Temporal differentiation and conditions of reproduction of chum salmon <Emphasis Type="Italic">Oncorhynchus keta</Emphasis> (Salmoniformes,Salmonidae) from the Tugur River basin (Khabarovsk Krai)

Chum salmon Oncorhynchus keta from the Tugur River of Khabarovsk krai was studied. It was found that, in the river basin, chum salmon of two ecological groups reproduces: the first spawns at river sections with a pronounced underflow and the second spawns at river sections with a groundwater outlet. In the first anadromous migration, usually three maxima of the number of approaches of migrating fish are observed. The first in time maximum is represented by early (or summer) chum salmon, the second includes migrants of early and late (or autumn) form, and the third is represented by chum salmon of only late form. Forms of chum salmon from the Tugur River basin different in dates of the run and the spawning sites differ not only in the ecology of reproduction but also in biological indices. The later chum salmon is larger; its gonads at entry into the river for spawning are less mature as compared with early chum salmon. The autumn form of chum salmon reproducing at key spawning grounds is distributed more widely than the summer form. It is found along the Asian coast from Chukotka in the north to Japan and Korea in the south. Summer chum salmon occupies only part of the range of the autumn form.     

Genetic evidence for lacustrine spawning of the non‐anadromous Atlantic salmon population of Little Gull Lake, Newfoundland

Allozyme markers were used to investigate the spatial boundaries of the non‐anadromous Atlantic salmon population of Little Gull Lake, Eastern Canada. Mixed year class samples of juvenile salmon were analysed from inlet streams, upstream lakes, the outlet river and sites downstream. No evidence was found that individuals from the Lake's non‐anadromous population were present in these samples, strongly suggesting that the non‐anadromous population is confined to the lake and has a lacustrine spawning habit.     

Calcium metabolism and osmoregulation during sexual maturation of river running Atlantic salmon

As Atlantic salmon return from the ocean to undertake the anadromous spawning migration up the river of origin, profound changes in calcium metabolism and osmoregulation take place. Using tartrate resistant acid phosphatase as a marker, scale osteoclast activity was found to increase throughout sexual maturation and spawning migration. Thus, the participation of osteoclasts in the elevated scale resorption observed during this phase is established. As calcium was simultaneously accumulated in the female gonads, it is proposed that the scales are resorbed in order to provide calcium for the growing ovaries. Plasma oestradiol-17 β levels were elevated in females during sexual maturation, and had decreased at the time of spawning. Plasma testosterone levels were similar in males and females during the first part of the upriver migration, but had increased in males and decreased in females at spawning. In addition to the role of these sex steroids in the gonadal growth, their possible involvement in the increased scale resorption during this phase is discussed. Plasma growth hormone and thyroxine levels were elevated in both sexes at spawning, with the triiodothyronine/thyroxine (T₃/T₄) ratio declining sharply, indicating possible roles for these hormones in the maturational process. The relatively low gill Na⁺, K⁺-ATPase activity of salmon caught in the estuary implies that the fish had already adapted to a hypoosmotic environment. During the upriver migration, the gill Na⁺, K⁺-ATPase activity decreased further, indicating that the hypoosmoregulatory ability was suppressed further during sexual maturation and spawning migration.     

Life-history effects of migratory costs in anadromous brown trout

Mean size of sexually mature anadromous brown trout (sea trout) Salmo trutta in south-east Norway increased significantly with migratory distance ( D ) between the feeding area at sea and the spawning area in fresh water, from 32 cm for those spawning close to the river mouth to 43 cm at the spawning grounds 40 km inland. This was largely due to an increased size of the smallest anadromous spawners with increasing D . The raised mean size of the long-distance migrants is paralleled by an increased age at sexual maturity. Body mass at the same length of sea trout decreased with D in fresh water, meaning that the fish moving far inland was slimmer than those spawning near the coast. Gonadal mass of first-time spawning anadromous males declined significantly with D , and the fecundity and the ratio of fecundity over mean mass of the individual eggs adjusted for variation in fish mass, increased with D . There was no clear relationship between the ratio of anadromous to resident fish and D , probably because more variables than D , influence this relationship in the study streams.     

Experimental study of the European river lamprey, <Emphasis Type="Italic">Lampetra fluviatilis</Emphasis> (L.), migratory behavior in the period of initial dispersion of juveniles

Mechanisms of the initial dispersion of early larvae resident and anadromous forms of the European river lamprey have been experimentally explored. The flow (channel flow and underflow), type of bottom, and illumination are the key factors influencing the beginning and the end of downstream migration. Larvae leave the pebble ground of the spawning area when there is underflow and illumination is reduced to the night level, and then they migrate downstream in the channel stream. It is shown that the individuals dispersing from the spawning areas have switched to exogenous feeding. The end of the dispersion occurs after larvae reach oozy habitats of the riverbed, where they can find optimal feeding conditions.     

Investigations of spawning migration and assessment of abundance of the Kamchatka steelhead (<Emphasis Type="Italic">Parasalmo mykiss</Emphasis>) from the Utkholok River by means of Didson dual-frequency identification sonar

Quantative estimates of abundance of anadromous Kamchatka steelhead Parasalmo mykiss in the period of its maximum spawning migration in the shallow Utkholok River by the instrumental method using Didson dual-frequency identification sonar are presented. Equipment and methods are described for the stationary observation of spawning run of Kamchatka steelhead through the river cross section and mobile surveys of the water area of the Utkholok R. As a result of stationary observations, the abundance and size composition of Kamchatka steelhead migrating to the upper reaches of the river are determined. Peculiarities of the diurnal dynamics of migration of Kamchatka steelhead characterized by two sharp peaks of run intensity in the morning and evening were revealed. During the period of observations from September 22 to October 20, 2007, in the seasonal dynamics of migration two peaks of intensity of run of Kamchatka steelhead depending on an abrupt decrease in water temperature by 2°C were also registered. Mobile surveys supplied data on the abundance and size composition of Kamchatka steelhead in the surveyed water area of the river, on its spatial distribution, and dynamics of accumulation of fish in the river. The abundance of Kamchatka steelhead in the river almost doubled every three-four days and the spatial distribution was aggregated, producing maximum concentration mainly in areas of increasing depth and channel meandering before and after riffles. Comparison of the results of assessment of abundance of Kamchatka steelhead that arrived for spawning in 2006 and 2007 showed significant fluctuations in population abundance.     

Spawning migration behaviour of sea trout (Salmo trutta L.) in a boreal river system: effects of flow conditions and obstacles on migratory activity

In this study, radio telemetry was used to examine the upstream spawning migration behaviour of anadromous brown trout (sea trout), Salmo trutta L., in a boreal river system, the River Isojoki, western Finland. The aim was to study the movement activity and migration characteristics of trout during the upstream spawning migration, as well as to locate the important spawning habitats and study the spawning characteristics. Furthermore, the authors analysed how flow conditions and a hydropower dam, with adjacent fishways, affected the upstream spawning migration. Tagged trout spawned in both the main stem and four tributaries, with spawning taking place from early October to November. The movement activity of radio-tagged trout was influenced by a hydropower dam (Perus dam), with spring migrators spending prolonged periods at the dam area, postponing the migration upstream. Flow conditions affected the total time spent at the dam area, as well as the movement activity in the free-flowing sections above the dam, with increasing flow stimulating activity. In addition, time of river ascent and location of spawning area had a significant effect on the movement activity of tagged trout. These results are further evidence that synergistic effects of flow and migratory obstacles can negatively influence migrations of anadromous fish, regardless of constructed fishways. The management of flow regimes and the efficiency of fishways are vital, as climate change will likely influence the flow and increase the water temperature of boreal river systems, further aggravating issues caused by obstacles.     

Aquatic food‐web dynamics following incorporation of nutrients derived from Atlantic anadromous fishes

Changes in the isotopic composition (δ¹³C and δ¹⁵N) in biofilm, macro‐invertebrates and resident salmonids were used to characterize temporal dynamics of marine derived nutrients (MDNs) incorporation between stream reaches with and without MDN inputs. Five Atlantic rivers were chosen to represent contrasting MDN subsidies: four rivers with considerable numbers of anadromous fishes; one river with little MDN input. Rainbow smelt Osmerus mordax, alewife Alosa pseudoharengus, sea lamprey Petromyzon marinus and Atlantic salmon Salmo salar, were the primary anadromous species for the sampled rivers. Regardless of the spatial resolution or the pathway of incorporation, annual nutrient pulses from spawning anadromous fishes had a positive effect on isotopic enrichment at all trophic levels (biofilm, 1·2–5·4‰; macro‐invertebrates, 0·0–6·8‰; fish, 1·2–2·6‰). Community‐wide niche space shifted toward the marine‐nutrient source, but the total ecological niche space did not always increase with MDN inputs. The time‐integrated marine‐nutrient resource contribution to the diet of S. salar parr and brook trout Salvelinus fontinalis ranged between 16·3 and 36·0% during anadromous fish‐spawning periods. The high degree of spatio‐temporal heterogeneity in marine‐nutrient subsidies from anadromous fishes lead to both direct and indirect pathways of MDN incorporation into stream food webs. This suggests that organisms at many trophic levels derive a substantial proportion of their energy from marine resources when present. The current trend of declining anadromous fish populations means fewer nutrient‐rich marine subsidies being delivered to rivers, diminishing the ability to sustain elevated riverine productivity.     

The presence of anadromous twaite shad (Alosa fallax) in the Ebro River (western Mediterranean,Spain): an indicator of the population's recovery?

Although the Ebro River is one of the largest Mediterranean rivers, the presence of anadromous fish species was near to complete extirpation since the second half of the 20th century. However, many mature twaite shads (Alosa fallax) were captured at the lower reaches of this river (Xerta's weir site, 63 river km) in spring 2005, suggesting a natural recovery of this population. In this study, the first biological data on twaite shad from the Ebro River are presented, and their significance for the species conservation and recovery are discussed. The historical reasons for the decline of this species are reviewed, and data supports the proposal for urgent construction of a proper fish passage for anadromous species at the Xerta's weir. This action would allow fish access to a further 60 km of river upstream of the weir, representing a 200% increase in the amount of freshwater habitat and potential spawning areas. Improved access together with restrictive fishery regulations will assist with renewed interest in the fishery and conservation of the species.     

Maintenance of a small anadromous subpopulation of brown trout (Salmo trutta L.) by straying

1. Microsatellite and isozyme loci variation were used to study structure and dynamics of a brown trout (Salmo trutta) population heavily affected by damming. The downstream area accessible for spawning was drastically reduced to a stream 1 km long influenced by regulated discharge. 2. Stocking of hatchery‐reared juveniles failed and the population is entirely supported by anadromous adults from neighbouring populations. 3. Temporal genetic stability is reported here. Some punctual between‐river genetic differences are likely because of different contribution from each neighbouring river through years. 4. High anadromy‐mediated gene flow produces a lack of genetic substructure in the region. The role of anadromous brown trout on maintenance of endangered small populations is emphasised.     

Phylogeography,genetic structure,and conservation of the endangered Caspian brown trout, <Emphasis Type="Italic">Salmo trutta caspius</Emphasis> (Kessler, 1877), from Iran

The Caspian Sea, the largest inland closed water body in the world, has numerous endemic species. The Caspian brown trout (Salmo trutta caspius) is considered as endangered according to IUCN criteria. Information on phylogeography and genetic structure is crucial for appropriate management of genetic resources. In spite of the huge number of studies carried out in the Salmo trutta species complex across its distribution range, very few data are available on these issues for S. trutta within the Caspian Sea. Mitochondrial (mtDNA control region) and nuclear (major ribosomal DNA internal transcribed spacer 1, ITS-1, and ten microsatellite loci) molecular markers were used to study the phylogeography, genetic structure, and current captive breeding strategies for reinforcement of Caspian trout in North Iranian rivers. Our results confirmed the presence of Salmo trutta caspius in this region. Phylogenetic analysis demonstrated its membership to the brown trout Danubian (DA) lineage. Genetic diversity of Caspian brown trout in Iranian Rivers is comparable to the levels usually observed in sustainable anadromous European brown trout populations. Microsatellite data suggested two main clusters connected by gene flow among river basins likely by anadromous fish. No genetic differences were detected between the hatchery sample and the remaining wild populations. While the current hatchery program has not produced detectable genetic changes in the wild populations, conservation strategies prioritizing habitat improvement and recovering natural spawning areas for enhancing wild populations are emphasized.     

Sex-specific spawning behavior and its consequences in an external fertilizer

Identifying the target of sexual selection in externally fertilizing taxa has been problematic because species in these taxa often lack sexual dimorphism. However, these species often show sex differences in spawning behavior; males spawn before females. I investigated the consequences of spawning order and time intervals between male and female spawning in two field experiments. The first involved releasing one female sea urchin's eggs and one or two males' sperm in discrete puffs from syringes; the second involved inducing males to spawn at different intervals in situ within a population of spawning females. In both, fertilization success was measured as the fraction of eggs fertilized and the paternity share of each male. The results indicate that spawning after females imposes a cost on males but only during sperm competition. Further, the optimal interval between the initiations of male and female spawning depends on degree of sperm competition, distance between males and females, and water velocity. The results show that sex differences in spawning timing of marine invertebrates can be explained on the basis of the differential costs and benefits of spawning out of synchrony with the other sex and that the result of sexual selection on external fertilizers may be behavioral rather than morphological differentiation of the sexes.     

Poor water quality constrains the distribution and movements of twaite shad <Emphasis Type="Italic">Alosa fallax</Emphasis><Emphasis Type="Italic">fallax</Emphasis> (Lacépède, 1803) in the watershed of river Scheldt

Worldwide, river fragmentation is primarily responsible for the decline of populations of migrating fish. In particular, anadromous fish species, which necessarily migrate to fresh water to reproduce, are endangered since many are no longer able to reach their natural spawning sites. In addition, pollution of rivers effectively prevents upstream or downstream movements and blocks access to spawning grounds. This article investigates how poor water quality interferes with the life history cycle of twaite shad Alosa fallax fallax (Lacépède, 1803), an anadromous clupeid fish, in the watershed of River Scheldt, a heavily impacted environment in West Europe. We used two models based on known ecological and environmental information to explain past and present twaite shad distribution within the watershed and to make inferences about a future population recovery and juvenile habitat value. We demonstrated that historical spawning areas satisfy water quality conditions necessary to support spawning and successful development of early life history stages of the twaite shad. However, poor water quality conditions just upstream the freshwater–saltwater boundary still act as an effective migration barrier for upstream movement. As a consequence, spawning grounds are inaccessible and the population is dominated by seasonal adults occurring in the lower estuarine part of the watershed. This article provides testable and diagnostic information to the watershed management in that it identifies habitat and water quality requirements needed to support the expected recovery of an endangered anadromous fish population. Guest editors: S. Dufour, E. Prévost, E. Rochard & P. Williot Fish and diadromy in Europe (ecology, management, conservation)     

Variations of life history strategy of the arctic lamprey <Emphasis Type="Italic">Lethenteron camtschaticum</Emphasis> from the Utkholok River (Western Kamchatka)

Morphobiological traits of the lamprey from the Utkholok River (Western Kamchatka) are investigated for the first time. All present forms of lamprey (typically anadromous, anadromous early maturing forma praecox, resident) do not differ by diagnostic characters and belong to one species Lethenteron camtschaticum. Males prevail among mature specimens of various forms, smolts, and ammocoetes at later developmental stages. The forma praecox is represented by males by 92%. A hypothesis is suggested explaining the relationships of the forms and the mechanism of realization of different types of life history strategy in relation to energy resources of water bodies and the kind of feeding of different groups of ammocoetes (organic detritus or Pacific salmon carcasses decomposed after spawning).     

A novel sperm adaptation to evolutionary constraints on reproduction: Pre‐ejaculatory sperm activation in the beach spawning capelin (Osmeridae)

Reproduction of external fertilizing vertebrates is typically constrained to either fresh or salt water, not both. For all studied amphibians and fishes, this constraint includes immotile sperm that are activated after ejaculation only by the specific chemistry of the fertilizing medium in which the species evolved (fresh, brackish, or salt water). No amphibians can reproduce in the sea. Although diadromous fishes may migrate between salt and fresh water, they are shackled to their natal environment for spawning in part because of sperm activation. Here, we report for the first time among all documented external fertilizing vertebrates, that in the absence of any external media, sperm are motile at ejaculation in a marine spawning fish (Osmeridae, capelin, Mallotus villosus). To illuminate why, we evaluated sperm behavior at different salinities in M. villosus as well as the related freshwater spawning anadromous rainbow smelt (Osmerus mordax). Surprisingly, sperm performance was superior in fresh water for both species. M. villosus spend their entire life at sea but our results show that their sperm are deactivated by sea water, suggesting a freshwater ancestry. By circumventing constraining water chemistry, we interpret the unique pre‐ejaculatory sperm activation in this species as a novel adaptation that enables fertilization in the marine environment. These findings also contribute to understanding the persistence of anadromy, despite great energetic costs to adult fishes.     

Upstream passage problems for wild Atlantic salmon (<Emphasis Type="Italic">Salmo salar</Emphasis> L.) in a regulated river and its effect on the population

Due to hydropower development, the upstream migration of wild anadromous salmon and brown trout is impaired in many European rivers, causing negative effects on the long-term survival of natural salmonid populations. This study identified problems for Atlantic salmon during upstream migration in a regulated river in northern Sweden, Umeälven (mean flow: 430 m³ s^?1). Tagging from 1995 to 2005 involved radio tags (n = 503), PIT tags (n = 1574) and Carlin tags (n = 573) to study the spawning migration of salmon from the coast past the regulated section of the river to a fish ladder at the dam/spillway 32 km upriver. The results demonstrate that migration success from the coast to the fish ladder varied between 0% and 47% among years, indicating an average loss of 70% of potential spawners. Discharge from the turbines attracted the salmon away from the bypass route. Echo-sounding in the turbine outlet showed that salmon were normally found at 1–4 m depths. They responded with upstream and/or downstream movements depending on flow changes; increased spill in the bypass channel attracted salmon to the bypass. Once in the bypass channel, salmon could be delayed and had difficulties passing the first rapid at high spills. Additional hindrances to upstream migration were found at rapids and the area of the fish ladder, located further upstream in the regulated river section. The average migration duration was 44 days from the estuary to the top of the fish ladder, with large variation among individuals within years. Modelling the salmon population dynamics showed a potential population increase of 500% in 10 years if the overall migration success could be improved from the current 30% to levels near 75%. Consequently improved migration facilities at the regulated river section should be implemented to achieve a long-term sustainability of these threatened anadromous salmonids.     

Ontogenetic behavior of Siberian sturgeon, Acipenser baerii: A synthesis between laboratory tests and field data

We have reviewed field data and studies on the behavior and development of Siberian sturgeon at early-life intervals and related them to different ecologically relevant environmental factors that may play a role in the distribution, recruitment, and survival of young fish. Four behavioral phases (swimming-up, rheotactism, shoaling, and foraging) are observed from hatching to the juvenile phase. Each behavior is associated with an early-life interval and might allow fish to occupy different river habitats, directly influencing their distribution, survival, and recruitment. River current intensity, substrate typology, food resources, and predation pressure seem to be the most important factors affecting the distribution of Siberian sturgeon free embryos and larvae, while juveniles and adult fish disperse and migrate according to food abundance and reproduction. Mechanisms involved in regulating downstream migration during Siberian sturgeon early life stages are different than those observed in anadromous sturgeon species. In all large Siberian rivers, with the exception of the Lake Baikal, the Siberian sturgeon is represented by population continuums, and in many cases the foraging range also includes the spawning areas. Ontogenetic changes in Siberian sturgeon behavior could be interpreted as a species-specific mechanism to maintain the population continuums described in this species without significant mixture of local populations within the river.     

Changes of gonadotropin-releasing hormone receptor 2 during the anadromous spawning migration in <Emphasis Type="Italic">Coilia nasus</Emphasis>

Background

An increase in the activity of the pituitary-gonad axis (PG-axis) and gonad development are essential for the onset of spawning migration in teleosts. In the fish Coilia nasus, gonad development and spawning migration up the Yangtze River occurs by the end of each summer. We hypothesized that gonadotropin releasing hormones receptor 2 (GnRH-R2), which together produce a signal that interacts with the PG-axis, may help to regulate spawning migration processes.

Results

In this regard, we (1) characterized the gonadosomatic index (GSI) in the anadromous fish C. nasus; (2) analyzed the GnRH-R2 mRNA expression levels in ovary and brain, and concentrations in the serum; and (3) identified the GnRH-R2 protein distribution in the brain and ovaries. We found strong relationships between all of these indices.

Conclusions

The results indicate that GnRH-R2 could act together to promote spawning during the anadromous migration. There is some evidence that the GnRH-R2 gene expression levels and protein distributions change in association with the migratory behavior.

     

Movement of radio-tagged anadromous sea lamprey during the spawning migration in the River Mondego (Portugal)

The available spawning habitat for the anadromous sea lamprey, Petromyzon marinus L., population that enters the River Mondego has been drastically reduced in the last 20 years. The installation of a fish passage in the first impassable dam, the Açude-Ponte, would enable sea lamprey to recolonise the 34.6-km river stretch between the Açude-Ponte and Raiva dams. In order to assess the suitability of the upstream river stretches for this species, 10 radio-tagged sea lamprey were released upstream of the Açude-Ponte dam and tracked continuously throughout the entire migratory path. Lamprey were unable to pass over intact weirs that had been built for recreational purposes. Sea lamprey movements were more frequent during dusk and night than other periods. Increased river discharge at night, resulting from the operation of the Raiva power station, stimulated lamprey movements but reduced ground speeds recorded. After reaching a certain location, some of the animals maintained their position for several weeks, before undergoing a new movement. Two of the main tributaries of this river stretch were used by some sea lamprey, indicating that the animals were able to find these historical spawning grounds.     

On joint spawning of anadromous and resident mykiss <Emphasis Type="Italic">Parasalmo mykiss</Emphasis> in rivers of Western Kamchatka

The fact of joint spawning of anadromous and resident mykiss Parasalmo mykiss is reliably recorded in the basin of the Kol River (Western Kamchatka). The obtained data on relationships of these forms during spawning indicate to their conspecificity. This species should be protected as a whole, in all diversity of its forms.