It’s two hours to high tide, and the beach has been reduced to a half moon of smooth sand. The sea is grey, grey-blue and green, and even here in the estuary it’s lined with ragged swell. Winter swimmers in trunks and swimming costumes are wading in, hunching their bare shoulders.

Without much sand to run on, my four-year-old, Nina, decides we’re playing mountain goats on the boulders at the back of the beach. They are a jumble of awkward shapes and irregular angles: sometimes I have to hold a hand – sorry, hoof – but mostly she finds her own balance. A rock pipit (Anthus petrosus) seeps at us then flutters away. Nina bleats at it. A wren (Troglodytes troglodytes) whirrs away into a crevice, a tiny ball of fox-red feathers. Nina bleats again.

We reach the end of the rocks and, with the loudest bleat of all, bound down on to the sand. It’s scattered with scallop and razor clam shells, the prettiest of which we collect in a red bucket. There are seagrass plants torn up by yesterday’s storm, laid out neatly like botanical specimens. We soon come across my daughter’s favourite beach find: a mermaid’s purse (the old sailor’s term is so much more alluring than “egg case”). A few steps on we find another purse. And then another: I’ve never seen such a concentration.

‘Without much sand to run on, my four-year-old, Nina, decides we’re playing mountain goats on the boulders at the back of the beach.’ Photograph: Tom Allan

I record our haul on the Shark Trust app, where the data is added to their Great Eggcase Hunt project. The cases we’re finding were laid by spotted and thornback rays (Raja montagui and Raja clavata), two of the commonest species around British coasts. Mermaid’s purses are leathery and tough – so tough, in fact, that they can survive for months on the seabed after the pups have emerged. The green colouration and shiny look of these ones suggest they have recently hatched: spawning takes place in the summer, after which eggs incubate for five to six months. For larger species such as the flapper skate, found off northern Scotland, incubation can take as long as 18 months.

Mountain goat limbs have become weary. Nina drops a final mermaid’s purse into her bucket, and we head for the warmth of the beach cafe.