No abstract

No abstract

No abstract

No abstract

Twenty adult returningsalmon were tracked in theestuaryofthe R. Ribbleusingacousticoxygensensing transmitters during summer when a pronounced dissolved oxygen (DO) sag develops. Excluding two mortalities, 75% of these fish left the estuary to sea. Fish in the estuary moved to and fro with the tides over a 10 km amplitude experiencing cyclical changes in DO. Fish movement was inhibited below 40% of air saturation value (ASV) DO, and there was evidence of avoidance of DO below 55% ASV. The high rate of loss of fish to sea suggests that many of the fish in the estuary are non-Ribble fish en roure to other rivers. Adverse conditions in the Ribble therefore are likely to affect stocks in adjacent rivers. §Present address: Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, Merseyside L63 3JW, U.K.JIPresent address: Ministry of Agriculture Fisheries and Food, Room 367 Great Westminster House,