Freshwater Biology (1993) 30, 439-446 Dispersal of adult Trichoptera at a Danish forest brook ANNETTE SODE AND PETER WIBERG-LARSEN* Department of Nature and Aquatic Environment, Funcn County Council, Qrbxkvej 100, DK-5220 Odense S0, Denmark *Author to whom correspondence should be sent SUMMARY 1. The dispersal of adult Trichoptera at the small Danish forest brook, Stamperenden, was studied from 10 May to 6 October 1990 using Malaise traps placed across the brook, parallel to the brook at distances of 2, 20 and 40 m, and perpendicular to it at a distance of 1—3 m. 2. Of the total of 4175 specimens caught, lotic spedes breeding in Stamperenden accounted for 95.6%. The dominant species were Agapelus fuscipes, Lype reducia, Plectrocnemia conspersa, Potamophylax nigricornis and Silo pallipes. 3. The sex ratios in the catches of most species were uneven. Agapetus fuscipes males were in excess, probably because males fly more frequently than females. 4. The dominant species flew predominantiy above the surface of the brook. Whereas both sexes oi A. fuscipes,' L. reducta and S. pallipes were rarely found even as little as 20—40 m from the brook, the dispersal range of P. conspersa was much wider. 5. The implications of low dispersal for the biological recovery of damaged but subsequently restored streams and stream catchments are discussed. It is concluded that complete biological recovery may be a process requiring decades unless the original aquatic fauna is re-established artificially. Introduction A number of Danish streams have been or are being restored in order to re-establish habitats for stream organisms. Restoration activities are usually limited to improvement of the physical environment and water quality, it being anticipated that stream organisms will recover rapidly and spontaneously following stream restoration. However, as pointed out by Fuchs & Statzner (1990), the degree and speed of recovery depends on the availability of inocula, i.e. communities able to provide potential colonizers, within the catchment of the restored stream. With most stream insects, rapid recolon ization will probably occur if the species is present elsewhere in the catchment, the determining processes being larval drift and upstream migration and oviposition by females (e.g. Mulier, 1982). If the spedes does not occur in the catchment of the restored stream, how- ever, recolonization requires the immigration of females from elsewhere. Their recolonization poten- tial will depend on spedes-spedfic factors such as behaviour, flight strength and dispersal, physical factors such as the distance between the streams and/or catchments, and the weather. Clearly of great significance is the dispersal range, i.e. the distance that the spedes will normally venture from its habitat. A useful approach to the understanding of recolonization processes, therefore, is to examine spedes-specific dispersal around undisturbed natural stream habitats. The purpose of the present study was to elucidate dispersal of adult Trichoptera at a small forest brook as a reflection of their colonization potential. Description of the Study Area The study was undertaken at a forest brook, Stam- perenden, situated in Northern Funen, Denmark (lOnO'E, 55''35'N). The brook is spring fed with a relatively constant discharge, running through glacial sandy deposits, the catchment being dominated by agriculture. The study area is an unregulated section of the brook situated in a dedduous forest (Fig. 1). 439