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Exposure of brown trout, Safmo frurfa, to zinc under continuous flow conditions over 96 h showed that both water hardness and pH exert major influences on the toxicity of the metal. 96-h LC50 values for total zinc ranged from cO.14mg I-' in alkaline soft water (PH 8; lOmg I-' as CaCO,) to 3.20 mg I-' in acidic hard water @H 5; 204 mg I-' as CaCOJ. A variable reduction in zinc toxicity in hard water compared with soft water over the pH range 4 9 was attributed to high external calcium. Zinc toxicity was positively correlated with decreasing acidity over the pH range 5-7, the metal being most toxic at pH 8-9 where metal complexes predominate. Below pH 5 metal toxicity also increased, irrespective of hardness. Water hardness and pH interacted with zinc toxicity in a complex manner, apparently dependent on physical and chemical transformations of the metal, and as changes in uptake, detoxification and excretion by the fish.

In January 2004, 46 wels catfish, Silurus glanis L., between 51 and 135 cm total length were tagged and released into a 0.4 ha recreational, catch and release, lake fishery in the UK, and their recapture by anglers monitored throughout the year. Of the 46 tagged fish, 16 were recaptured by anglers during 170 capture events. Some individual fish were captured up to 26 times. Catfish <75 cm appeared to be the least susceptible to capture and fish >100 cm most susceptible. Frequency of captures increased with temperatures >10°C and peaked between June and August. Selected recapture weight data were used to produce a mass specific growth equation and revealed specific growth rate was a decreasing function of body mass over the size range of the recaptured fish. Their growth was slower than fish from elsewhere in their distribution range. K E Y W O R D S : mass specific growth, non-native, recreational angling, stocking.