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Earlier spring warming and anadromous fish migrations prompted by climate change are linked to shorter freshwater residency. Impacts of phenological change on anadromous fish populations are poorly understood with limited studies focused on iteroparous non-salmonids. We assessed freshwater residence time and reproductive success in an iteroparous clupeid, alewife (Alosa pseudoharengus) using a pedigree analysis and otolith-based spawning dates from captured juveniles. The primary objectives were to (1) estimate adult spawning duration in a freshwater pond (freshwater residence time) and (2) evaluate adult freshwater residence time, arrival date, length, sex, and reproductive success across 2 years in one system. Estimated freshwater residence times varied widely (1–64 days), and longer residence times were associated with earlier arrival dates, higher reproductive success, and more mating events. Longer freshwater residence times may allow alewife to spawn with more mates, produce more gametes, and experience a range of spawning and nursery conditions. Plasticity in alewife freshwater residence time could support earlier and shorter migration periods but may result in lower reproductive output if adults spend less time in freshwater ponds. [ABSTRACT FROM AUTHOR]

The involvement of the preoptico- or hypothalamo-hypophysial neurosecretory system (HHNS) in the initiation of migration processes in fish has been discovered, based on the results of ecologo-histophysiological studies using light microscopy, immunohistochemistry, electron microscopy and morphometry, thus confirming the hypothesis proposed by N.L. Gerbilsky and A.L. Polenov about the nature of the migratory impulse in fish. It has been demonstrated for the first time that in genetically distant but ecologically similar anadromous migratory fish species with different breeding seasons (spring-spawning Russian sturgeon, stellate sturgeon, and fall-spawning pink salmon), the synthesis of neurosecretory products is activated in neurosecretory cells at the beginning of the migration process, followed by their transport and accumulation in the neurohypophysis. At the same time, neurosecretory products have been shown to be secreted into the cerebrospinal liquid of the 3rd ventricle. Thus, the HHNS has a complex synchronous effect, which includes: (1) the neurotropic action of nonapeptide neurohormones on behavior centers of the central nervous system (CNS), causing a dominant state of CNS excitation originally called the "migratory impulse", (2) osmoregulatory imbalance during the marine "fattening" period, and (3) the cessation of the known antigonadotropic action of nonapeptide neurohormones, thus promoting the switching of the organism to an energy-intensive type of energy metabolism. A comparative analysis of our own and literature data confirms the communality of this metabolic mechanism in fish, thus allowing it to be considered as the most important phylogenetic adaptation (aromorphosis) aimed at achieving biological progress of species. A leading role in the subsequent navigational mechanisms of homing, widely covered in the world literature, is played by the luliberinergic centers of the hypothalamus. The ecological diversity of these mechanisms at the population level and the presence of straying in fish allow considering them as microevolutionary specializations. Further analysis of HHNS functions during fish migrations and spawning will help establish the basic principle of integration of these processes at the given ontogenetic stage, which is of prime importance for the existence of species. [ABSTRACT FROM AUTHOR]

Dredging is considered a major threat/impedance to anadromous fish migrating to spawning habitat. Due to the perceived threat caused by dredging, environmental windows that restrict dredge operations are enforced within many rivers along the east coast. However, it is generally unknown how anadromous fish react to encountering an active dredge during spawning migrations. Atlantic sturgeon (ATS) are an endangered, anadromous species along the Atlantic slope of North America. To determine if and how an active dredge may affect ATS spawning migration, a Vemco Positioning System array was deployed around an active hydraulic-cutterhead dredge that adult ATS must traverse to reach spawning habitat in the James River, VA. Telemetry data showed that all ATS that entered the study area survived. ATS that migrated upstream during dredge operations (N = 103) traversed the dredge area and continued upstream to spawning habitat. Many ATS made multiple trips through the study area during dredge operations. There was no noticeable difference in swim behavior regardless of whether the dredge was absent or working within the study area. We suggest that dredging in the lower James River does not create a barrier for adult ATS migrating to spawning habitat or cause adults to significantly modify swim behavior. This is the first study to utilize fine-scale telemetry data to describe how an organism moves in relation to an active dredge. This methodology could be used to describe dredge-sturgeon interactions on different life stages and in other locations and could be expanded to other aquatic organisms of concern. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Repeat spawners of anadromous salmonids may contribute significantly to population resilience by providing multiple cohorts to both seawater and freshwater life stages. In this study, winter survival of sea trout (Salmo trutta Linnaeus, 1758) post spawners (kelts) was 89%. Sea survival increased linearly with female length with a return probability between 30% and 50%, whereas males attained a maximum return probability of 60% at 520 mm. Of the returning sea trout, 40% skipped spawning and they had significantly lower condition factor as kelts compared with those who returned after one summer. These results suggest that sex-specific differences in individual post-spawning growth–survival trade-off exist and that energetic status of descending kelts may influence the probability to skip spawning. We discuss to what extent hydropower may reduce post-spawning survival due to blocking of river descent opportunities, potentially altering the fitness landscape and favouring new life-history adaptations. From a management perspective, it is concluded that it is crucial to maintain connectivity in regulated rivers, allowing fish to complete repetitive spawning and feeding migrations, thus contributing to population productivity. [ABSTRACT FROM AUTHOR]

Anadromous fishes often use various survival tactics while migrating through main stem rivers to successfully reach spawning grounds and reproduce. Mixed-stock assemblages of anadromous adult summer steelhead Oncorhynchus mykiss re-enter the Columbia River from late spring through fall including the period of peak summer water temperatures, and previous studies suggest that stocks alter migratory behaviour in response to warm temperatures by seeking cool water refuges. We combined parentage-based tagging with mixed stock analyses to test whether steelhead use a non-natal tributary (Deschutes River, OR, USA) as a thermal refuge and if this migratory behaviour is associated with stock-specific run-timing. Results collected over two migration years indicated that out-of-basin fish in the Deschutes River were disproportionately from specific stocks in the Snake River (Salmon and Grande Ronde) that had migrated through the main stem Columbia River when water temperatures exceeded 21 °C. Stocks migrating through the main stem river during cooler temperature periods were either less frequent (Clearwater River), or not encountered (lower Snake River) in the Deschutes River. This study facilitates an improved understanding of stock-specific migratory characteristics associated with environmental conditions in this system. Results potentially affect fisheries management, hatchery protocols, cool-water refuge maintenance, and conservation of wild Deschutes River populations. [ABSTRACT FROM AUTHOR]

ABSTRACT Migrations are characterized by periods of movement that typically rely on orientation towards directional cues. Anadromous fish undergo several different forms of oriented movement during their spawning migration and provide some of the most well-studied examples of migratory behaviour. During the freshwater phase of the migration, fish locate their spawning grounds via olfactory cues. In this review, we synthesize research that explores the role of olfaction during the spawning migration of anadromous fish, most of which focuses on two families: Salmonidae (salmonids) and Petromyzontidae (lampreys). We draw attention to limitations in this research, and highlight potential areas of investigation that will help fill in current knowledge gaps. We also use the information assembled from our review to formulate a new hypothesis for natal homing in salmonids. Our hypothesis posits that migrating adults rely on three types of cues in a hierarchical fashion: imprinted cues (primary), conspecific cues (secondary), and non-olfactory environmental cues (tertiary). We provide evidence from previous studies that support this hypothesis. We also discuss future directions of research that can test the hypothesis and further our understanding of the spawning migration. [ABSTRACT FROM AUTHOR]

Anadromous fish could have intraspecific groups with different life histories; identification of these is very important for management and conservation. Geometric morphometrics represents a quick and cost-effective method to identify such intraspecific groups. In this study, geometric morphometrics of Pontic shad (Alosa immaculata) migrants from the Danube River were investigated, as was Fulton's condition factor (K), hepatosomatic index (HSI) and gonadosomatic index (GSI). Pontic shad specimens were collected during April and May of 2004 and 2006. The sample comprised four groups (65 males and 56 females in 2004; 110 males and 78 females in 2006). Geometric morphometrics revealed differences in shape that were statistically significant for both year (nested analysis of variance: F = 13.84; p < 0.001) and sex (nested analysis of variance: sex nested in year, F = 1.71; p < 0.01). The major differences between years were in head shape and position of fins. The most significant difference between females and males was in the dorsolateral expansion of the mid-body section with emphasis on the higher dorsal part of the body. Values for K in this work (0.65–0.70) were the lowest recorded for Pontic shad in the Danube River. Additional investigations involving both geometric morphometrics and molecular genetics are necessary to confirm the presence of races. This could be important information for conservation purposes as Pontic shad is listed as Vulnerable on the International Union for Conservation of Nature list. [ABSTRACT FROM AUTHOR]

Dams fragment watersheds and prevent anadromous fishes from reaching historic spawning habitat. Sedgeunkedunk Stream, a small tributary to the Penobscot River (Maine), has been the focus of efforts to reestablish marine-freshwater connectivity and restore anadromous fishes via the removal of two barriers to fish migration. Currently, Petromyzon marinus (Sea Lamprey) is the only anadromous fish known to spawn successfully in the stream downstream of the lowermost dam. Here, we describe the distribution and abundance of a spawning population of Sea Lamprey in Sedgeunkedunk Stream, prior to and in anticipation of habitat increase after the completion of one barrier removal. In 2008, we estimated the abundance of Sea Lamprey and its nests using daily stream surveys and an open-population mark-recapture model. We captured 47 Sea Lamprey and implanted each with a PIT tag so that we could track movements and nest associations of individual fish. The spawning migration began on 18 June, and the last living individual was observed on 27 June. We located 31 nests, distributed from head-of-tide to the lower- most dam; no spawners or nests were observed in the tidally influenced zone or upstream of this dam. Mean longevity in the stream and the number of nests attended were correlated with arrival date; early migrants were alive longer and attended more nests than later migrants. Males were more likely to be observed away from a nest, or attending three or more nests, than were females, which attended usually one or two nests. We observed a negative association between nest abundance and substrate cover by fine sediment. Based on their observed movements in the system, and the extent of their habitat use, we anticipate that spawning Sea Lamprey will recolonize formerly inaccessible habitat after dam removals. [ABSTRACT FROM AUTHOR]

Capturing adult anadromous fish that are ready to spawn from a self sustaining population and transferring them into a depleted system is a common fisheries enhancement tool. The behaviour of these transplanted fish, however, has not been fully evaluated. The movements of stocked and native anadromous alewife, Alosa pseudoharengus (Wilson), were monitored in the Ipswich River, Massachusetts, USA, to provide a scientific basis for this management tool. Radiotelemetry was used to examine the effect of origin (native or stocked) and release location (upstream or downstream) on distribution and movement during the spawning migration. Native fish remained in the river longer than stocked fish regardless of release location. Release location and origin influenced where fish spent time and how they moved. The spatial mosaic of available habitats and the entire trajectory of freshwater movements should be considered to restore effectively spawners that traverse tens of kilometres within coastal rivers. [ABSTRACT FROM AUTHOR]

The timing and duration of the estuarine phase during seaward migration were addressed for allis shad and twaite shad, closely related sympatric species in the Gironde system. Otolith Sr:Ca and daily growth increments were used to retrospectively estimate the age at entrance in the estuary, the time spent in estuary and the age at seaward exit from returning adults. The two species exhibited different seaward migration strategies: allis shad entered the estuary at an older age than twaite shad (88 days vs. 59 days), stayed shorter in the estuary (11 days vs. 17 days) and exited to the sea at an older age (99 days vs. 77 days). In the Gironde system, the estuarine phase is very short and occurs within a particular time ‘window’ for the juveniles that will become future spawners. In this context, it might be suggested that the estuarine phase is a bottleneck for these sympatric shad populations. [ABSTRACT FROM AUTHOR]