Idris Prothero was ten years old and he watched the water. His aunt Nell was the skipper of the drifter Nancy Rose, and she took him out in the summer, and he got a share of the haul like everybody else aboard. His job was the top of the net drum, which is a flat steel lid a long way up, where you can see for miles and nobody can see you. When the feed came he called it, and his aunt set the nets.

A herring shoal gives itself away three ways. The water goes blue-grey instead of green. Gannets go into it from high up, straight down like stones, and stay under three seconds. Nothing comes out of the top. Herring hold together and stay down. Idris had a brass stopwatch that had belonged to his grandfather. He timed the birds and he counted the fish that came out of the water. He chalked each trip on the galvanised lid of the fish box, in three columns: BIRDS, JUMPS, WATER.

At the fish market there is a harbour book. It has one line for every landing, with the vessel, the day, the fish and the weight in kilos. It is a record. Nobody rubs anything out of the harbour book.

The herring came in the second week of August, and they were fat, and they filled eleven boxes in eleven nights. Then on the fifteenth the water went a dirty green and the gulls came instead. Idris called the feed. His aunt set the nets at four in the morning, because that is the hour you fish for herring. They hauled at seven, and they had nine boxes of mackerel.

Mackerel go soft. Herring do not. A box of mackerel has to be bled and iced the hour it comes aboard, and the ice on that boat was herring ice. Two boxes went over the side as fish meal before noon. The harbour book wrote NINE BOXES MACKEREL, 780 KILOS.

It went on like that for four weeks. Gulls, and no gannets. Green water. Idris calling the feed at four in the morning. Mackerel every night, twenty-eight nights of it. His aunt never said a word about the watching, which was worse than shouting. His share of twenty-eight nights of mackerel was smaller than his share of eleven nights of herring, and he could do that sum without a pencil.

Then he went over the chalk on the box lid. The spray had taken half of it, but it was still there under the tar. He had written BIRDS in the first column on every single trip. He had never written which birds. So he went back through it and sorted the columns by what the birds were doing. Gulls hang and drop and fight each other on the surface. They never go under. Gannets go in from thirty feet and come up with a fish.

The eleven nights with gannets in them are the eleven nights the harbour book says herring. The twenty-eight nights with gulls in them are the twenty-eight nights it says mackerel. He had been calling gulls gannets because they were birds, and the herring nights had the same birds over them, with something underneath.

On the last Saturday in September he watched for an hour and ten minutes on his grandfather's stopwatch. Gannets. Three of them. Blue-grey water. Nothing came out of the top. “Gulls,” he said, after a while. “And green water.” “Gannets, then,” said his aunt. “And blue. Set the gear.” They landed twenty-two boxes of herring and two boxes of mackerel before the sun was properly up. The harbour book wrote it down.

The paper for the season says thirty-three boxes of herring and sixty-six boxes of mackerel. Twenty-eight of those nights were not mackerel nights, and a harbour book is a record, and a record of a wrong night stays a record of a wrong night. His aunt read the line for the last Saturday and put her thumb on it. “I will take that,” she said. “Do not let it go to your head.”

The salt has taken most of the chalk and run the mackerel weeks into one long grey stripe, so there is no way now to say which night was which night. On the last line he wrote two things, and one of them is not a number of fish. He wrote 1 H 10 M beside the birds. His grandfather had timed herring in the net with that stopwatch. Idris timed gannets with it. They were not the same measurement, and the harbour book was never going to know that.

The boat came in at six. The box lid went down on the deck with the chalk face up, and the crew stacked fish boxes on it, because that is what it was for. Nobody wiped it off. It went white again under the salt.