Arild Folkvord

How species genetically adapt to new environments is a central question in evolutionary biology. Here whole-genome sequencing combined with functional analysis is used to dissect how Atlantic herring, a marine fish, has adapted to the brackish Baltic Sea. Genes involved in reproduction and early development emerge as primary targets of natural selection, with key changes in a sperm-specific anion channel ( LRRC8C2 ), a zona pellucida protein ( ZPBA1 ), a cluster of three genes for fish transglutaminase ( FTG1-3 ), and a copy number expansion of a fish hatching enzyme gene ( HE1C ). The large diameter of LRRC8C2 homomers facilitates transport of ions and osmolytes, likely preventing swelling of sperm when spawning in low salinity. Altered ZPBA1 sequence together with modified FTG1-3 enzyme activity produces a harder egg envelope that prevents egg swelling in brackish waters, while the enhanced activity of the adapted HE1C enzyme enables larvae to digest this reinforced egg envelope during hatching. Baltic Sea herring populations reproducing in brackish water are fixed or nearly fixed for variant alleles at these four unlinked loci, each carrying multiple amino acid substitutions compared to the alleles prevalent in the Atlantic Ocean populations. The alleles at two of these loci ( FTG1-3, and HE1C ) have been introgressed from the sister species Pacific herring. These findings reveal concrete molecular mechanisms by which a marine species has adapted to a novel, low-salinity environment.

Sexual size dimorphism (SSD) and skewed sex ratios are common in the animal kingdom, and marine fishes, such as the Atlantic cod (Gadus morhua), are no exception. This study utilized newly discovered sex‐linked genetic markers to investigate young‐of‐the‐year cod in the Skagerrak system, comprising approximately 2700 individuals from North Sea and fjord ecotypes. Overall, no significant difference in sex ratio was observed overall (51% females, 49% males), with consistent findings across 14 locations and three sheltering degrees. Although slightly more females were found in exposed areas, and slightly more males in sheltered regions, these differences were not significant. Size variation between female and male cod was generally negligible. These results indicate a likely purely genetic sex determination system in Atlantic cod and a lack of prematuration growth differences between sexes. This research enhances our understanding of the Atlantic cod, providing more insight into the Atlantic cod population in the Skagerrak and emphasizing the importance of genetic factors in sex determination.

Fjords provide valuable research opportunities for marine scientists. They are excellent natural infrastructure for climate impact studies associated with hypoxic episodes and consequences for mesopelagic and deep‐sea ecosystems involving oceanographic circulation processes and basin water renewals. Repeated sampling from the same populations is possible, making fjords excellent systems for developing time series of data for climate impact studies. We provide an overview of the 14 years of data from Norwegian West Coast fjords, focusing on Masfjorden, and report major findings from Oslofjorden in Eastern Norway, exhibiting recurrent hypoxia in the basin waters. We document that the oxygen levels in Masfjorden decreased rapidly by over 60% at 450 m depth in < 8 years, which is much faster than the average rate of deoxygenation in the global ocean. We also discuss the increase in the deep‐sea and low‐light‐adapted coronate jellyfish Periphylla periphylla in view of altered optical conditions of the basin water potentially related to deoxygenation. We argue that fjords like Masfjorden and Oslofjorden are not only macrocosms for ecological processes but also are likely an accelerated version of deep oceans with respect to climate impacts.

Sustainable fisheries management is important for the continued harvest of the world's marine resources, especially as they are increasingly challenged by a range of climatic and anthropogenic factors. One of the pillars of sustainable fisheries management is the accurate identification of the biological units, i.e., populations. Here, we developed and implemented a genetic baseline for Atlantic herring harvested in the Norwegian offshore fisheries to investigate the validity of the current management boundaries. This was achieved by genotyping > 15,000 herring from the northern European seas, including samples of all the known populations in the region, with a panel of population‐informative SNPs mined from existing genomic resources. The final genetic baseline consisted of ~1000 herring from 12 genetically distinct populations. We thereafter used the baseline to investigate mixed catches from the North and Norwegian Seas, revealing that each management area consisted of multiple populations, as previously suspected. However, substantial numbers (up to 50% or more within a sample) of herring were found outside of their expected management areas, e.g., North Sea autumn‐spawning herring north of 62° N (average = 19.2%), Norwegian spring‐spawning herring south of 62° N (average = 13.5%), and western Baltic spring‐spawning herring outside their assumed distribution area in the North Sea (average = 20.0%). Based upon these extensive observations, we conclude that the assessment and management areas currently in place for herring in this region need adjustments to reflect the populations present. Furthermore, we suggest that for migratory species, such as herring, a paradigm shift from using static geographic stock boundaries towards spatial dynamic boundaries is needed to meet the requirements of future sustainable management regimes.

Liver condition is hypothesized to be a better condition index for juvenile cod than other somatic condition indices, since cod store most of their excess energy in their liver. It is suggested that population changes in liver condition can be used as a proxy for overwinter mortality. Here, we have collected 0- and 1-group cod from the same cohort in the Barents Sea before and at the end of the winter season. We have taken measurements for somatic- and liver-based condition indices, as well as enumerating nematode endoparasites found in each individual cod. The average wet weight of sampled cod increased by 85% (approx. 0.4% day⁻¹) from 6.0 to 10.1 g from September to February, while the average liver wet weight decreased by 15%. The relative water content of the liver also increased significantly over the winter, resulting in a 42% reduction in average dry liver weight from 0.116 to 0.067 g over the 4–5-month period between samplings. About 30% of the 1-group cod surviving the winter had less than 20% of the liver energy content of same-sized 0-group cod. The liver condition was also 5–10% lower in cod with high nematode endoparasite load than the cod without nematode endoparasites. The significance of the loss of liver energy content is discussed in relation to the marked seasonality combined with climate change and the challenges this imposes on the survival of 0-group cod in high-latitude environments during their first winter.

Early life survival is critical to successful replenishment of fish populations, and hypotheses developed under the Growth‐Survival Paradigm (GSP) have guided investigations of controlling processes. The GSP postulates that recruitment depends on growth and mortality rates during early life stages, as well as their duration, after which the mortality declines substantially. The GSP predicts a shift in the frequency distribution of growth histories with age towards faster growth rates relative to the initial population because slow‐growing individuals are subject to high mortality (via starvation and predation). However, mortality data compiled from 387 cases published in 153 studies (1971–2022) showed that the GSP was only supported in 56% of cases. Selection against slow growth occurred in two‐thirds of field studies, leaving a non‐negligible fraction of cases showing either an absence of or inverse growth‐selective survival, suggesting the growth‐survival relationship is more complex than currently considered within the GSP framework. Stochastic simulations allowed us to assess the influence of key intrinsic and extrinsic factors on the characteristics of surviving larvae and identify knowledge gaps on the drivers of variability in growth‐selective survival. We suggest caution when interpreting patterns of growth selection because changes in variance and autocorrelation of individual growth rates among cohorts can invalidate fundamental GSP assumptions. We argue that breakthroughs in recruitment research require a comprehensive, population‐specific characterization of the role of predation and intrinsic factors in driving variability in the distribution and autocorrelation of larval growth rates, and of the life stage corresponding to the endpoint of pre‐recruited life.

Atlantic herring in International Council for Exploration of the Sea (ICES) Divisions 6.a, 7.b–c comprises at least three populations, distinguished by temporal and spatial differences in spawning, which have until recently been managed as two stocks defined by geographical delineators. Outside of spawning the populations form mixed aggregations, which are the subject of acoustic surveys. The inability to distinguish the populations has prevented the development of separate survey indices and separate stock assessments. A panel of 45 single-nucleotide polymorphisms, derived from whole-genome sequencing, were used to genotype 3480 baseline spawning samples (2014–2021). A temporally stable baseline comprising 2316 herring from populations known to inhabit Division 6.a was used to develop a genetic assignment method, with a self-assignment accuracy greater than 90%. The long-term temporal stability of the assignment model was validated by assigning archive (2003–2004) baseline samples (270 individuals) with a high level of accuracy. Assignment of non-baseline samples (1514 individuals) from Divisions 6.a, 7.b–c indicated previously unrecognized levels of mixing of populations outside of the spawning season. The genetic markers and assignment models presented constitute a ‘toolbox’ that can be used for the assignment of herring caught in mixed survey and commercial catches in Division 6.a into their population of origin with a high level of accuracy.

Atlantic herring in ICES Divisions 6.a, 7.b-c comprises at least three populations, distinguished by temporal and spatial differences in spawning, which have until recently been managed as two stocks defined by geographic delineators. Outside of spawning the populations form mixed aggregations, which are the subject of acoustic surveys. The inability to distinguish the populations has prevented the development of separate survey indices and separate stock assessments. A panel of 45 SNPs, derived from whole genome sequencing, were used to genotype 3,480 baseline spawning samples (2014-2021). A temporally stable baseline comprising 2,316 herring from populations known to inhabit Division 6.a was used to develop a genetic assignment method, with a self-assignment accuracy >90%. The long-term temporal stability of the assignment model was validated by assigning archive (2003-2004) baseline samples (270 individuals) with a high level of accuracy. Assignment of non-baseline samples (1,514 individuals) from Division 6.a, 7.b-c indicated previously unrecognised levels of mixing of populations outside of the spawning season. The genetic markers and assignment models presented constitute a ‘toolbox’ that can be used for the assignment of herring caught in mixed survey and commercial catches in Division 6.a into their population of origin with a high level of accuracy.

Climate change will increase the magnitude and duration of warming events and the variability in the phenology and abundance of available prey to the early life stages of fish. These factors influence physiological, behavioral and ecological processes, impacting growth, development and survival. Using a fully factorial design with two prey-availability treatments (1200 prey items L⁻¹ (high prey abundance) or 40 prey items L⁻¹ (low prey abundance)) under three temperature regimes (8, 10 and 12°C), the swimming kinematics of 6-week old spring-spawning Atlantic herring larvae were examined using silhouette video photography. Higher temperatures combined with food limitation significantly decreased the growth and swimming kinematics of larval herring, with the most negative effect observed in larvae reared at 12°C and exposed to low food abundances. Specifically, larvae displayed reduced locomotory behaviors and reduced vertical movements. By contrast, larvae reared at high prey abundance and at 12°C displayed more active swimming and exploratory behavior, as evidenced by an increase in both locomotory behavior and vertical and horizontal turn angles, suggesting increased motivation to search for food. This research highlights the importance of determining to what degree fish larvae are sensitive to changes in temperature and how these changes might be further influenced by food availability.

The estimation of growth rates in young herring larvae (Clupea harengus) in the field can be difficult because the primary increments in the otoliths may not be discernible or formed at a daily level. Likewise, the estimation of mortality rates of fish larvae in the field is very difficult to achieve, especially in a rigorous quantitative manner. In this study, the authors suggest the use of a stage‐based proxy of feeding success, growth and potential survival or mortality risk of field‐caught larvae. The stage‐based proxy is derived based on observations from previous laboratory studies where larvae successfully completing start‐feeding on external food sources will advance through the early development stages, whereas those that do not (unsuccessful larvae) remain and accumulate in the development stage preceding first feeding. The relative occurrence of larvae in the early development stages is therefore expected to reflect feeding conditions of the larvae, with higher ratios of unsuccessful larvae indicative of poor feeding success and higher mortality risk. Using field data on Norwegian spring spawning herring, the authors document that the relative occurrence of larvae in the late non‐feeding stage is significantly higher at lower average zooplankton concentrations, in line with the predictions of the authors that this novel approach of using a stage‐based proxy could be a useful indication of feeding success, growth and mortality in the field. Further, there was a significant interaction effect with ambient temperature, with the ratio being higher at low zooplankton concentrations at higher temperatures. This study also suggests that these findings are not population specific as the same accumulation of non‐feeding larvae in the late non‐feeding stage was observed in laboratory‐reared larvae of both autumn and spring spawning herring populations.