Short-term hydrographic variability in a stratified Arctic fjord

Fjords in the Arctic often have a more complex circulation pattern than the classical two dimentional estuarine circulation. This is due to the effects of the Earth’s rotation on stratified waters in wide fjords. Observations from a semi-enclosed fjord basin, Van Mijenfjorden on Spitsbergen, show that the hydrography and circulation vary considerably on short time scales (hours) in the summer season. The depth and distribution of the low salinity upper water layer respond quickly to changes in the wind field. The Coriolis effect has an essential impact on the circulation, inducing eddy-like flow patterns, and strong cross-fjord adients. Within the upper layer, the lowest salinity values and highest temperatures were found on the northern side of the fjord in calm wind periods. When the wind was strong from west the cross-fjord gradients were reversed. Internal wave activity contributes to large vertical displacement of water below the upper layer. Knowledge of such strongly variable hydrographic conditions in fjords are important for sampling strategy and interpretation of data, for instance of primary production and sedimentation processes, and for the understanding of fjords as depositional systems

Ice extent in sub-arctic fjords and coastal areas from 2001-19 analysed from MODIS imagery

Results examining variations in the ice extent along the Norwegian coastline based on the analysis of Moderate Resolution Imaging Spectroradiometer (MODIS) images from 2001 to 2019, February through May, are presented. A total of 386 fjords and coastal areas were outlined and grouped into ten regions to assess seasonal and long-term trends in ice extent. In addition, three fjords were examined to investigate how ice extent may vary over short distances (5 km2 of ice at least once between 2001 and 2019. Over this span of time, no statistically significant trend in ice extent is found for all ten regions; however, variations between regions and years are evident. Ice extent is assessed through comparison to three weather variables – freezing degree days (FDD), daily new snowfall and daily freshwater supply from rainfall plus snowmelt. Six out of ten regions are significantly positively correlated (p < 0.05) to FDD. In addition, ice in two regions is significantly positively correlated to daily new snowfall, and in one region negatively correlated to rainfall plus snowmelt. The importance of fjord geometry and bathymetry as well as other weather variables including wind is discussed.publishedVersio

Ice extent in sub-arctic fjords and coastal areas from 2001-19 analysed from MODIS imagery

Results examining variations in the ice extent along the Norwegian coastline based on the analysis of Moderate Resolution Imaging Spectroradiometer (MODIS) images from 2001 to 2019, February through May, are presented. A total of 386 fjords and coastal areas were outlined and grouped into ten regions to assess seasonal and long-term trends in ice extent. In addition, three fjords were examined to investigate how ice extent may vary over short distances (5 km2 of ice at least once between 2001 and 2019. Over this span of time, no statistically significant trend in ice extent is found for all ten regions; however, variations between regions and years are evident. Ice extent is assessed through comparison to three weather variables – freezing degree days (FDD), daily new snowfall and daily freshwater supply from rainfall plus snowmelt. Six out of ten regions are significantly positively correlated (p < 0.05) to FDD. In addition, ice in two regions is significantly positively correlated to daily new snowfall, and in one region negatively correlated to rainfall plus snowmelt. The importance of fjord geometry and bathymetry as well as other weather variables including wind is discussed

Effects of mortality changes on biomass and production in Calanus spp. populations

Calanus species are the main link between primary producers and higher trophic-level organisms in the Barents Sea. The natural mortality rate is an essential parameter for determining the standing stock of Calanus, but it is also one of the most uncertain parameters in present knowledge. The level of human activity, and the associated risk of pollution, is increasing in the Barents Sea, and knowledge of the Calanus population response to increased mortality is crucial for management of the ecosystem. In the present study, we estimated natural mortality rates of Calanus, based on available field data from the Barents Sea, and performed numerical simulation experiments with a coupled physical–biological model, testing the response of Calanus populations to changes in mortality rates, and other related ecological parameters. The field-based estimates of natural mortality showed high variability. The model simulations showed that the 2 Calanus species modelled, C. glacialis and C. finmarchicus, respond differently to increased mortality, and that in­creased mortality alters both the timing of peak Calanus production and biomass relative to peak primary production. These simulations illustrate the potential for a mismatch between peak food availability and Calanus population dynamics in the Barents Sea as a consequence of natural or human-induced perturbations. We suggest that the observed differences in the 2 Calanus species’ responses to perturbations relates to each species’ life cycle and habitat characteristics. The present study illustrates how models can be used to assess key parameters affecting species’ population dynamics and some potential consequence of external forcing factors affecting mortality

An effective method for the recapture of escaped farmed salmon

The search for effective strategies to prevent and mitigate accidental releases of aquaculture fishes is on-going. To test a new recapture strategy and evaluate the individual dispersal behaviour of escaped farmed Atlantic salmon Salmo salar L. at the northern limit of its range, 39 adult salmon (mean ± SD fork length and weight: 85.5 ± 5.0 cm and 7.4 ± 1.4 kg, respectively) were implanted with depth-sensing acoustic tags and released in a north Norwegian fjord during the spring of 2007. The fish were released from 2 aquaculture sites in the Altafjord system and tracked using both mobile and fixed receivers. The coastal marine bag-net fishery, in combination with inriver angling, was tested as a potential recapture strategy. Immediately following the simulated escape event, the fish dove to near-bottom depths, subsequently returning to surface levels within the following days. The fish dispersed rapidly (9.5 ± 19.2 km d–1), traveling outward to coastal waters along the edges of the fjord. The bag-net fishers and anglers recaptured 79% of the escaped fish within 1 mo post-release, 90% of which were from bag nets. While most of the fish left the fjord, 7 tagged fish (18%) entered the Alta River estuary (3 of which later migrated up the Alta River), and 1 returned to the Altafjord the following year, presumably to spawn. The results showed that recapture efforts need to be immediate and widespread to mitigate farm-escape events. Coastal bag nets were effective at recapturing escaped farmed salmon, compared to previously tested methods, and would be especially useful in areas where gill-netting is not permitted

Homing behaviour of Atlantic salmon (Salmo salar) during final phase of marine migration and river entry

Little is known about Atlantic salmon behaviour during the last phase of the marine homing migration and subsequent river entry. In this study, 56 adult Atlantic salmon in the Alta Fjord in northern Norway were equipped with acoustic transmitters. Salmon generally followed the coastline, but their horizontal distribution was also affected by wind-induced spreading of river water across the fjord. Mean swimming depth was shallow (2.5–0.5 m), but with dives down to 30 m depth. Timing of river entry was not affected by river flow, diel periodicity, or tidal cycles. Movements during the last part of the marine migration and river entry were unidirectional and relatively fast (mean 9.7 km·day−1). However, migratory speed slowed as salmon approached the estuary, with a significantly lower speed in the innermost part of the estuary than in the open fjord. Migration behaviour seemed not to be affected by handling and tagging, as there were no behavioural differences between newly tagged fish and those captured and tagged 1 year before their homing migration

Modelled salmon lice dispersion and infestation patterns in a sub-arctic fjord

Salmon lice infestation is a major challenge for the aquaculture industry in Norway, threatening wild salmonid populations and causing welfare problems for farmed salmon. Lice dispersion and infestation patterns are simulated by combining a high-resolution hydrodynamic model for the Norwegian coast and fjords with an individual-based model for salmon lice. We here present results from Altafjorden, a sub-arctic fjord with large stocks of wild salmonids, where the inner part is protected as a National Salmon Fjord. The outer part of the fjord hosts several fish farms, and our simulations demonstrate how ocean currents can disperse lice between farms as well as into the protected part of the fjord. The relative contributions from the farms in the different parts of the fjord depends on their locations relative to the currents and circulation patterns in the fjord. Knowledge of how the highly variable water currents disperse salmon lice within fjord systems is necessary for managing farm locations and production quotas, if the goal is to minimize infestation pressure on wild salmonids and between fish farms.publishedVersio