Last week I blogged about adaptive responses in pink salmon, possibly in response to current climate change.
This week, another paper has come out demonstrating a potentially plastic response in age at maturity in a confamilial species to pink salmon, the Atlantic salmon.
This time researchers from Norway showed that the proportion of salmon returning to over 50 Norwegian rivers from 1991 to 2005, that spent just one winter at sea feeding was on the decline. Instead, the populations examined displayed a higher proportion of multiple sea winter fish (MSW) relative to one sea winter fish (1SW). This trend results in an overall increase in age at maturity for a particular population.
The increase in age at maturity, an especially important trait for evolutionary fitness in Atlantic salmon, was linked to increasing sea surface temperatures (SST) in the North-East Atlantic ocean. These increasing SSTs are known to be having major impacts on the pelagic food web in the North Atlantic, notably causing a reduction in Copepod size thus leading to reduced lipid availability. Lipid storage is known to be a major limiting factor in the completion of maturity (i.e. gonad development) in Atlantic salmon, thus providing a plausible causal mechanism to explain the observed trend. The particular experimental design used in the study did not allow researchers to rule out the alternative explanation that MSW fish simply had higher survival than 1SW fish. This alternative would have to be ruled out before the observed trend could be conclusively linked to the causal mechanism mentioned.
An additional interesting feature about this study in relation to the pink salmon paper are the conclusions that are possible as a result of experimental design. Due to the lack of a genetic experimental component in the Atlantic salmon paper, it is not possible to rule out that the current life history changes are solely explicable via phenotypic plasticity without genetic change, whereas the pink salmon paper demonstrated actual directional frequency changes in alleles associated with the traits measured. This difference in study design means that only the pink salmon paper can describe the observed changes as a result of climate change in an evolutionary context with any confidence that they are of actual consequence. Although it is doubtful that such major changes in the life history of populations of Atlantic salmon, as observed in Otero et al., would not be of evolutionary importance, until evidence of genetic change is observed, the parsimonious conclusion is that the changes are a plastic response.