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The Enhancement of Drained Salmonid Rivers in Ireland 9 these drainage schemes involved a dredging of channels both vertically and horizontally to achieve the desired objectives. A small number of embanked schemes were undertaken. A limited number of partial drainage schemes were undertaken from the late 1800's to 1950. Subsequently many of Ireland's major catchments were subjected to arterial drainage from 1950 to 1985 (fig. 1). Fish stocks in virtually all of the drained river catchements in question are domi- nated by salmonids. There are two in- digenous species in question, atlantic salmon (Salmo salar L.) and brown trout (Sa/mo trutta L.). The water- sheds in question have drainage basin areas up to 4,375 km2. Channel gradients in the main channels and first order tributaries of these catche- ments are generally within the range 0.05 % to 0.3 % (fig. 2). Since the 1980's there has been an increased recognition in Ireland that its salmonid riverine catchments are a valuable natural resource and a sig- nificant source of tourist revenue (Whelan and Marsh, 1988). Unlike many European nations Ireland's riverine catchments continue to func- tion as salmonid waters. For example migratory atlantic salmon stocks, while reduced in numbers in some cases, have not been eliminated in any catchment in Ireland in the last century. Over the last decade the authors have been involved jointly in under- taking post-drainage fishery enhance- ment programmes in a number of drained catchments. We have drawn on our experiences in this area to pre- sent a strategy, with hindsight, in re- lation to how drainage engineers and biologists might work together to en- sure that the drainage of salmonid catchments will cause the minimum damage to the functioning of these channels from a fisheries perspective. The key elements in each stage of such a strategy are outlined in this paper. Comment is also provided on drainage maintenance in relation to reducing its impact on fisheries. Some of the positive and negative effects of drainage on Irish rivers are also out- lined. 2. THE PLANNING STAGE Initially when a catchment is selected for drainage detailed physical, hydro- logical and agricultural studies are re- quired in order to design the scheme and thereafter a cost benefit analysis can be carried out. Assuming that the latter is favourable and the scheme is to proceed then the physical and hy- drological information compiled by the engineers in the course of the plan- ning stage should be used by the bi- ological team as a starting point for their involvement. A schematic outline of the type of information compiled by Irish drainage engineers in the course of their studies is presented (fig. 3). In the authors experience salmonid stock densities in Irish rivers are de- termined in the first instance by chan- nel gradient and influenced thereafter principally by discharge level, sub- strate type and the bank cover regime in circumstances where good water