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shows body water content in the 4 age groups after 4 and 24 h of exposure to freshwater (0 ppt) and different salinities. Significant effects of time (p = 0.001), age (p = 0.001) and salinity (p = 0.001) were identified, as well as significant interac
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The North Sea houting Coregonus oxyrinchus is an endangered anadromous salmonid belonging to the European lake whitefish complex. The last remaining indigenous population of North Sea houting is found in the River Vidaa, Denmark. Despite legislative protection and numerous stocking and habitat restoration programmes, including a €13.4 million EU Li...
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... Anadromous fish species reproduce in freshwater habitats, while a major part of their somatic growth takes place in marine environments . This strategy can be advantageous as migrants often benefit from faster growth rates and exhibit elevated reproductive output (Bohlin et al. 2001); however, migrants may also experience elevated levels of predation risk (Gowans et al. 2003;Wright et al. 2007), disease exposure (Halttunen et al. 2018), osmoregulatory requirements (Jensen et al. 2015b), and energetic costs (Forseth et al. 1999;Boel et al. 2014), as well as difficult access to the spawning areas Svendsen et al. 2004; Thorstad et al. 2008). ...
... Previous reports have indicated that adult North Sea houting migrate into rivers to reproduce during the autumn and winter months. Larvae hatch in the early spring and presumably rely on lentic nursery areas within the river system for foraging (Jensen et al. 2015b). Juvenile North Sea houting stay in the river until they reach 3-5 cm in body length. ...
... Juvenile North Sea houting stay in the river until they reach 3-5 cm in body length. At this point, they develop the ability to hypo-osmoregulate and presumably migrate into the Wadden Sea (Jensen et al. 2015b). After a period of 2-4 years at sea, the adults return to the natal river to spawn. ...
Understanding migratory behavior, mortality, and physiology is essential for conservation of many species, particularly anadromous fish. In this study, freshwater and marine migrations of the endangered salmonid North Sea houting (Coregonus oxyrinchus (Linnaeus, 1758)) were investigated using telemetry. Furthermore, physiological samples were collected from North Sea houting and from resident and anadromous populations of the closely related European whitefish (Coregonus lavaretus (Linnaeus, 1758)) to compare hypo-osmotic tolerances. On average, North Sea houting spent 193 days at sea where the mortality was 36%. Most fish returned from sea in the autumn, and river entry correlated inversely with river temperature and positively with discharge. Fish spent an average of 49 days in the estuarine area. Artificial lakes negatively affected migration speeds. Migration speeds did not differ consistently between individuals (i.e., not a repeatable trait) but correlated positively with water temperature. Fish arrived at spawning areas in November. In the post-spawning state, Na ⁺ /K ⁺ -ATPase activities were elevated in North Sea houting and anadromous whitefish compared with resident whitefish, while osmolality was elevated only in North Sea houting. Our study provides important information for conservation planning related to the Habitat Directive of the European Union that lists the North Sea houting as critically endangered.
... Although the restoration project as such is finalized by now, the lack of biological knowledge implies continuing research and management questions. Different studies have been conducted, for example investigating reproduction patterns, migration behaviour and habitat requirements (Jensen et al., 2015) as well as impact of predators on mortality (Jensen et al., 2017). Jensen's et al. (2015 and2017) research results, although not directly mentioning the use of ENA, imply the usefulness of ENA indices as additional decision-making guidance for future conservation actions. ...
... Different studies have been conducted, for example investigating reproduction patterns, migration behaviour and habitat requirements (Jensen et al., 2015) as well as impact of predators on mortality (Jensen et al., 2017). Jensen's et al. (2015 and2017) research results, although not directly mentioning the use of ENA, imply the usefulness of ENA indices as additional decision-making guidance for future conservation actions. The measurement of Interaction Strength could for example be used as an indicator to better understand and estimate predation risk. ...
The Water Framework Directive (article 2, paragraph 21) as well as the Marine Strategy Framework Directive (MSFD, Descriptor 4) stress the need for assessing the quality of the structure and the functioning of ecosystems. The MSFD also underlines the urgent need for development, testing, and validation of ecosystem state indicators. Holistic function-based criteria and indicators as provided by Ecological Network Analysis (ENA) could be used to define and assess the 'Good Environmental Status' of marine ecosystems. This approach also feeds Ecosystem Based Management (EBM). ENA generally analyses the fluxes' quality of a single medium such as here the carbon fluxes in a food web and produces a number of useful metrics that indicate, inter alia, the total carbon flow through the system, the quality of the functioning of the system or the trophic efficiency of system. A short list of indices [i.e. Detritivory over Herbivory ratio (D/H), Connectance Index (CI), Transfer Efficiency (TE) over trophic levels, System Omnivory Index (SOI), Finn's Cycling Index (FCI), relative Redundancy (R/DC), Average Mutual Information (AMI) and Interaction Strength (IS)] is proposed for practical use. This paper presents a first framework for OSPAR Regional Sea Convention food web indicators based on ENA. These are presented here focusing on their applicability and what is needed for implementation, illustrating their potential use by case studies.
... Previous studies have indicated that adult North Sea houting migrate from the Wadden Sea into adjacent rivers to spawn in the autumn and winter (Borcherding, Breukelaar, Winter, & König, 2014;Jepsen et al., 2012). Eggs hatch in February and March, and larvae and juveniles presumably stay in lentic freshwater habitats until reaching 30-50 mm in body length, upon which they migrate into the Wadden Sea to forage (Jensen et al., 2015;Poulsen et al., 2010Poulsen et al., , 2012. Using stable isotopes from scale samples, Borcherding, Pickhardt, Winter, and Becker (2008) examined migration strategies in a reintroduced population of North Sea houting in the Dutch Lake IJsselmeer. ...
... Migration strategies of the indigenous population in River Vidaa have not been examined, but could reveal important riverine habitats, which would add valuable knowledge for future conservation plans. Restoration projects have created artificial lakes in River Vidaa to support the early life stages of North Sea houting (Jensen et al., 2015), but possible effects of the artificial lakes on the migration of adult North Sea houting remain uncertain. ...
... To date, it remains unknown if migration of North Sea houting correlates with temperature. In this context, migration between river and marine areas may be particularly interesting, because mechanisms to switch from hyper-osmoregulation to hypo-osmoregulation (Jensen et al., 2015) are influenced by temperature (Finstad, Staurnes, & Reite, 1988). ...
Life history theory predicts a trade-off between migration and residency where migration is favoured when it infers elevated fitness. Although migration to more favourable environments offers higher growth rates, migrants often experience increased mortality due to predation. Here, we investigated mortality and migration behaviour of the North Sea houting (Coregonus oxyrinchus), an anadromous salmonid endemic to the Wadden Sea. We used acoustic telemetry to map the migration of the only remaining indigenous population by applying stationary hydrophones combined with manual tracking. Data suggested a total mortality of 26%, with 30% of the total mortality attributed to predation by great cormorants (Phalacrocorax carbo sinensis), highlighting that North Sea houting conservation could be jeopardised by increased cormorant predation. Risk of cormorant predation was size-dependent, with smaller fish suffering higher risk of predation. The study found North Sea houting to exhibit disparate migration strategies and identified a lentic area in the estuary as an important habitat. Two newly established artificial lakes within the river system significantly reduced the migration speeds, possibly indicating constrained navigation through the lakes. The migration into the Wadden Sea correlated with temperature perhaps indicating osmoregulatory constraints of sea entry. Unlike most salmonid species, migration occurred both day and night. Moreover, fish exhibited repeatable individual differences in diel activity patterns, suggesting that individuals differ consistently in their migratory activity throughout the 24-hr period. Our study provides novel information on salmonid migration, which is crucial for the development of science-based conservation strategies.
... One potential integrative solution is the creation of wetlands across the hydrographic network (Jensen et al., 2015;Koed et al., 2006;Poulsen et al., 2012), because wetlands maintain water reserves while reducing the need for more intrusive water barriers. However, if planned incorrectly, these natural lentic ecosystems might still weaken the river continuum and, therefore, prevent restoration objectives from fully being accomplished (Braukmann et al., 2010;Koed et al., 2006). ...
Agriculture is widespread across the EU and has caused considerable impacts on freshwater ecosystems. To revert the degradation caused to streams and rivers, research and restoration efforts have been developed to recover ecosystem functions and services, with the European Water Framework Directive (WFD) playing a significant role in strengthening the progress.
Analysing recent peer-reviewed European literature (2009–2016), this review explores 1) the conflicts and difficulties faced when restoring agriculturally impacted streams, 2) the aspects relevant to effectively reconcile agricultural land uses and healthy riverine ecosystems and 3) the effects and potential shortcomings of the first WFD management cycle.
Our analysis reveals significant progress in restoration efforts, but it also demonstrates an urgent need for a higher number and detail of restoration projects reported in the peer-reviewed literature. The first WFD cycle ended in 2015 without reaching the goal of good ecological status in many European water-bodies. Addressing limitations reported in recent papers, including difficulties in stakeholder integration and importance of small headwater streams, is crucial. Analysing recent developments on stakeholder engagement through structured participatory processes will likely reduce perception discrepancies and increase stakeholder interest during the next WFD planning cycle.
Despite an overall dominance of nutrient-related research, studies are spreading across many important topics (e.g. stakeholder management, land use conflicts, climate change effects), which may play an important role in guiding future policy. Our recommendations are important for the second WFD cycle because they 1) help secure the development and dissemination of science-based restoration strategies and 2) provide guidance for future research needs.
... where larger individuals appear more susceptible than smaller ones, were opposite of that found for some species of sturgeons, where the adult fish were able to withstand salinities above 20 ppt better than juveniles (Altinok et al., 1998;LeBreton & Beamish, 1998;McEnroe & Cech, 1985). However, it has also been shown for other species that different life stages respond differently to varying environmental conditions (Banks et al., 1991;Glover et al., 2012;Jensen et al., 2015). The reason why adults should fare worst is, however, still elusive. ...
The Mozambique tilapia, Oreochromis mossambicus, is a teleost fish native to estuarine waters that vary in salinity between fresh water (FW) and seawater (SW). The neuroendocrine system plays a key role in salinity acclimation by directing ion uptake and extrusion in osmoregulatory tissues such as gill. While most studies with O. mossambicus have focused on acclimation to steady-state salinities, less is known about the ability of adult fish to acclimate to dynamically-changing salinities. Plasma osmolality, prolactin (PRL) levels, and branchial gene expression of PRL receptors (PRLR1 and PRLR2), Na⁺/Cl⁻ and Na⁺/K⁺/2Cl⁻ co-transporters (NCC and NKCC), Na⁺/K⁺-ATPase (NKAα1a and NKAα1b), cystic fibrosis transmembrane conductance regulator (CFTR), and aquaporin 3 (AQP3) were measured in fish reared in FW and SW steady-state salinities, in a tidal regimen (TR) where salinities changed between FW and SW every six hours, and in fish transferred from FW or SW to TR. Regardless of rearing regimen, plasma osmolality was higher in fish in SW than in FW fish, while plasma PRL was lower in fish in SW. Furthermore, branchial gene expression of effectors of ion transport in TR fish showed greater similarity to those in steady-state SW fish than in FW fish. By seven days of transfer from steady-state FW or SW to TR, plasma osmolality, plasma PRL and branchial expression of effectors of ion transport were similar to those of fish reared in TR since larval stages. These findings demonstrate the ability of adult tilapia reared in steady-state salinities to successfully acclimate to dynamically-changing salinities. Moreover, the present findings suggest that early exposure to salinity changes does not significantly improve survivability in future challenge with dynamically-changing salinities.
