The European eel (Anguilla anguilla L.) has a catadromous life cycle, with a single panmictic population that colonises continental Europe and northern Africa yet migrates 5000 to 9000 km to spawn in the Atlantic Ocean. It is unknown how this continental migration is organised so individual eels arrive in time for spawning with conspecifics. This meta-analysis combined tracking data from 18 water bodies in freshwater and transitional systems distributed along the southwest-northeast axis of Europe, resulting in a dataset of 2306 eels, making it the most comprehensive in terms of geographical coverage and number of eels tagged. The eels were tracked using acoustic telemetry and the Nedap Trail System and allowed us to study the eel's migration phenology at a continental scale. The findings reveal that the day when eels arrive at sea varies significantly with latitude, with northern eels arriving earlier. Migration speed differs between tidal and non-tidal habitats, suggesting that tidal currents facilitate faster movement. However, despite these patterns, we observed substantial variability in arrival at sea time and migration speed within water bodies, suggesting that the eel's migration phenology is considerably plastic. The presence of water regulating structures such as weirs, pumping stations and hydropower plants can impact migration timing and speed, potentially delaying eels, but is likely dependent on local hydrological conditions which can be water body specific.

Atlantic salmon Salmo salar typically enter fresh water several months prior to spawning and just as pools can provide areas of refuge in river systems, lakes may also provide important refuge habitat during the spawning migration. Using acoustic telemetry we examined the spatial and temporal movements of wild and a ranched strain of Atlantic salmon in a freshwater lake where the main spawning areas were located upstream of the lake. Over the study period (2011-2014), returning adult wild salmon spent an average of 228 days in fresh water and 90% of that time in the lake. On entering the lake, most wild salmon moved quickly to the northern part of the lake, close to the main inflow, spending an average 76% of the time in this location. The average number of days wild fish were absent from the lake during the main spawning period varied between years, ranging from 10 to 26 days for females and 32 to 35 days for males. Seventy four per cent (17/23) of salmon spawners returned to the lake and two salmon subsequently died in the lake post-spawning. Atypically, two salmon were resident in the lake for the whole period in 2013/14. During the study, wild salmon were detected at depths within the top 5 m for 73% of the time. Median depths post-spawning were greater than in the pre-spawning period, when salmon were found to spend extensive periods at depths in excess of 10 m. In July 2013, when the lake was stratified, thermal regulation behaviour was observed in wild salmon, whereby salmon moved to cooler deeper water when water temperatures at 1 m exceeded 20°C. In contrast to wild salmon, the majority of ranch salmon returned to the traps downstream of the lake prior to the spawning period, which would be expected as they were released as smolts below the freshwater lake. Ranch fish spent an average 80% of the time in the vicinity of receivers in the south of the lake and an average 98% of the time within the top 5 m. However, two ranch females were resident in the lake until the following spring and one ranch female moved upstream into the river during the spawning period. Clearly, in this catchment the lake provides an important habitat for migrating adult salmon. In the context of climate change, where thermal and hydrological regimes in rivers are expected to change in response to changes in air temperature and precipitation patterns, the availability of deep lakes that stratify in the summer and cool water refuges in river systems is likely to play a key role in the sustenance and conservation of salmonid species. Information about the migration patterns of Atlantic salmon in undisturbed freshwater systems may also assist in resolving issues associated with fish passage in impacted rivers and inform management decisions.

Acoustic telemetry is widely used to investigate aquatic animal movement. Pulse position modulation (PPM) is an acoustic telemetry method that allows multiple unique identification codes to be transmitted at a single acoustic frequency, typically in the 69 kHz range. However, because the potential number of unique identification codes (i.e. tags) is ultimately limited by the number of pulses in the PPM signal, this poses a practical limitation. In addition, different manufacturers have developed different approaches to encoding the transmitted data, hampering compatibility across brands. A lack of broad compatibility across telemetry systems restricts users to a single manufacturer and operating system, reduces market competition and limits innovation. As the aquatic animal tracking research community organises towards networks of devices and data, incompatibility becomes more problematic and jeopardizes the unique scientific benefits offered by the networking approach. Here, we make a plea for collaboration among the manufacturers globally and propose a set of open protocols to ensure equipment interoperability as a medium-term solution.

Salmonids are some of the most widely studied species of fish worldwide. They span freshwater rivers and lakes to fjords and oceans; they include short- and long-distance anadromous migrants, as well as partially migratory and non-migratory populations; and exhibit both semelparous and iteroparous reproduction. Salmonid life-history strategies represent some of the most diverse on the planet. For this reason, salmonids provide an especially interesting model to study the drivers of these different life-history pathways. Over the past few decades, numerous studies and reviews have been published, although most have focused on ultimate considerations where expected reproductive success of different developmental or life-history strategies are compared. Those that considered proximate causes generally focused on genetics or the environment, with less consideration of physiology. Our objective was therefore to review the existing literature on the role of physiology as a proximate driver for life-history strategies in salmonids. This link is necessary to explore since physiology is at the core of biological processes influencing energy acquisition and allocation. Energy acquisition and allocation processes, in turn, can affect life histories. We find that life-history strategies are driven by a range of physiological processes, ranging from metabolism and nutritional status to endocrinology. Our review revealed that the role of these physiological processes can vary across species and individuals depending on the life-history decision(s) to be made. In addition, while findings sometimes vary by species, results appear to be consistent in species with similar life cycles. We conclude that despite much work having been conducted on the topic, the study of physiology and its role in determining life-history strategies in salmonids remains somewhat unexplored, particularly for char and trout (excluding brown trout) species. Understanding these mechanistic links is necessary if we are to understand adequately how changing environments will impact salmonid populations.

We report on concentrations of polybrominated diphenylethers (PBDEs), polychlorinated biphenyls (PCBs), dichlorodiphenyldichloroethylene (p,p'-DDE) and hexachlorobenzene (HCB) measured in the adipose fins of returning adult Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) to the river Tees in the Northeast of England. Overall, higher concentrations of these contaminants were found in sea trout samples, where detected congeners reflected the more widely used commercial formulations, in particular for the PBDEs. Our results suggest that these fish could be bioaccumulating persistent organic pollutants via diet during their migratory routes (North Sea and the Norwegian Sea) and, in addition, some level of re-mobilisation of these compounds could still be occurring in the UK eastern coastal areas. The use of adipose fin of returning salmonids could be further developed as a non-lethal approach to assess whether persistent contaminants are being accumulated during the juvenile to adult phase of salmonids originating from UK rivers.

The potential effects of a hydropower scheme on the migratory behaviour of Atlantic salmon Salmo salar smolts was studied on the River Frome, southern England. The potential delay to migration at the intake of the hydropower scheme was assessed, together with the effects of passage through the turbine on the temporal and spatial migration of the fish in the river and estuary. The migratory behaviour of the emigrating S. salar smolts was monitored using miniature acoustic transmitters and an array of acoustic receivers positioned at the hydropower scheme and in the river and estuary. The majority of the smolts bypassed the hydropower scheme with only 8.1% of the fish moving downstream through the turbine. Movement was nocturnal and occurred during elevated river flows. There was no apparent delay at the turbine intake or at the adjacent weir. The subsequent migration of all smolts through the estuary of the River Frome occurred during both day and night and there was a distinct ebb-tide migration through the estuary and into the coastal zone. There was no difference in the rate of migration between smolts that moved through the turbine or over the weir. The detection of smolts during both the freshwater migration and the transition from the freshwater to the marine environments was high (91.8 and 73.3%, respectively). A laboratory investigation on the de-scaling of smolts indicated that removal of 1, 5 and 10% of scales had no significant effect on saltwater survival or the measured physiological parameters (gill Na+-K+-ATPase activity, plasma osmolality and chloride concentrations). Smolt passage through the turbine was assessed and resulted in either no damage to the integument or scale loss or between 20 and 80% of total body area of recaptured smolts. It is estimated that 1.53% of the smolt population would suffer significant damage after passage through the turbine. The implications of the hydropower scheme on the population of salmon in the River Frome are discussed.

European eels (Anguilla anguilla) migrate between the southwestern Sargasso Sea and the European and Mediterranean coasts. In a recent paper in Current Biology, Naisbett-Jones et al. [1] claim to “provide the first evidence that they [eels] derive positional information from the Earth’s magnetic field” and that this information guides their migration. The evidence reported by Naisbett-Jones et al. [1] in support of this conclusion was derived from eels collected in the Severn River (UK), approximately 50 km upstream of the estuary (i.e. not “in the Severn Estuary” as stated by the authors). Eels collected this far into rivers are benthic and fully adapted to freshwater; that is, they are late-stage glass eels (∼ 2 years old), not the pelagic leptocephalus (larval) life stage that actually undertakes the trans-Atlantic migration. The entire interpretive framework for the Naisbett-Jones et al. [1] study rests on the assumption that the behaviour of these late-stage freshwater glass eels, and their responses to magnetic fields, can be used as a proxy for the responses of eel leptocephali. The authors present no evidence in support of this key assumption.

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Fisheries Management and EcologyVolume 23, Issue 2 p. 169-171 Management and Ecological Note A preliminary study on the movements of smelt, Osmerus eperlanus, in two East Anglian rivers A. Moore, Corresponding Author A. Moore Cefas – Salmon and Freshwater Team, Lowestoft, Suffolk, UKCorrespondence: Andy Moore, Cefas – Salmon and Freshwater Team, Lowestoft, Suffolk NR33 0H, UK (e-mail: andy.moore@cefas.co.uk)Search for more papers by this authorM. Ives, M. Ives Cefas – Salmon and Freshwater Team, Lowestoft, Suffolk, UKSearch for more papers by this authorP. Davison, P. Davison Cefas – Salmon and Freshwater Team, Lowestoft, Suffolk, UKSearch for more papers by this authorL. Privitera, L. Privitera Cefas – Salmon and Freshwater Team, Lowestoft, Suffolk, UKSearch for more papers by this author A. Moore, Corresponding Author A. Moore Cefas – Salmon and Freshwater Team, Lowestoft, Suffolk, UKCorrespondence: Andy Moore, Cefas – Salmon and Freshwater Team, Lowestoft, Suffolk NR33 0H, UK (e-mail: andy.moore@cefas.co.uk)Search for more papers by this authorM. Ives, M. Ives Cefas – Salmon and Freshwater Team, Lowestoft, Suffolk, UKSearch for more papers by this authorP. Davison, P. Davison Cefas – Salmon and Freshwater Team, Lowestoft, Suffolk, UKSearch for more papers by this authorL. Privitera, L. Privitera Cefas – Salmon and Freshwater Team, Lowestoft, Suffolk, UKSearch for more papers by this author First published: 13 December 2015 https://doi.org/10.1111/fme.12150Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Volume23, Issue2April 2016Pages 169-171 RelatedInformation

Migrating silver European eels were exposed for 5days in a laboratory to an environmental level of tributyl phosphate (TBP), tagged with acoustic transmitters and released below the Tange hydropower station, on the River Gudenaa, Denmark. The subsequent movements of the eels were monitored as they migrated through Randers Fjord and into the Kattegat Sea using an array of acoustic receivers. In laboratory-based studies, exposure of eels for 5days to a nominal concentration of 0.5 mu g center dot l(-1) TBP significantly affected plasma glucose concentration and reduced plasma levels of sodium and chloride both in freshwater and 3days after transfer to saltwater. However, there were no mortalities when the eels were transferred to full strength sea water. Exposure to TBP did not appear to have a significant effect on the subsequent movements of the eels in the river or fjord. The eels moved rapidly through the fjord predominantly at night. The results of the study are discussed in relation to the impact of diffuse pollution on the behaviour and physiology of European eels.

The migratory behavior of the anadromous Atlantic salmon (Salmo salar L.) has intrigued anglers, scientists and fi shery managers for centuries. In his treatise on angling, The Compleat Angler published in 1653, Izaak Walton, reports: “Sir Francis Bacon observed, the age of a salmon exceeds not ten years…much of this has been observed by tying a ribbon or some known tape or thread, in the tail of some young salmons, which have been taken in weirs as they have swimmed toward the salt water, and then by taking a part of them again with the known mark at the same place at their return from the sea, which is usually about six months after....which has inclined many to think, that every salmon usually returns to the same river to which it was bred.”

Short-term Al-exposure and moderate acidification increased initial marine mortality in migrating post-smolts, and can thereby reduce viability of Atlantic salmon stocks. The delayed impact of short-term aluminium (Al) exposure on hatchery-reared Atlantic salmon smolt in moderately acidified freshwater (pH 5.88-5.98) was investigated during the first 37 km of the marine migration. Smolts were tagged with acoustic tags and exposed to low (28.3 +/- 4.6 mu g l(-1) labile Al, 90 h) or high (48.5 +/- 6.4 mu g l(-1) labile Al, 90 or 48 h) Al concentrations within the hatchery. Thereafter their movements, together with a control group, were monitored throughout the marine fjord. Al-exposure resulted in increased gill-Al and compromised hypoosmoregulatory capacity, as shown by elevated mortality in laboratory seawater challenge tests and reduced Na+, K+-ATPase activity levels. Further, Al-exposure resulted in decreased plasma concentrations of growth hormone (GH), while the insulin-like growth factor (IGF-I) was unaffected. There was a significant mortality in the 90 h high-Al group during exposure, and those surviving until release died during the first 3.6 km of the marine migration. Physiological stress and mortality were not only a result of the AI-concentrations, but also dependent on exposure duration, as shown by results from the 48 h high-Al group. Elevated mortality was not recorded in freshwater or after entering the sea for this group, which highly contrasts to the 100% mortality in the 90 h high-Al group, despite both groups having similarly high gill-Al levels. The low-Al group showed a 20% higher mortality compared to the control group during the first 10 km of the marine migration, but during the next 28 km, mortality rates did not differ. Hence, post-smolts surviving the first 10 km subsequently showed no differences in mortality compared to controls. At least one third of the mortality in both the low-Al and control groups were due to predation by marine fishes, indicating that the proximate cause for elevated mortality due to Al-exposure may have been predation. Migration speeds over 3.6, 9.6 or 37.1 km from the release site was not affected by Al-exposure. (C) 2013 Elsevier Ltd. All rights reserved.

Wild sea trout (Salmo trutta L.) smolts, tagged with miniature acoustic transmitters, were tracked in the River Conwy, North Wales, to describe the freshwater and estuarine patterns ofmigration. Migration in freshwater was predominantly noctumal, although there were changes in this pattern in the lower reaches <>fthe estuary with fish moving during both the day and night. The noctumal pattern ofmigration in freshwater would appear to be the result ofa rhythm ofswimming activity which results in the smolts moving up into the water column after dusk and migrating seawards. Smolts tagged earlier in the study spent significantly longer in the river before migrating into coastal waters than fish tagged later in the study. The movement of smolts through the estuary was indicative ofa selective ebb tide transport pattern of migration. All the smolts migrated seawards on an ebb tide elose to the surface and within the fastest moving section ofthe water column. Smolt migration in the lower portion ofthe estuary was indicative ofactive directed swimming and there was no apparent period ofaccIimation required when moving from fresh to saltwater. The behaviour ofthe smolts is discussed in relation to the possible environniental and physiological cues controlling estuarine migration.

Advances in the field of telemetry techniques during the last few decades have greatly expanded our knowledge on migratory behaviour and provided the opportunity to obtain practically useful data for the conservation and management of salmonid populations. However, applying this information to the development of much needed population-based migration models has been limited. Furthermore, this research has generally been restricted to assessing the role of river flow on fish movement. Models derived from a 6-year telemetry study on the movements of Atlantic salmon, Salmo salar L., and sea trout, Salmo trutta L., in the River Tyne are presented together with an assessment of counter data from the same river to highlight the importance of additional environmental and physiological parameters in regulating fish movement. The data are discussed in relation to the advantages of telemetry data, the need to develop predictive models, the statistical methods used and the potential direction of future work in this area.

The anadromous life cycle of Atlantic salmon Salmo salar involves long migrations to novel environments and challenging physiological transformations when moving between salt-free and salt-rich waters. In this article, (1) environmental factors affecting the migration behaviour and survival of smolts and post-smolts during the river, estuarine and early marine phases, (2) how behavioural patterns are linked to survival and (3) how anthropogenic factors affect migration and survival are synthesized and reviewed based on published literature. The timing of the smolt migration is important in determining marine survival. The timing varies among rivers, most likely as a consequence of local adaptations, to ensure sea entry during optimal periods. Smolts and post-smolts swim actively and fast during migration, but in areas with strong currents, their own movements may be overridden by current-induced transport. Progression rates during the early marine migration vary between 0.4 and 3.0 body lengths s(-1) relative to the ground. Reported mortality is 0.3-7.0% (median 2.3) km(-1) during downriver migration, 0.6-36% (median 6.0) km(-1) in estuaries and 0.3-3.4% (median 1.4) km(-1) in coastal areas. Estuaries and river mouths are the sites of the highest mortalities, with predation being a common cause. The mortality rates varied more among studies in estuaries than in rivers and marine areas, which probably reflects the huge variation among estuaries in their characteristics. Behaviour and survival during migration may also be affected by pollution, fish farming, sea lice Lepeophtheirus salmonis, hydropower development and other anthropogenic activities that may be directly lethal, delay migration or have indirect effects by inhibiting migration. Total mortality reported during early marine migration (up to 5-230 km from the river mouths) in the studies available to date varies between 8 and 71%. Hence, the early marine migration is a life stage with high mortalities, due to both natural and human influences. Factors affecting mortality during the smolt and post-smolt stages contribute to determine the abundance of spawner returns. With many S. salar populations in decline, increased mortality at these stages may considerably contribute to limit S. salar production, and the consequences of human-induced mortality at this stage may be severe. Development of management actions to increase survival and fitness at the smolt and post-smolt stages is crucial to re-establish or conserve wild populations.

Locating and differentiating the marine feeding areas used by adult salmon (Salmo salar) is essential to stock-based management and conservation, but traditional tagging studies are limited and influenced by the uneven distribution of the fisheries or research vessel surveys. Here, a novel approach is used, based on the observation that the isotopic composition of animal tissues is intrinsically linked to the environmental conditions during tissue growth, which allows for the distinction of pelagic fish feeding in different locations. This isotopic approach is applied using archived collections of salmon scales and shows that (i) salmon act as size-structured pelagic predators, (ii) adult salmon from different natal origins within the UK (and hence components of the southern European stock complex) feed in different oceanic regions before their return, (iii) one-sea-winter (1SW) and multi-sea-winter salmon returning to some rivers in the UK are separated in their marine feeding areas, whereas those from others are not, and (iv) salmon from the rivers sampled are not feeding in regions of the Northwest Atlantic used by 1SW salmon returning to rivers in Newfoundland. Therefore, the stable isotope approach allows for retrospective investigations of marine diet, location, and migration at stock- and cohort-specific levels.

In terms of the spawning migration of adult salmon, Salmo salar L., water flow is often considered the primary factor controlling river entry and fluctuations in flow controlling when the fish subsequently migrate upstream. However, water temperature has also been suggested to modify the spawning migration of salmon, particularly their movements within estuaries and the timing of freshwater entry. Freshwater temperature is more likely to impact salmonid biology than flow, particularly in relation to temperature dependant metabolic costs, time of spawning and fecundity. Therefore, temperature may be more of a factor regulating salmonid populations in fresh water than flow itself. This study focuses on two aspects of the impact of temperature on salmonids in fresh water: first, how salmon may modify their behaviour to adapt to changes in temperature and second the potential relationship between temperature, environmental conditions (e.g. water quality) and physiology (e.g. maturation and olfaction) in regulating adult migration.

Knowing the distribution of marine animals is central to understanding climatic and other environmental influences on population ecology. This information has proven difficult to gain through capture-based methods biased by capture location. Here we show that marine location can be inferred from animal tissues. As the carbon isotope composition of animal tissues varies with sea surface temperature, marine location can be identified by matching time series of carbon isotopes measured in tissues to sea surface temperature records. Applying this technique to populations of Atlantic salmon (Salmo salar L.) produces isotopically-derived maps of oceanic feeding grounds, consistent with the current understanding of salmon migrations, that additionally reveal geographic segregation in feeding grounds between individual philopatric populations and age-classes. Carbon isotope ratios can be used to identify the location of open ocean feeding grounds for any pelagic animals for which tissue archives and matching records of sea surface temperature are available.

Although many studies have shown that trout farm effluents can affect water quality and macro-invertebrate populations downstream of the farm, few studies have investigated effects on fish. Previous work has suggested that trout farm effluents can affect salmonid parr and embryos but there is no data as to whether they affect salmonid smolts. In this experiment, Atlantic salmon smolts were caged upstream and downstream of trout farm effluents for 3days on two UK Rivers and compared against hatchery controls in two years (2005–2006). No persistent effects on plasma osmolality, plasma sodium, plasma chloride, condition factor and hepato-somatic indices were observed, although there were variations in responses between years. No effects of the effluents were observed on gill Na+K+ATPase activity or plasma thyroid hormones. There was evidence that fish placed downstream of the fish farms had modified plasma potassium regulation, although the relative influence of the fish farm effluents on the physiology compared to other compounds in the river has not been determined. Only one of the smolts (5%) caged downstream of the effluents died and there was little effect on plasma ionoregulatory and osmoregulatory indices, but mortalities increased when the smolts were given a 48h seawater challenge. However, it is possible that other compounds within the river in addition to the fish farm effluents may have influenced the survival of the smolts.

Magnetization measurements of the European eel Anguilla anguilla demonstrated the presence of magnetic material concentrated in the region of the mandibular canals of the lateral line system. The data suggest that the material is magnetite, has a size suitable for magnetoreception and is of biogenic origin. The presence of magnetic particles in the lateral line system is discussed in relation to their possible role in allowing the fish to orientate with respect to the geomagnetic field during their extensive oceanic spawning migrations.