Temporal Variability of Co-Occurring Calanus finmarchicus and C. helgolandicus in Skagerrak

The congeneric copepods Calanus finmarchicus and C. helgolandicus co-occur in the northern North Sea and Skagerrak where they play important roles as prey for higher trophic levels. This study analyses a 26-year time series (1994-2019) with a ~ two-week temporal resolution for Calanus spp. at a fixed monitoring station in Skagerrak, off southern Norway. Seasonal variation, inter-annual variability and long-term trends for the two species were examined. Strong differences in the species-specific seasonality were revealed, with C. finmarchicus dominating in spring and C. helgolandicus in autumn. The seasonal peak of C. finmarchicus was associated with relatively low temperatures (6-8°C) and high chlorophyll a concentration, while C. helgolandicus displayed its seasonal maximum at higher temperatures (11-16°C). C. finmarchicus was found to produce one dominant annual generation (in spring), but two or more generations are considered likely. Contrasting long-term trends in abundances were found for the two species, suggesting that their population sizes were affected by different mechanisms. The abundance of C. helgolandicus showed an increasing trend over the time period studied, apart from the last years. The abundance of C. finmarchicus was more variable and displayed no unidirectional long-term trends over the time series. The study revealed a shift in the phenology of Calanus spp. over the last 25 years at this site. Since 1994 the timing of the annual peaks in both C. finmarchicus and C. helgolandicus have advanced by about one month. Moreover, the seasonal pattern of C. helgolandicus, switched from a unimodal to a bimodal pattern around 2002, with a small additional peak also appearing in spring. The results suggest that the proximity to the Norwegian Trench influences the demography and abundance of C. finmarchicus in this coastal area, both as a gateway for the advective supply, as well as a habitat for local overwintering.publishedVersio

Trans-polar drift-pathways of riverine European microplastic

High concentrations of microplastic particles are reported across the Arctic Ocean–yet no meaningful point sources, suspension timelines, or accumulation areas have been identified. Here we use Lagrangian particle advection simulations to model the transport of buoyant microplastic from northern European rivers to the high Arctic, and compare model results to the flux of sampled synthetic particles across the main entrance to the Arctic Ocean. We report widespread dispersal along the Eurasian continental shelf, across the North Pole, and back into the Nordic Seas; with accumulation zones over the Nansen basin, the Laptev Sea, and the ocean gyres of the Nordic Seas. The equal distribution of sampled synthetic particles across water masses covering a wide time frame of anthropogenic influence suggests a system in full saturation rather than pronounced injection from European sources, through a complex circulation scheme connecting the entire Arctic Mediterranean. This circulation of microplastic through Arctic ecosystems may have large consequences to natural ecosystem health, highlighting an ever-increasing need for better waste management

Feeding ecology of Atlantic mackerel (Scomber scombrus) in the Norwegian Sea

Previous studies on Atlantic mackerel (Scomber scombrus) feeding during the highly migratory summer period are scarce. In this study we present detailed diet analyses and investigations of prey selection for Atlantic mackerel in relation to zooplankton distribution and hydrographical conditions in the Norwegian Sea in July 2004. A wide variety of prey organisms were found in the stomachs of mackerel in all water masses. According to dry weight was Limacina retroversa the principal prey in Coastal and Atlantic water masses. L. retroversa was presumably abundant in the Norwegian Sea and taken as secondary prey in the quest for more nutritious prey. Calanus finmarchicus was less important in all water masses than previously documented, also when combined with calanoidae copepod remainders, indicating that their descend to overwinter in deeper waters had already started in late July. Mackerel showed a size selective feeding behaviour and preferred larger prey species over smaller prey. Small copepods (< 1 mm) were numerous in the zooplankton samples in all water masses, especially Coastal and Atlantic, but were not present in the stomach samples. Furthermore, the three largest copepodite stages of C. finmarchicus (CIV – CVI) were selected over the three smallest stages (CI – CIII). Pronounced prey and size selection strongly suggests that mackerel performed particulate feeding. The potential feeding competition between mackerel and herring was also investigated. There was a clear difference in the diets of the two species. C. finmarchicus and L. retroversa were the main prey of mackerel, whereas herring preferred euphausiids and amphipods. These latter species normally swim in deeper waters and only enter the upper layers for feeding when protected by the dark. Moderate feeding by herring at the end of their feeding season and more active feeding by mackerel could explain the pronounced differences in the diet and lack of feeding competition between these two important planktivorous species

Patterns, efficiency and ecosystem effects when fishing Calanus finmarchicus in the Norwegian Sea—using an individual-based model

Due to the important role of Calanus finmarchicus as key prey for the abundant pelagic fish stocks (Northeast Atlantic mackerel, Norwegian spring spawning herring, blue whiting) in the Norwegian Sea, an increase in the quota of C. finmarchicus has raised public concern. Here, 2 vessel types were implemented in an individual-based model within the NORWECOM.E2E ecosystem model, one ordinary vessel similar to the vessels used in the real fishery, the other with perfect knowledge of the C. finmarchicus distribution to account for possible future development of the fishery. The perfect vessels were free to move among all grid cells, whereas the other vessel type was restricted. Differences between the vessel types were large in terms of catch per unit effort and hotspots for catches. Operating with 10 perfect vessels, the catches were on average 54000 t yr-1, almost 3 times higher than for the ordinary vessels. Increasing to 60 perfect vessels, catches increased to 349000 t yr-1. The vessels with full knowledge of distribution and concentration located new hotspots distant from the traditional fishing grounds. Due to area restrictions in the current quota, allowing only 3000 t caught within the 1000 m depth contour, the perfect vessels shifted their activity offshore. In the simulated ecosystem, no ecosystem effects were found, neither on the C. finmarchicus biomass nor on the Norwegian spring spawning herring biomass. This finding indicates that the proposed quota of C. finmarchicus supports a sustainable fishery.publishedVersio

Comparison of two multiple plankton samplers: MOCNESS and Multinet Mammoth

To ensure an optimal continuation of a long time series of zooplankton monitoring surveys, two types of equipment for depth-stratified mesozooplankton sampling were compared. The Institute of Marine Research (Norway) has applied the MOCNESS with good results since 1985, but recent events have made it necessary to change to the Multinet Mammoth. During a cruise in March 2019, both sampling devices were calibrated before 17 paired deployments of the 2 gears were undertaken. During each deployment, three nets and depth-strata covering ~ 425–200, 200–100, and 100–0 m were sampled. All samples were size-fractionated or taxonomically fractionated into 10 different biomass categories. The results revealed no significant differences between the two gears when comparing total depth-integrated biomass (2.46 ± 0.36 vs. 2.61 ± 0.59 gDW m−2) or depth-integrated biomass of any specific biomass category. Running paired t-tests separately for all combinations of biomass categories and nets, the differences were only significant for zooplankton biomasses in the 180–1000 μm size fraction and only for Net 2. Possible reasons for this result are discussed in the paper. Gears produced similar catches whether sampling during day or night. We conclude that the MOCNESS and Multinet Mammoth in this study provided comparable results regarding abundances of various zooplankton categories.publishedVersio

Impacts of a Changing Ecosystem on the Feeding and Feeding Conditions for Atlantic Salmon During the First Months at Sea

During the last decades, many wild Atlantic salmon populations have declined dramatically. One hypothesis for an observed reduction in salmon marine growth and survival is reduced abundance of prey. However, the effect of spatial and temporal variation in marine prey abundance on post-smolt feeding conditions is poorly understood. Here we use stomach content data from 2572 salmon postsmolts sampled during 25 years in the Northeast Atlantic Ocean to examine spatial and temporal changes in diet and stomach fullness. Sandeel larvae west of Scotland and Ireland and in the northern North Sea, herring larvae in the eastern part of the Norwegian Sea, and amphipods in the western part of the Norwegian Sea were particularly important prey species. There was a reduction of fish larvae in the postsmolt stomachs over a large geographic area when comparing the period 1995–2004 to 2008–2019. This may be result of a bottom-up driven process, as increasing extent of Arctic Water masses and zooplankton abundance were positively correlated with postsmolt stomach fullness in the Norwegian Sea. Furthermore, the interspecific competition for fish larvae between post-smolts and mackerel may have increased with a larger mackerel stock expanding the feeding migrations northwards since 2007. Norwegian Sea, fish larvae, zooplankton, mackerel, oceanography, dietpublishedVersio

Seamounts in the OSPAR maritime area - from species to ecosystems

This report was prepared by the Institute of Marine Research, Norway, for the Norwegian Environment Agency, as part of Norway´s contribution to OSPAR. The report summarizes the latest knowledge on species and habitats associated with seamounts in the OSPAR Regions I, IV and V. Knowledge was sought from published literature, reports and online marine data archives. The global bathymetry model of Harris et al. (2014) predicts that 161 seamounts occurr within the OSPAR maritime area. Not all of these have been charted or studied, and the literature and bathymetry database review in this report resulted in a much shorter list of 100 seamounts or seamount-like features within the deep seas of OSPAR, i.e. regions I, IV and V. Published literature from the OSPAR area documented that there is knowledge of planktonic organisms for 11 seamounts, information on benthic species for 24 seamounts, and of fish from 16 seamounts. The best described component is the benthos with a total of 49 peer-reviewed papers. The global knowledge of seamounts (and a few studies from the NE Atlantic) suggests that seamounts are inhabited by species from the regional species pool within the biogeographical zones they occur. At individual seamounts the structure of species assemblages and production patterns are variable over time and modified by factors such as the local and regional hydrography and circulation which are sometimes modified by the seamounts themselves. Other significant factors causing variability are the varying depths of slopes and summits in relation to the depth of the euphotic zone, the depth of summits relative to mesopelagic scattering layers, and presumably the distance from continents, islands and wider areas such banks and ridges. Furthermore, seamounts represent isolated shallows in the deep-sea and may have several important local and regional functions. However, within the OSPAR maritime area, few studies have produced more than descriptive data, hence a major shortage is the lack of quantitative information on species occurrences as well as studies measuring processes and documenting functions. The roles of seamounts at regional scales, e.g. as stepping stones for species across wider ocean areas, have only been incompletely studied. Seamounts apparently constitute patches of suitable habitats for aggregating commercially valuable fish species that are relatively easy to locate and target, and if not properly controlled, to overexploit. This is known from the OSPAR region V, e.g. from historical depletion of orange roughy west of the British Isles and sharp declines of alfonsino aggregations on seamounts north of the Azores. Also, seamounts are features likely to have Vulnerable Marine Ecosystems (VMEs) (sensu FAO, 2009), primarily in the form of structure-forming coral and sponge aggregations. These require special protective action such as called for by the UN General Assembly and OSPAR, and accordingly several nations, the EU and the Northeast Atlantic Fisheries Commission (NEAFC) implemented measures to prevent further signficant adverse impacts from bottom fishing. Studies at many seamounts in OSPAR have shown that many summits have rich occurrence of VME indicator taxa and probably VMEs. Significant adverse impacts of past bottom fishing have been well documented in some slope and shelf habitats, but studies on seamounts have been scattered, and as yet there is not enough information to assess the overall status of VMEs on seamounts in Region I, IV and V.publishedVersio

Arctic and Atlantic waters in the Norwegian Basin, between year variability and potential ecosystem implications

The ocean climate of the southern Norwegian Sea - the Norwegian Basin - is largely set by the relative amount of Atlantic Water in the eastern and Arctic Water in the western region. Here we utilized hydrographic data from repeated sections, together with annually gridded survey data of the upper 1000 m, to resolve the main hydrographic changes over the period 1995-2019. Based on integrated heat -and freshwater content, we divide into three periods. The first period 1995-2005, denoted Arctic, is characterized by relative fresh and cold Atlantic Water overlaying Arctic Intermediate Water that basically covers the whole Norwegian Basin. Differently, the conditions during the period 2006-2016, denoted Atlantic, are warmer and more saline, and the extent and thickness of Arctic Intermediate Water is greatly reduced. During the most recent period denoted Fresh, 2017-2019, there has been a major freshening of the Atlantic waters, the layer of Arctic Intermediate Water has not recovered, but instead a layer of warmer but relative fresh Arctic Water has expanded. We find that increased abundance of the Arctic zooplankton Calanus hyperboreus in the southern and eastern Norwegian Basin coincides with increased extent of Arctic Water. We also note that the overall mesozooplankton biomass in the Norwegian Basin is significantly higher during periods of relative high amount of Arctic Water. Furthermore, we show that both nitrate and silicate winter (pre-bloom) concentrations are significantly higher in the Arctic Water compared to Atlantic Water, and that there is a reduction in nutrients from the Arctic period compared subsequent Atlantic and Fresh periods. Since these nutrients can be interpreted as the potential for new production, changes in the influx of western Arctic waters are expected to have a bottom-up effect on the Norwegian Sea. Hence, this study indicates that the amount of Arctic waters and their concentration of nutrients and zooplankton are more important for the Norwegian Basin ecosystem functioning rather than the temperature of the Atlantic waters.publishedVersio

Vitenskapelig bakgrunnsmateriale for forvaltningsplan for raudåte

A management plan for Calanus finmarchicus has been completed by the Institute of Marine Research and the Directorate of Fisheries, in line with orders from the Ministry of Trade, Industry and Fisheries. The work is presented as a background report with advice from IMR and a management plan for C. finmarchicus from the Directorate of Fisheries and IMR. The background report discusses important aspects of a fishery such as biological conditions, population definition, life cycle and ecology, population variation in time, distribution, monitoring, biomass and production, ecological effects of fishery, harvesting advice and ultimately quota advice. Important knowledge gaps that must be filled to improve future advices is highlighted. This is the first management plan that has been prepared for a plankton population and shows the desire to develop the management of our populations in an ecological direction. The management plan cover the Norwegian Sea and its adjacent areas. This is because this is the area where the current fishing is taking place and where we have the best data basis. This area can be expanded if the plan is updated in the future. The plan is the Director General of Fisheries’ management plan for Calanus finmarchicus and has emerged as a collaboration between the Directorate of Fisheries and IMR.I tråd med bestillinger fra Nærings- og fiskeridepartementet har Havforskningsinstituttet (HI) og Fiskeridirektoratet (F.dir.) laget en forvaltningsplan for raudåte. Arbeidet foreligger som en bakgrunnsrapport med råd fra HI og en forvaltningsplan for raudåte fra F.dir. og HI. Bakgrunnsrapporten omhandler viktige aspekter ved et fiske slik som biologiske forhold, bestandsenhet, raudåtas livssyklus og økologi, bestandsendringer over tid, utbredelse, overvåkning, biomasse og produksjon, økologiske effekter av fiske, høstingsregler og til slutt kvoteråd. Det pekes også på viktige kunnskapshull som må fylles dersom en i framtiden skal kunne forbedre våre råd. Selve forvaltningsplanen er den første som er utarbeidet for en planktonbestand og viser ønsket en har om å utvikle forvaltningen av våre bestander i en økologisk retning. Vi har valgt å lage denne forvaltningsplanen for Norskehavet med tilliggende områder. Det er både fordi det er der fisket nå skjer og fordi det er fra dette området vi har det beste datagrunnlaget. En kan tenke seg at dette området utvides ved framtidig oppdatering av planen. Selve forvaltningsplanen er Fiskeridirektørens forvaltningsplan for raudåte og er framkommet som et samarbeid mellom F.dir. og HI

Feeding ecology of Benthosema glaciale across the North Atlantic

The glacier lanternfish Benthosema glaciale is a key myctophid with a wide distribution in the northern Atlantic. It is a species that has a strong vertical migration capacity and have the potential to move between the surface waters and mesopelagic depths in a diel cycle (DVM), mainly depending on ambient light conditions. We investigated the feeding ecology of B. glaciale across the Norwegian, Iceland, Irminger and Labrador Sea basins. An important component of Benthosema diet was various types of calanoid copepods, but with some additional variability across the deep basins. ‘House’s’ of Appendicularia were only found in stomachs from the Labrador basin and were positively selected for here. The large calanoid Calanus hyperboreus was strongly selected for in the Iceland Sea, while its smaller counterpart C. finmarchicus was negatively selected for here. Fish from the Irminger, Labrador and Norwegian Seas displayed a high number of empty stomachs while no fish stomachs were found empty in the Iceland Sea. Contrary to expectation due to being located at the highest latitude, Benthosema in the Iceland Sea had significantly higher condition factor (for both small and large fish) and liver indices compared to fish from other basins, but the abundance of small fish in the sampled population here was very low. This contrasts with the “light environment exclusion” (LEE) hypothesis, which propose that the extreme light environment at higher latitudes restricts feeding opportunities for mesopelagic fish at high latitudes. It is suggested that improved classification of prey through stomach analyses should aim to allow bioluminescent organisms to be separated from non-bioluminescent prey if feasible. This would allow improved resolution to analyse stomach contents and certainly progress our understanding of the success of myctophids across variable habitats.publishedVersio