Assessing the patchiness of early life stage of a fish stock (Gadus morhua) and its contribution to the stock recruitment

Patchiness, defined as spatial heterogeneity in distribution of organisms, is a common phenomenon in zooplankton including ichtyoplankton. In heterogeneous landscapes, depending on the scale of prey and predatory distributions, individuals in patches may experience distinct differences in the survival rate compared to individuals distributed more homogeneously outside patches. In this study, we focused on drifting eggs and larvae of Northeast Arctic (NEA) cod, one of the largest exploited fish stock in the world. The eggs and larvae are largely distributed along the north-western coast of Norway and northern Russia. We ask to what degree individuals are located in patches contribute to the species recruitment. For this purpose, we developed a patch recognition method to detect the existence of patches in particle tracking simulations using a connected-component labeling algorithm. We then assessed the contribution of individuals in detected patches to the total recruitment. Our results showed that depending on year, day of year, and resolution scale for detection of patches, recruits present in patches can vary between 0.6% and 38.7% with an average of 20.4% of total recruitment. The percentage decreased with increasing day of year in the drifting season but increased with decreasing patch resolution scale, down to the finest investigated scale of 8 km. On the basis of these results, we advise field recruitment studies of NEA cod to at least resolve an 8-km spatial scale to capture effects of spatial heterogeneity in the survival rate on the species recruitment.publishedVersio

Wind-Driven Atlantic Water Flow as a Direct Mode for Reduced Barents Sea Ice Cover

Variability in the Barents Sea ice cover on interannual and longer time scales has previously been shown to be governed by oceanic heat transport. Based on analysis of observations and results from an ocean circulation model during an event of reduced sea ice cover in the northeastern Barents Sea in winter 1993, it is shown that the ocean also plays a direct role within seasons. Positive wind stress curl and associated Ekman divergence causes a coherent increase in the Atlantic water transport along the negative thermal gradient through the Barents Sea. The immediate response connected to the associated local winds in the northeastern Barents Sea is a decrease in the sea ice cover due to advection. Despite a subsequent anomalous ocean-to-air heat loss on the order of 100 W m22 due to the open water, the increase in the ocean heat content caused by the circulation anomaly reduced refreezing on a time scale of order one month. Furthermore, it is found that coherent ocean heat transport anomalies occurred more frequently in the latter part of the last five decades during periods of positive North Atlantic Oscillation index, coinciding with the Barents Sea winter sea ice cover decline from the 1990s and onward.publishedVersio

Polar cod in jeopardy under the retreating Arctic sea ice

The Arctic amplification of global warming is causing the Arctic-Atlantic ice edge to retreat at unprecedented rates. Here we show how variability and change in sea ice cover in the Barents Sea, the largest shelf sea of the Arctic, affect the population dynamics of a keystone species of the ice-associated food web, the polar cod (Boreogadus saida). The data-driven biophysical model of polar cod early life stages assembled here predicts a strong mechanistic link between survival and variation in ice cover and temperature, suggesting imminent recruitment collapse should the observed ice-reduction and heating continue. Backtracking of drifting eggs and larvae from observations also demonstrates a northward retreat of one of two clearly defined spawning assemblages, possibly in response to warming. With annual to decadal ice-predictions under development the mechanistic physical-biological links presented here represent a powerful tool for making long-term predictions for the propagation of polar cod stocks.publishedVersio

Fish Assemblages of a Sub-Arctic Fjord Show Early Signals of Climate Change Response Contrary to the Benthic Assemblages

Arctic benthic ecosystems are facing high-speed environmental changes, such as decreased sea ice coverage, increased temperature and precipitations, as well as the invasion by non-indigenous species. Few sub-arctic fjords have the particularity to have an inner-most part forming a basin in which water remains very cold. Those fjords may offer a refugee for cold-water arctic species as well as a small-scale “laboratory” of the changes that arctic assemblages located at higher latitudes might face soon. The Porsangerfjord in Northern Norway is a sub-arctic fjord with an inner arctic part and face red king crabs Paralithodes camtchasticus invasion since the end of the 1990s. It offers a case study of the dynamics of arctic ecosystems facing multiple stressors, i.e., climate change and invasive species. Based on a time series of megabenthic invertebrates and bentho-demersal fishes over 2007–2019, a complex multivariate analysis (STATICO) was used to identify the trends in the relationship between taxa and the environment. We showed the main environmental changes in the fjord were the freshening of the water, the increase of the seabed current, and the decrease of the maximum sea ice extent. A strong along-fjord gradient was visible for both benthic and fish assemblages. Species richness and Shannon diversity of fishes significantly increased into the fjord, due to the arrival of warm-water species over time that overlapped with cold-water species that have seen their biomass significantly reduced. No significant decrease in the biomass of the cold-water benthic species was visible, which could indicate an efficient refugee effect of the inner fjord. Yet, this refugee effect could be unbalanced by the red king crab invasion as it is a predator of several arctic species. In the Porsangerfjord, fish species thus respond to climate change while megabenthic assemblages are more threatened by invasive species.publishedVersio

Fish Assemblages of a Sub-Arctic Fjord Show Early Signals of Climate Change Response Contrary to the Benthic Assemblages

Arctic benthic ecosystems are facing high-speed environmental changes, such as decreased sea ice coverage, increased temperature and precipitations, as well as the invasion by non-indigenous species. Few sub-arctic fjords have the particularity to have an inner-most part forming a basin in which water remains very cold. Those fjords may offer a refugee for cold-water arctic species as well as a small-scale “laboratory” of the changes that arctic assemblages located at higher latitudes might face soon. The Porsangerfjord in Northern Norway is a sub-arctic fjord with an inner arctic part and face red king crabs Paralithodes camtchasticus invasion since the end of the 1990s. It offers a case study of the dynamics of arctic ecosystems facing multiple stressors, i.e., climate change and invasive species. Based on a time series of megabenthic invertebrates and bentho-demersal fishes over 2007–2019, a complex multivariate analysis (STATICO) was used to identify the trends in the relationship between taxa and the environment. We showed the main environmental changes in the fjord were the freshening of the water, the increase of the seabed current, and the decrease of the maximum sea ice extent. A strong along-fjord gradient was visible for both benthic and fish assemblages. Species richness and Shannon diversity of fishes significantly increased into the fjord, due to the arrival of warm-water species over time that overlapped with cold-water species that have seen their biomass significantly reduced. No significant decrease in the biomass of the cold-water benthic species was visible, which could indicate an efficient refugee effect of the inner fjord. Yet, this refugee effect could be unbalanced by the red king crab invasion as it is a predator of several arctic species. In the Porsangerfjord, fish species thus respond to climate change while megabenthic assemblages are more threatened by invasive species.publishedVersio

Key processes regulating the early life history of Barents Sea polar cod

The polar cod (Boreogadus saida) in the Barents Sea is one of the main stocks of this species in the Arctic, reaching a total biomass of almost 2 million tonnes in some years. It has been fluctuating considerably in abundance, and in recent years, it has been at a low level. Only small catches have been taken from the stock over the last four decades, and consequently, the observed variation in abundance must be caused by natural (environmental and/or biological) changes in the ecosystem. Sea temperatures have been rising in the Barents Sea in recent years, possibly causing changes to the living conditions of this true Arctic stock. Consequently, there is a need for investigating how the observed changes might affect polar cod in this area. One important aspect of the environmental impact on the stock is possible effect on the recruitment, which has been varying considerably from year to year. In this modelling study, we thus recreate and analyse the environmental and developmental histories of the observed 0-group individuals in the Barents Sea (young of the year), with emphasis on the importance of ice cover, ice breakup time, maximum temperature, and spawning stock biomass. Our simulations indicate that the environmental conditions experienced by individuals successfully “recruited” to the 0-group are characterized by high ice concentration well into summer, and low temperatures throughout the pelagic juvenile phase, and any perturbations from the Arctic ocean climate typically found in the northern and eastern Barents Sea appears to be detrimental to stock recruitment. In light of the projected warming of the Barents Sea in the next decades and the potential reduction in ice cover, this will entail, the mechanisms investigated herein might lead to future marginalization of polar cod in the Barents Sea.publishedVersio

Egg mortality of Northeast Arctic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus)

This is a pre-copyedited, author-produced PDF of an article accepted for publication in ICES Journal of Marine Science following peer review. The definitive publisher-authenticated version ICES J. Mar. Sci. (2013) is available online at: http://dx.doi.org/10.1093/icesjms/fst00

Utskifting av bassengvatn i djupe terskelfjordar

Med sikte på å kartlegge tidsperioden mellom vassutvekslingsepisodar i terskelfjordar, er det i denne rapporten bruka et analytisk modelloppsett basert på dei observerte vassmassane langs kysten. Metoden som vurderinga er basert på er generisk, og utfører ei systematisk vurdering basert på informasjon om batymetri, tidevatn og hydrografiske forhold i vassmassane på kysten. Opphaldstida vart estimert for fjordar med en maksimal terskeldjup grunnare enn 100 m og et bassengdjup djupare enn 100 m. Den estimerte utskiftinga var vurdert for to periodar: 1960 - 1990 og 1990 - 2020. For mange fjordar vart vatnet skifta ut sjeldnare i den siste perioden.publishedVersio

Ecological Effects and Ecosystem Shifts Caused by Mass Mortality Events on Early Life Stages of Fish

Mass mortality events (MMEs) are a key concern for the management of marine ecosystems. Specific stages and species are at risk and the causes may be single or cumulative pressure from a range of sources including pollutants, anthropogenic climate change or natural variability. Identifying risk and quantifying effects of plausible scenarios including MMEs are key to stakeholders and a quest for scientists. MMEs affect the whole ecosystem, but are traditionally only studied in relation to specific species, disregarding ecological feedbacks. Here we use an end-to-end ecosystem model adapted to the Nordic and Barents seas to evaluate the species-specific and ecological impacts for 50 years following an MME. MMEs were modeled as 10, 50, or 90% reduced recruitment for cod, herring and haddock, individually or in combination. The MME scenarios were compared to a base case model run that includes the current fishing mortality. All species showed declines in population biomass following an MME, increasing in duration and severity with increasing mortality. Cod biomass rebounded to the base case level within 3–13 years post the MME independent of scenario, while neither haddock nor herring fully rebounded to base case levels within the considered time horizon. Haddock responded much more variably to the mortality scenarios than cod or herring, with some scenarios yielding much higher levels of biomass than the base case. Herring responded negatively to all scenarios, leading to lower herring biomass and a steeper decline of the species than seen in the base case due to persistent harvest pressure. Corresponding responses showed that the demersal guild biomass increased substantially, while the pelagic guild biomass declined. Few effects were observed on the other guilds, including the top predators. Ecosystem effects as measured by ecological indicators were greatest after 5 years, but persisted through the entire model run. Fishery indicators showed the same features, but the responses were stronger than for the ecosystem indicators. Taken together this indicates long-term, ecological response to MMEs that can be described as regime shifts, highlighting the importance of using ecosystem models when evaluating effects of MMEs.publishedVersio

Combined effects of fishing and oil spills on marine fish: Role of stock demographic structure for offspring overlap with oil

It has been proposed that the multiple pressures of fishing and petroleum activities impact fish stocks in synergy, as fishing-induced demographic changes in a stock may lead to increased sensitivity to detrimental effects of acute oil spills. High fishing pressure may erode the demographic structure of fish stocks, lead to less diverse spawning strategies, and more concentrated distributions of offspring in space and time. Hence an oil spill may potentially hit a larger fraction of a year-class of offspring. Such a link between demographic structure and egg distribution was recently demonstrated for the Northeast Arctic stock of Atlantic cod for years 1959–1993. We here estimate that this variation translates into a two-fold variation in the maximal proportion of cod eggs potentially exposed to a large oil spill. With this information it is possible to quantitatively account for demographic structure in prospective studies of population effects of possible oil spills.acceptedVersio