This protocol was discontinued in 2014 due to funding cuts.
Annual electric fishing surveys were employed to assess the abundance, age structure and standing stock of salmonid populations at each stream site and at the outflow streams immediately downstream from each lake site. Fishing took place between mid-September and mid-October. Three local laboratories contributed (see collaborators). Marine Scotland and UCL/ECRC provided central coordination of the fish surveys.
The sampling procedure involved isolating within a 500 m section, three 50 m reaches of the stream with stop nets. Where topographical conditions permit these reaches are at least 50 m apart. Each 50 m reach was fished using electric fishing apparatus, usually from the downstream to the upstream net. The fishing was repeated three times, or more if no clear drop off in numbers has occurred. Fish were anaesthetised and the fork length and weight of each individual fish are recorded. All fish are retained in holding boxes in the stream and returned alive at the cessation of electric fishing.
It is possible to determine age of smaller fish from length-frequency distributions, but scales were taken from the larger fish (>65mm length) and from a selection of the smaller ones to determine age. Scales were taken from the flank of the fish just behind and below the dorsal fin. Historically quality control in the determination of fish age was coordinated by the Centre for Ecology & Hydrology and by a scale exchange exercise among the fish scientists involved in the Network.
Fish catches at each site are summarised by age/class, length and weight and together with the population estimates are recorded in UK UWMN annual data reports.
References
- Carle, F. L. & Strub, M. R. (1978) A New Method for Estimating Population Size from Removal Data. Biometrics, 34, 621-630.
- Clarke, R. T. (1988) Remove - a program for population assessment. Institute of Freshwater Ecology, Internal Paper.
- Zippin, C. (1956) An Evaluation of the Removal Method of Estimating Animal Populations. Biometrics, 12, 163-189.