Feeding of juvenile sterlet (<Emphasis Type="Italic">Acipenser ruthenus</Emphasis>, Acipenseridae) in the Danube River midstream

The present paper provides the results of the study on feeding of juvenile (underyearlings and age 2+) sterlet (Acipenser ruthenus) caught in the Danube River in 2002–2003. The full spectrum of food items found in the fish stomachs was identified. The seasonal patterns of food composition are revealed. It is shown that juvenile sterlet in the Danube feeds mainly on larvae of chironomids and trichopterans, amphipods, and leeches. The organisms typical for lithophilic and lithorheophilic biocenoses play considerable role in the feeding of sterlet. Mean daily gains of body length and weight in the sterlet underyearlings are calculated.     

Patterns of migration between feeding and spawning sites in a coral reef surgeonfish

Many coral reef fishes exhibit regular localised migrations between feeding and spawning areas, but the factors affecting these migration patterns, such as the distance, frequency and spawning site fidelity are poorly understood. The aim of this study was to investigate the patterns of migration to spawning sites of the surgeonfish, Ctenochaetus striatus (Acanthuridae). We explored relationships amongst an individual’s size and sex, the distance and frequency it migrated from its feeding area to spawning sites, fidelity to particular spawning sites and the number of individuals that aggregated to spawn. In order to achieve this, 406 C. striatus were captured and tagged on inshore reefs in Kimbe Bay (5°30′S 150°6′E), New Britain, Papua New Guinea. Tagged individuals were consistently observed within spatially discrete but overlapping feeding areas (maximum diameter averaging <13 m). The mean distance migrated was 58 m (ranging from 2 to 291 m). No tagged individuals were witnessed spawning at more than one site. Whilst most individuals (n = 88) migrated to the spawning site that was closest to their feeding areas, those that migrated to sites further away (n = 9) always spawned at sites where the number of conspecifics aggregating was larger. Neither the size nor the sex of individuals limited migration distance. However, males migrated significantly more frequently than females (on average once every 2 days vs. once every 3 days), and migration frequency was positively correlated with size in females. Migration distance did not affect the frequency with which individuals spawned. Whether patterns of migration are determined by cost-benefit optimisation, tradition, or an alternative mechanism is unknown.     

Sturgeon re‐introduction in the Upper and Middle Danube River Basin

All six sturgeon species naturally occurring in the Danube River from the Black Sea (BS) to the upstream reaches in Germany today are under severe pressure and classified as either vulnerable, critically endangered or already extinct in the Danube River Basin (DRB). This sharp decline of populations is the result of two major drivers: habitat alteration and unsustainable harvest. The Danube River Basin can be divided in three functionally separated sections by its hydromorphology and character: The Lower Danube (LD) and BS, now also separated ecologically from the Middle Danube (MD) by two hydropower stations at the Iron Gate gorge, 800 km from the sea, currently restricting the remaining anadromous sturgeon species (A. gueldenstaedtii, A. stellatus, Huso huso) to the LD and BS also restraining migrations of potamodromous species. The Upper Danube (UD) is nowadays disconnected from the approx. 850 km long MD by Gab?ikovo Dam. The UD itself, as well as many tributaries along the Danube, are fragmented by numerous barriers, leaving no more than a total of 300 km free flowing river out of 700 km. Overharvest led to disappearance of most sturgeon species in the UD already in the Middle Ages, with populations in the MD following in the subsequent centuries. Today the potamodromous sterlet (A. ruthenus) still exists in small decreasing populations in the UD and MD basins, where its occurrence is supported by stocking. Most probably the potamodromous ship sturgeon (A. nudiventris) is extirpated in the whole basin. The Danube basin comprises 19 countries, being the most international river basin in the world, rendering coordinated conservation efforts a challenge. Strategic guidelines for the conservation of the Danube sturgeons are available in form of several action plans, but implementation remains insufficient as of yet. This paper attempts an analysis of the potential for sturgeon restoration in the UD and MD basin and highlights the need for concerted approaches when it comes to ex situ actions and reintroductions.     

Effects of feeding rate on the growth performance of gynogenetic albino sterlet,Acipenser ruthenus (Linnaeus, 1758) larvae

The feeding rate effects were studied on the growth performance of gynogenetic diploid larvae of sterlet Acipenser ruthenus during the first 4 weeks of exogenous feeding. The experimental rearing was conducted from 7 to 38 days post‐hatch (dph) in a circulation system. This was set up in four groups with three replicates (440 individuals/replicate), viz: AC‐control larvae fed Artemia sp., CFC‐control larvae fed compound feed, AG‐gynogenetic larvae fed Artemia sp., and CFG‐gynogenetic larvae fed compound feed. The larvae were reared in glass tanks (44 L volume, 10 individuals/L) with the temperature maintained at 18 ± 0.5°C, photoperiod of 12L:12D and water flow regime of 1‐L/min and fed 50%, 25%, 25%, and 9% of their total biomass/day during feeding. Highest TL and WBW of gynogenetic diploid larvae (AG) were observed with 50.6 ± 1.2 mm and 607.3 ± 36.1 mg (n = 30) at 38 dph. Highest TL and WBW of control larvae (AC) were recorded with 49.5 ± 3.8 mm and 600.8 ± 88.0 mg (n = 30), respectively, with 73.1% ± 11.4% survival; the lowest survival rate was at 46.4% ± 7.1% (n = 30) for the CFG group. The results indicate that the gynogenetic and normal larvae of sterlet fed with live food (Artemia nauplii) from 7 dph can achieve higher growth and survivability compared to the larvae fed with formulated test feed. Results of this study suggest that the effective rearing of sterlet larvae from 7 to 38 dph strongly depends upon the types of feed rather than the genome manipulation performed.     

Timing matters: species‐specific interactions between spawning time,substrate quality,and recruitment success in three salmonid species

Substratum quality and oxygen supply to the interstitial zone are crucial for the reproductive success of salmonid fishes. At present, degradation of spawning grounds due to fine sediment deposition and colmation are recognized as main factors for reproductive failure. In addition, changes in water temperatures due to climate change, damming, and cooling water inlets are predicted to reduce hatching success. We tested the hypothesis that the biological effects of habitat degradation depend strongly on the species‐specific spawning seasons and life‐history strategies (e.g., fall‐ vs. spring‐spawners, migratory vs. resident species) and assessed temperature as an important species‐specific factor for hatching success within river substratum. We studied the species‐specific differences in their responses to such disturbances using egg‐to‐fry survival of Danube Salmon (Hucho hucho), resident brown trout (Salmo trutta fario), and migratory brown trout (Salmo trutta lacustris) as biological endpoint. The egg incubation and hatching success of the salmonids and their dependence on temperature and stream substratum quality were compared. Hatching rates of Danube salmon were lower than of brown trout, probably due to higher oxygen demands and increased interstitial respiration in spring. Increases in maximum water temperature reduced hatching rates of resident and migratory brown trout (both fall‐spawners) but were positively correlated with hatching rates of Danube salmon (a spring‐spawner). Significantly longer incubation periods of resident and migratory brown trout coincided with relatively low stream substratum quality at the end of the egg incubation. Danube salmon seem to avoid low oxygen concentrations in the hyporheic zone by faster egg development favored by higher water temperatures. Consequently, the prediction of effects of temperature changes and altered stream substratum properties on gravel‐spawning fishes and biological communities should consider the observed species‐specific variances in life‐history strategies to increase conservation success.     

Temporal plasticity in thermal‐habitat selection of burbot Lota lota a diel‐migrating winter‐specialist

In this study, animal‐borne telemetry with temperature sensors was coupled with extensive habitat temperature monitoring in a dimictic reservoir, to test the following hypotheses: behavioural thermoregulation occurs throughout the year and temperature selection varies on a diel and seasonal basis, in a winter‐specialist diel‐migrating fish. Burbot Lota lota demonstrated nightly behavioural thermoregulation throughout the year, with a large seasonal shift between selection for very cold temperatures (<2° C) optimal for reproduction during the spawning period and selection for warmer temperatures (12–14° C) optimal for hunting and feeding during non‐reproductive periods. During daylight hours, while L. lota avoided habitats warmer than optimal for reproduction and feeding during the spawning and non‐reproductive periods, respectively, active selection was limited to selection for 4–6° C habitat during the prespawning period. Although behavioural thermoregulation explained the night‐time migration, behavioural thermoregulation only partially explained daytime behaviour, indicating that diel migration is best explained by a combination of factors. Thus, thermal‐habitat selection was a good predictor of night‐time habitat occupancy in a diel‐migrating species. Together, these results show that thermal‐habitat selection by fishes may be important throughout the year and a more seasonally plastic behaviour than previously recognized.     

Long-term investigations of migratory behaviour of asp (Aspius aspius L.) in the middle part of the Elbe River, Germany

The migratory behaviour, habitat use, home range and temporal and spatial variability of migrations of the potamodromous asp (Aspius aspius L.) in the large Elbe River, Germany, were studied in the years 1997–2000. Fifty‐three asp were tagged with surgically implanted radio transmitters and individually hand‐tracked usually once per week for about 1 year; 50% of the asp exclusively inhabited the main channel, 22% moved for short visits into other habitats and 24% changed between winter habitats in a harbour or oxbow and summer habitats in the main channel where they spawned in late March/April. The migratory behaviour of asp was highly variable; 34 asp observed for more than 1 year lived in home ranges of 1 to >100 stream kilometres (skm) mostly near their capture site. Longest observed migration from summer habitat in the Tidal Elbe back to the spawning ground in the Middle Elbe was 166 skm. After the spawning period or the spawning period thereafter, nine of these 34 fish left their home ranges for a long downstream migration without returning during the observation period. From these results we conclude that populations of rheophilic, potamodromous fish use a surprisingly long river section and that weirs have a strong impact on migration patterns and, ultimately, on population dynamics of these species.     

Timing and pattern of annual silver eel migration in two European watersheds are determined by similar cues

Many animals perform long‐distance migrations in order to maximize lifetime reproductive success. The European eel migrates several thousand kilometers between their feeding habitats in continental waters (fresh‐, brackish, and sea water) and their spawning area in the Sargasso Sea. Eels residing in freshwaters usually initiate their spawning migration as silver eels during autumn, triggered by diverse environmental cues. We analyzed the time series of silver eel downstream migration in Burrishoole, Ireland (1971–2015), and Imsa, Norway (1975–2015), to examine factors regulating the silver eel migration from freshwater to the sea. The migration season (90% of the run) generally lasted from 1 August to 30 November. Environmental factors acting in the months before migration impacted timing and duration of migration, likely through influencing the internal processes preparing the fish for migration. Once the migration had started, environmental factors impacted the day‐to‐day variation in number of migrants, apparently stimulating migration among those eels ready for migration. Both the day‐to‐day variation in the number of migrants and the onset of migration were described by nearly identical models in the two rivers. Variables explaining day‐to‐day variation were all associated with conditions that may minimize predation risk; number of migrants was reduced under a strong moon and short nights and increased during high and increasing water levels. Presence of other migrants stimulated migration, which further indicates that silver eel migration has evolved to minimize predation risk. The onset of migration was explained mainly by water levels in August. The models for duration of the migration season were less similar between the sites. Thus, the overall migration season seems governed by the need to reach the spawning areas in a synchronized manner, while during the actual seaward migration, antipredator behavior seems of overriding importance.     

Different growth trends of whitefish (Coregonus lavaretus) forms in the northern Baltic Sea

The sea growth of two whitefish forms, anadromous (Coregonus lavaretus lavaretus) and sea‐spawning (Coregonus lavaretus widegreni), was analysed using samples collected from the commercial sea catch in the Gulf of Bothnia (GoB) in the northern Baltic Sea during 1998–2014. In the GoB area, these two forms are possible to identify because the gill‐raker number and size at maturity vary between forms. The growth rate of the forms is linked to their feeding area. Sea‐spawning whitefish, which has a feeding migration near its home site, was shorter in the northern GoB (66°N–64°N) at the ages of 3–11 than those in the southern GoB (64°N–60°30′N). In the data, most whitefish were caught with gill nets in the GoB. The mesh sizes of gill nets capturing the anadromous form were mostly 35–45 mm, while those capturing the sea‐spawning form were <35 mm in the northern GoB. It is likely that the different growth trends for small and large whitefish were connected with differences in their recruitment for fishing. The length of anadromous females at the age of four sea years increased significantly, but the length of six‐year‐old anadromous female whitefish decreased over the catch years from 1998–2014. In contrast, the length of slow‐growing sea‐spawning whitefish of six years or older increased significantly in relation to the catch year in the gill‐net catch. The increase in the growth of young age groups in both forms was probably associated with the increasing temperature and the low fishing pressure on small fish. The decreasing age at capture for both forms and the depression of the mean size of old anadromous whitefish are signs of high fishing pressure with a high gill‐net effort that selectively removes the largest and oldest individuals of both forms.     

Morphometrics of two morphs of sterlet, Acipenser ruthenus L., in the middle course of the Danube River (Serbia)

Two morphs of the sterlet, Acipenser ruthenus, were clearly recorded in two samplings from the middle course of the Danube River in Serbia. The pooled samples comprised 47.9% pointed‐snout morphs and 52.1% blunt‐snout morphs. The most obvious differences were snout shape and length, mainly the differences in the snout area in front of the barbels. In addition to this snout difference, the two morphs also differed with regard to head lengths and the pre‐ocular, pre‐dorsal and post‐dorsal areas. The determination of morphs is not exclusively related to either location or body size as sources of this mode of variability, although it seems that it is easier to detect these in larger size sterlet.     

Migrations and resources of sterlet <Emphasis Type="Italic">Acipenser ruthenus</Emphasis> (Acipenseridae) from the lower reaches of the Volga River

Based on long-term materials, spawning, postspawning, and downstream migrations of the Lower Volga sterlet Acipenser ruthenus were studied. The dates, extent, pattern (two-step and one-step) and main parameters of the spawning and postspawning migrations of the previously separated three populations of the Lower Volga starlet—Volgograd and eastern and western delta ones were established. The main differences of the downstream migration of this potanadromous species from that of anadromous species of acipenserids were revealed. It was found that, after downstream migration, juveniles of sterlet form a continuous chain of aggregations from the dam of the Volgograd Hydraulic Power System to the Caspian Sea. Each link of this chain acts as a center of formation of local groups of sterlet that are components of the distinguished populations. It is shown that the observed distribution of sterlet in the lower reaches of the Volga is determined by the location of the spawning grounds of individual populations and the extent of downstream migrations of prolarvae and larvae. The total extent of migrations of sterlet is small (it does not exceed 250 km), which is a cause of the relative isolation of the previously distinguished populations. The stocks of Volgograd sterlet are exhausted at the present time; the recovery of its stock is possible, first of all, at the expense of a decrease in illegal fishing. The resources of the eastern and western delta populations of sterlet are in a satisfactory condition.     

Nutritional status of adult sea lamprey (Petromyzon marinus Linnaeus, 1758) during spawning migration in the Minho River,NW Iberian Peninsula

The anadromous sea lamprey (Petromyzon marinus) stop feeding during spawning migration; following the migration effort, the energy reserves of their tissues are expected to be mobilised and depleted. The present work aimed at testing if mobilisation of nutrients occurred in sea lampreys during the 2011 spawning run in the Minho River. Professional fishermen at three sampling sites captured the individuals used: two at the beginning of the freshwater spawning migration (one at the river mouth and one upstream the estuary) and one near the spawning grounds (35 km upstream the estuary limit). The total lipids (Folch extraction), protein (nitrogen conversion), energy content (direct calorimetry) and fatty acids (GC/EI‐MS‐SIM) were determined in the carcass (n = 19) and female liver (n = 8) and gonads (n = 8). The results indicated that a reduction of carcass dry matter during migration was mainly caused by a lipid decrease (51.0 ± 2.46% downstream and 38.0 ± 2.92% upstream, LSmean ± SE). In females, a decrease in the measured amounts of liver lipids was also observed (4.28 ± 2.637 g downstream and 0.64 ± 2.206 g upstream, LSmean ± SE) and the monounsaturated fatty acids consistently composed the larger fraction of lipids in all tissues (ranging between 42.9 ± 4.20% and 54.9 ± 3.43% of total fatty acids, LSMean ± SE). The results suggest a dynamic nutrient mobilisation and the major importance of liver fatty acids (such as 14:1n‐5 and 16:1n‐7) as possible energy sources for embryonic development or combustion during spawning migration. The P. marinus individuals captured at the intermediate sampling site (approximately 35 km upstream river mouth) in the middle of the season seemed as ready to spawn as the upstream sea lampreys.     

Current velocity shapes co‐existence patterns among invasive Dikerogammarus species

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Migration patterns of a potamodromous piscivore,asp (Leuciscus aspius), in a river–lake system

Potamodromous fishes require safe migration routes between spawning, feeding and wintering habitats to complete their life cycle. As knowledge on asp migrations is restricted, this work investigated the movements of adult asp tagged with acoustic transmitters for 3 years in the large Peipsi-Emajõgi-Võrtsjärv lake–river system, Estonia, which is free of migration barriers. Asp showed complex migration patterns, moving between and within different waterbodies (lakes, river, tributaries) in all seasons, but with a tendency to repeat habitat use patterns between years. Lakes were mainly used for feeding during spring and summer (after spawning 65% of the fish migrated to Lake Peipsi), and more so by large fish. The majority (80–96%) of the fish spent the winter in the rivers, mostly close to their subsequent spawning area. Spawning areas were in swift-flowing waters in tributaries and the main river. The results indicate that asp may benefit from an extensive and diverse complex of habitats, and any migration barrier during any season may restrict the natural habitat use by asp. Maintenance and restoration of habitat heterogeneity and connectivity is critical to protect behaviourally diverse fish populations and increase resilience in rivers negatively impacted by various human activities.     

Morphometric recognition of two morphs in sterlet (Acipenser ruthenus) population induced by different reproductive behaviour

Four species of sturgeons appear in the Yugoslav part of the Danube River. Among them three are anadromous, beluga ( Huso huso ), Russian sturgeon ( Acipenser gueldenstaedtii ) and stellate sturgeon ( Acipenser stellatus ) and migrate to Danube from the Black Sea to spawn. Only sterlet ( Acipenser ruthenus ) is exclusively freshwater. There are obvious differences between vernal and winter races of anadromous sturgeons, due to their different migratory patterns. Russian ichthyologist Berg (1934) claimed that there are two races (morphs) in the starlet population as well. This stimulated long discussion in Russian and Yugoslav literature, and it was the starting point for our research on sterlet in the Yugoslav part of the Danube River.
Morphometric analysis was preformed on 159 individuals caught in Danube River among 1123 and 1173 km, on four localities during September and October 2002. Analysis included 24 morphological and 3 meristic characters. K‐means cluster analysis indicated existence of two different clusters (groups), one with 95 (59·75%) and other with 64 (40·25%) samples. Significant differences were found for preorbital, preoral and prebarbel length. Mean values (in % of head length) for first group were 50·13 ± 1·71, 57·61 ± 1·87, 39·74 ± 2·20 and for second group 53·50 ± 2·46, 62·07 ± 2·20 and 43·30 ± 2·08 respectively. Findings were confirmed by Principal Components & Classification Analysis. These results speak in favour of existence of two morphs in sterlet population, one with long and pointed rostrum (vernal) and other with short rostrum (winter). According to the literature that is probably influenced by different reproductive behaviour (time separation in spawning).     

The ontogenetic development of the digestive tract and accessory glands of sterlet (Acipenser ruthenus L.) larvae during endogenous feeding

The process of differentiation of digestive tract structures in the sterlet Acipenser ruthenus (L.) larvae was studied from hatching to the beginning of exogenous feeding [9 dph (day post hatching)] using histological procedures. On the day of hatching the digestive tract was closed and completely filled with nutrients (the yolk platelets) that were successively utilized during development. A liver primordium was present in the ventral region of the yolksac. The pancreas was observed on the 2 dph. At the same time, the mouth opening took place. Glandular and nonglandular stomach and anterior and intermediate intestine developed from the yolksac walls. Gastric glands became visible on the 7 dph. The primary intestine developed into the spiral intestine. At the moment of onset of exogenous feeding the yolk material was completely exhausted and there was not mixed feeding observed in sterlet larvae. The fish started exogenous feeding on the 9 dph, which was accompanied with evacuation of melanin plug. At the end of endogenous feeding the digestive tract of sterlet larvae was developed and functional, so they could properly utilize food.     

Maternal and environmental influences on egg size and juvenile life‐history traits in Pacific salmon

Life‐history traits such as fecundity and offspring size are shaped by investment trade‐offs faced by mothers and mediated by environmental conditions. We use a 21‐year time series for three populations of wild sockeye salmon (Oncorhynchus nerka) to test predictions for such trade‐offs and responses to conditions faced by females during migration, and offspring during incubation. In years when their 1100 km upstream migration was challenged by high water discharges, females that reached spawning streams had invested less in gonads by producing smaller but not fewer eggs. These smaller eggs produced lighter juveniles, and this effect was further amplified in years when the incubation water was warm. This latter result suggests that there should be selection for larger eggs to compensate in populations that consistently experience warm incubation temperatures. A comparison among 16 populations, with matching migration and rearing environments but different incubation environments (i.e., separate spawning streams), confirmed this prediction; smaller females produced larger eggs for their size in warmer creeks. Taken together, these results reveal how maternal phenotype and environmental conditions can shape patterns of reproductive investment and consequently juvenile fitness‐related traits within and among populations.     

Spatial genetic features of eastern oysters (Crassostrea virginica Gmelin) in the Gulf of Mexico: northward movement of a secondary contact zone

The eastern oyster (Crassostrea virginica Gmelin) is an economically and ecologically valuable marine bivalve occurring in the Gulf of Mexico. This study builds upon previous research that identified two divergent populations of eastern oysters in the western Gulf of Mexico. Allelic and genotypic patterns from 11 microsatellite markers were used to assess genetic structure and migration between the previously described oyster populations in Texas. The main findings are as follows: (1) there are two distinct populations (F_ST = 0.392, P < 0.001) of oysters that overlap in the Corpus Christi/Aransas Bay estuarine complex in Texas, (2) the distribution of genotypes among individuals in the contact zone suggests limited hybridization between populations, (3) the variables of salinity, temperature, dissolved oxygen, turbidity and depth are not correlated with allele frequencies on reefs in the contact zone or when analyzed across Texas, and (4) there is little evidence of directional selection acting on the loci assayed here, although patterns at four markers suggested the influence of balancing selection based on outlier analyses. These results are consistent with long‐term historical isolation between populations, followed by secondary contact. Recent hydrological changes in the area of secondary contact may be promoting migration in areas that were previously inhospitable to eastern oysters, and observed differences in the timing of spawning may limit hybridization between populations. Comparison of these findings with the results of an earlier study of oysters in Texas suggests that the secondary contact zone has shifted approximately 27 km north, in as little as a 23‐year span.     

Assessing the effect of Separation Levee Project on Chinese sturgeon (<Emphasis Type="Italic">Acipensor sinensis</Emphasis>) spawning habitat suitability in Yangtze River,China

Chinese sturgeon (Acipenser sinensis) is a protected anadromous fish species. The migration pattern of the fish has been blocked by the construction of Gezhouba Dam, reducing the natural spawning site length to less than 7 km along the Yangtze River. However, the fish has since established an alternative spawning ground in the narrow reach downstream of Gezhouba Dam. To enhance navigation, a Separation Levee Project (SLP) was implemented in the new-found spawning habitat of the fish. To therefore evaluate the effect of the SLP on Chinese sturgeon spawning habitat suitability, the conditions in the spawning habitat were simulated using River2D (a two-dimensional hydrodynamic model). Two main approaches (habitat kinetic energy and circulation metrics) were used in the simulation. The study showed that SLP only slightly changed the physical conditions in the spawning habitat. Using hydrodynamic simulation, the weighted usable area (WUA) before and after the SLP construction was also computed and habitat preference curve developed for water depth and velocity. On the average, SLP reduced WUA—a finding that was consistent with field-measured data. Based on WUA, the habitat conditions were more sensitive to SLP proximity than metrics based on velocity gradients. SLP posed detrimental impacts on the suitability of spawning habitats of Chinese sturgeon. The findings in this study provide further basis for the protection and restoration of Chinese sturgeon spawning habitats in especially the lower reach of Yangtze River.     

Distinct seasonal migration patterns of Japanese native and non‐native genotypes of common carp estimated by environmental DNA

Understanding behavioral differences between intraspecific genotypes of aquatic animals is challenging because we cannot directly observe the animals underwater or visually distinguish morphologically similar counterparts. Here, we tested a new monitoring tool that uses environmental DNA (eDNA), an assemblage of DNA in environmental water, to specifically detect Japanese native and introduced non‐native genotypes of common carp (Cyprinus carpio) in Lake Biwa, Japan, and estimated differences between the two genotypes in the use of inland habitats. We monitored the ratios of native and non‐native single nucleotide polymorphism alleles of a mitochondrial locus of common carp in a lagoon connected to Lake Biwa for 3 years using eDNA. We observed seasonal dynamics in the allele frequency showing that the native genotype frequency peaked every spring, suggesting that native individuals migrated to the lagoon for spawning and then returned to the main lake, whereas non‐native individuals tended to stay in the lagoon. The estimated migration patterns corresponded with the estimates of a previous study, which were based on commercial fish catch data. Our findings suggest that eDNA‐based monitoring can be useful tool for addressing intraspecific behavioral differences underwater.