The Cove Awakens

June 2026

It is easy to look out from Lean point across the waters of Treen or Rose-an-Hale cove and think that little changes, except for the weather - the swells that build and subside, the wind that whips up spindrift, the squalls that pass through on a brisk northwesterly.

Underwater though, in spring, it is all change. Just as on the cliffs where the blackthorn bursts into bloom, and thrift and squill colour the rocky back drop, underwater the seaweeds come out of dormancy and begin to grow.

Beneath the waves, sand that was washed offshore during winter months, slowly begins to accumulate in the cove. On the rocky reefs, the regrowth of kelp is dramatic. The winter storms removed everything except the kelp holdfasts, but in spring these start to grow - oarweed, furbelows, golden kelp and sugar kelp. When conditions are ideal - with clear water and good sunlight, they can grow 2 cm a day. Within a month or two, they’ll form a canopy 2m high.

In the shallow water around Rose-an-Hale's ‘gull-rock’, little brown buttons on horizontal rock surfaces start to sprout from the center - they are the bases of thong weed. Later in the summer their long brown tresses will float on the surface - and wrasse, mullet and bass will slip silently beneath them.

Mid summer kelp forest

Thong weed revealed by a low spring tide

 Just like the emergence of insects that feed on the cliff plants above, the surge in seaweed growth is followed by an influx of grazers. Iridescent blue rayed limpets graze the kelp stipes - they hollow out little pits into which they retreat for protection when it gets rough.

Blue rayed limpets are one of the most striking molluscs. They feed on kelp stipes (stems). They chew holes into the stipe into which they can retreat in rough weather.

An iridescent jewel of a mollusc barely 10mm in length - the blue rayed limpet is so small that it is ofter overlooked.

On the vertical gully walls, edible sea-urchins, find purchase thanks to thousands of tube feet. They have travelled up from the depths where they’ve spent the winter and now graze the seaweed turf.

Forty years ago, in Cornwall, they were the basis of an industry. Divers would fill sacks with urchins, collecting thousands in a day. Once scrubbed clean to remove their spines,  the colourful dried shells would be fashioned into trinkets and lamp shades for tourists.

Edible urchins grazing the seaweed turf at the base of the kelp forest.

Spring is when sea hares can be seen in the cove's tidepools. They are shell-less molluscs closely related to garden snails and slugs. They come into the shallows to feed on the new growth of red seaweeds.

Lift them from the water and they release a defensive purple dye. The Romans feared them as they thought the dye highly toxic, which could cause pregnancies to fail and your hair to fall out. In fact the dye works as a chemical defence on several levels. It can jam a predator’s sense organs, or fill the water around it with such high concentrations of amino acids that a predator is confused into thinking there's a bigger meal close by and drops the sea hare in its search for the tasty alternative.

Sea hares live fast and die young - when food is plentiful they can double their weight in ten days. They live for just a year - coming into the cove’s warmer water to feed and mate before dying - leaving behind them ribbons of pink spawn trailing through the kelp forest.

Cuttlefish come inshore in April to display and mate - which they do ‘head to head’. They lay eggs which can sometimes be seen on a low spring tide - entwined in clumps at the base of seaweeds. They are black and the size of marbles with a pointed tip - and known as ‘sea-grapes’. Cuttle fish die after egg laying and only their white, floating 'cuttle-bones' will wash up on the shore, to show they'd ever been here.

Cuttlefish mate head to head so the male left can transfer a package of sperm, the spermatophore, to the smaller female (right)

This May, we’ve twice seen their chief predators, Risso’s dolphins, follow them into the cove. Risso’s were once rare. Previously, I’d seen them only once in the cove in thirty years. This year though, around Cornwall, they have been spotted more regularly.

You can often tell when they’ve been in the area, as the sea is filled with remnants of the cuttlefish they’ve been feeding on. Why they seem to be more numerous is likely due occurence of large numbers of common octopus.

Risso's dolphin feed predominantly on cephalopods, including cuttlefish and octopus. They are large mottled dolphins which, at a distance, can be mistaken for orca.

Last year, there was an octopus bloom along the south Cornish coast. A Newlyn beam trawler landed 26 tons from a single trip. They’ve been a mixed blessing for fishermen though as they enter pots to eat the lobsters and crabs, so shellfish catches have been badly hit - in some places they’ve decreased by 90%. However, octopus can fetch a good price and with up to a dozen being caught in a single crab-pot, they are rapidly becoming an important source of income.

This year, much to the local shell-fishermen’s concern, they seem to be appearing on the north coast as well. They’ve been recorded in quantities from both west and east of us - at Sennen and Newquay, so it can only be a matter of time before they’re seen in Rose-an-Hale cove.

In the past, they’ve bloomed like this every few decades, but with warming seas and plenty of prey for them (crustaceans and scallops) it looks like they might be here to stay and the ecosystem will have to adapt.

They are not the only species on the rise - conger eels have always been common in the cove, particularly where there is rocky ground which has plenty of holes and crevices for them to hide in. The smaller 2-5 kg ‘strap’ eels are often caught in lobster pots which they enter to feed on the bait. Over the last year though they’ve become much more common. They are natural predators of octopus so it might well be that they help stabilise the octopus population.

One of the reasons Rose-an-Hale reserve was set up was to monitor the change in species as the climate shifts. I expected this might be more obvious on land, but we are seeing marked change in the ocean. Last year there was a bloom of salps and mauve stinging jellyfish (Pelagia noctiluca). Blue sharks have become much more numerous. We are also seeing big shoals of pilchards (now rebranded by the fishing industry as ‘Cornish Sardines’!) - a southern species that used to be common here in the 18th century and on which a whole industry was based. There was a pilchard seining operation based in the cove, but this will be the subject of another post.

This time though they were followed by Bluefin tuna, in numbers that have not been seen before.  The Bluefin’s range has been expanding northwards with the warming ocean.

Once common species are now rare - we are seeing fewer basking shark now as their copepod prey has moved north. Bluefin tuna are undeniably one of the most spectacular fish in the ocean though and it is wonderful to have them here.

Salty is intrigued by the new arrivals.