Maren Oke was nine years old and she kept the rock pool book. The pool sat on a slate ledge below the slipway at Kilve, and it was the last rock pool for a mile. She went down to it every day after school, at four o'clock, and she wrote down what was in it. She wore a red anorak to her knees and green wellingtons, and the cuffs were wet through when she finished.

The book had six columns. Date. Time. Tide. Depth on the staff. Water temperature. Then four counts: beadlet anemones, whelks, shore crabs and blennies. Mr Trewin came on the first Sunday of the month and took the book away in a plastic bag. He was a volunteer with the county wildlife trust. He copied the numbers into a shore survey for a bigger office. The pool was one line in that survey, and his line said MAREN OKE, DAILY. In April the line for anemones said forty-four.

By June it said thirty-one. By July it said nineteen. Mr Trewin wrote a question mark in the margin of the June page and nothing at all for July. Maren kept going down to the pool and told herself she was counting worse. Then the trust wrote back about the path. The council was widening the slipway path and wanted a survey of the pool first, because a healthy pool had to be dug around. The sheet came back with the pool marked in red.

The anemones were the last of their kind on that shore. Fresh water ran off the road above, and where it ran they stayed. Gridded over and filled with rubble, that was the end of them.

Maren read her book again on a Monday night, torch under the blanket, and got to the tide column first. It said LOW. Every page. LOW, in her own hand. She had written LOW on the first page and copied it after that, because that is what goes in the tide column. She had never checked it.

Low water comes about fifty minutes later every day. A fortnight is eleven hours and forty minutes of that. So four o'clock does not stay four o'clock. It walks the whole way round the tide and comes back twice a month. In April, four o'clock was low water, and the pool was forty centimetres deep. In July, four o'clock was six hours after low water. She had written the depth in her own book: forty centimetres in April, twelve in July.

By then the pool was a scrape, and the anemones had shut tight like closed fists. The thermometer in its copper case read twenty-seven degrees, in water that is fourteen degrees at Kilve at eight in the morning. She had been counting a pool that was not there, and she had written the number down.

So she changed the hour. She read the time of low water off the tide table on the kitchen wall and set the alarm. Then she went down to the pool in the cold and the dark, and got herself home before school. The first cold morning she counted forty-four. The next morning forty-six. The next, forty-nine. Her fingers were too cold to write after twenty minutes at the water, so she did the counts in the porch.

Mr Trewin came on the Sunday with the big book open on the tailgate of the van. “There is no loss,” he said. “There is a book with the wrong hours in it.” He drew one line through the red pool on the survey sheet and wrote POOL HEALTHY, WATCH. The council put the filling off until the spring. It was not cancelled. Somebody wrote the word again on the form, small, in pencil.

Then Maren went through the old pages with a pencil and wrote wrong hour beside every count that was too low. Twenty-six of them. She got to page twenty-two and stopped. Page twenty-two was a Saturday in May. Her aunt had walked down with her at ten in the morning and they had counted together. That was a good hour, and the count was fifty-one. She had written wrong hour on it, so she rubbed it out with her sleeve. The rubber left a grey smear straight across the five.

On the last page she wrote the date, the time of low water, the depth, the temperature, which was nineteen degrees, and the four counts. The number that went to the trust was fifty-one. Fifty-one is on page twenty-two, in May, under a smear about the width of her thumb. Mr Trewin reads it as fifty-one. Maren read it as forty-one once, when the page was wet, and wrote that down in the back with the date.

She walks up the slipway with the book under her arm. The spray has got into its corners, and she still does not know whether the pool holds fifty-one anemones or forty-one.