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Diadromous fish use both freshwater and marine habitats to complete their life cycles, but dams restrict those connections, and as a result, many populations are historically low across their range. The Penobscot River, Maine, USA, once had large populations of diadromous fish and has recently been the focus of large restoration projects. Our objective was to assess the river restoration impacts in the estuary using mobile, multi-frequency echosounders (SIMRAD EK60 split-beam 38 and 120 kHz) to construct a time series of fish distribution using metrics of fish length and fish density, both seasonally and pre-restoration (2012–2014) versus post-restoration (2015–2019). Seasonally, fish size was the smallest in summer, while density and biomass were greater as compared to spring and fall. Between pre- and post-restoration years, fish length did not differ, and with a median of 4.7 cm, median density increased from less than 0.5 fish m−2 to greater than 1 fish m−2. Fish biomass was approximately three times greater in post-restoration years where it ranged 23 to 316 mt per survey than pre-restoration years where it ranged from 9 to 114 mt per survey. These patterns are consistent with a system undergoing restoration of migratory fish where larger adult fish migrate in the spring and their juveniles in the summer and fall. This study demonstrates the ecological impacts of river restoration on its estuary by quantifying multiple metrics (fish biomass, density, and size).

Ecosystem fragmentation is a serious threat to biodiversity and one of the main challenges in ecosystem restoration. River continuity restoration (RCR) has often targeted diadromous fishes, a group of species supporting strong cultural and economic values and especially sensitive to river fragmentation. Yet it has frequently produced mixed results and diadromous fishes remain at very low levels of abundance. Against this background, this paper presents the main challenges for defining, evaluating and achieving effective RCR. We first identify challenges specific to disciplines. In ecology, there is a need to develop quantitative and mechanistic models to support decision making, accounting for both direct and indirect impacts of river obstacles and working at the river catchment scale. In a context of dwindling abundances and reduced market value, cultural services provided by diadromous fishes are becoming increasingly prominent. Methods for carrying out economic quantification of non-market values of diadromous fishes become ever more urgent. Given current challenges for rivers to meet all needs sustainably, conflicts arise over the legitimate use of water resources for human purposes. Concepts and methods from political science and geography are needed to develop understandings on how the political work of public authorities and stakeholders can influence the legitimacy of restoration projects. Finally, the most exciting challenge is to combine disciplinary outcomes to achieve a multidisciplinary approach to RCR. Accordingly, the co-construction of intermediary objects and diagrams of flows of knowledge among disciplines can be first steps towards new frameworks supporting restoration design and planning.

Tidal estuaries provide important spawning, nursery and feeding habitats for a wide diversity of species and serve as transit areas for diadromous fishes. However, they are globally among the most impacted aquatic ecosystems, pressured by anthropogenic alterations and global change. Assessing local fish communities and their temporal dynamics is crucial to determine the ecological baseline of these ecosystems and to serve as an indicator of their ecological status. Here, we studied the fish community in a typical estuary of the Wadden Sea in central Europe, the strongly tidal and highly anthropised River Ems. Weekly bycatch samples from a commercial stow net fishery were analysed regarding fish community composition, individual length and biomass covering fourteen consecutive months. Thirty‐nine diadromous, estuarine, marine and freshwater species were recorded, amending previously reported numbers upward. Total fish abundance peaked in November 2020 and was lowest in the first half of the year 2021, corroborating previous observations of a cyclical intra‐annual community pattern in estuaries. The community structure changed significantly throughout the year, coinciding with variation in water temperature, dissolved oxygen concentrations, salinity and atmospheric pressure. During most months, European smelt ( Osmerus eperlanus ) and flounder ( Platichthys flesus ) dominated in abundance, and sand gobies ( Pomatoschistus spp.), European eel ( Anguilla anguilla ), river lamprey ( Lampetra fluviatilis ), three‐spined stickleback ( Gasterosteus aculeatus ) and European perch ( Perca fluviatilis ) occurred in substantial shares during single months. Some historically common species such as twaite shad ( Alosa fallax ), houting ( Coregonus cf. oxyrinchus ) and Atlantic salmon ( Salmo salar ) showed very low abundances. The non‐native Western tubenose goby ( Proterorhinus semilunaris ) was first recorded in this area. Intra‐annual changes in abundance and length composition of species of commercial or conservational interest, e.g., smelt and river lamprey, are discussed in detail. We show that high sampling resolution and complete annual coverage are crucial for a full picture of the fish community and conclude that the ecological status of the River Ems remains poor.

The decline and collapse of populations have been reported for a large range of taxa. Diadromous fishes migrate between fresh water and the sea and encounter many anthropogenic pressures during their complex life cycle. In spite of being of ecological, cultural and economic interest, diadromous fishes have been in decline for decades in many parts across the world. In this study, we investigated the change in five diadromous fish counts in France over a 30-year period using 43 monitoring stations located in 29 rivers across 18 catchments. Our hypothesis was that the counts of these species evolved in a contrasting way between catchments. We also tested the effect of five drivers potentially contributing to the observed trends: catchment, latitude, presence of commercial fisheries, improvement of ecological continuity and salmon stocking. We found contrasting trends in fish counts between species at the national scale, with some taxa increasing ( Anguilla anguilla and Salmo trutta ), some showing a slight increase ( Salmo salar ) and some decreasing ( Alosa spp. and Petromyzon marinus ). For each taxon, except Anguilla anguilla , we highlighted a significant catchment effect indicating contrasting trends between catchments and stations. However, we found no significant effect of catchment characteristics for any of the studied taxa. Le déclin et l'effondrement des populations ont été signalés pour un large éventail de taxons. Les poissons amphihalins migrent entre les eaux douces et la mer, et subissent de nombreuses pressions anthropiques au cours de leur cycle de vie complexe. En dépit de leur intérêt écologique, culturel et économique, les poissons amphihalins sont en déclin depuis des décennies dans de nombreuses régions du monde. Dans cette étude, nous avons étudié l'évolution des comptages de cinq taxons amphihalins en France sur une période de 30 ans en utilisant les données de 43 stations de comptage situées dans 29 rivières et 18 bassins versants. Notre hypothèse est que les comptages de ces espèces ont évolué de manière contrastée entre les bassins versants. Nous avons également testé l'effet de cinq facteurs susceptibles de contribuer aux tendances observées : le bassin versant, la latitude, la présence de pêcheries commerciales, l'amélioration de la continuité écologique et la présence d'un programme de déversement pour le saumon. Nous avons trouvé des tendances contrastées dans les comptages de poissons entre les espèces à l'échelle nationale, certains taxons étant en augmentation ( Anguilla anguilla et Salmo trutta ), certains ne montrant qu'une légère augmentation ( Salmo salar ) et d'autres étant en déclin ( Alosa spp. et Petromyzon marinus ). Pour chaque taxon, à l'exception d' Anguilla anguilla , nous avons mis en évidence un effet bassin versant important indiquant des tendances contrastées entre les bassins ou les stations de comptage. Cependant, nous n'avons trouvé aucun effet significatif des caractéristiques du bassin versant pour aucun des taxons étudiés.

In Arctic Alaska, Indigenous and rural residents depend on wild‐harvested foods for sustenance and the subsistence lifestyle is integral to their culture. Marine and diadromous fishes, which often occupy coastal lagoon habitats, are relied upon for subsistence harvest, particularly Pacific salmon, whitefishes (Coregoninae), cods (Gadidae), and flounder species (Pleuronectidae). However, little research has been conducted that assesses the energetic condition of these fishes, which are harvested by the tens of thousands annually. The effects of accelerating climate change and human development on the metabolic rates, diet, body condition, and energy density of fish is of great concern because these effects will, in turn, affect the people who depend on these species for food security. Consequently, we characterized energy density and percentage of lipid, water, and protein content of 10 fish species in four coastal lagoons within Cape Krusenstern National Monument, Alaska. We found that whitefishes, particularly Bering Cisco Coregonus laurettae and Least Cisco C. sardinella exhibited the highest energy density, percentage of lipid, and percentage of protein content, significantly greater than that of marine cod and flounder species, Fourhourn Sculpin Myoxocephalus quadricornis, freshwater Pond Smelt Hypomesus olidus, and values from other regional species that have been in published literature. Additionally, when the relative mean abundance and body weight of each species was taken into account, total energy value by availability in the lagoons was highest for whitefishes when compared with marine taxa and the freshwater Pond Smelt. Given the impending effects of climate change and coastal construction in the region, it will be important to monitor the energy condition of lagoon fishes to ensure their quality and availability for subsistence harvesters.

Despite the global crisis facing migratory benthic fishes, conservation genetic knowledge of these species remains scarce. In this study, we conducted a population genetic analysis using seven microsatellite loci to obtain basic information for determining conservation units and priorities of Cottus hangiongensis in Shiretoko, a mountainous peninsula where sculpin habitats are thought to be in decline throughout the region. The genetic structure was clearly divided between west and east coastal populations, and there was little recent migration between them. The western populations, which are closer to the center of the species’ range, had significantly higher genetic diversity than the eastern populations. However, a bottleneck analysis and the inference of demographic history using approximate Bayesian computation showed that only the west group had experienced a significant recent bottleneck, probably due to recent habitat losses. These results suggest that the western and eastern populations should be different conservation units and that the western populations should be prioritized for conservation despite their high genetic diversity. This study contributes to the conservation genetics of diadromous sculpin and reiterates the importance of analyzing not only the current levels but also temporal changes in assessing genetic diversity.

Optimizing the initial live feed is critical to success in fish larviculture in terms of both larval health and productivity. It is also vitally important due to the significant cost of provisioning live feeds. Glaxiids are high-value diadromous fish species found in parts of the Southern Hemisphere, which are wild-harvested and sold for human consumption in their larval form. In the emerging aquaculture of galaxiids, the live feed makes up a great proportion of the production cycle and is therefore a significant factor in achieving the economic viability of their production. In New Zealand, the endemic galaxiid species giant kōkopu (Galaxias argenteus) is considered suitable for aquaculture; however, little is known of their larval-feeding requirements. This study compares the growth performance and survival of giant kōkopu larvae over a four-week period when fed different proportions of first-molt Artemia nauplii (instar-I) and enriched, second-molt-onward Artemia (instar-II+) for different durations. The larvae in the treatment group which received the greatest proportion of Instar-I Artemia for the longest duration had the lowest mean wet weight and survival, leading to the production of the lowest total biomass of fish larvae when compared to the other feed treatments. The feed treatment that received only instar-II+ Artemia achieved an equally best total productivity. While the treatment fed a partial initial duration of mixed instar-I and instar-II+ Artemia achieved the same total productivity, the individual larvae were heavier and their body length was longer. The latter feed treatment appears to offer the best balance between the availability of feed particles in the optimum size range and a higher level of digestibility given that instar-I Artemia had higher levels of highly unsaturated fatty acids and polyunsaturated fatty acids.

All around the world, diadromous fish are facing multiple anthropogenic pressures resulting in a global decline in these migratory species. In Gironde, the allis shad populations which were the most abundant in Europe are no exception and have dramatically declined since 2000. Failure in reproduction success is one of the proposed explanations for this collapse. In light of this, we studied the reproductive strategy of the allis shad populations in the Gironde watershed. We used an original methodology to define the thermal behavior of allis shad during reproduction using an electivity index for 14 years of field monitoring in two rivers, composed of daily reproduction activity and temperature. For the first time, this study deeply explores and defines the thermal preference of a shad species during the reproduction. A temperature preference between 14.5 °C and 23 °C by spawners during the reproduction was observed. Despite annual fluctuations, an overall similarity of temperature ranges between the two rivers was observed. The thermal preference matched with the thermal tolerance of early stages (16.2 °C to 24.8 °C) and confirms that allis shad spawners adopt behavioral rules of reproduction in order to maximize the survival of their offspring. The similarity of shad thermal behavior may indicate the phylogenetic basis of thermal preference for fish species.

Many species traverse multiple habitats across ecosystems to complete their life histories. Degradation of critical, life stage-specific habitats can therefore lead to population bottlenecks and demographic deficits in subpopulations. The riparian zone of waterways is one of the most impacted areas of the coastal zone because of urbanisation, deforestation, farming and livestock grazing. We hypothesised that sink populations can result from alterations of habitats critical to the early life stages of diadromous fish that use this zone, and tested this with field-based sampling and experiments. We found that for Galaxias maculatus, one of the most widely distributed fishes of the southern hemisphere, obligate riparian spawning habitat was very limited and highly vulnerable to disturbance across 14 rivers in New Zealand. Eggs were laid only during spring tides, in the highest tidally influenced vegetation of waterways. Egg survival increased to >90% when laid in three riparian plant species and where stem densities were great enough to prevent desiccation, compared to no survival where vegetation was comprised of other species or was less dense. Experimental exclusion of livestock, one of the major sources of riparian degradation in rural waterways, resulted in quick regeneration, a tenfold increase in egg laying by fish and a threefold increase in survival, compared to adjacent controls. Overall, there was an inverse relationship between river size and egg production. Some of the largest rivers had little or no spawning habitat and very little egg production, effectively becoming sink populations despite supporting large adult populations, whereas some of the smallest pristine streams produced millions of eggs. We demonstrate that even a wide-ranging species with many robust adult populations can be compromised if a stage-specific habitat required to complete a life history is degraded by localised or more diffuse impacts.

Increasing habitat fragmentation is a major contributing factor to dramatic reductions in populations of migratory species worldwide. Diadromous fish species in particular are affected by this anthropogenic disturbance, resulting in historically low population abundances. Despite a plethora of management measures and considerable investment, desired results are often lacking. Here, we highlight five important considerations –the “Big Five” –for diadromous species management: removal of barriers to migration, installation of fish passages, habitat restoration, restocking, and fisheries management. We review current management measures and their effectiveness, and propose a way forward. Current management of diadromous fish populations largely focuses on mitigation of migration barriers, but management will likely fail if other fundamental aspects of diadromous species’ life cycles are overlooked or disregarded. We therefore propose an integrated management strategy that takes into account the five major factors influencing diadromous fish species, with the ultimate goal of restoring their populations.