pups larger at birth than many species of shark are fully grown!

There’s little data on Basking Shark reproduction. But pregnancy is thought to last around 14 months. There’s only ever been one reported catch of a pregnant female (1943), who was carrying 6 pups. This suggests that Basking Sharks give birth in areas of low, or no fishing pressure.

BEHAVIOUR:
Basking Sharks are quite social. They can be seen on their own, in small groups, or, schools of hundreds. There are many reports of same size and sex groups. Suggesting a strong sexual and age segregation within the species.

Despite their size, Basking Sharks are capable of leaping clear out of the water. A behaviour known as breaching. They seem to breach most when in large groups and during courtship, so this may act as a social or sexual function. It could also help to dislodge external parasites.
Info from sharktrust.org/about-basking-sharks

Basking Sharks are long lived, slow growing and produce few young. This makes them extremely vulnerable to human impacts.  Although Basking Sharks are now one of the most heavily protected sharks in UK and EU waters, they continue to face threats from human activities:

ENTANGLEMENT
Basking Sharks easily become entangled in fishing nets and ropes. Unless fishermen are on hand to quickly release them, they often die. Although some do manage to disentangle themselves. You can sometimes see scarring and abrasions caused by nets on their dorsal fin.

BOAT-STRIKE
Propeller and boat strikes remain a serious danger for Basking Sharks. Particularly in summer months when they’re feeding at the surface. Basking Sharks rarely evade approaching boats. So it’s common for them to have scarring and sometimes horrific injuries from collisions.

HARASSMENT
Basking Sharks are very sensitive to disturbance and harassment by people. In all the excitement of seeing Basking Sharks, boats and jet-ski’s often end up striking them. As well as causing physical harm, water-users can also disrupt their natural behaviour. Such as feeding, courting and mating.

FISHERIES
Basking Shark fisheries worldwide have all but collapsed. Although in some parts of the world they continue, driven by demand for shark fins. Basking Sharks are also still caught as bycatch in nets intended for other species.

Info from sharktrust.org/basking-shark-threats

To learn how you can help basking sharks go to sharktrust.org/how-can-you-help-basking-sharks

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S is for Sturgeon

Sturgeon    noun

Oxford Dictionary definition:  large edible fish yielding caviar.

Our definition:  Sturgeons are native to subtropical, temperate and sub-Arctic rivers, lakes and coastlines of Eurasia and North America.  They are distinctive for their elongated bodies, lack of scales, and occasional great size: sturgeons ranging from 7–12 feet (2-3½ m) in length are common, and some species grow up to 18 feet (5.5 m). Most sturgeons are anadromous (migrating up rivers to spawn) bottom-feeders, spawning upstream and feeding in river deltas and estuaries.  While some are entirely freshwater, a very few venture into the open ocean beyond near coastal areas.

Sturgeon are primarily benthic  feeders (feeding on the river bed or ocean floor), with a diet of shells, crustaceans and small fish. They feed by extending their syphon-like mouths to suck food from the benthos.  Having no teeth, they are unable to seize prey, though larger individuals can swallow very large prey items, including whole salmon.  Sturgeons feed non-visually.  They are believed to use a combination of sensors, including olfactory sensors, tactile chemosensory cues on the 4 barbules, and passive electroreceptors (ampullae of Lorenzini).

Many sturgeon leap completely out the water, usually making a loud splash which can be heard half a mile away on the surface and probably further under water. It is not known why they do this, but suggested functions include group communication to maintain group cohesion, catching airborne prey, nuptial behaviour, or to help shed eggs during spawning. Other plausible explanations include escape from predators, shedding parasites, or to gulp or expel air. Another explanation is that it “simply feels good”.

Sturgeon can live 100 years and have been around since the dinosaurs.  Because of their long reproductive cycles, long migrations, and sensitivity to environmental conditions, many species are under severe threat from overfishing, poaching, water pollution, and damming of rivers.  There is also a noticeable decline in sturgeon populations as the demand for caviar increases (see Roe on the R page). According to the IUCN, over 85% of sturgeon species are classified as at risk of extinction, making them more critically endangered than any other group of species.

Click here for the S page, and here for the rest of the dictionary 🙂