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.

Temporal and spatial patterns of sea lice levels on sea trout in western Scotland in relation to fish farm production cycles

The relationship between aquaculture and infestations of sea lice on wild sea trout (Salmo trutta) populations is controversial. Although some authors have concluded that there is a link between aquaculture and lice burdens on wild fish, others have questioned this interpretation. Lice levels have been shown to be generally higher on Atlantic salmon farms during the second years of two-year production cycles. Here we investigate whether this pattern relates to lice burdens on wild fish across broad temporal and spatial axes. Within Loch Shieldaig across five successive farm cycles from 2000 to 2009, the percentage of sea trout with lice, and those above a critical level, were significantly higher in the second year of a two-year production cycle. These patterns were mirrored in 2002-2003 across the Scottish west coast. The results suggest a link between Atlantic salmon farms and sea lice burdens on sea trout in the west of Scotland.

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.

Over-summering behaviour of Atlantic salmon Salmo salar returning to rivers in the Cromarty Firth, north-east Scotland

In 2005, a high percentage (50%) of Atlantic salmon Salmo salar fitted with acoustic transmitters, which entered rivers in the Cromarty Firth area of north-east Scotland, dropped back to the estuary and ascended adjacent rivers after 33-80 days of freshwater residence. Fish residing in non-natal rivers generate mixed stock fisheries, and movements of fish between rivers could increase the risk of disease transmission between catchments.

Habitat use and dispersal of post-smolt sea trout Salmo trutta in a Scottish sea loch system

Post-smolt anadromous brown trout Salmo trutta, sea trout, from two Scottish west coast rivers, the Balgy and Shieldaig, flowing into adjacent sea lochs were tracked simultaneously using arrays of moored acoustic receivers to determine dispersal patterns and loss rates. Fish tended to stay close to their natal rivers for the first 14 day after entering the sea, during which time about half the fish were lost to the study. Although initially the overall pattern of dispersal was similar for individual fish from both rivers, towards the end of the study the groups had converged into one of the loch basins. There were also pronounced individual differences in habitat use with all those fish detected for >42 days exhibiting different patterns of habitat use. Loss rates were similar between the two rivers despite differences in the range of air-breathing predators to which the fish were initially exposed. These findings suggest that any management of predators or other mortality agents should be targeted towards mouths of rivers during and immediately following smolt emigration.

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.