Ola Håvard Diserud

Non-native pink salmon ( Oncorhynchus gorbuscha ) have increased rapidly in Norwegian rivers since 2017, yet their marine ecology remains poorly understood. To assess variation in marine growth, we analysed scales from 476 individuals returning to northern (Barents Sea region) and central (Atlantic region) Norway in 2019 and 2021. Clear differences in growth and body size emerged between regions and years, with fish returning in 2019 experiencing the poorest growth and reaching the smallest adult size. Differences in growth histories indicate that pink salmon captured in the two regions may have experienced distinct marine feeding resource conditions, possibly because they followed different migratory pathways, beginning in the first weeks at sea. Approximately one-third of individuals experienced distinct periods of depressed growth during their first spring and summer at sea, and these events were more common among fish returning to central Norway. Mid- to late-season growth during the first year at sea strongly influenced adult size, highlighting the importance of favourable conditions during this period. At the same time, it was shown that growth after the winter could more than compensate for poor growth up until then. The pronounced variability in growth performance suggests that environmental conditions during the marine phase strongly shape adult size and may influence the future invasion potential of pink salmon in the North Atlantic. As this invasive species continues to expand, continued monitoring of growth and improved understanding of their marine ecology and environmental drivers of growth will be essential for anticipating invasion dynamics and impacts on native ecosystems.

The migratory behaviour of Atlantic salmon kelts is poorly understood. Due to the Atlantic salmon's considerable socio‐economic and ecological importance, their precipitous population abundance declines, and the population resilience potential of salmon kelts, addressing these knowledge gaps is important. In this study, directional swimming behaviours and position choice were quantified for the swimming trajectories of 48 acoustically tagged kelts in a regulated river. The kelts exhibited positive rheotaxis under high flow velocities, though their response to turbulence was more varied. The kelts exhibited clear preferences for moving between positions with similar hydraulic conditions and avoided moving to positions where the flow velocity or turbulence changed rapidly. The extent to which the kelts avoided moving to positions with rapid changes in hydraulic conditions varied by individual experience, individual body size, how active the individual was during daytime and how often an individual was detected near a conspecific. This study is the first to combine data on intrinsic individual characteristics with extrinsic hydraulic factors to conduct a detailed analysis of kelt behaviour during migration in a regulated river. By examining these behaviours across different hydraulic conditions, our findings bear important implications for both hydropower development and river management practices.

Genetic introgression of domesticated plants and animals into wild populations occurs globally. Such introgression disrupts adaptive potential, reduces fitness in wild populations and threatens intraspecific genetic variation. The best‐documented case of farmed introgression into wild populations is that of the Atlantic salmon ( Salmo salar ). Norway is the world's largest producer of farmed Atlantic salmon, and the industry is growing in Iceland and other countries. In Norway, genetic introgression resulting from farmed escapees breeding with wild conspecifics has been documented in approximately two‐thirds of 250 salmon populations studied. This comprehensive quantification has been possible due to a panel of genetic markers diagnostic of farmed introgression. Improved genomic resources, continued selection and genetic drift in the farmed breeding lines, as well as new breeding lines in commercial production, call for an updated tool to quantify farmed genetic introgression. Here, we present second‐generation panels of genetic markers diagnostic of farmed introgression in Norway and the first panels of genetic markers diagnostic of farmed introgression in Iceland. We show that these diagnostic markers provide increased power to detect introgression compared to the first‐generation panel, as well as increased power compared to a genome‐wide marker set. Improved accuracy will benefit the ongoing monitoring of farmed introgression and facilitate research into the ecological and functional effects of farmed introgression in wild populations.

Sea trout, the anadromous phenotype of brown trout (Salmo trutta), is severely affected by high salmon lice infestation pressure that occurs in areas of Atlantic salmon (S. salar) aquaculture. Yet, and in contrast to Atlantic salmon, there is currently no effect indicator implemented for sea trout for regulating the Norwegian aquaculture industry. Here, we present such an indicator centred on the concept of fitness. Our numerical example, based on previous published estimates of reduced survival and reduced marine feeding time of sea trout due to spillback of salmon lice from aquaculture, demonstrates that the indicator is readily implementable. There are several benefits of our indicator: (i) multiple effects of salmon lice on sea trout can be implemented in a single biologically meaningful indicator; (ii) the indicator can be customized and simplified depending on data availability and knowledge; (iii) the simplifications provide a conservative yet effective indicator; (iv) the indicator is on the same scale as the already implemented Atlantic salmon indicator, making it possible to use the same management threshold values; and (v) the changes in fitness measured by the indicator can provide scenarios for the evolution of anadromy.

Marine aquaculture of Atlantic salmon (Salmo salar) is a relatively new industry where breeding programs have led to rapid genetic change in the captive populations that were built up alongside conspecific wild individuals. Throughout its 50-years history, marine aquaculture of Atlantic salmon has been associated with escapes, and studies have shown that escapees may enter rivers, spawn successfully, and this may lead to farmed-to-wild genetic introgression and maladaptation in wild populations. Yet, an open question is what factors can best explain the variability in the proportion of farmed escapees in wild populations, and when present, which additional factors lead to introgression. Here, we combine two large-scale data sets from monitoring escaped farmed salmon and introgression in Norwegian rivers between 2006 and 2018 to model how anthropogenic, environmental, and population factors influence proportion of escapees and level of introgression. We found that increasing farming intensity and river discharge increase the expected proportions of escaped farmed salmon in rivers, whereas a larger wild salmon population size reduces the expected proportion of escapees despite increasing the expected absolute numbers of escaped farmed salmon. On a large scale, introgression is primarily a function of proportions of escaped farmed salmon, and only to a minor extent a function of local environmental factors or salmon population characteristics. This suggests that as long as salmon aquaculture is based on technologies where non-sterile fish can escape, all anadromous wild Atlantic salmon populations are at risk. Large marine protected areas without salmon aquaculture may slow down the rate of intrusion and introgression by increasing the distance between intensive aquaculture and wild populations.

The timing of seaward migration is a key life‐history trait for many anadromous fish species, with growth and survival at sea depending on a match/mismatch scenario between the timing of the sea entry and optimal conditions. Based on a 25‐year study with 15,226 individually tagged brown trout (Salmo trutta) in a Norwegian river, we analysed how the within‐season timing of sea migration impacted growth and survival. In both first‐time and veteran migrants, marine growth was highest for early migrating individuals, large individuals, and those with a low condition factor when entering the sea. Survival was highest for individuals entering the sea early in the season. In first‐time migrants, survival increased with body length. Survival also increased with the number of other smolts migrating simultaneously. As the early smolts were the most successful, it may seem strange that many smolts migrate later in the season. We suggest that late‐migrating smolts may not be of a size and/or physiological state suitable for early marine conditions, and may make the best of a bad situation.

Early and late life history and growth are affected by farmed salmon introgression across 105 wild populations of Atlantic salmon.

Consistent individual differences in behavior have been demonstrated for many animals, but there are few studies of consequences of such repeated behavior in the wild. We tested consistency in migration timing to and from the sea among anadromous Arctic char (Salvelinus alpinus) and brown trout (Salmo trutta), using data from a study period of about 25 years, including more than 27,000 uniquely Carlin‐tagged individuals that migrated to sea for feeding in the spring and returned to the river in late summer for up to 13 successive years. Consistency was found between individuals across time in timing of the seaward migration. Individuals migrating early during their first migration tended to migrate early the following years, and late migrants tended to migrate late. The same pattern was found also at ascent to freshwater. Hence, this study demonstrated that individual fish in nature can differ in behavior related to migration timing and that these differences can be consistent during their lifetime. Early migrants increased their mass more than late migrants and had a higher specific growth rate. Early migrating Arctic char, but not brown trout, experienced a longer life after the first migration to sea than late migrants. In both species, maturity occurred earlier in individuals that migrated early. For brown trout, but not for Arctic char, fecundity was significantly correlated to the timing of smolt migration. Hence, the repeatable individual variation in migration timing seemed to have ecological and fitness consequences in terms of growth, longevity, timing of maturity, and lifetime fecundity.

The Changeable Hawk-Eagle Nisaetus cirrhatus complex is represented by two taxa in mainland India: N. c. cirrhatus in the northern plains and peninsula and N. c. limnaeetus in the Himalayan foothills. Traditionally these taxa have been regarded as subspecies of one species, but recently they have been proposed to be different species. Here, we use an integrative taxonomic approach based on considerations of plumage, biometrics, genetics and vocalizations. Several plumage characters are significantly different between the two taxa, but crest length was the only one of 56 characters that was diagnostically different, with no overlap. About 30% of the birds had intermediate crest lengths, suggesting that they are hybrids or backcrosses, as also supported by the microsatellite results. PCAs of adult plumage show many intermediate individuals, irrespective of whether these birds were collected near a putative contact zone. There is restricted gene flow between the two taxa, presumably as a result of their largely allopatric distributions. On current knowledge, reproductive isolation appears to be weak at best, and we therefore recommend continuing to regard limnaeetus and cirrhatus as conspecific.