Life history changes in Atlantic salmon from the River Dee, Wales

The change in life history of Atlantic salmon (Salmo salar L.) on the River Dee over the last 60 years is described. Over the last 60 years, salmon have shown a change in run timing, the majority currently entering the river between August and October compared with prior to June. This has coincided with a change in the sea age composition, which was dominated by multi-sea winter salmon prior to the 1980s after which the proportion of 1sea-winter fish increased until they now dominate the mature population. Growth rates of salmon in fresh water remained relatively stable until the mid-1980s and then increased. By the end of the 1990s juvenile salmon were, by the end of their first and second year, respectively, ∼60 and ∼19%, on average, larger than they were between the late 1930s and mid-1980s. This has been reflected in a change in the age composition of smolts where the mean smolt age has declined from ∼2 years prior to the 1980s to ∼1.6 years in the late 1990s. There was no observed trend in post-smolt (marine) growth for salmon. Size at return for 1SW salmon appeared stable while there is some evidence of an increase in mean length of 2SW salmon at the end of the 1990s. A steady state life history model was developed which suggests an increase in the instantaneous rate of mortality by 2.9% from 1.495 year−1 in 1937/1938 to 1.538 year−1 in 1967/1969 and by 21.6% to 1.870 year−1 in 1997/1999. This is considered to explain the shift in mean age at maturity from 5.2 to 4.8 to 3.9 years for the three periods examined. There is close agreement between the observed mean age at maturity and that predicted by the model suggesting optimal lifetime reproductive success.