Sea trout migration in the Rhine delta

Sea trout Salmo trutta migration in the Rhine delta, The Netherlands, was studied using the NEDAP TRAIL System^®, consisting of a chain of fixed detection stations and transponders, each transmitting a unique code. From 16 December 1996 to 31 December 2000, 195 (34%) out of 580 tagged sea trout were detected. Inland migration of sea trout was observed through the sluices in the dam between the Wadden Sea and Lake IJsselmeer (Afsluitdijk), the Nieuwe Waterweg and the sluices in the Haringvliet Dam. Seventy-five per cent were detected for the first time at one of the detection stations within 30 days after tagging. Inland migration through the sluices in the Afsluitdijk, the Nieuwe Waterweg and the sluices in the Haringvliet Dam was observed for 34, 103 and 70 sea trout, respectively. Based on the number of tagged fish released in the adjacent coastal areas, however, migration through the Afsluitdijk was most important. During migration in fresh water, sea trout was predominantly active during daylight. Fish entering the Rhine delta through the Nieuwe Waterweg or the sluices in the Haringvliet Dam chose eight and six different routes, respectively, for upstream migration. Differences in current velocity in the Rhine distributaries seemed to be an important factor in the choice of migration routes.