Site fidelity, homing and spawning migrations of flounder Platichthys flesus in the Tamar estuary, South West England

Brand-marked flounder Platichthys flesus (L.) were used to study migrations, site fidelity and homing ability of individuals in the Tamar estuary, SW England, between 1976 and 1980. A total of 1308 recaptures were made, of 7401 flounder marked at 13 stations along a 29 km length of the estuary and 2 stations in Plymouth Sound. A further 1667 fish were marked on the spawning grounds in 1976, 57 being recaptured. Recaptured fish were released again, with individuals being recaptured on up to 6 occasions. In all but 118 cases, the recaptures within the estuary or Plymouth Sound were from the original capture sites, despite the displacement of 681 fish, after marking, to a variety of locations. Most flounder in the middle estuary did not move > 200 m along the estuary until they left to spawn. Flounder in the upper estuary also showed high site fidelity, but were temporarily displaced by adverse conditions such as river spates. Ripe, estuary-marked flounder were recaptured at sea from 10 to 35 km west of Plymouth in water depths of 35 to 55 m. Most individuals returned to their original estuarine range after spawning. Twelve percent failed to return to the Tamar post-spawning, all migrating eastwards. A total of 200 Tamar fish were released 200 km eastwards along the coast. Many of these migrated towards Plymouth, 2 reaching the estuary, although some returned to the release site post-spawning. The results are applicable to the management of flounder stocks and of estuaries, emphasising the value of retaining intertidal mud flats in estuary development plans.