This study of the fish and fishery of Stocks Reservoir, Lancashire, is the result of contract work undertaken by the researcher for North West Water (NWW). In an attempt to describe the location of the study, relevant information covering the catchment, local geology, reservoir construction and flora and fauna is included.The Authority's remit suggested a study of three facets of the catchment, namely, a study of the native fish populations, a monitoring of the recently opened fishery and an analysis of operational filter plate impingement.In order to minimise disturbance of the sport fishery, native fish population work was necessarily limited to the reservoir's three major afferent streams, the River Hodder, Hasgill Beck and Bottoms Beck.As a preliminary measure of tributary stream status, a simple invertebrate site study was undertaken by the researcher in 1985.Fish population work based on catch per unit effort (CPUE) was pursued in the spring, summer and winter of 1985, 1986 and 1987 at 8 sites, employing D. C. electric fishing equipment, whilst Carle and Strub's (1978) MWL Method was adopted for population estimations. The validity of the electric fishing survey and age determination are discussed in the text.The native species of brown trout, bullhead, stone loach and minnow were encountered, as was the introduced rainbow trout. Species densities and the population structure of the native brown trout were examined in detail for each survey site. The River Hodder was revealed to be the least populous tributary, whilst Hasgill Beck exhibited the greatest fish densities. Spawning migrations of native brown trout were evident, with fry recruitment at its optimum at site 4 on Hasgill Beck and site 8 on Bottoms Beck. The waterfall on Bottoms Beck might well have precluded upstream access to the head waters of this tributary. Observed mean brown trout length for age data were similar to those recorded by authors researching other upland stream locations.The history of Stocks Reservoir as a sport fishery is outlined prior to the present leaseholder's opening of the reservoir as a day ticket fly fishery for the 1985 season. The water was stocked predominantly with rainbow trout, together with some brook trout and brown trout before fishing commenced.The present study covering the seasons 1985 to 1987 was based primarily on data abstracted from catch return forms, which displayed a notably high rate of submission, and stocking consent data provided by NWW. The validity of return form data is discussed.Over the three seasons studied, angler patronage was observed to decline by 16%, whilst the number of fish caught and taken also declined by 34.8% and 20.5% respectively. Angler success was similarly observed to decline in accord with the decrease in patronage and catches. Interestingly, there was an increased reliance on introductions of rainbow trout over the period, including larger fish, and by 1987 a cessation in the stocking of other trout species.From correlations observed between environmental parameters and angler patronage, anglers appeared to prefer fishing in dry, sunny conditions, but decreases in angler success occurred during periods of increased water turbidity. Such declines in success also displayed congruity with decreases in angler patronage.From a comparison undertaken with a cross-section of English and Welsh stillwater trout fisheries, Stocks Reservoir was judged to rate poorly, returning the lowest performance data in the upland stocked category.An examination of the stomach and hind gut contents of 127 rainbow trout, 7 brook trout and 8 brown trout caught by anglers, was undertaken in the 1985 and 1986 seasons, and was compared with the reservoir fauna data of Mills, M. L. (1971).A description of the water treatment plant and its operation is delineated, and a pertinent collection of fish impingement and screening literature is included.Impingement data were collated from lst March 1985 to 31st December 1987 from routine and emergency cleaning of the filter plates. After storage in a freezer, the thawed fish were examined chronologically, identified, measured and weighed. During the examinations a random sample of stomach and hind guts was procured, and scales from brown trout were removed for possible future reference.The total annual impingement was observed to vary considerably, although brown trout habitually exhibited the greatest losses, comprising 71%, 64% and 89% of fish impinged annually. Of the introduced species, rainbow trout and brook trout, brook trout were the more susceptible to impingement, but remarkably few rainbow trout were lost considering the number stocked.Rainbow trout and brook trout of medium (150mm to 300mm) and large (>300mm) length classes were impinged, whereas many smaller (<150mm) brown trout were lost, a phenomenon concurrent with the recruitment of juvenile stream fish to the reservoir population.Brown trout in particular exhibited an annual dissimilarity in rates of impingement, probably suggesting that seasonal migration was not causative of their increased impingement.Impingement of rainbow trout showed limited correlation with environmental parameters. In 1985 and 1986, increased impingement of both brown trout and brook trout was significantly correlated with low reservoir levels, and to some extent might be linked to rising values of water turbidity.The collecting of stomach and hind gut samples from impinged fish was discontinued after 1985 because of problems in collection associated with delays in sampling and probable regurgitation of stomach contents. The problem of eye fluke infestation in impinged fish was noted and enumerated as sampling progressed.A brief discussion of further routes of operational fish loss from the reservoir is included.