Salmon spawning gravels in the chalk streams
of Southern England and their impact on egg survival.
In recent years declines in Atlantic salmon numbers and catches have given cause for concern in many of the southern chalk rivers. Several potential causes for such changes have been suggested, including factors operative in both the marine and freshwater phases of the life of the salmon. There is, however, a strong viewpoint that problems at the early freshwater stages could be of major significance.
Whilst the composition and quality of salmon spawning areas have been reasonably well studied in North America, in the UK there have been very few published investigations in the UK on gravel composition that is suitable for Atlantic salmon.
Over most of the stream beds of the chalk rivers of Southern England, the gaps between the larger gravels are filled by finer sediments and in many places, there is an overlying blanket of sand or silt, often associated with growths of aquatic plants. There is a seasonal cycle of low-flow (spring/summer) sediment deposition followed by high-flow (autumn/winter) wash out. In general, by the time that salmon spawn (normally December onwards), the shallower, faster flowing reaches of river which they select for redd construction will have been swept free of fine sediment by the seasonal increases in discharge. In addition, the female fish disturb the gravel and, in the process, winnow away clogging interstitial material. In normal conditions this redd construction creates an open structure which persists for sufficient time to permit adequate irrigation of the egg pockets by flowing, oxygenated water, thus promoting egg development and, ultimately, enabling emergence of fry If, however, the flushing flows are inadequate or conversely, if the sediment load is greatly increased, it may be that the areas of suitable spawning gravel are greatly reduced or degraded. In these situations, if the female fish are unable to loosen, excavate and/or winnow redds or the interstices of redds become blocked by fine particles during development and prior to emergence, then the spawning will be a partial or complete failure. In the case of blockage of interstices during development, it is known that when fine (less than 2 mm) sediment is more than 20% by weight and/or less than 1 mm sediment is more than 15% by weight, reduced survival is likely.
Several factors are known to contribute to increases in fine sediment loading of streams. Impacts from abstraction of water, reduced weed growth, forestry operations, enhanced erosion of the land surface, aquaculture e.g. discharge from fish farms and cress beds, poor agricultural practice, etc. are all likely to have detrimental effects on the river’s fine sediment load. Recent changes in agricultural practices are causing particular concern, increased growing of maize as a silage or biofuel crop (a 2,500% increase in acreage over