Exorbitant mortality of hatchery-reared Atlantic salmon smolts Salmo salar L., in the Meuse river system in the Netherlands Niels Brevé & Hendry Vis & Igor Spierts & Gerard de Laak & Franklin Moquette & André Breukelaar Received: 21 August 2012 / Revised: 18 January 2013 / Accepted: 18 January 2013 # Springer Science+Business Media Dordrecht 2013 Abstract Stocking Atlantic salmon Salmo salar L. in the Meuse river system is unsuccessful, since hardly any adults return upstream. To investigate problems during seaward migration, individual salmon smolts were tracked in the Dutch part of the river Meuse, using the Nedap Trail® system. The study included a comparison for two seasons, one with high (2010) and one with low (2011) discharge conditions. Cultivated smolts (Loire-Allier strain, n = 100 per year) were implanted with telemetry tags and released in the tributary Roer in March, at the beginning of the natural smolt run. The study area was split into four river sections each characterized by different conditions: tributary Roer, main river Meuse extensively dammed, main river Meuse free flowing and the estuary. Mortalities differed consider- ably between sections. In the free flowing river Meuse the mortalities were relatively low (10 and 25 %). Mortalities were high in the tributary (44 and 45 %), the dammed river (46 and 49 %) and in the estuary (89 and 90 %). Only 2 and 3 % of the smolts escaped into the North Sea. Results are discussed in relation to environmental factors light and discharge and the presence of man-made obstacles: weirs, hydropower plants and a sea lock. A lack of current and delays at man-made obstacles result in disorientation of fish, a higher risk of predation and disturbance of the smolt run timing. Mortality causes must decrease for sustaining a salmon population in the Meuse. Keywords Atlantic salmon . Salmo salar L. . Smolt mortality . Downstream migration . River Meuse . Nedap Trail® system Introduction With the aim of reintroducing the Atlantic salmon (Salmo salar L.), eggs and young salmon of various life stages have been released since 1984 into the tributaries of the rivers Meuse and Rhine (Prignon et al. 1999; Rhein 2020 Lachs (2004). Both rivers have estuaries in the Netherlands which nowadays are heavily modified and water flow is controlled with dams and sea locks. In the Rhine system the manage- ment target for a long-term stable, expanding population of salmon is a return rate of 3 % (Schneider 2009, presump- tions: sex ratio 1:1, reproductive success 75 %; MUNLV – NRW 2006). Thiel and Magath (2011) found a minimum return rate of 2.75 % for the river Weser. At the time being (2012), this rate is well below 1 % for wild smolts in the Rhine and for fish stocked as smolts (age 1) it is only 0.20 – 0.25 %. Obviously these rates are too low for a self- sustaining population (Rhein 2020 Lachs 2004; MKULNV 2011). For the Meuse, the number of returning adult salmon is much lower. From 2003 to 2011 only a few dozen adult salmon found their way back into the tributaries each year (Philippart et al. 2010). In tributary Roer (in German: Rur) the number of returning salmon was 6 in 2009, 8 in 2010, 10 in 2011 + 1 kelt and 20 in 2012 + 2 kelts (pers. comm. N. Brevé (*) : G. de Laak : F. Moquette Royal Dutch Angling Organization - Sportvisserij Nederland (abbr. SNL), Leijenseweg 115, 3721 BC, Bilthoven, the Netherlands e-mail: breve@sportvisserijnederland.nl H. Vis Fish Advice - VisAdvies BV, Nieuwegein, the Netherlands I. Spierts ATKB, fish research, Geldermalsen, the Netherlands A. Breukelaar The implementing body of the Ministry of Transport, Public Works and Water Management, Rijkswaterstaat Waterdienst (abbr. RWS), Lelystad, the Netherlands J Coast Conserv DOI 10.1007/s11852-013-0237-4