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

Abstract

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 mid1980s. 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.

Evidence for long‐term change in length, mass and migration phenology of anadromous spawners in French Atlantic salmon Salmo salar

“…The observed delay in migration timing ( c . 5–10 days per decade) is also congruent with previous findings in Scotland (Youngson et al ., ; Todd et al ., ), England (Aprahamian et al ., ) or Spain (Valiente et al ., ).…”

“…In particular, a decline in the proportion of late maturing fish has been reported for many European and North American stocks (Baglinière et al ., ; Otero et al ., ; ICES, ; Jonsson et al ., ). Those changes have been accompanied by a decline in length and mass of fish and a variation in age at maturity (Aprahamian et al ., ; Bacon et al ., ; Jonsson et al ., ) together with a delay in adult migration timing (Valiente et al ., ; Todd et al ., ). For instance, higher proportions of 1 sea‐winter (1SW) fish in the late 20th century and more multi‐sea‐winter (MSW) fish in this century have been reported by Jonsson et al .…”

Getting into Hot Water? Atlantic Salmon Responses to Climate Change in Freshwater and Marine Environments

“…Narayanan et al 1995 ) and may comprise a fundamental population response to climate change. For example, over the past 60 years in the River Dee, Wales, Aprahamian et al (2008) showed that most adults now re -enter between August and October as opposed to prior to June. This coincided with a shift from MSW domination prior to the 1980s to the present prevalence of the 1SW maturity grouping.…”

Influence of climate and abundance on migration timing of adult Atlantic salmon (Salmo salar) among rivers in Newfoundland and Labrador

“…; Aprahamian et al. ) may also have contributed to later runs. Reduced condition was indirectly associated with warming temperatures that may have contributed to decreased prey availability (Beaugrand & Reid ) such that salmon in some European rivers may be remaining at sea longer as a result of lower swimming stamina (Valiente et al.…”