Pancreas disease (PD) is an emerging disease in salmon farming caused by the salmonid alphavirus (SAV). SAV is evidently spread horizontally between neighbouring salmon farms, but whether such transmission occurs by passive drift in the water current or via fomites is not known. We tested whether hydrodynamic modelling contributes to explain the spread of PD, in which case SAV is likely to spread by passive drift. We present a simple logistic regression model that accounts for the effect of PD in the neighbourhood on the probability of acquiring PD in cohorts of farmed salmonids from an area on the west coast of Norway between 2005 and 2008. For a given cohort, we calculated infection pressure (IP) based on Euclidean distance, seaway distance or estimated water contact to sites with PD, and compared the amount of variance explained in the regression model by the different variants of IP. Water contact between a discharging farm site and a receiving site was calculated by simulating particle discharge using a hydrodynamic model. IP estimated by water contact was the best predictor of PD cases and controls in the model, which performed significantly better than IP estimated by seaway distance or Euclidean distance. Since the spread of PD in the study area was best explained by modelled water velocity, we conclude that PD is likely to be spread by passive drift of SAV in the water current.

The Pacific oyster, Crassostrea gigas, was introduced to Europe for aquaculture purposes, and has had a rapid and unforeseen northward expansion in northern Europe. The recent dramatic increase in number of C. gigas populations along the species’ northern distribution limit has questioned the efficiency of Skagerrak as a dispersal barrier for transport and survival of larvae. We investigated the genetic connectivity and possible spreading patterns between Pacific oyster populations on the southern Norwegian coast (4 localities) and Swedish and Danish populations by means of DNA microsatellite analysis of adult oysters, and by simulating larvae drift. In the simulations we used a 3D oceanographic model to explore the influence of recent climate change (1990–2010) on development, survival, and successful spreading of Danish and Swedish Pacific oyster larvae to Norwegian coastal waters. The simulations indicated adequate temperature conditions for development, survival, and settlement of larvae across the Skagerrak in warm years since 2000. However, microsatellite genotyping revealed genetic differences between the Norwegian populations, and between the Norwegian populations and the Swedish and Danish populations, the latter two populations being more similar. This patchwork pattern of genetic dissimilarity among the Norwegian populations points towards multiple local introduction routes rather than the commonly assumed unidirectional entry of larvae drifted from Denmark and Sweden. Alternative origins of introduction and implications for management, such as forecasting and possible mitigation actions, are discussed.

This study investigates the dissipation rates and flow conditions at the Drøbak Sill in the Oslofjord. The area was transected 13 times with a free-falling microstructure shear probe during 4 days in June 2011. At the same time, an ADCP was deployed inside the sill. During most tidal cycles, internal hydraulic jumps with high dissipation rates were found on the downstream side of the sill. However, the internal response varied strongly between different tidal cycles with similar barotropic forcing. In the beginning of the observational period, ebb tides had no hydraulic jumps, and in the end one of the flood tides did not have a hydraulic jump. During the same period, the mean baroclinic exchange flow changed from inflow to outflow in the bottom layer. The authors conclude that the conditions at the sill are on the edge of forming hydraulic jumps and that the mean baroclinic exchange may push the flow above or below the limit of a hydraulic jump depending on the situation. This conclusion is supported by two-layer hydraulic theory. The volume-integrated dissipation rates within 500 m from the sill crest compare well with estimates of energy loss in the lower layer calculated from the Bernoulli drop under the assumption of no energy loss in the upper layer. Finally, the mean dissipation rate at the sill was compared with the radiation of internal tidal energy away from the sill, and it was found that about 60%-90% of the total energy loss was dissipated locally.

Purpose The current study was conducted in order to assess the release of dissolved contaminants from sediment relocation works in Oslo harbour, Norway, whilst operations were being carried out, and to assess the potential for spreading into the wider fjord area. Materials and methods This was achieved by comparing the concentration of metals and organic contaminants in the vicinity of the disposal site, and at strategic locations in the adjacent fjord area. In total, 14 stations were chosen, with passive sampling devices, diffusive gradients in thin films for metals and semipermeable membrane devices for organic compounds, deployed at various depths at each station for a period of 1 month. Results and discussion The highest concentrations were generally found closer to the harbour area where trace metals and organic contaminants are supplied from several sources including river discharges and old, contaminated sediments. Near the deposition site, the concentrations of polychlorinated biphenyls were low and the concentration gradients for Cd, Zn and Cu indicated removal of these metals from the water column, probably due to association with sulphides present at the deposition site or in the discharged sediments. Elevated concentrations of Pb and polycyclic aromatic hydrocarbons (PAHs) revealed that the disposal acted as a source of these compounds, but maximum concentrations were similar to or less than those predicted before the deposition begun. The estimated vertical transport and spreading to areas outside of the semi-enclosed sill basin was negligible. Conclusions Strictly, the validity of the results presented here are limited to a 1-month period of the disposal operations which lasted nearly 3 years. During this period, release of contaminants was compound-specific, but even for the most labile PAHs and Pb, the release appeared to be in the same order of magnitude or smaller than the release from other local sources not limited in time to the duration of the remediation operation.