The Enhancement of Drained Salmonid Rivers in lreland - A Bioengineering Perspective

The authors, a fisheries biologist and drainage engineer have been involved in enhancing drained salmonid rivers in lreland over the last decade. Our objective has been to increase the extent of productive salmon (Salmo salar L.) and trout (Salmo trutta L.) water in rivers which have been subjected to arterial drainage schemes. This paper draws on the experience of the authors to describe, with hindsight, how one should ideally plan such a study from the drainage design phase through to the postdrainage fisheries enhancement programme. Such a programme must be regarded as a bio-engineering exercise if it is to prove successful. Information required in this area will include detail in relation to channel and bank substrate types, a longitudinal profile of all channels, bank heights and hydrographs. In relation to the latter area particular attention should be paid to both peak flood, mean (x) and minimum low flow discharges. The fisheries biologist should ideally compile a detailed ecological profile of the catchment encompassing information on bankside and instream vegetation, macroinvertebrate fauna and fish numbers with particular reference to defining the extent to which, and the reasons why, particular channel lenghts function effectively as salmonid spawning, nursery or adult areas. When the drainage scheme is being designed, the biologist, using the aforementioned data base, should examine the proposals, at draft stage, with a view to adjusting same to the advantage of fishery interests. The various ways in which the pre-drainage bio-engineering data base can be used to minimise the impact of drainage and hasten recovery are described. Ideally, post-drainage, most instream physical works designed for fisheries enhancement should not be undertaken for two years after the dredging is completed. The logic for this approach is outlined. Some of the physical enhancement works programmes undertaken by the authors are outlined and their relative success is documented. Collaborative bio-engineering ventures in relation to minimising the ecological impact of drainage design are also discussed. The objectives of the drainage engineer and the fisheries biologist are very different. However, the authors would contend that the co-operation of both disciplines in this area can help to minimise the disruptive influence of drainage programmes and, there-after, restore the productivity of channels in fisheries terms