Horizontal distribution and overlap of planktivorous fish stocks in the Norwegian Sea during summers 1995–2006

The Norwegian Sea harbours several large pelagic fish stocks, which use the area for feeding during the summer. The period 1995–2006 had some of the highest biomass of pelagic fish feeding in the Norwegian Sea on record. Here we address the horizontal distribution and overlap between herring, blue whiting and mackerel in this period during the summers using a combination of acoustic, trawl and LIDAR data. A newly developed temperature atlas for the Norwegian Sea is used to present the horizontal fish distributions in relation to temperature. The centre of gravity of the herring distribution changed markedly several times during the investigated period. Blue whiting feeding habitat expanded in a northwestern direction until 2003, corresponding with an increase in abundance. Strong year classes of mackerel in 2001 and 2002 and increasing temperatures throughout the period resulted in an increased amount of mackerel in the Norwegian Sea. Mackerel was generally found in waters warmer than 8°C, while herring and blue whiting were mainly found in water masses between 2 and 8°C. The horizontal overlap between herring and mackerel was low, while blue whiting had a large horizontal overlap with both herring and mackerel. The changes in horizontal distribution and overlap between the species are explained by increasing stock sizes, increasing water temperature and spatially changing zooplankton densities in the Norwegian Sea

Bioenergetics modeling of the annual consumption of zooplankton by pelagic fish feeding in the Northeast Atlantic

The present study uses bioenergetics modeling to estimate the annual consumption of the main zooplankton groups by some of the most commercially important planktivorous fish stocks in the Northeast Atlantic, namely Norwegian spring-spawning (NSS) herring (Clupea harengus), blue whiting (Micromesistius poutassou) and NEA mackerel (Scomber scombrus). The data was obtained from scientific surveys in the main feeding area (Norwegian Sea) in the period 2005–2010. By incorporating novel information about ambient temperature, seasonal growth and changes in the diet from stomach content analyses, annual consumption of the different zooplankton groups by pelagic fish is estimated. The present study estimates higher consumption estimates than previous studies for the three species and suggests that fish might have a greater impact on the zooplankton community as foragers. This way, NEA mackerel, showing the highest daily consumption rates, and NSS herring, annually consume around 10 times their total biomass, whereas blue whiting consume about 6 times their biomass in zooplankton. The three species were estimated to consume an average of 135 million (M) tonnes of zooplankton each year, consisting of 53–85 M tonnes of copepods, 20–32 M tonnes of krill, 8–42 M tonnes of appendicularians and 0.2–1.2 M tonnes of fish, depending on the year. For NSS herring and NEA mackerel the main prey groups are calanoids and appendicularians, showing a peak in consumption during June and June–July, respectively, and suggesting high potential for inter-specific feeding competition between these species. In contrast, blue whiting maintain a low consumption rate from April to September, consuming mainly larger euphausiids. Our results suggest that the three species can coexist regardless of their high abundance, zooplankton consumption rates and overlapping diet. Accordingly, the species might have niche segregation, as they are species specific, showing annual and inter-annual variability in total consumption of the different prey species. These estimates and their inter-annual and inter-specific variation are fundamental for understanding fundamental pelagic predator-prey interactions as well as to inform advanced multispecies ecosystem models.publishedVersio

The diet of juvenile Atlantic mackerel (Scomber scombrus) feeding in new northern nursery areas along the Norwegian coast

The North-east Atlantic mackerel stock size increased substantially from 2006–2014 coinciding with high recruitment. This resulted in a pronounced northerly geographic expansion of mackerel, followed by an influx of juvenile mackerel into Norwegian waters. The objective of this work was to study the diet and feeding intensity of juvenile mackerel at the new nursing grounds along the Norwegian coast during the summer. Juvenile mackerel were feeding as far north as 70°N. Stomach content was analysed for the first time from co-occurring juvenile and adult mackerel at the same locations. Almost 80% of all juvenile mackerel had prey in their stomachs, and juveniles had similar stomach fullness as adult mackerel in the same areas. The juveniles preyed on a wide variety of prey species and seemed to utilize both passive filter feeding and active particulate feeding. The most abundant prey group was Appendicularia, accounting for 31% of the stomach content by weight. Juveniles fed on similar prey species as adults, but their diet niche differed somewhat as adult mackerel fed more on krill. Juvenile mackerel can thus successfully survive and feed on various prey in high latitudes and can potentially be a feeding competitor to other planktivorous fish species in the area.publishedVersio

Diet analysis of Atlantic salmon (Salmo salar) post-smolts after the ecological regime shift in the Northeast Atlantic

Transition from freshwater to saltwater presents multiple challenges for anadromous Atlantic salmon, and survival during this critical life-stage is thought to influence adult population abundance. Despite this, the role of feeding, which influences growth and therefore survival, is poorly studied. Here, we analyzed the diet of 580 post-smolts captured in four Norwegian fjords in 2018 and 2019. Post-smolt diet mainly consisted of fish larvae (Teleostei), krill (Euphasiidae), planktonic amphipods, and insects. However, diet varied among fjords and years. For example, post-smolts in Altafjord in northern Norway displayed a higher frequency of fish larvae in their diet compared to post-smolts from fjords in western Norway, although this effect was less clear in 2019 than in 2018. Post-smolts consuming fish larvae and/or krill displayed substantially higher feeding ratios, and these fish were on average 0.52 cm longer. This observation underpins results from earlier studies suggesting that consumption of fish larvae is important for marine growth and ultimately survival. The dietary observations reported here may therefore have implications for spatial and temporal patterns in Atlantic salmon marine survival rates in this region. Furthermore, we did not detect any clear differences in diet between post-smolts analyzed here in comparison with post-smolts collected in the same region approximately 20 years earlier. As there has been a well-documented ecological regime shift in the Northeast Atlantic between the present and earlier studies, we conclude that it has not had a large impact on post-smolt feeding conditions within Norwegian fjords. Diet Fish larvae Coastal ecosystem Migration EstuarypublishedVersio

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

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

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

Geographic distribution, abundance, diet, and body size of invasive pink salmon (Oncorhynchus gorbuscha) in the Norwegian and Barents Seas, and in Norwegian rivers

We report for the first time the geographic distribution, abundance, diet, and body size of invasive pink salmon (Oncorhynchus gorbuscha) in the Norwegian and Barents Seas and Norwegian coast and rivers. We found that pink salmon have spread throughout the Norwegian Sea and along the Norwegian coast, and abundance increased by several orders of magnitude in 2017, with no signs that it has peaked. Marine pink salmon diet comprised mainly fish larvae, amphipods, and krill, but their relative importance varied with geographic distribution. North of 67.5◦N, Amphipoda, herring, and saithe were more important, while south of 67.5◦N, Euphausiidae and mesopelagic fish abounded. Pink salmon body size was larger in the northern rivers, and to the north of the Norwegian Sea and the Barents Sea, relative to the southern rivers and sea areas. Pink salmon were feeding in the ocean during the winter and spring, and in coastal areas immediately before return to the rivers, but not after they had entered the rivers. There was no geographical pattern in the seasonal timing of river ascent. The geographic pattern in abundance and diet of pink salmon, as reported here, offer a measure of the ecological effect of the invasion. diet, distribution, invasive species, northeast Atlantic, pink salmon.publishedVersio

Poor feeding opportunities and reduced condition factor for salmon post-smolts in the Northeast Atlantic Ocean

During the last few decades, many wild Atlantic salmon populations have declined dramatically. One possible contributing factor for the decline is reduced prey availability at sea. Here, we examine post-smolt diet and investigate if post-smolts show signs of selective feeding based on 2546 post-smolts sampled from west of Ireland to the northern Norwegian Sea over a 25-year period. We also test for changes over time in stomach fullness, diet, condition factor and body length. There was a clear reduction in condition factor for post-smolts sampled in the Norwegian Sea in the period 2003–2012. The post-smolt stomach fullness was also reduced in the same period. The reduction in condition factor is partly explained by reduced stomach fullness, including a reduction of highly energetic fish larvae and Amphipoda in the diet. Feeding on other prey, such as meso-zooplankton and insects, cannot substitute the high-quality fish larvae and Amphipoda in the post-smolt diet. This is the first study to document how salmon post-smolts feeding in the Norwegian Sea are affected by reduced feeding conditions. Possible causes for the observed changes in post-smolt feeding are ocean warming, decreased primary productivity, and reduced recruitment of important fish larvae.publishedVersio