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22 M.-F. O'Grady and J. Curtin figures are very conservative esti- mates of the increases which might be achieved. Significantly higher values have been noted at a number of sites in the Boyne catchment in as little as 2 years after works (O'Grady, unpublished). In addition the figures provided take no account of the large numbers of salmonid fry which are likely to be surplus to the carrying capacity of the section. Some of these young fish will emigrate and are likely to colonise other channel areas. The value of the fish produced were cal- culated on the following basis. It was assumed that the production costs of a fish farm smolt was £ 1.00. It has been established that there is three to fourfold difference in the proportion of wild smolts which return as adult salmon compared to farm reared fish (Mills & Piggins, 1983). Consequently the commercial value of the former type is £ 3-£ 4 per fish. Similarly there is a significantly greater survival of wild trout compared to stocked fish. For the purposes of this calculation the differential in survival was calcu- lated at 100 %. The value of the wild trout produced are therefore calcu- lated at twice the current fish farm production costs. This too is a con- servative figure (O'Grady, 1981). Despite the very conservative basis of calculation in this case it is clear that, in terms of fish production alone, there is a complete return on investment in as little as five years i.e. an allowance is made for a slow initial colonisation rate. Monitoring of some enhanced sites indicate that this is a generous allowance. Post-drainage in-stream and ripa- rian enhancement programmes result in complex ecological change over time of benefit to water quality, fishe- ries, wildlife and general conservation interests. Therefore a simple cost be- nefit in fisheries terms for a specific site falls far short of describing the full benefits. 6. THE DRAINAGE MAINTENANCE MANAGEMENT AREA Many of Ireland's catchments have now been drained and thereafter, in the longterm, must be maintained. Currently there is very little informa- tion available to indicate the ways and extent to which such programmes af- fect fisheries. In 1990 the O.P.W. in conjunction with the Central Fisheries Board have commenced a five years research programme to study this area (O'Grady and King 1991 and 1992a). This exercise is being funded by O.P.W. The objectives are to study a wide range of specific site types in channels which normally require maintenance and, following works, monitor the effects on fish stocks. This will involve the "creation" of experi- mental and control zones in each hab- itat type to review the effects of a standard maintenance exercise rela- tive to both unmaintained areas and zones where an alternate main- tenance approach is adopted. It is hoped that this programme will enable