Reintroducing Atlantic salmon in the river Rhine for decades: Why did it not result in the return of a viable population (original) (raw)

Historical rise of waterpower initiated the collapse of salmon stocks

The collapse of Atlantic salmon (Salmo salar) stocks throughout NorthWestern Europe is generally ascribed to large-scale river regulation, water pollution and over-fishing in the 19 th and 20 th century. However, other causes have rarely been quantified, especially those acting before the 19 th century. By analysing historical fishery, market and tax statistics, independently confirmed by archaeozoological records, we demonstrate that populations declined by up to 90% during the transitional period between the Early Middle Ages (c. 450–900 AD) and Early Modern Times (c. 1600 AD). These dramatic declines coincided with improvements in watermill technology and their geographical expansion across Europe. Our extrapolations suggest that historical Atlantic salmon runs must have once been very abundant indeed. The historical perspective presented here contributes to a better understanding of the primary factors that led to major declines in salmon populations. Such understanding provides an essential basis for the effective ecological rehabilitation of freshwater ecosystems. Marine fishing has resulted in severe and wide ranging ecological consequences dating back to prehistoric times 1–3. To date, comparable early anthropogenic impacts on fish stocks of freshwater ecosystems have not been established in quantitative terms. The main contributor to the decline in ecological integrity of the larger river systems in NorthWestern Europe, viz. the rivers Rhine, Meuse, Thames and Seine, is assumed to be 19 th and 20 th century pollution, over-fishing and river regulation 4. During the previous two centuries, migratory species like common sturgeon (Acipenser sturio), houting (Coregonus oxyrinchus), allis shad (Alosa alosa), twaite shad (Alosa fallax) and Atlantic salmon (Salmo salar) have become extinct or endangered 5,6. This decline has resulted in the designation of the Atlantic salmon as a flagship species and ambassador for ecological restoration in the River Rhine and its tributaries 4. However, despite the application of several rehabilitation measures aimed at mitigating and compensating for the adverse effects of pollution, fishing and river regulation, restocking of Atlantic salmon appears far from effective throughout Europe. A previously ignored and potentially limiting factor may be the fact that declines in salmon populations began much earlier in history than is currently recognized, i.e. well before the start of large-scale river regulation 7,8. Essential components may thus be overlooked in rehabilitation projects accounting for the limited success of restocking programs. Indeed, many pre-19 th-century sources appear to perceive salmon stocks to have been " much larger in preceding times ". However, these anecdotal observations have not been substantiated with quantitative evidence. Although it is accepted that watermills had a tremendous impact on the geomorphology of river and stream habitat 9 , the potential negative impacts of watermills has not been thoroughly analysed in quantitative terms in ecological research. Generally, the few scientific articles describing the historical effects of watermills on aquatic ecosystems do this in a qualitative or anecdotal manner, at local geographical scales 7,10 or examine relatively short periods in history 11. In this article we examine potential watermill related salmon declines in NorthWestern Europe at a temporal scale of many hundreds of years, and demonstrate the existence of a relationship between historical economic activities and the degradation of stream ecosystems. Quantitative proxies for Atlantic salmon