Spatial and temporal variations in fish populations in the upper Thames estuary

Between February 1989 and August 1990, the upper Thames estuary contained 2 3 species of fish. Fish numbers were higher and relatively constant in the uppermost part of the estuary. Number of species was augmented in summer fr om fresh water and from downstream, coinciding with high temperature, low f low and high salinity. The eight most abundant species contributed to 98.5% of the total number. Flounder Pleuronectes flesus, dace Leuciscus leuciscu s and perch Perca fluviatilis, recruited from May to August, and common gob y Pomatoschistus microps, roach Rutilus rutilus and chub Leuciscus cephalus , from August to November. The upper estuary (salinity 0.34-2.96 p.s.u.) fo rmed a species transition area between the freshwater but salinity-resistan t roach, chub, and gudgeon Gobio gobio upstream, and the estuarine eurhyhal ine common goby and flounder downstream. The three-spined stickleback Gaste rosteus aculeatus and cyprinids were more abundant at upstream while perch was more abundant at downstream sites. High abundances of gudgeon, chub and roach were associated with high transparency and dissolved oxygen and low salinity, while high abundances of perch were associated with high salinity and low transparency. Dace and three-spined stickleback were associated wi th high dissolved oxygen and low pH, and common goby with high pH. Flounder showed no clear preferences. (C) 1999 The Fisheries Society of the British Isles.