Six years' data from the River Coquet together with published information for other British rivers show that upstream migrant salmon and sea trout tend to be intercepted at counting fences and be caught on rod and line when river flows are somewhat higher than average and not when they are at their extreme values, though there is no particular river flow favoured all the year round. In years of highest river discharge the number of fish entering the River Coquet and the number of fish caught on rod and line are highest but the efficiency of angling is lowest.

The survival of Atlantic salmon srnolts on exposure to constant concentrations of ammonia has been measured under laboratory conditions. At concentrations of dissolved oxygen close to the air-saturation value, the 24-h LC50 of un-ionised ammonia is 0.15 mg NH, 1 -I in fresh water (hardness 264 mg I-' as CaCO,) and 0.3 mg NH,I-l in 30% sea water; at concentrations of dissolved oxygen of 3.5 mg 1-i in fresh water and 3.1 mg 1 -I in 30% sea water, the 24-h LC50 is 0.09 mg NH, 1-1 and 0.12 mg NH, 1-1 respectively; for fish acclimated for 1 day to a concentration of ammonia close to the 24-h median for un-acclimated fish, the median is increased between 38 and 79%, depending on test conditions.

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