Literature Survey Into the Possibility of Restocking the River Rhine and Its Tributaries with Atlantic Salmon (Salmo Salar)

Literature survey into the possibility of restocking the River Rhine and its tributaries with Atlantic salmon (Salmo salar). Dr. S.J. de Groot NETHERLANDS INSTITUTE FOR FISHERY INVESTIGATIONS (RIVO) Haringkade 1 - P.O. Box 68 - 1970 AB IJmuiden - The Netherlands Tel +31 2550 64646 Publications and reports vol. 1989-11 of the project "Ecological Rehabilitation of the river Rhine". Participating institutes: On behalf of the Ministry of Transport and Public Works: Institute for Inland Water Management and Waste Water Treatment (DBW/RIZA), P.o.box 17, NL 8200 AA Lelystad. On behalf of the Ministry of Housing, Physical Planning and the Environment: National Institute for Public Health and Environmental Protection (RIVM), P.o.box 1, NL-3720 BA Bilthoven. On behalf of the Ministry of Agriculture and Fisheries: Netherlands Institute for Fishery Investigations (RIVO), P.o.box 68, NL-1970 AB IJmuiden. Report (RIVO): MO 88-205/89.2 Date of publication: April 1989 S.\I..M(lN-\VATCI!i:i; s T'iWKIi ON TUI". ItllIXK. Rhine basin: 185,000 km2, incl. 25,000 km2 in the Netherlands Length of the river: Konstanz-Lobith 862 km Lobith-Kampen* 133 km Lobith-Hagestein 84 km Lobith-Vuren 89 km Vuren-Hook of Holland 79 km Average discharge* 2,200 m3/s Highest discharge recorded 13,000 m3/s (1926) Lowest discharge recorded 620 m3/s (1947; open river) 575 m3/s (1929; river frozen over) Including 8.6 km of meandering river bed straightened by hydraulic engineering works. Based on data collected over the period 1901-1975 at the Lobith measuring station. Contents: Foreword List of abbreviations Summary 1. Ecology 1.1 Life cycle (Figs. 1.1, 1.2) 1.2 Spawning grounds (Figs. 1.4, 1.5, Table 1.2) 1.3 Nursery areas 1.4 Population dynamics and production 2. History of the salmon fishing industry up to 1940 2.1 Size of the salmon population in the Rhine 2.2 Variations in the number of salmon caught 2.3 Distribution of salmon in the Dutch part of the Rhine basin 2.4 The effect of human intervention 2.4.1 Channelisation and flood control 2.4.2 Closing off the delta and the construction of weirs 2.4.3 Sand and gravel winning 2.4.4 Waste water discharges 2.4.5 The impact of the fishing industry on salmon stocks 2.5 Developments in Western Europe and the N.E. Atlantic 2.5.1 Germany 2.5.2 The United Kingdom and Ireland 2.5.3 France 2.6 Salmon stocks at the end of the 19th century 3. Developments since 1940 3.1 Size of the population 3.2 Production of smolts or juvenile fish 3.3 Fluctuations in numbers 3.4 Channel works and the construction of weirs and locks 3.5 Additional factors likely to limit the size of salmon stocks 3.5.1 Minimum water quality standards for salmon and trout 4. Future perspective 4.1 Increasing existing salmon stocks and further rehabilitation measures 4.2 Accessibility of the Rhine for fish returning to spawn 4.3 Expected dispersion within the Rhine basin 5. Assessment of future prospects 5.1 Feasibility of restocking operations 5.2 Recommendations for further study 6. References Foreword This report contains the findings of a literature survey into the possibility of restocking the Rhine and its tributaries with the atlantic salmon. It principally deals with the possibility of reintroducing salmon into the Rhine basin. The report has been prepared as part of a joint study undertaken by the Institute for Inland Water Management (DBW) of the Dutch Department of Public Works (Rijkswaterstaat) in Lelystad and the Netherlands Institute for Fishery Investigations (RIVO) in IJmuiden. Special thanks are due to a number of researchers who have contributed to this study, in particular Dr. L.P. Hansen (Trondheim), Prof. A.D. Hawkins (Aberdeen), Dr. D.H. Mills (Edinburgh), Dr. J.E. Thorpe (Pitlochry) and Mr. A. bij de Vaate (Lelystad). IJmuiden, 5 April 1989. S.J. de Groot List of abbreviations A Anal fin D Dorsal fin DAFS Department of Agriculture and Fisheries, Scotland DBW Institute of Inland Water Management (of RWS) EC European Community EEFAC European Inland Fisheries Advisory Commission ICES International Council for the Exploration of the Sea IFREMER Institut français de recherche pour l'exploitation de la mer IMP Multi-year Plan of RWS LL Scales along the lateral line MO Environmental Research Department (of RIVO) NASCO North Atlantic Salmon Conservation Organisation RIVO Netherlands Institute for Fishery Investigations RIZA State Institute for Waste Water Management RWS Dutch Department of Public Works (Rijkswaterstaat) SUMMARY The Atlantic salmon (Salmo salar L.), which is an anadromous migratory fish that reaches maturity at sea and returns to spawn in fresh water, was once of major importance to the fishing industry along the River Rhine. Salmon can return to their natal streams to spawn after only spending a single winter at sea. Such fish usually begin their ascent of the river system between June and August, while salmon that have been at sea for two winters tend to re-enter fresh water somewhat earlier in the year, between May and July. Those salmon that have spent even longer periods at sea are likely to delay their return until September or October. The transition to fresh water is greatly facilitated if the temperature difference between the river and the sea is small. After entering the river system, salmon generally swim upstream for several kilometres and wait for a period of time, which may be a matter of hours or even as long as two months. They select a part of the river where the current is minimal and remain almost stationary to acclimatise. This allows them to reorientate and provides an opportunity for certain physiological changes to take place. Although the salmon's ability to recognise characteristic smells in the river is considered to be essential for locating the spawning grounds, other factors are thought to assist in the general recognition process. While ascending the river, salmon only swim at night and therefore seek shelter during the day. This contrasts with their behaviour at sea where salmon also remain active during the day. After reaching the spawning grounds, salmon lay their eggs in clear cool water (2-6°C) and bury them in gravel. In the case of the Rhine, the traditional spawning grounds lie outside the Dutch territory in the river's many tributaries in Germany, France and Switzerland. After about 110 days, larvae emerge from the eggs and disperse. Good salmon rivers contain upwards of 20 larvae per 100 m2. Less well-populated streams have about 5 larvae per 100 m2. By dispersing in this way, salmon larvae make the best use of the limited food resources available and eventually develop into fully adapted freshwater fish. At this stage of their life cycle, salmon are known as parr. After remaining in the nursery area for 1, 2 or 3 years, most parr undergo a further transformation into smolts, which forces them to leave their natal streams for the sea. However, a certain number of male parr remain behind eventually becoming sexually mature without undergoing smolting. Smolts usually swim downstream to the sea in March and April. Unlike parr, which are only active at night, smolts also travel during the day. Several factors have been put forward to explain why smolts decide to migrate to the sea. These include the higher discharge rates that occur in Spring, the corresponding reduction in pheromone levels, as well as the action of light and temperature changes. On encountering brackish water, smolts undergo further physiological changes to prepare them for the marine environment. Where the transition from fresh water to sea water is rather abrupt, the pockets of fresh water that are normally found in estuaries can facilitate the process of adaptation. Evidence has also emerged to suggest that the time of year at which such changes take place is quite critical. Up to about forty years ago, when salmon still populated the Rhine, they would swim down the river and enter the North Sea via outlets along the Dutch coast. After drifting along with the residual currents, they eventually headed in a northwesterly direction towards the seas between Greenland and Iceland. This type of behaviour is typical of the post-smolt phase, in which salmon attempt to conserve their energy reserves and maximise their growth rates. Depending on the prevailing conditions, Atlantic salmon will stay in the seas around Greenland for 1, 2 or more years before returning to their spawning grounds. Their physiological condition is thought to be one of the key factors which determine when they decide to return. The importance of salmon fishing along the Rhine is discussed in detail in this report by reviewing the Dutch and German salmon catches over the periods 1863-1957 and 1875-1950, respectively. Even up to the end of the last century, it was not uncommon for Dutch and German fishermen to land 100,000 salmon a year. However, factors such as the increased use of locks and weirs along the Rhine, coupled with the growth in pollution, soon led to a rapid decline in numbers. By 1933, the salmon fishing industry in the Netherlands had virtually ceased to exist. Analysis of the available catch statistics suggests that the decline in the salmon population could already have started before official records began. Under normal circumstances, the probability that an egg develops into a mature salmon which returns to its natal stream to spawn is rather small. Moreover, the chance of this happening has become even more remote due to actions taken by man.

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