Atlantic salmon (Salmo salar)
The River Spey is famous for its salmon population and the fishery it supports. The River Spey has been designated a Special Area of Conservation (SAC) as it supports one of the largest salmon populations in Scotland. The salmonids are one of the most economically important families of fish in the world, and wild populations were heavily exploited in the past due to their excellence as a food resource. Nowadays wild Atlantic salmon are more important as a sporting resource with anglers coming from all over the world to fish the Spey.
Atlantic salmon are an anadromous species, that is they spawn in freshwater but migrate to sea to feed. Spawning occurs between October to December with adults females burying their eggs in redds created in the river gravel. As embryo development is largely temperature controlled the incubation period can last for over 6 months in upland areas with the emergence of the free swimming fry from the gravel in April and May. The young salmon, known as fry in their first year then parr, spend between 1 and 5 years in the river, before they become smolts in the spring and migrate down the river to enter the sea. Once at sea they generally spend between 1 and 3 years feeding and growing in the northern Atlantic Ocean before returning to spawn in the river of their birth.
The migratory habits of salmon and its reliance on a range of different habitats mean that salmon are exposed to a range of threats. Within the river the main threats to adult salmon are exploitation by anglers, predation, disease, barriers to upstream migration and pollution. Barriers to upstream migration can be physical structures, e.g. a weir or dam, culvert etc., but low flows can also limit or prevent fish migration. Low flows occur naturally but a more insidious threat is abstraction which can permanently reduce flows making parts of the catchment unsuitable for salmon.
During the incubation period in the gravel eggs are reliant on there being an adequate flow of water through the gravel to bring oxygen and to remove waste products. Excessive siltation can block the voids in the gravel matrix suffocating eggs or entombing the alevins. In a healthy river system with a robust salmon population egg survival is high and after emergence there will be a period of intense competition when the fry compete to establish a territory. By the end of the first summer only the fittest fry; those able to establish and protect a territory and survive predation, will still be alive. The survival rate of these surviving fry through to the smolt stage determines the productivity of the river.
Spey salmon generally become smolts when 2 or 3 years with the majority returning after two years at sea. If they return after only one year at sea they are known as grilse. Grilse are generally smaller, ranging from 2lb to 11lb for the later returning fish. The Marine Scotland official catch returns, which run from 1952, show that from at least the mid 1960s the percentage of grilse caught by rod on the Spey increased with grilse forming 40% of the catch in the 80s. However only in 1988 did the Spey grilse catch surpass the salmon catch. In recent years the proportion of grilse appears to be declining, with a resurgence in salmon numbers, although 2010 was a notably good grilse year. This recent trend towards a greater proportion of salmon to grilse has been observed elsewhere. Data from the Conon, where there is an adult fish trap shows a switch from being a river dominated by grilse to salmon over the last few years.
Long term studies of salmon catches on many rivers have shown that salmon runs change over long cycles. The graphs on the salmon catches page show that on the Spey the spring catch has declined massively in the last 60 years. However the graph above appears to show an inverse correlation between spring salmon and grilse catches. Will the recent trend in reduced grilse catches herald a return to a period of spring salmon dominance? Time will tell but an increase in the highly valued spring salmon catches will be welcomed by most Spey anglers.