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Mitigation measures for downstream migratory Atlantic salmon smolts (Salmo salar L.) at migration barriers usually turn out to be insufficient to enable safe and quick passage, probably due to a lack of knowledge about their behavioural responses. Combining manual 2D tracking with hydrodynamic modelling has been rarely performed but might be useful to highlight environmental factors influencing smolt behavioural tactics and the choice of a migration route. We investigated the smolt downstream migration at a hydropower plant that offers five migration routes, including a Kaplan turbine and a fish-friendly Archimedes screw. Four behavioural tactics were defined to describe the smolt expressed behaviour, which was mainly complex and hesitant. The majority of the smolts approached more than one migration route before crossing the site and the Kaplan turbine turned out to be the most approached route, contrary to the Archimedes screw. Hydrodynamic modelling highlighted that flow velocity and water depth were used as hydraulic cues in the selection of a migration route, as the smolts preferred higher flow velocities and water depths. The comprehension of the factors influencing the research behaviour at hydropower plants may be useful to design attractive mitigation measures and to guide the smolts efficiently towards safe routes.

Disruption to migration is a growing problem for conservation and restoration of animal populations. Anthropogenic barriers along migration paths can delay or prolong migrations, which may result in a mismatch with migration-timing adaptations. To understand the interaction of dams (as barriers along a migration path), seasonally changing environmental conditions, timing of Atlantic salmon ( Salmo salar ) downstream migration, and ultimate migration success, we used 10 years of river temperature and discharge data as a template upon which we simulated downstream movement of salmon. Atlantic salmon is a cool-water species whose downstream migrating smolts must complete migration before river temperatures become too warm. We found that dams had a local effect on survival as well as a survival effect that was spatially and temporally removed from the encounter with the dam. While smolts are delayed by dams, temperatures downstream can reach lethal or near-lethal temperatures; as a result, the match between completion of migration and the window of appropriate migration conditions can be disrupted. The strength of this spatially and temporally removed effect is at least comparable to the local effects of dams in determining smolt migration success in the presence of dams. We also considered smolts from different tributaries, varying in distance from the river mouth, to assess the potential importance of locally adapted migration timing on the effect of barriers. Migration-initiation temperature affected modeled smolt survival differentially across tributaries, with the success of smolts from upstream tributaries being much more variable across years than that of smolts with a shorter distance to travel. As a whole, these results point to the importance of broadening our spatial and temporal view when managing migrating populations. We must consider not only how many individuals never make it across migration barriers, but also the spatially and temporally removed consequences of delays at the barriers for those individuals that successfully navigate them.

Many salmon populations in both the Pacific and Atlantic Oceans have experienced sharply decreasing returns and high ocean mortality in the past two decades, with some populations facing extirpation if current marine survival trends continue. Our inability to monitor the movements of marine fish or to directly measure their survival precludes experimental tests of theories concerning the factors regulating fish populations, and thus limits scientific advance in many aspects of fisheries management and conservation. Here we report a large-scale synthesis of survival and movement rates of free-ranging juvenile salmon across four species, 13 river watersheds, and 44 release groups of salmon smolts (>3,500 fish tagged in total) in rivers and coastal ocean waters, including an assessment of where mortality predominantly occurs during the juvenile migration. Of particular importance, our data indicate that, over the size range of smolts tagged, (i) smolt survival was not strongly related to size at release, (ii) tag burden did not appear to strongly reduce the survival of smaller animals, and (iii) for at least some populations, substantial mortality occurred much later in the migration and more distant from the river of origin than generally expected. Our findings thus have implications for determining where effort should be invested to improve the accuracy of salmon forecasting, to understand the mechanisms driving salmon declines, and to predict the impact of climate change on salmon stocks.

There is increasing evidence that predation by harbour seals on out-migrating salmon smolts may be responsible for the low return of adult coho and Chinook salmon in the Salish Sea. However, little attention has been given to understanding where and when this predation occurs and the extent to which it might be conducted by few or many seals in the population. We equipped 17 harbour seals with data loggers to track seal movements and used accelerometry to infer prey encounter events (PEEs) following the release of ~384 000 coho (May 4, 2015) and ~3 million Chinook (May 14, 2015) smolts into the Big Qualicum River. We found a small proportion (5.7%) of all PEEs occurred in the estuary where salmon smolts entered the ocean—and that only one-quarter of the seals actively fed there. PEE counts increased in the estuary after both species of smolts were released. However, the response of the seals was less synchronous and occurred over a greater range of depths following the release of the smaller-bodied and more abundant Chinook smolts. Harbour seals feeding in the estuary appeared to target coho smolts at the beginning of May but appeared to pursue predators of Chinook smolts in mid-May. PEE counts in the estuary increased as tide height rose and were higher at dusk and night—especially during full moonlight. Such fine-scale behavioural information about harbour seals in relation to pulses of out-migrating smolts can be used to design mitigation strategies to reduce predation pressure by seals on salmon populations.

High salmon lice density is a threat to wild and farmed salmonid fish in Norway. To assess and identify areas for high salmon lice infestation pressure, continuous monitoring is necessary. The national Norwegian salmon lice monitoring program has until now been based on sampling and counting of salmon lice on wild salmonids and smolts in sentinel cages. The number of lice eggs hatched into the water masses, the relatively long-lasting pelagic life stages and the high spatiotemporal variability of the ocean currents all have a major influence on the local infestation pressure. Thus, a new monitoring system including a numerical ocean model with high temporal and spatial resolution has been established. The plan is that the model will complement, direct or replace parts of the logistically demanding and costly field-based monitoring program. In this study, we evaluate the model’s ability to realistically simulate the spread and density of pelagic salmon lice. Results from a 4 yr model run are presented, and the simulated density compared to the mean abundance on smolts in sentinel cages. The comparison demonstrates that the modeled salmon lice density corresponds well with the observational data. Within a slight shift in space, the model matches the observed lice infestation class values in 78% of the cases. Using the modeled lice density, a binary forecast system is proposed to predict areas of elevated lice infestation pressure. For the 2015 test case, the prediction system is correct (elevated/non-elevated) in 32 of 36 cases (89%).

The high level of escapes from Atlantic salmon farms, up to two million fishes per year in the North Atlantic, has raised concern about the potential impact on wild populations. We report on a two-generation experiment examining the estimated lifetime successes, relative to wild natives, of farm, F1 and F2 hybrids and BC1 backcrosses to wild and farm salmon. Offspring of farm and 'hybrids' (i.e. all F1, F2 and BC1 groups) showed reduced survival compared with wild salmon but grew faster as juveniles and displaced wild parr, which as a group were significantly smaller. Where suitable habitat for these emigrant parr is absent, this competition would result in reduced wild smolt production. In the experimental conditions, where emigrants survived downstream, the relative estimated lifetime success ranged from 2% (farm) to 89% (BC1 wild) of that of wild salmon, indicating additive genetic variation for survival. Wild salmon primarily returned to fresh water after one sea winter (1SW) but farm and 'hybrids' produced proportionately more 2SW salmon. However, lower overall survival means that this would result in reduced recruitment despite increased 2SW fecundity. We thus demonstrate that interaction of farm with wild salmon results in lowered fitness, with repeated escapes causing cumulative fitness depression and potentially an extinction vortex in vulnerable populations.

The assessment report of the 4th International Panel on Climate Change confirms that global warming is strongly affecting biological systems and that 20-30% of species risk extinction from projected future increases in temperature. It is essential that any measures taken to conserve individual species and their constituent populations against climate-mediated declines are appropriate. The release of captive bred animals to augment wild populations is a widespread management strategy for many species but has proven controversial. Using a regression model based on a 37-year study of wild and sea ranched Atlantic salmon (Salmo salar) spawning together in the wild, we show that the escape of captive bred animals into the wild can substantially depress recruitment and more specifically disrupt the capacity of natural populations to adapt to higher winter water temperatures associated with climate variability. We speculate the mechanisms underlying this seasonal response and suggest that an explanation based on bio-energetic processes with physiological responses synchronized by photoperiod is plausible. Furthermore, we predict, by running the model forward using projected future climate scenarios, that these cultured fish substantially increase the risk of extinction for the studied population within 20 generations. In contrast, we show that positive outcomes to climate change are possible if captive bred animals are prevented from breeding in the wild. Rather than imposing an additional genetic load on wild populations by releasing maladapted captive bred animals, we propose that conservation efforts should focus on optimizing conditions for adaptation to occur by reducing exploitation and protecting critical habitats. Our findings are likely to hold true for most poikilothermic species where captive breeding programmes are used in population management.

Artificial propagation of salmonids in Latvia has a long-standing history, spanning nearly 140 years, periodically interrupted and resumed after the First and Second World War. Between 1939 and 1974, a cascade of three hydroelectric power plants was built on Latvia’s most important salmon river, the Daugava, thereby preventing access to spawning areas. To maintain salmon stocks at an economically significant level, a scientifically based restocking programme of salmon stocks has been established and continues to this day. Over the years, salmonid rearing techniques developed, initially growing them to the larval stage, then to the juvenile stage, and finally to physiologically mature, saltwater-adapted smolts. The artificial propagation of salmon was not interrupted even during the collapse of the Soviet Union. To compensate for the loss of salmonid resources, the Institute of Food Safety, Animal Health, and Environment BIOR performs salmonid rearing, supplementing natural fish stocks with more than one million salmon and sea trout juveniles annually. The salmon stock in the Daugava River basin has been successively artificially maintained for more than fifty years, and the national restocking programme supports salmon stocks in the Gauja and Venta rivers with expectation of the wild stock stabilisation.

1. Temperature-driven life-history modifications by adaptation occur in ectotherms, and therefore, life-history modifications by adaptation need to be taken into consideration when predicting population responses to the climate change. 2. Partial migration is a common form of life-history diversity in which a population contains both migratory and resident behaviours. Salmonid fish exhibit a wide range of life-history diversity and, in particular, partial migration. We evaluated the effect of temperature-driven life-history modifications on population dynamics in partially migratory masu salmon (Oncorhynchus masou) by field observations and theoretical models. 3. Field observations revealed that spatial patterns of alternative migratory tactics were associated with temperature gradients. The occurrence of resident males increased, whereas the proportion of migrant males and the proportion of delayed migrants including both sexes decreased with increasing temperature and, thereby, with improved early growth conditions. 4. The expected fitness for each migratory tactic was computed in a life-history model with early growth conditions as a function. Individual fitness was maximized by adopting resident tactics under favourable early growth conditions, early migrant tactics under intermediate early growth conditions and delayed migrant tactics under unfavourable early growth conditions. The results suggest that individuals exhibited a status-dependent conditional strategy, that is, the adoption of alternative migratory tactics is influenced by the status of individuals to make the best of a situation. 5. A simulation model suggests that increased residency by males to increased temperature leads to a substantial decrease in the number of migrants. Moreover, the decrease in the number of delayed (older) migrants with increasing temperature magnified fluctuations in abundance. Our findings indicate the importance of temperature-driven life-history modifications for predicting dynamics of natural populations under climate warming.

Multiple dam passage during seaward migration is thought to reduce the subsequent survival of Snake River Chinook salmon. This hypothesis developed because juvenile Chinook salmon from the Snake River, the Columbia River’s largest tributary, migrate >700 km through eight hydropower dams and have lower adult return rates than downstream populations that migrate through only 3 or 4 dams. Using a large-scale telemetry array, we tested whether survival of hatchery-reared juvenile Snake River spring Chinook salmon is reduced in the estuary and coastal ocean relative to a downstream, hatchery-reared population from the Yakima River. During the initial 750-km, 1-mo-long migration through the estuary and coastal ocean, we found no evidence of differential survival; therefore, poorer adult returns of Snake River Chinook may develop far from the Columbia River. Thus, hydrosystem mitigation efforts may be ineffective if differential mortality rates develop in the North Pacific Ocean for reasons unrelated to dam passage.