River Spawning Research Articles

Individual return patterns of spawning flannelmouth sucker (Catostomus latipinnis) to a desert river tributary

Tributaries provide temporal and spatial habitat heterogeneity in river networks that can be critical for parts of the life history of a species. Tributary fidelity can benefit individual fish undergoing spawning migrations by reducing time and energy spent exploring new areas and leveraging previous experience, but anthropogenic activities that fragment or degrade these systems can eliminate those benefits. We used multistate models based on passive integrated transponder (PIT) detection data from 2013 to 2023 to estimate the proportion of flannelmouth suckers (Catostomus latipinnis) migrating to a tributary, McElmo Creek, from the mainstem San Juan River for spawning. Survival varied among years and among states. The top model for migration probability included sex, with males slightly more likely to migrate (0.93 vs 0.90), and the next model identified interannual variation in migration probability ranging from 0.875 to 0.999 across years, indicating high site fidelity. Individuals showed consistency in relative arrival timing across years, with the highest correlation generally during years with greater spring discharge and extended tributary residence time. Successful tributary spawning may be important for the maintenance of the mainstem San Juan River flannelmouth sucker population, but site fidelity may be maladaptive where tributaries are vulnerable to human alterations.

Diatom Diversity from Watercourses of North-Eastern Kamchatka with Description of One New Species

A study of diatoms in eight watercourses (four spawning rivers and four streams) in the area of the Ozernovsky Mining and Metallurgical Complex (MMC) on the Kamchatka Peninsula was carried out for the first time. A total of 174 taxa were identified, and a comparative analysis of periphyton species diversity at the sampling stations was carried out. A new species for science was proposed: Gomphonema anissimovae Glushchenko, Kezlya & Kulikovskiy sp. nov. The composition and quantitative characteristics of plankton were analysed only in samples collected from rivers. It was shown that all rivers are oligotrophic in terms of phytoplankton biomass. The work includes lists of taxa with indication of their abundance, as well as illustrative material of found diatom taxa, which will provide an opportunity to monitor changes in planktonic and periphyton microalgae communities in the studied watercourses in the context of potentially high anthropogenic load from industry.

Generation abundance dynamics of peled at the lower reaches of the Ob’ River in 1981–2021

The generation abundance of peled has been assessed at the lower reaches of the Ob’ River. Based on data on the number of larvae migrated downstream from spawning grounds, obtained in 1981–2021 on the main spawning rivers, current population structure of peled at the lower reaches of the Ob’ River is described, the influence of anthropogenic factors and of the water content of the Ob’ River floodplain on peled reproduction is analyzed. The Severnaya Sos’va River definitely plays a decisive role in the reproduction of peled. In recent years, there has been an increase in the contribution of the Voikar and Sob’ rivers. Seven-year cycles have been identified (between the peaks in the larvae number); their maximums coincide with the peaks of water content at the lower reaches of the Ob’ River with a 1-year lag, which supports the world’s largest river peled population abundance. The presence of such cycles makes it possible to predict phases of high fish abundance. An equation describing the dependence of the generation size of peled at the lower reaches of the Ob’ River on the average water level of Ob’ River in June–August at the Salekhard gauging level in the previous year is proposed.

Population characteristics and movements of striped bass Morone saxatilis (Walbaum, 1792) in the Mira River estuary, Cape Breton Island, Nova Scotia, Canada

The occurrence of striped bass outside the immediate vicinity of known spawning rivers in Canada is neither widely understood nor well studied. Striped bass in Canada are managed and assessed within three distinct units, the Bay of Fundy, the Gulf of St. Lawrence, and the St. Lawrence River; but stocks that may occur outside these units are unrecognized. We document a previously unstudied aggregation of striped bass in the Mira River estuary (MRe), Cape Breton Island (46° 01′N, 60° 03′W), a location on the east coast of Nova Scotia omitted from present management units but which has been long reported to host an aggregation. From July 2012 to November 2014, 62 striped bass within MRe were sampled and 31 were surgically implanted with VEMCO acoustic transmitters. Striped bass ranged in size from 31.6 to 125.0 cm total length and age 3 to 24 years. Acoustic telemetry from 2012 to 2015 elucidated residency and fidelity to the MRe with mid-estuary overwintering every year, freshwater residency of the adult population during spring, and a summer through autumn aggregation in the lower estuary. Of the 31 acoustically tagged striped bass, 24 remained in MRe throughout the study, six exhibited mid-summer departures to the Atlantic Ocean but returned by mid-autumn, while one left the MRe and was never detected again. Mira River SB with acoustic tags were never detected at nearby Ocean Tracking Network telemetry infrastructure. Striped bass stocks exhibit similar residency and fidelity patterns to their natal rivers and estuaries elsewhere in its Atlantic coast range which suggests the Mira River aggregation constitutes a possible distinct stock yet unrecognized by Canadian fisheries managers.

Temporal shifts in the marine feeding of individual Atlantic salmon inferred from scale isotope ratios.

Given the limited information on prey use during the marine residency period for Atlantic salmon, scales were collected from salmon at return to the River Namsen (Norway) for spawning after 1 year at sea, and scale material from the first and second summer marine feeding periods was analysed using stable isotope methods to understand dynamics of their trophic ecology. As the salmon increased in size from the first to second summer, they reduced their feeding niche and specialised more (narrowed the δ13C range) and increased their dependency on higher tropic level (δ15N) prey, likely fish. Changes in δ13C indicated a consistent pattern of movement towards the north and west between summer feeding periods. Hence, salmon during their first year at sea may have a migration route roughly resembling that of previous spawners, as inferred from earlier tagging studies. Feeding conditions and nutrient composition during the last summer at sea, i.e. in the months before returning to the river for spawning, impacted final body size and within-season timing of return. Fish undergoing the largest trophic niche shift (δ13C and δ15N combined) between summer feeding periods, returned earliest. The earliest returning fish had the fastest specific growth rates at sea. Hence, salmon encountering abundant high-quality fish food during the marine migration, particularly during the last months, may reach a size and energetic state whereby it is better to return early to a safer environment in freshwater than risk being eaten by a big predator at sea. Both trophic status (δ15N), resource use (δ13C) and growth rates were significantly correlated between feeding periods. Nutrient composition during the first summer at sea did not impact the fish body length after the following winter, but growth conditions during the first summer evidenced carry-over effects from the first to the second summer of feeding.

Inadequate Sampling Frequency and Imprecise Taxonomic Identification Mask Results in Studies of Migratory Freshwater Fish Ichthyoplankton

In South America, knowledge of major spawning sites is crucial for maintaining migratory fish populations. In this study, we aimed to understand the spatio-temporal distribution of fish eggs in the upper São Francisco River using high sampling frequency and DNA metabarcoding identification. We evaluated the possible effects of the non-molecular identification of eggs and decreased sampling frequency on the determination of spawning sites and major breeding periods. Collections were carried out every three days from November 2019 to February 2020. We found that, if we had assumed that all of the free and non-adhesive sampled eggs belonged to migratory species, as is usual in the literature, this assumption would have been wrong for both the spawning sites and the breeding periods. Moreover, any decrease in the frequency of sampling could dramatically affect the determination of the major spawning rivers, and the spawning events of some of the migratory species may not have been detected. Therefore, without the proper identification and adequate sampling frequency of eggs, important spawning sites may be overlooked, leading to ineffective or inappropriate conservation measures.

The seasonal movement of sediment-associated marine-derived nutrients in a morphologically diverse riverbed: the influence of salmon in an Interior British Columbia river

PurposeThis study (1) investigated the extent to which flocculation and the hydrological and morphological attributes of an interior salmon-bearing river regulate the seasonal storage of marine-derived nutrients (MDN) and (2) compared the contribution of MDN to the fine bed sediment relative to other nutrient sources to the river.MethodsPrevious research has determined that the co-existence of re-suspended fine sediment, generated by salmon redd construction, with salmonid excretion and decay products in the water column creates ideal conditions for the flocculation of these inorganic and organic particles. Stored and suspended fine bed sediment was sampled from seven sites with varying morphologies and bed substrate down the length of a large spawning river in the interior of British Columbia over a 12-month period. MDN contributions to the sediment was tracked using aggregated versus dispersed particle size, carbon-to-nitrogen ratios, stable carbon and nitrogen isotopes, and MixSIAR modeling.Results and discussion(1) There was a significant longitudinal spatial distinction of nutrient retention between sites upstream and downstream of a large seasonally inundated floodplain; (2) the MDN isotopic signal in the surficial stored bed sediment in this sample year was short term; and (3) upstream spawner numbers, substrate size, stream morphology, and discharge were relevant to both the magnitude and retention time of sediment-associated MDN.ConclusionA cumulative magnification of MDN was correlated with the distance from the headwaters and the number of upstream spawners. The relationship between MDN retention in interior rivers, and possible multi-year accumulation, was influenced by variability in channel morphology, substrate size, and the presence of an inundated floodplain halfway down the river.

Potential of Otolith Microchemistry to Distinguish Nursery Areas of Salmon within River Simojoki

Draining into the northern Baltic Sea, River Simojoki is an important spawning river for Atlantic salmon. The present study aimed to preliminary explore the potential of analysing the elemental composition of otoliths to distinguish the within-river nursery area of origin for salmon. Parr were sampled at three nursery areas in the river and smolts of unknown origin were sampled near the river mouth during the migration to the sea. The concentrations of multiple elements and the strontium 87Sr/86Sr isotopic ratio in the otoliths were analysed using single- and multi-collector laser ablation inductively coupled plasma mass spectrometry, respectively. Based on the otolith elemental variables, parr could be reclassified to the sampled nursery areas using discriminant function analysis and random forest with a success rate of 53.3% and 63.3%, respectively. However, for parr sampled at the uppermost nursery area in the river, the success rates were 90% and 100%, respectively. Applying the classification models trained on the parr samples to determine which nursery area sampled smolt originated from was constrained by the limited sampling of parr, both in sample sizes and the coverage of the nursery areas found in the river.

Assessing the occurrence of egg stranding for trout and salmon in a regulated river

AbstractA key challenge in many regulated rivers is to define adequate flow levels to protect aquatic organisms. Provisioning of suitable flow can be pivotal bottlenecks for fishes such as salmon and trout that use the riverbed as an incubation habitat. Additionally, the locations where females spawn will define the probability that embryos will be dewatered during the incubation period in a regulated flow regime. We investigated the water flow, dewatering, and incubation mortality in Atlantic salmon and brown trout in natural nests over 19 years in the regulated Bjoreio River in western Norway. During the study period, different flow strategies were applied to mitigate the dewatering of incubating salmon and trout embryos. Average survival in nests sampled in late winter ranged from 54% to 92% among years and was significantly correlated with the minimum water flow occurring during the incubation period. Mortality was significantly higher in nests in shallow areas, reflecting nests exposed to dewatering. The results demonstrate a strong link between incubation mortality and managed flow regimes for river spawning salmonids. Using detailed information on the nest location and incubation mortality, we estimate minimum flow requirements for this river and demonstrate an approach to effectively mitigate the impact of river regulations on embryo survival in Atlantic salmon and brown trout.

Fish resources of the early life history stages in the Fuling section of the Three Gorges Reservoir and their responses to hydrological and thermal characteristics

三峡水库调度运行改变了库区江段的水文情势,影响漂流性卵苗在库区江段内的时空分布特征。在水库低水位运行时,具有流水生境特征的库中和库尾江段是产漂流性卵鱼类在整个三峡库区内重要的产卵江段。了解三峡库中和库尾江段产漂流性卵鱼类的早期资源状况及其对水文和水温特征的响应关系,对于采取适宜的保护措施促进鱼类早期资源的恢复具有重要的意义。本文通过2017-2020年5-7月在库中涪陵断面进行的逐日早期资源监测,了解三峡库中和库尾江段产漂流性卵鱼类早期资源的种类组成、产卵规模、产卵场分布及其年际变动特征,并采用随机森林(RF)模型,分析鱼类逐日产卵规模变动与水文、水温格局及其过程的关系,辨识影响不同种类产卵规模变动的关键水文及水温要素。结果表明:2017-2020年共采集到漂流性鱼卵21种,包括长江上游特有鱼类鱼卵3种;以圆筒吻鮈Rhinogobio cylindricus、蒙古鲌Chanodichthys mongolicus、贝氏Hemiculter bleekeri等6种鱼类的鱼卵为主,占总采集鱼卵数的88.76%;各年的鱼卵规模分别为83.04×10⁸、14.29×10⁸、20.43×10⁸和22.83×10⁸ 粒;涪陵以上干流江段分布有6个鱼类的集中产卵场,是圆筒吻鮈、长鳍吻鮈R. ventralis、长薄鳅Leptobotia elongata等长江上游特有鱼类重要的产卵场所;逐日产卵规模主要受产卵时透明度及流量大小、产卵前流量或水位上涨过程以及产卵前积温大小的影响,且影响不同鱼类自然繁殖的关键水温、水文因素在不同种类间存在差异。研究结果可为三峡库区鱼类资源保护措施特别是生态调度措施的实施提供科学依据。;Reservoir operation resulting from river damming has altered the flow regimes of the river sections of reservoirs, inducing profound impacts on the spatiotemporal distribution of the early resources of fish species that spawn drifting eggs (FSDE) in the river sections of reservoirs. During the period of low water-level operation stage of the reservoir, the river section with flowing-water habitats is an important spawning river section compared those other sections of the reservoir. Fully understanding the status of early resource of FSDE and their response to hydrological and thermal characteristics in a reservoir's middle and tail reaches is of great significance for the restoration and promotion the scale of early resource. This paper intended to reveal the inter-annual variation in the species composition of eggs and larvae collected, as well as the spawning scale and distribution of fish spawning grounds in the middle and tail reaches of the Three Gorges Reservoir (TGR), through the daily sampling for eggs and larvae being conducted at the Fuling section from May to July in 2017-2020. The random forest model was further performed to clarify the relationships between the variations in the daily spawning scales and the hydrological, thermal patterns and process conditions, aiming to identify the key hydrological and thermal factors affecting the variations of spawning scales of different species. The results showed that: 1) 21 species of eggs were collected, including 3 species of fish eggs that endemic to the upper reaches of the Yangtze River, and the amounts of eggs of 6 fish species, including Rhinogobio cylindricus, Chanodichthys mongolicus, and Hemiculter bleekeri, etc., were dominant, which accounted for 88.76% of total number of eggs collected. 2) The scale of fish eggs each year was 83.04×10⁸ ind., 14.29×10⁸ ind., 20.43×10⁸ ind., and 22.83×10⁸ ind., respectively. 3) There were 6 main spawning grounds distributed in the upstream of Fuling sampling site in the TGR, where is an important spawning place for endemic fish species (e.g. R. cylindricus, R. ventralis, and Leptobotia elongata) in the upper reaches of the Yangtze River. 4) The daily spawning scales were mainly affected by the amounts of transparency and flow discharge during spawning, as well as flow or water level rising rate and accumulated water temperature before spawning, and the key hydrological and thermal factors that affect the natural reproduction were different among different species. This research provides data support for the implementation of fish resource protection measures, especially ecological regulation measures, in the TGR.

Эколого-морфологическая характеристика нерестового стада посольской популяции байкальского омуля Coregonus migratorius Georgi в современный период (по данным 2009–2020 гг.)

The timing of the start and end of the spawning migration of the Baikal omul is specific for each of its populations, is closely related to the temperature regime in their areals and fluctuates from year to year in accordance with the variability of climatic conditions in the basin of the lake. Since the thermal and water regime of Lake Baikal is changing under the influence of global warming, the omul populations are adapting to shifts in the rhythms of the functioning of the lake's ecosystem. The purpose of this work is to analyze changes in the current ecological and morphological composition of spawning stocks of omul on the example of the posolskaya population in connection with the recorded changes in the timing of its migration for spawning in the 2000s. To analyze the nature of the variability in the timing and dynamics of spawning migration in 2009–2019, we used data from calendar records of the catch by a fixed river seine of mature individuals entering spawning rivers – tributaries of Posolsky Sor Bay. For ecological and morphological analysis, we used data on the morphometry of the omul that entered the Bolshaya Rechka River (one of the main tributaries of the Posolsky Sor. Bay) in different periods of the spawning season of 2020. The results of measurements of spawning migrants in 2020 indicate that, against the background of a general delay in the timing of migration, the dynamics of the morpho-ecological composition of spawning shoals is identical to that in the 1960s and 1980s and is typical for the population under the study. At the same time, data on the morphometry of fish that entered the river in different periods of the spawning season of 2020 indicates a change in the ecological and morphological structure of the population, namely, the quantitative ratio of spawning migrants from various subpopulations changes towards the predominance of "pelagic predators" – inhabitants of the deep-sea areas of Lake Baikal.

River‐of‐origin assignment of migratory Striped Bass, with implications for mixed‐stock analysis

AbstractObjectiveThe Striped Bass Morone saxatilis is an anadromous teleost with a native range extending north from the Gulf of Mexico into Canadian waters. Far‐ranging coastal migrations support one of the most popular recreational fisheries in the United States. Identifying the underlying population genetic structure of the spawning populations and the genetic markers capable of differentiating among them advances our understanding of these economically and ecologically important fish and enables more targeted management to occur.MethodsWe used a restriction site‐associated DNA sequencing approach to identify neutral and adaptive single‐nucleotide polymorphisms (SNPs), and we determined the population genetic structure of 438 adult Striped Bass sampled from nine spawning locations along the Atlantic coast from the Roanoke River, United States, to the Miramichi River, Canada.ResultThe two Canadian populations (Shubenacadie and Miramichi rivers) were genetically distinct from U.S. populations and from each other. Neutral loci differentiated Striped Bass from U.S. waters into four genetically distinct populations: Roanoke River, Hudson–Kennebec River, Upper Chesapeake Bay–Potomac River–Delaware River, and Choptank River (eastern Chesapeake Bay). Outlier loci further differentiated the Delaware River from the Chesapeake Bay tributaries, suggesting that there may be local adaptation in the face of gene flow. We identified 1300 highly informative SNPs (the top 10% [with respect to the genetic differentiation index FST] of the full suite of 13,361 SNPs in our study) capable of assigning fish with at least 90% accuracy to their river of origin; through simulations, we established their applicability for conducting robust mixed‐stock analyses of the coastal migratory Striped Bass fishery.ConclusionThis study demonstrated that neutral and adaptive loci together provide evidence for fine‐scale population structure of migratory Striped Bass, and these loci provide the most informative genetic panel for mixed‐stock analysis of Striped Bass to date, capable of assigning fish to their spawning river of origin.

Ural (Zhayik) River Spawning Grounds of the Sturgeon (Acipenseridae) in the Republic of Kazakhstan: Modern Situation

Ural (Zhayik) River Spawning Grounds of the Sturgeon (Acipenseridae) in the Republic of Kazakhstan: Modern Situation

Результаты многолетних исследований зообентоса реки Мегры (бассейн Белого моря)

The results of long-term studies of the zoobenthos for the downstream of the Megra salmon spawning river (White Sea Basin) have been presented. The benthic fauna of the rapids and riffles for the lower reaches of the river is represented by 25 taxa of invertebrates. The abundance and biomass of zoobenthos in June-July vary within a wide range. Based on the results of the analysis of the abundance and biomass of invertebrates for all the years of observation the highest rates were found to be at the upper stations in June - early July. By the end of July, the situation changed: the maximum abundance and biomass were noted at the lowest station, while the abundance and biomass of invertebrates decreased sharply at the uppermost section. A more gradual decrease was noted at the intermediate station.

Антропогенное воздействие на популяции атлантического лосося (Salmo salar) в Арктическом бассейне Российской Федерации

The global problems of anthropogenic impact on the planet Earth and marine ecosystems, pollution in the 21st century of the World Ocean, including the Arctic seas, negatively affecting the reproduction of marine biological resources are considered. For more than half a century, the intensively developing exploitation of the bioresources of the World Ocean shelf has led to a de-crease in the number of mammals, fish, mollusks, crustaceans, algae and other valuable aquatic organisms. In many developed countries, due to economic activity and industrial pollution of coastal territories, there is a depression in the number and a de-crease in intraspecific diversity, due to the loss of populations of passing marine "wild" Atlantic salmon and other aquatic organ-isms. Since the late 1980s, natural Atlantic salmon stocks have declined significantly. In the main spawning rivers of the Mur-mansk region, the content of pollutants in the spring is marked on the scale as high and extremely high levels of pollution. The oxygen content and water quality in rivers decrease, the number of saprophytic bacteria increases, the species diversity of zoo-plankton and phytoplankton decreases. Diseases and death of populations of spawning Atlantic salmon occur. A program has been developed to comprehensively investigate the causes of mass disease and death of Atlantic salmon populations, and to de-velop measures to restore a genetically healthy salmon population in the Arctic region of Russia.

Chub movement is attracted by the collision sounds associated with spawning activities

Cyprinids (carps, chubs and minnows) possess well-developed hearing and high sensitivity to sound pressure. The sensitive hearing may assist cyprinids with searching for food, territory defense, and mating behavior. Many paired fishes violently shake in sand and gravel while spawning in rivers. However, no study has examined the ecological importance of the collision sound made by the behavior. This study examined whether cohabitated chubs (Opsariichthys evolans and Zacco platypus) use the collision sound as a signal to locate spawning events so they can be a male satellite or egg eater. Three types of sounds (i.e., collision sound, music noise and ambient noise) were played with or without jerkbaits at the midstream of the Keelung River, Taiwan during the spawning season in 2018 and 2019. Generalized linear mixed models were then built to examine the effects of the sound types and the presence of jerkbaits on the number of individuals that the two chubs attracted. Results showed significantly different levels of attractiveness among the three sound types, with the collision sound attracting most fishes, including both females and males, followed by music noise and ambient noise. The presence of jerkbaits increased the number of fishes attracted, but the effect was only statistically marginally significant. These results suggest that the collision sound as an acoustic signal may be more important than a visual signal for the chubs to locate spawning events of other mating pairs, probably because of the longer transmission distance of the former. The present study demonstrates the ecological meanings of the collision sounds made in association with spawning activities of the chubs and implies that the native chub's spawning activities may be affected by the introduced Z. platypus. More studies on the interactions between these cohabitated chubs will benefit the conservation of native chubs.

Seasonal Movements and Spatial Overlap of Juvenile and Adult Lake Sturgeon in Lake Champlain

AbstractThe lake sturgeon Acipenser fulvescens is a large, long‐lived, potamodromous species that is widely distributed throughout freshwater systems in the central part of North America. In this study, we used acoustic telemetry to examine seasonal distribution and movement patterns of endangered Lake Sturgeon in Lake Champlain, Vermont. Acoustic tags were implanted in 29 juvenile Lake Sturgeon (453–874 mm TL) and 19 adults (1,215–1,615 mm TL) from the Winooski River and nearby areas of Lake Champlain between 2015 and 2019; tags were detected with 23 passive acoustic receivers deployed in the river and delta area and an additional 34 receivers deployed throughout Lake Champlain. Home range analysis using a lattice‐based density estimator indicated that juvenile home range sizes were the same as adult home range sizes in spring and summer but were statistically larger than adult home ranges in winter. Cumulative home range analysis showed that juvenile and adult home ranges overlapped in shallow (<10‐m) water in the summer and fall. In winter and spring, cumulative home ranges from juveniles included deepwater sites (>25 m), while adults remained in shallow water near the mouth of their spawning river. Seven juveniles made long‐range movements (18–34 km) during the winter and spring months, and 13 juveniles moved back into the lower section of their natal river after overwintering in Lake Champlain. This study is the first to directly compare adult and juvenile Lake Sturgeon distribution, home range size, movements, and habitat use in a large lake system and provides a baseline for further research on the movement ecology of Lake Sturgeon in Lake Champlain.

Physiological, morphological and transcriptomic responses of Tibetan naked carps (Gymnocypris przewalskii) to salinity variations

Gymnocypris przewalskii is a native cyprinid fish that dwells in the Lake Qinghai with salinity of 12–13‰. It migrates annually to the freshwater rivers for spawning, experiencing the significant changes in salinity. In the present study, we performed the physiological, morphological and transcriptomic analyses to understand the osmoregulation in G. przewalskii. The physiological assay showed that the osmotic pressure of G. przewalskii was almost isosmotic to the brackish lake water. The low salinity reduced its ionic concentrations and osmotic pressure. The plasticity of gill microstructure was linked to the salinity variations, including the presence of mucus and intact tight junctions in brackish water and the development of the mitochondria-rich cells and the loosened tight junctions in freshwater. RNA-seq analysis identified 1926 differentially expressed genes, including 710 and 1216 down- and up-regulated genes in freshwater, which were enriched in ion transport, cell-cell adhesion, and mucus secretion. Genes in ion uptake were activated in low salinity, and mucus pathways and tight junction showed the higher transcription in brackish water. The isosmoticity between the body fluid and the environment suggested G. przewalskii was in the metabolic-saving condition in the brackish water. The decreased salinity disrupted this balance, which activated the ion uptake in freshwater to maintain osmotic homeostasis. The gill remodeling was involved in this process through the development of the mitochondria-rich cells to enhance ion uptake. The current finding provided insights into the potential mechanisms of G. przewalskii to cope with salinity alteration.

Role of tide and lunar phases on the migration pattern of juvenile Hilsa shad (Tenualosa ilisha) within a meso-macrotidal estuary.

Tide and lunar phases often influence the behaviour and life cycle of different fishes, especially migratory species. In the Hooghly River estuary, Hilsa shad is an anadromous fish species that migrates from the adjacent sea to the estuary and rivers for spawning. After spawning, the juveniles remain in the rivers and estuary for few months then start their downstream migration towards the adjacent sea. However, the pattern of their downstream migration has not been studied in detail so far. This study investigates the role of tide and lunar phases on the juvenile Hilsa shad migration pattern. In this study, we have estimated the rate of juveniles migrating through the river channel (no. m-2 h-1 ) during high tide and low tide in all of the lunar phases. The number of juvenile Hilsa shad fishes is found to be much higher during low tides in most of the observations and there is a significant difference (t=11.904, P < 0.001) between the high tide and low tide catches in the entire study region. Among the eight lunar phases, the number of juveniles is also observed to be higher during the new moon and full moon, and there is also a significant difference in juvenile catch among the lunar phases (F=64.372, P < 0.001) in the entire stretch of the study area. These observations enabled us to develop a plausible mechanism of the downstream migration of Hilsa shad juveniles.

Life history strategies of Mekong pangasiid catfishes revealed by otolith microchemistry

Fourteen species of pangasiid catfish are found in the Mekong River. They are key components of both capture fisheries and aquaculture, although little is known about their migratory ecology, which is particularly important as some species are in decline. In order to examine the life-history strategies of key pangasiid catfishes in the Mekong River, we collected otoliths from 126 specimens belonging to four species in the Lower Mekong Basin. We analysed the otolith microchemical structure using two methods (Laser Ablation – Inductively Coupled Plasma Mass Spectrometry and Scanning X-ray Fluorescence Microscopy) to identify evidence of diadromy (movements between the ocean and upstream waters). We followed an established method of examining patterns of strontium to calcium (Sr:Ca) and barium to calcium (Ba:Ca) ratios along otolith transect profiles to interpret whether fish were in fresh or salt water at various stages of their life. We confirmed anadromy in two species (Pangasius krempfi and P. mekongensis; spawning in river but growing in marine waters) with at least three migratory tactics identified for each species, thus highlighting more diverse life history strategies than previously thought. Based on locations where we collected fish, we can infer these species migrate up to 700 km (P. mekongensis) and 1400 km (P. krempfi) from the sea into the Mekong River for spawning. The other two catfishes (P. elongatus and P. bocourti) appeared to be freshwater species that occasionally strayed into the estuary. Long-distance migratory species require free flowing rivers to connect critical habitats used at different life stages. Any activity disrupting these migratory pathways and disconnecting these habitats must be carefully considered. Conservation and management of transboundary species require international agreements to protect them from the impacts of river development, and the evidence presented here is pertinent for catfish in the Mekong River.