Seasonal Update (May 2024)

A brown hare resting in a field of short grass and clover. - Credit: Marc Baldwin

Much like March, April had some nice spring-like spells, but they were short-lived and there was a procession of low pressure systems that brought unseasonably strong wind and heavy rain across the whole country during the first half of the month including two named storms. The latter of these (“Pierrick”) resulted in widespread flooding in southern England and damaging winds in the south-west, peaking at 76 mph (122 kph) at St Ives, resulting in people being evacuated in parts of Sussex and Hampshire. Overall, it remained quite mild for the time of year, though, with parts of East Anglia seeing temperatures in the low 20s Celsius (low 70s F) over the first weekend. Things were more settled for about a week in the second half of April before wet and windy conditions returned to see the month out. Outside of the UK there was unprecedented heat (and some record cold, in Finland, for example) across much of Europe and Africa, where temperatures were regularly in the mid-30s Celsius (mid-90s F).

Website news

A few minor updates have been made to the red fox section, along with a new part of the Chinese water deer article looking at their feeding behaviour, and a new Speed Read covering the grass snake.

News and discoveries

Wight-clawed conservation. The Hampshire and Isle of Wight Wildlife Trust have opened their first conservation centre for the white-clawed crayfish at the Wildheart Animal Sanctuary on the island. The centre will oversee captive breeding of crayfish and represents an important part of the Trust's Southern Chalk Stream Project, which aims to protect Hampshire's chalk streams and re-establish the species in rivers across the county. White-clawed crayfish are thought to have declined by up to 80% across their range and are recognized as a species of conservation concern in England.

A white-clawed crayfish - Credit: Natural England/Jenny Wheeldon (CC BY-NC-ND 2.0 DEED)

Snail success. It's fair to say that, as zoo exhibits go, those containing snails probably don't pull in the crowds like lion cubs or newborn gorillas do, but that hasn't stopped Askham Bryan Wildlife Park, on the outskirts of York, starting work on an outdoor reserve for the glutinous snail (Myxas glutinosa). Glutinous pond snails are one of the most critically endangered animals in Europe owing to their strict requirement for very clean fresh water, which is uncommon nowadays. Indeed, there was great excitement in 2015 when footage emerged of an individual simply grazing on the side of a tank, something hitherto never witnessed. The population at the Wildlife Park has been founded from animals collected from Llyn Tegid in Snowdonia back in March, with the intention to form a breeding colony.

Saving seagrass. The condition of the marine environment in the Solent has declined significantly in recent decades. Just over half the salt marsh has been lost since the 1860s, seagrass beds are considered to be in poor condition, and native oyster populations fell so significantly in response to over-fishing, pollution, and invasive species that the fishery collapsed in 2013. The Solent Seascape Project aims to redress some of the balance, however, by restoring seven hectares (17 acres) of seagrass along the Hampshire and Isle of Wight coast. In its first year, the project has collected 30,000 seagrass seeds that are being held dormant at Plymouth University ahead of being sown this spring.

Chemical crisis. There has been much in the news in recent months about the quality of Britain's aquatic ecosystems, and specifically the amount of sewage that has been released over the very wet autumn and winter we've just experienced. The issue runs deeper than sewage, however, and a recent study of marine mammal strandings has found that nearly half of the carcasses carried unsafe levels of pollutants. Data for 1,070 marine mammals that stranded on UK shores between 1990 and 2018 were assessed to look at the presence of several persistent environmental pollutants (PCBs, DDT, Dieldrin, PBDEs, HCHs and HCBs) in their blubber. Of particular note was that pollutant concentrations were highest in long-lived, apex odontocetes (e.g., killer whales, bottlenose dolphins, and white-beaked dolphins) and were significantly higher in animals that stranded on more industrialized coastlines. When comparing the last five years' worth of strandings to those in the first five years, 48% of animals demonstrated concentrations above those known to be toxic. Killer whales were found to be 30 times over the safe threshold for PCBs.

Seasonal highlight – The basking shark (Cetorhinus maximus)

The group of marine predators that we refer to as sharks are among the most feared animals on the planet. Say the word “shark” and most people conjure up an image from Jaws of a great white attacking a boat or, worse, a human. The 1975 film Jaws did little to improve shark public relations but, contrary to popular misconception, far from being a contrived Sci-Fi notion it represented pretty much all that was known about the white shark at the time and had many heavyweights in the world of shark research as consultants. More importantly, it inspired countless generations (of which I am part) to want to learn more about sharks and to see them as a group of biologically fascinating and ecologically essential animals. We now know that there are more than 450 different species of shark, the vast majority of which grow to less than two metres (6.5 ft.) in length. One exception to that last statement is a giant that visits Britain's shores during the spring and summer months: the basking shark.

The basking shark's binomial name, Cetorhinus maximus, comes from the Greek ketos, for "marine monster" or "whale", and rhinos, meaning "nose", while maximum is from Latin for "greatest". - Credit: jidanchaomian (CC BY-SA 2.0 DEED)

The basking shark—so named because early observers mistook its slow surface swimming while feeding for basking in the sunshine—is the second largest extant shark species known to science. I confess to being pedantic over the size of this animal, which is often cited simply as the “second largest fish” or “second largest shark” in the world. The world's largest shark/fish species is the whale shark (Rhincodon typus), recorded to reach a whopping 21 m (just shy of 70 ft.), so technically the second largest fish in the world is apt to be another whale shark. Hence, it's more accurate to refer to the basking shark as the second largest fish species in the world. Anyway, pedanticism aside, there's no denying that basking sharks are large animals: historic records put them anywhere up to 15 m (49 ft.), but most of the empirical data we have suggest 10 m (33 ft.) is likely to be closer to the maximum size and very few animals (i.e., fewer than 10%) recorded in British waters exceed eight metres (26ft).

Key to how basking sharks can grow so large is in the food they eat. You could be forgiven for thinking that a shark with a mouth almost 1.5 m (5 ft.) wide and containing up to 480 hook-like teeth could eat pretty much anything it wanted. These teeth are tiny, however, and the sharks are planktivorous - in other words, they feed on the microscopic plants (phytoplankton) and animals (zooplankton) that drift on ocean currents. Plankton is filtered from the water by a series of cartilaginous gill rakers located inside the gill slits. The shark swims, mouth agape, at a speed of roughly two knots (about 2.5 mph or 4 kmph) diverting water into its mouth and out through the gills as it goes. Plankton gets trapped by mucous secreted onto the gill rakers. Periodically, the shark will close its mouth, collapsing the gill rakers and scraping the plankton off into the mouth. The resulting plankton-mucous ball is then swallowed. This is a highly efficient feeding mechanism, and the sharks are capable of filtering more than 2,000 tons of water per hour. In other words, they could filter an Olympic swimming pool every couple of hours. Moreover, plankton are an abundant food source, meaning that basking sharks have access to vast quantities of these tiny prey and this large energy supply is part of how they can reach such considerable sizes (see Q/A: How do Whale and Basking sharks grow so big eating such small food?).

A basking shark swimming near the surface with mouth agape, filter-feeding. - Credit: yohancha (CC BY-ND 2.0 DEED)

So, what is it that draws basking sharks to Britain's coasts? Well, breeding seems to be part of the equation, and there have been reports of courtship behaviour (including nose-to-tail following, the female in the lead) from around our coast between May and July. That said, very little is known about breeding biology of these marine giants and while they may breed here, it is most likely that they are initially attracted by food.

Plankton availability varies considerably with water conditions and, therefore, basking sharks are highly migratory. Towards the end of spring an area of warmer water from the Atlantic Ocean pushes its way into the cooler coastal waters off southern Britain. Where these warm and cold waters mix, an area known as the Ushant front, high nutrient levels promote phytoplankton growth. When phytoplankton bloom, so too do the species of animal (i.e., zooplankton, largely calanoid copepods) that feed on them. When the conditions are calm, these critters form visible 'planktonic slicks' on the surface; it is these blooms that the sharks are attracted to. Consequently, basking sharks are generally first seen in the western English Channel and the southern Celtic Sea during May. During “invasion years”, shoals of several hundred animals can be seen off the coast, although numbers vary considerably from year to year; largely in accordance with sea surface temperature rather than zooplankton density, as might have been assumed.

Shark sightings start in south-western England, prime spotting sites including the Lizard Peninsula, Penzance, and Falmouth, the animals moving north into the Irish Sea and up to the Firth of Clyde and Sea of Hebrides as the summer progresses. We know that the sharks can track the zooplankton density across large areas, although we currently don't know exactly how they do it - suggestions range from an inherited memory to the sharks being able to smell compounds released by the zooplankton while they're feeding on algal blooms. However the sharks manage to find the blooms, they seem to stick around until the zooplankton density drops below a critical level, which occurs towards the end of summer; come November, the sharks head out to deeper water. Indeed, until relatively recently the movements of basking sharks over the autumn and winter were a mystery.

Like all sharks, baskers have many rows of tiny (vestigial) teeth in their mouths. Their feeding apparatus is, however, long strips of keratin in the gill arches called rakers that trap plankton. - Credit: Hannah Keogh (CC BY-NC-ND 2.0 DEED)

In 1953 Dutch zoologists Antonius van Deinse and Marcus Adriani published a paper in the journal Zoologische Mededelingen in which they described how four of seven basking sharks stranded on beaches in The Netherlands 1950 and 1952 lacked gill rakers. This curious lack of feeding apparatus caused a stir in the scientific community, posing the question of how the sharks fed themselves during the winter. In a paper to the Proceedings of the Zoological Society of London during May of the following year, Hampton Parker (at the British Museum) and Marinus Boeseman (at the National Museum of Natural History in The Netherlands) suggested that the sharks moved into deep water during the autumn where they shed their gill rakers and stopped feeding while the rakers re-grew. Eight years after Parker and Boeseman's paper, British zoologist and Scientific Director of the Zoological Society of London Leonard Matthews developed the theory further and, based on his calculation that a seven-metre-long (23 ft.) basking shark would require 663 calories simply to keep swimming for an hour, concluded that the winter plankton density could not provide these. In his paper to New Scientist, Matthews suggested that these sharks solve the problem of a winter plankton paucity by:

“... throwing away its rakers worn by a season's use, refraining from feeding, sinking to the bottom and hibernating. At this moment there are probably great schools of these enormous fish quietly resting on the bottom of the sea, perhaps in the heads of the canyons at the edge of the continental shelf, with their metabolism running at its lowest level while they grow their new gill rakers ready for browsing on next summer's crop of plankton.”

The shark enthusiast in me smirks at the thought of schools of torpid basking sharks littering the sea floor waiting for spring, but at the time the notion was entirely plausible in a biological sense. Moreover, basking sharks had long been fished—indeed, over-fished—for their liver oil, which had various commercial uses, including as a lamp fuel, and the livers of this species were some of the largest around. (Without swim bladders to keep them afloat, sharks and their kin have evolved large livers filled with low-density oil that helps them remain buoyant.) So, the theory was that the sharks could 'tough out' the winter months by living off the oil accumulated during the spring and summer; not unlike hibernating bears or hedgehogs living off the fat reserves they'd built up earlier in the year. Subsequent sightings of basking sharks during the winter hinted that Matthews' theory was incorrect but, quite unbelievably, it was not until almost 40 years later that the mystery was solved.

Basking sharks are harmless filter-feeders, but the Shark Trust have issued a Code of Conduct for interacting with them. - Credit: candiche (CC BY 2.0 DEED)

Matthews' calculations suggested that basking sharks would lose out if plankton densities dropped below about 1.4 grams per litre - so, if there was less than this much plankton in the water, the sharks would burn more energy filtering it out than they'd recover digesting it. In a 1999 paper to the Proceeding of the Royal Society of London, however, David Sims, at the time at the University of Aberdeen (now at Plymouth University), reassessed the calculations based on his studies of these sharks off Plymouth. Sims calculated that, in theory, a shark can make a living until plankton levels drop below about 0.55 grams per litre, although if feeding as part of a group levels must remain above 0.74 g per litre. Sims' observations of basking sharks foraging in plankton densities as low as 0.48 g per litre support his theoretical calculations. If we consider that the waters off Plymouth often support 0.62 g per litre of plankton during the winter months, it seems that the sharks would have no need to hibernate. Furthermore, given that this winter density is below the 'group threshold', Sims' theory probably also explains why only the odd one or two sharks are spotted off our coasts during the winter. So, if the sharks don't hibernate, where do they go? Sims has provided some clarification here too, having radio-tagged several sharks off Plymouth during the summer and followed their movements over the year.

The fin of a basking shark. These sharks frequently swim close to the surface while feeding (hence their common name, as the sharks were thought to be basking in the sunshine), during which their dorsal fin breaks the surface. These fins are frequently mistaken for great whites, despite the profile being quite different. - Credit: Rich MacDowell (CC BY-NC-ND 2.0 DEED)

When we add Sims' data to that of Cornish Wildlife Trust naturalist Colin Speedie, who has been tagging basking sharks off Britain for more than a decade, a picture slowly begins to emerge. Some of the sharks that Sims tagged in the English Channel stayed in continental shelf waters, while others migrated offshore into deeper 'shelf-break' waters off the west coast of Ireland down to the Bay of Biscay. The biologists think the sharks continue feeding here, where plankton densities are higher, making surface to depth migrations following their prey.

It seems that some sharks may go even further afield, crossing the Atlantic. It had long been considered that North American and European populations of basking sharks were isolated. Indeed, a paper in 1976 suggested there were actually four species that were geographically distinct, but more recently we have found data to suggest otherwise. In a brief but fascinating paper to Biology Letters, a team led by Marine Conservation International biologist Mauvis George reported on the travels of an eight-metre (26 ft.) female tagged off the Isle of Man in June 2007. The shark moved south through the Irish Sea and, at the start of July, left the tip of Cornwall heading out into the Atlantic. When the tag detached, 82 days later, the shark had arrived in the waters off Newfoundland in Canada, a straight-line distance of almost 9,600 km (just under 6,500 miles) and, during the journey, she was recorded to dive to around 1,300m (almost a mile), the deepest dive record currently held for this species.

Despite these glimpses into the world of the second largest shark species, we still have so much to learn. So, if you find yourself on the western coasts of the UK this spring and summer, keep an eye out for basking sharks and please report any sightings via The Shark Trust's website, which also contains details of the Basking Shark Code of Conduct, for anyone planning on swimming with basking sharks or approaching them in a boat. The code is aimed at minimising disturbance to the animals and making the experience enjoyable for all concerned. Finally, it's worth mentioning that these sharks often swim close to the surface, with their dorsal fin breaking the surface, and are frequently mistaken for great white sharks (Carcharodon carcharias) - further details can be found in my are there white sharks in British waters article.

For a round-up of Britain's seasonal wildlife highlights for late spring, check out my Wildlife Watching - May blog.

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