Hierarchical analysis of wild Atlantic salmon (Salmo salar) fecundity in relation to body size and developmental traits.

Using data from wild Atlantic salmon Salmo salar returning to spawn in seven Scottish rivers, we developed a model of fecundity based on individual body size and key developmental traits. We used a novel approach to model selection which maximises predictive accuracy for application to target river stocks to select the best from a suite of Bayesian hierarchical models. This approach aims to ensure the optimal model within the candidate set includes covariates that best predict out-of-sample data to estimate fecundity in areas where no direct observations are available. In addition to body size, the final model included the developmental characteristics of age at smolting and years spent at sea. Using two independent long-term monitoring datasets, the consequences of ignoring these characteristics was revealed by comparing predictions from the best model with models that omitted them.

Precision and accuracy of Dahl-Lea back-calculated smolt lengths from adult scales of Atlantic salmon Salmo salar.

Using tagged and recaptured Atlantic salmon Salmo salar (n = 106) the present analysis shows that the most commonly applied linear back-calculation method for estimating past length, the Dahl-Lea method, resulted in overestimation of the length of large smolts and underestimation of small smolts. A correction equation (y = 0.53x + 6.23) for estimating true smolt length (y) from lengths back-calculated from adult scale measures (x) to account for these systematic discrepancies is proposed.

Is the composition of dissolved organic carbon changing in upland acidic streams?

The quantity and composition of dissolved organic carbon (DOC) exported from upland soils to surface waters is a key link in the global carbon cycle and economically important for treating potable waters. The relationship between ultraviolet (UV) absorbance and DOC concentrations can be used to infer changes in the proportion of hydrophobic (aromatic, recalcitrant) carbon and hence biodegradability of DOC. This study describes a significant change in the relationship between UV absorbance and DOC over 22 years at two upland moorland catchments in Scotland, UK. Despite increases in long-term DOC concentrations, analysis suggests that the proportion of hydrophobic material has declined. A statistical mixed-effect modeling approach was used to examine the likely mechanisms that could explain these observations. Annual nonmarine sulfate load was the only significant forcing factor that could explain the observed long-term trend in the UV absorbance-DOC relationship at both sites. It is hypothesized that enhanced heterotrophic decomposition of organic matter and increased solubility of carbon compounds in soils where sulfate driven acidification is being reversed are the dominant mechanisms behind this change in DOC composition. These trends will impact on carbon substrate dynamics by potentially increasing biodegradability of exported organic matter, influencing carbon cycling in terrestrial and aquatic ecosystems.

Functional and aggregative responses of harbour seals to changes in salmonid abundance.

There is intense debate over the potential impact of seal predation on declining salmon stocks in both the Pacific and Atlantic oceans. However, efforts to model such interactions have been constrained by a lack of data on the functional and numerical responses of these predators. Based upon theory, and data from small-scale terrestrial and freshwater systems, a type 3 functional response is expected to best describe predation by generalist pinnipeds. Similarly, theory also predicts that seal numbers should increase with salmon density in rivers following an aggregative response of predator to prey. We tested these predictions by studying the diet and local density of harbour seals in relation to seasonal variations in the abundance of salmonid in a Scottish river system. As predicted, the abundance of seals in the river was directly related to the abundance of returning salmon, and dietary data supported the type 3 functional response to changes in salmonid abundance. These studies provide empirical support for the use of type 3 response in modelling studies.