Tuning and Development of an Individual-Based Model of the Herring Spawning Migration

Norwegian spring spawning herring is a migratory pelagic fish stock that seasonally navigates between distant locations in the Norwegian Sea. The spawning migration takes place between late winter and early spring. In this article, we present an individual-based model that simulated the spawning migration, which was tuned and validated against observation data. Individuals were modelled on a continuous grid coupled to a physical oceanographic model. We explore the development of individual model states in relation to local environmental conditions and predict the distribution and abundance of individuals in the Norwegian Sea for selected years (2015–2020). Individuals moved position mainly according to the prevailing coastal current. A tuning procedure was used to minimize the deviations between model and survey estimates at specific time stamps. Furthermore, 4 separate scenarios were simulated to ascertain the sensitivity of the model to initial conditions. Subsequently, one scenario was evaluated and compared with catch data in 5 day periods within the model time frame. Agreement between model and catch data varies throughout the season and between years. Regardless, emergent properties of the migration are identifiable that match observations, particularly migration trajectories that run perpendicular to deep bathymetry and counter the prevailing current. The model developed is efficient to implement and can be extended to generate multiple realizations of the migration path. This model, in combination with various sources of fisheries-dependent data, can be applied to improve real-time estimates of fish distributions.

Population Genetics and Spawning Time of Lake Taupo Rainbow Trout

<p>The rainbow trout (Oncorhynchus mykiss) of Lake Taupo, New Zealand provide an exceptional opportunity to explore the contemporary adaptation of an introduced aquatic species. Recently it has become evident that their spawning migration time has shifted to later in the season. I investigated the genetic basis of these changes in spawning time by (1) using genetic markers to determine the origins of Taupo trout in California, (2) determining the pattern and extent of spatial population genetic variation throughout the Lake Taupo catchment and in comparison to nearby Lake Tarawera in the Rotorua district, (3) analysing genetic variation at the OtsClock1b spawning time gene in temporal replicates from several sites from Taupo, and (4) comparing contemporary genetic variation at this gene and microsatellite markers to genetic variation from three Taupo tributaries in 1980s. I compared the ability of single nucleotide polymorphism (SNP) and microsatellite markers to determine the origins of Lake Taupo rainbow trout, translocated from California around 120 years ago. Data were collected from 15 microsatellite and 93 SNP markers, using samples from the Lake Taupo population and ten populations throughout California, which included all historically indicated populations of origin. Results revealed that the Lake Taupo population has significantly diverged from Californian populations at both microsatellite and SNP loci. These analyses also showed that the Lake Taupo population was probably derived from several sources in California (the most likely origins being the McCloud River and Lake Almanor), and an indeterminate California coastal population. This conclusion was supported with simulations of founder events, which suggested that the genetic patterns of a single source of introduction would still be detectable 100 years post-founding, but with multiple introductions exact source populations become more difficult to detect. Approximately 50 individuals from 10 locations throughout the catchment were then analysed using 15 microsatellite loci to determine if there was any spatial population genetic differentiation. There was no significant difference in genetic distance between locations within Lake Taupo, although there was a significant difference between these populations and Rotorua and Waipakihi, which are isolated by geographic barriers. Lake Taupo rainbow trout do appear to diverge at markers potentially under selection, though, because genotyping of the poly-Q region of the timing locus OtsClock1b shows significant differentiation between individuals sampled at different times in the Waipa River. Two other sites, however, did not show the same pattern of significant seasonal variation in OtsClock1b allele frequencies. This suggests that genotypes at this locus could be influencing spawning migration time, but that this variation could also be site specific, and therefore have a strong environmental component. Scale samples from the 1980s show no significant divergence at 5 microsatellites and OtsClock1b, indicating that allele frequencies have not changed significantly over the last 20 years at neutral markers or markers under selection. I therefore conclude that while Taupo rainbow trout have diverged from their origins in California, they have only slightly diverged within their new environment, and do not show a consistent pattern of genetic change over time. This information will contribute not only to the management of the Taupo fishery but also to the current understanding of the population genetic structuring of introduced salmonids.</p>

Population Genetics and Spawning Time of Lake Taupo Rainbow Trout

<p>The rainbow trout (Oncorhynchus mykiss) of Lake Taupo, New Zealand provide an exceptional opportunity to explore the contemporary adaptation of an introduced aquatic species. Recently it has become evident that their spawning migration time has shifted to later in the season. I investigated the genetic basis of these changes in spawning time by (1) using genetic markers to determine the origins of Taupo trout in California, (2) determining the pattern and extent of spatial population genetic variation throughout the Lake Taupo catchment and in comparison to nearby Lake Tarawera in the Rotorua district, (3) analysing genetic variation at the OtsClock1b spawning time gene in temporal replicates from several sites from Taupo, and (4) comparing contemporary genetic variation at this gene and microsatellite markers to genetic variation from three Taupo tributaries in 1980s. I compared the ability of single nucleotide polymorphism (SNP) and microsatellite markers to determine the origins of Lake Taupo rainbow trout, translocated from California around 120 years ago. Data were collected from 15 microsatellite and 93 SNP markers, using samples from the Lake Taupo population and ten populations throughout California, which included all historically indicated populations of origin. Results revealed that the Lake Taupo population has significantly diverged from Californian populations at both microsatellite and SNP loci. These analyses also showed that the Lake Taupo population was probably derived from several sources in California (the most likely origins being the McCloud River and Lake Almanor), and an indeterminate California coastal population. This conclusion was supported with simulations of founder events, which suggested that the genetic patterns of a single source of introduction would still be detectable 100 years post-founding, but with multiple introductions exact source populations become more difficult to detect. Approximately 50 individuals from 10 locations throughout the catchment were then analysed using 15 microsatellite loci to determine if there was any spatial population genetic differentiation. There was no significant difference in genetic distance between locations within Lake Taupo, although there was a significant difference between these populations and Rotorua and Waipakihi, which are isolated by geographic barriers. Lake Taupo rainbow trout do appear to diverge at markers potentially under selection, though, because genotyping of the poly-Q region of the timing locus OtsClock1b shows significant differentiation between individuals sampled at different times in the Waipa River. Two other sites, however, did not show the same pattern of significant seasonal variation in OtsClock1b allele frequencies. This suggests that genotypes at this locus could be influencing spawning migration time, but that this variation could also be site specific, and therefore have a strong environmental component. Scale samples from the 1980s show no significant divergence at 5 microsatellites and OtsClock1b, indicating that allele frequencies have not changed significantly over the last 20 years at neutral markers or markers under selection. I therefore conclude that while Taupo rainbow trout have diverged from their origins in California, they have only slightly diverged within their new environment, and do not show a consistent pattern of genetic change over time. This information will contribute not only to the management of the Taupo fishery but also to the current understanding of the population genetic structuring of introduced salmonids.</p>

Migrating silver eels return from the sea to the river of origin after a false start

The European eel's singular spawning migration from European waters towards the Sargasso Sea remains elusive, including the early phase of migration at sea. During spawning migration, the movement of freshwater resident eels from river to sea has been thought to be irreversible. We report the first recorded incidents of eels returning to the river of origin after spending up to a year in the marine environment. After migrating to the Baltic Sea, 21% of the silver eels, tagged with acoustic transmitters, returned to the Narva River. Half returned 11–12 months after moving to the sea, with 15 km being the longest upstream movement. The returned eels spent up to 33 days in the river and migrated to the sea again. The fastest specimen migrated to the outlet of the Baltic Sea in 68 days after the second start—roughly 1300 km. The surprising occurrence of returning migrants has implications for sustainable management and protection of this critically endangered species.