Caroline M. F. Durif

Current knowledge on the continental shelf and transatlantic spawning migration of the European eel ( Anguilla anguilla ) is based on silver eels outmigrating from fresh water. However, an unknown proportion of eels grow in marine coastal waters during the continental part of their lifecycle and therefore start their spawning migration while they are already in marine waters. To study the onset of the European eel's spawning migration in marine waters, we tagged yellow eels in Sandnesfjord, Norway. One hundred yellow and pre‐migratory eels (stage FII or FIII) were tagged with acoustic transmitters in August, September or October in 2018–2019 and their subsequent movements were recorded within a hydrophone array until May 2023. Twenty‐two eels left the fjord in a period spanning from early October to early December, which partly coincides with the usual downstream migratory period among European eels. These eels were never detected again and had thus presumably transitioned to the silver eel stage. This transition occurred from a few months to up to 3 years after tagging. In all silver eels, an increase in vertical and horizontal movements within the fjord preceded their exit from the fjord. This sudden increase in activity occurred only for a few days before they left the fjord in most individuals, but this period was as long as 1 month in other individuals. Five of the silver eels undertook migrations up to several kilometres to a tidal stream before leaving the fjord. Eel migration was largely nocturnal, independent of moon or tidal phase. We conclude that the relatively short migration season and individual migration duration within the fjord, combined with the absence of significant environmental, physiological or anthropogenic stressors, may contribute to higher survival rates during the continental phase of the spawning migration compared to eels that start their spawning migration from fresh water, but this needs to be investigated in future studies.

Lumpfish (Cyclopterus lumpus) spawn in coastal waters in the North Atlantic Ocean, yet little is known about the distribution outside of the spawning season. Using data from an extensive pelagic trawl survey that takes place annually in the Nordic Seas, we examine the distribution and abundance of different life stages of lumpfish during summer. Utilising generalised additive models, random forest and support vector machine, we investigate environmental variables that potentially could influence their distribution. Our results reveal that lumpfish have an extensive distribution covering the Irminger Sea, Denmark Strait and Norwegian Sea. However, their frequency of capture is lower south of Iceland and in the North Sea in comparison with other coastal and oceanic areas. Temperature emerged as a primary constraint on the distribution. Lumpfish were caught at over 90% of the stations when temperature at 20‐m depth ranged between 4°C and 9°C but less frequently when temperature was outside of this range. Other environmental variables, such as salinity, mixed layer depth and measures of primary and secondary production, had low influence on the distribution of lumpfish.

Atlantic cod ( Gadus morhua ) and haddock ( Melanogrammus aeglefinus ) are found on both sides of the North Atlantic and often their spawning overlaps in time and space. Yet, haddock has a much more variable recruitment than cod in areas where they are sympatric, a difference that is consistent across ecosystems. At the larval stages, differences in feeding behavior have consequences for growth and survival, potentially contributing to the differences in recruitment between these species. We tested whether haddock displayed different swimming behavior to cod under abundant food conditions, while also comparing their growth every 7 days until 38 days post hatching (dph). Swimming kinematics were quantified from the three‐dimensional trajectories of larvae filmed in an aquarium at 11, 21, and 32 dph. Cod swam faster than haddock starting at 21 dph. Differences in swimming speed between species were consistent with body morphology. Haddock was heavier than cod at a given age and body length. At 11 dph, haddock exhibited longer pauses and wider horizontal and vertical turn angles than cod, indicating a moderately larger visual search field compared to cod. Longer pause duration indicates that haddock are searching for prey for longer periods than cod at 11 dph. The differences in prey search disappeared at 21 and 32 dph. The differences in feeding and swimming behavior reported here could play an important role for survival, with consequences for the recruitment of these species.

Static magnetic fields (B-fields) are introduced in marine ecosystems by anthropogenic sources such as subsea power cables. Larvae of the gadoids Atlantic haddock (Melanogrammus aeglefinus) and Atlantic cod (Gadus morhua) disperse over the continental shelf and may encounter B-fields, which could alter their behavior. We tested the behavior of 184 gadoid larvae while they were being exposed to a B-field (22–156 μT), which is in the intensity range of that produced by a high-voltage direct current cable, for a duration of 10 min in a raceway tank. We present a reanalysis of published data on 92 haddock larvae combined with newly collected data on 92 cod larvae tested under the same settings. Exposure did not affect the spatial distribution of the larvae. Both species displayed the same proportion of nonexploratory (78%) and exploratory (22%) individuals. Exposure significantly reduced the average swimming speed of the nonexploratory larvae by 32% and the average acceleration by 30%. Exploratory larvae were not affected. These results indicate that the swimming behavior of a high proportion of gadoid larvae would be reduced by weak static B-fields. Consequences of exposure on dispersal and survival at the population scale awaits further work using biophysical-coupled models.

Anguillid eels were once considered to be the classic example of catadromy. However, alternative life cycles have been reported, including skipping the freshwater phase and habitat shifting between fresh, brackish, and saltwater throughout the growth phase. There is a lack of knowledge regarding these alternate life strategies, for example, the proportion of individuals in the population that adopt them compared to classic catadromy. We provide a description of these alternate life cycle strategies in temperate anguillids, their possible drivers, and the methods available to investigate them. These methods (lethal and non-lethal), include otolith microchemistry, fatty acid and stable isotope analyses, parasite identification, blood transcriptomics, and electronic tags. We argue that since the current management framework for the European eel and other temperate eels is based mainly on the freshwater component of the population, it ignores eels growing in saline waters. Many of the factors that are thought to be responsible for the precipitous decline of the eel population are more prevalent in freshwater systems. Therefore, the contribution of saline eels may be more important than currently estimated. The habitat-shifting ability of eels may be all the more crucial for the persistence and recovery of those species that are endangered.

Lumpfish (Cyclopterus lumpus) is a transatlantic marine fish displaying large population sizes and a high potential for dispersal and gene-flow. These features are expected to result in weak population structure. Here, we investigated population genetic structure of lumpfish throughout its natural distribution in the North Atlantic using two approaches: I) 4,393 genome wide SNPs and 95 individuals from 10 locations, and II) 139 discriminatory SNPs and 1,669 individuals from 40 locations. Both approaches identified extensive population genetic structuring with a major split between the East and West Atlantic and a distinct Baltic Sea population, as well as further differentiation of lumpfish from the English Channel, Iceland, and Greenland. The discriminatory loci displayed ~2–5 times higher divergence than the genome wide approach, revealing further evidence of local population substructures. Lumpfish from Isfjorden in Svalbard were highly distinct but resembled most fish from Greenland. The Kattegat area in the Baltic transition zone, formed a previously undescribed distinct genetic group. Also, further subdivision was detected within North America, Iceland, West Greenland, Barents Sea, and Norway. Although lumpfish have considerable potential for dispersal and gene-flow, the observed high levels of population structuring throughout the Atlantic suggests that this species may have a natal homing behavior and local populations with adaptive differences. This fine-scale population structure calls for consideration when defining management units for exploitation of lumpfish stocks and in decisions related to sourcing and moving lumpfish for cleaner fish use in salmonid aquaculture.

Submarine power cables carry electricity over long distances. Their geographic distribution, number, and areal coverage are increasing rapidly with the development of, for example, offshore wind facilities. The flow of current passing through these cables creates a magnetic field (MF) that can potentially affect marine organisms, particularly those that are magnetosensitive. The lumpfish (Cyclopterus lumpus) is a migratory species that is widely distributed in the North Atlantic Ocean and Barents Sea. It migrates between coastal spawning grounds and pelagic offshore feeding areas. We tested whether lumpfish respond to MFs of the same intensity as those emitted by high voltage direct current (HVDC) submarine power cables. Laboratory experiments were conducted by placing juvenile lumpfish in an artificial MF gradient generated by a Helmholtz coil system. The intensity of the artificial MF used (230 µT) corresponded to the field at 1 m from a high-power submarine cable. The fish were filmed for 30 min with the coil either on or off. Swimming speeds, and presence in the different parts of a raceway, were extracted from the videos and analyzed. Juvenile lumpfish activity, defined as the time that the fish spent swimming relative to stationary pauses (attached to the substrate), and the distance travelled, were unaffected by exposure to the artificial MF. The swimming speed of juvenile lumpfish was reduced (by 16%) when the coil was on indicating that the fish could either sense the MF or the induced electric field created by the movement of the fish through the magnetic field. However, it seems unlikely that a 16% decrease in swimming speed occurring within 1 m of HVDC cables would significantly affect Atlantic lumpfish migration or homing.

Many anguillid eel species display facultative catadromy. Some eel spend their entire life cycle in marine coastal areas, but the geographical extent of this, especially at the extremes of their distributional ranges, is unknown. We analysed otolith Sr:Ca and Ba:Ca from yellow-stage European eel ( Anguilla anguilla) sampled along the coast of Norway and in several freshwater lakes (58°N–63°N), to infer their initial settlement and later life movement patterns with regards to habitat salinity. Most eel (80%) sampled in marine habitats ( n = 371) had settled and remained in marine water, but 20% had moved between marine and freshwater habitats and were hence classified as inter-habitat shifters. Among freshwater sampled eel ( n = 99), 80% had settled and remained in fresh water, but 20% were classified as inter-habitat shifters. The average growth rates for marine water residents, inter-habitat shifters, and freshwater residents were 35, 27, and 17 mm·year^–1, respectively. Northern European shallow marine habitats may serve as important yellow eel growth habitats and may be critical to buffer the European eel population against the general decrease in continental recruitment.