Elevated river discharge enhances the immigration of juvenile catadromous and amphidromous fishes into temperate coastal rivers

Anthropogenic alterations to river flow regimes threaten freshwater biodiversity globally, with potentially disproportionate impacts on species that rely on flow cues to trigger critical life history processes, such as migration for diadromous fishes. This study investigates the influence of river discharge on the abundance of juvenile fish moving into rivers by four temperate catadromous or amphidromous species (common galaxias Galaxias maculatus, spotted galaxias Galaxias truttaceus, climbing galaxias Galaxias brevipinnis and the threatened Australian grayling Prototroctes maraena). Fyke netting or fishway trapping was used to catch juvenile fish moving from estuaries into freshwater in five coastal waterways in south-eastern Australia during the spring migratory period. There was a positive relationship between the probability of high catch rates and mean discharge in September. We also found a positive relationship between discharge and the number of recruits captured 22–30 days later in a flow stressed system. In addition, day-of-year had a strong influence on catch rates, with the peak abundance of juveniles for three species most likely to occur midway through the sampling period (spotted galaxias in October, climbing galaxias in late October and Australian grayling in late October and early November). Our study shows that higher magnitudes of river discharge were associated with increased catches of juvenile catadromous and amphidromous fishes. With a limited supply of environmental water, environmental flows used to enhance immigration of these fishes may be best targeted to maintain small amounts of immigration into freshwater populations in waterways or years when discharges are low and stable. When there are natural, large discharge volumes, relatively large numbers of juvenile fish can be expected to enter coastal waterways and during these times environmental flows may not be required to promote immigration.     

FRESHWATER FISH OF THE ADDO ELEPHANT NATIONAL PARK

Summary

This study aimed to determine the distribution and relative abundance of freshwater fish in the Addo Elephant National Park. A total of 1578 fish specimens of 12 species were collected during surveys carried out in the Sundays River and four tributaries during 1996 and 1997. The redfin minnow Pseudobarbus afer comprised 81.6% of the total catch. Length frequency analysis of P. afer populations indicated recent successful recruitment in several rivers. Differences between rivers in the concentrations of dissolved and suspended solids and pH are possible reasons for different species complements. These surveys show the distribution of P. afer and Barbus aeneus in the Eastern Cape Province to be wider than previously reported. Management action is required to minimise opportunities for further establishment and spread of alien and translocated fish species and to conserve indigenous fish assemblages within the Park.     

Fish communities in temporarily open/closed estuaries from the warm- and cool-temperate regions of South Africa: A review

The majority of estuaries along the coastline of southern Africa are termed temporarily open/closed estuaries (TOCEs) and are closed off from the sea for varying periods by a sandbar which forms at the mouth. It is therefore important to understand the processes occurring within TOCEs and their importance to fishes in order to make sound management recommendations. Estuaries along the coast of South Africa and their associated fish assemblages are biogeographically distinct and occur in either a subtropical, warm-temperate or cool-temperate zone. There are 125 TOCEs found within the cool-temperate and warm-temperate zones. Most fish species found in TOCEs are the juveniles of marine taxa that breed at sea. Permanently open estuaries generally have a higher diversity of species than TOCEs, but TOCEs still provide important nursery areas for many marine species and numerically often have a higher proportion of estuarine resident species. Important taxa in terms of abundance and biomass in warm-temperate TOCEs include the sparids Rhabdosargus holubi and Lithognathus lithognathus, several mugilid species, estuarine residents (particularly Gilchristella aestuaria and Atherina breviceps) and the freshwater cichlid Oreochromis mossambicus. The diversity of fishes in cool-temperate TOCEs is low when compared with warm-temperate systems and Liza richardsonii tends to dominate catches by number and mass in most systems. Several species recorded in TOCEs show clear longitudinal distribution trends. For example Atherina breviceps is generally more abundant in the lower reaches of estuaries. Mouth state, particularly the frequency, timing and duration of mouth opening plays a key role in determining species richness, composition, diversity and abundance in TOCEs. Mouth state is directly linked to freshwater input. Reduced river inflow leads to prolonged mouth closure and shorter open phases, which inhibits immigration and emigration of marine fish species between estuaries and the sea. Understanding of the effects of various processes occurring within these systems, particularly variation in freshwater input, on the biota of these important systems facilitates the development of informed management recommendations.     

Comparison of inshore and offshore fish larval assemblages within the wider Yangtze River estuary (China)

It is hypothesized that fish larvae undertake vertical movements in estuarine waters. To test this hypothesis, a sampling period was selected during times when many spring‐spawning fish reach maximum abundance in coastal waters so that spring and summer variations in larval fish assemblages could be determined in the Yangtze River estuary. Six oceanographic surveys were conducted across the salinity gradient of an inshore (freshwater) and offshore area (brackish intersection) during spring (May) and summer (August) between 2010 and 2012. The fish larval community was dominated by species of Engraulidae, Gobiidae, Champsodontidae and Mugilidae. The pre‐flexion and flexion larval stages of euryhaline marine species, which are dependent on estuaries as nursery areas, were common. The brackish and marine larval assemblage was the most abundant with taxa such as Coilia mystus and Engraulis japonicus accounting for more than 57.3% of the total catch. Spatial differences in the taxonomic composition of larval fish assemblages were evident between the inshore and offshore areas. Additionally, the Yangtze River runoff regulatory functions as affected by the Three Gorges Reservoir operational mode (hydrological alternating operations) showed weak influences on fish and habitat environments. Low salinities from high freshwater inflow limited bay anchovy production in the inshore area.     

Effects of flow on lateral interactions of fish and shrimps with off-channel habitats in a large river-floodplain system

Off-channel habitats play a crucial role in the life-cycles of many riverine fish species, but lateral movements of fish into these habitats are poorly understood. We tested how flow dynamics affects the movement of fish and shrimps between the main river channel and different types of off-channel habitats: a riverine lake and a wetland. Our study site was the lower Waikato River, North Island, New Zealand, where there are numerous off-channel habitats. Fish were sampled using directional fyke nets. Shortfin eel (Anguilla australis) migrated mostly into the wetland at night, particularly during high river flows. Common bullies (Gobiomorphus cotidianus) were most abundant during the day and in low-discharge conditions, moving mostly into the riverine lake, whereas freshwater shrimp (Paratya curvirostris) moved mostly into the wetland. High numbers of non-native larval common carp (Cyprinus carpio) moved out of the wetland with retreating flood water. This study emphasises the importance of lateral connectivity and flooding in functioning of this river system where numerous native fish, but also exotic fish, used off-channel habitats. Floodplain management strategies should promote ‘controlled connectivity’ measures that provide access for native species at key times while limiting opportunities for introduced species to utilise their favoured off-channel habitats.     

Why are there so few freshwater fish species in most estuaries?

The freshwater fish assemblage in most estuaries is not as species rich as the marine assemblage in the same systems. Coupled with this differential richness is an apparent inability by most freshwater fish species to penetrate estuarine zones that are mesohaline (salinity: 5·0–17·9), polyhaline (salinity: 18·0–29·9) or euhaline (salinity: 30·0–39·9). The reason why mesohaline waters are avoided by most freshwater fishes is difficult to explain from a physiological perspective as many of these species would be isosmotic within this salinity range. Perhaps, a key to the poor penetration of estuarine waters by freshwater taxa is an inability to develop chloride cells in gill filament epithelia, as well as a lack of other osmoregulatory adaptations present in euryhaline fishes. Only a few freshwater fish species, especially some of those belonging to the family Cichlidae, have become fully euryhaline and have successfully occupied a wide range of estuaries, sometimes even dominating in hyperhaline systems (salinity 40+). Indeed, this review found that there are few fish species that can be termed holohaline (i.e. capable of occupying waters with a salinity range of 0–100+) and, of these taxa, there is a disproportionally high number of freshwater species (e.g. Cyprinodon variegatus, Oreochromis mossambicus and Sarotherodon melanotheron). Factors such as increased competition for food and higher predation rates by piscivorous fishes and birds may also play an important role in the low species richness and abundance of freshwater taxa in estuaries. Added to this is the relatively low species richness of freshwater fishes in river catchments when compared with the normally higher diversity of marine fish species for potential estuarine colonization from the adjacent coastal waters. The almost complete absence of freshwater fish larvae from the estuarine ichthyoplankton further reinforces the poor representation of this guild within these systems. An explanation as to why more freshwater fish species have not become euryhaline and occupied a wide range of estuaries similar to their marine counterparts is probably due to a combination of the above described factors, with physiological restrictions pertaining to limited salinity tolerances probably playing the most important role.     

Species composition and dynamics of larval and juvenile fishes in the surf zone of Mauritius

The importance of the surf zone as a nursery ground for larval and juvenile fishes has been widely recognized, however the zone has yet to be studied in Mauritius. Recently, the coastal area of the island has been increasingly affected by human activities, especially by tourism. We collected fish samples with a hand pulled seine net during the period of August 2001 to March 2003 to clarify the fish fauna and the dynamics of fishes in the surf zone. Two sampling sites adjacent to river mouth areas and one sampling site adjacent to a mangrove area were selected for comparison of fish fauna in relation to environmental conditions. A total of 9,429 fish larvae and juveniles, representing at least 112 species from 48 families were collected. The abundant species were hardyhead silverside, Atherinomorus lacunosus, bluespot mullet, Valamugil seheli, and Ambassis spp., each contributing 16.2, 12.4, and 11.8% of the total number of individuals, respectively. Estuarine species dominated in the surf zone adjacent to the river mouth areas. Species composition and diversity changed seasonally. The number of fish increased during the rainy season. Species diversity increased at the turn of the seasons from the dry season to the rainy season. We conclude that species composition in each site was affected by environmental factors, such as the scale of the flux from the rivers, which is related to the precipitation. The results indicated that freshwater from the river is a trigger to aggregate larvae and juveniles in the surf zone.     

Prawn community structure in the subtropical Mfolozi–Msunduzi estuarine system,KwaZulu-Natal,South Africa

The life cycles of many marine and freshwater prawn species are closely associated with estuarine habitat. Here, the prawn community of the Mfolozi–Msunduzi estuarine system is described and the system's potential as an alternative nursery for prawns during prolonged closure of the adjacent St Lucia estuarine system is examined. Sampling was conducted at five localities in summer and winter in 2007–2012. The Mfolozi–Msunduzi Estuary is a river-dominated system with mouth condition, sediment grain size and turbidity being the most important factors affecting the structure of its prawn community. Twelve prawn species were recorded, including five marine penaeid and five freshwater palaemonid species. The prawn community was numerically dominated by freshwater Macrobrachium equidens and two penaeids, Fenneropenaeus indicus and Metapenaeus monoceros. Significant differences were recorded between summer and winter samples, with freshwater species dominating the catch in summer, while penaeid species were dominant in winter. The marked seasonal change in the prawn community was related to strong river flows during summer creating low-salinity conditions, unsuitable for penaeid postlarval development, throughout most of the system. Compared to St Lucia, the Mfolozi–Msunduzi Estuary can be regarded as a poor alternative nursery area for penaeids during the summer peak postlarval recruitment period.     

Effects of an experimental environmental flow release on the diet of fish in a regulated coastal Australian river

Environmental flow rules are developed to provide a flow regime necessary to maintain healthy river and floodplain ecosystems in rivers regulated for human uses. However, few studies have experimentally assessed potential ecological mechanisms causing declines in the health and productivity of freshwater fish assemblages in regulated rivers to inform the development of appropriate environmental flows. We tested whether an experimental flow release in a regulated tributary of the Hunter River, Australia, altered the diet of two widely distributed fish species (Australian smelt Retropinna semoni and Cox’s gudgeon Gobiomorphus coxii) compared with data from unregulated reference and regulated control tributaries. Neither species had significant differences in the number of prey taxa ingested, gut fullness or composition of gut contents due to the environmental flow release (EFR). The diet of R. semoni did not differ significantly between regulated and unregulated tributaries in either catchment. However, the diet of G. coxii differed in only one of the two pairs of rivers consistently across all sample times. Assuming the EFR was sufficient to alter the composition of prey available for consumption by the fish species studied, our findings imply that functional indicators, such as the diet of generalist higher-order consumers, may be more suitable indicators of long-term flow regime change rather than short-term flow events.     

Two centuries of spatial and temporal dynamics of freshwater fish introductions

Aim

Investigating major freshwater fish flows (translocations) between biogeographic regions and their temporal dynamics and also quantifying spatial patterns and temporal changes in the array of introduced species, and the emergence and distance between major donor and recipient regions.

Location

Global.

Time Period

1800–2020.

Major Taxa Studied

Freshwater fishes.

Methods

We analysed a global dataset on freshwater fish introductions (4241 events of 688 species). Freshwater fish flows were investigated with flow diagrams and χ² tests, while PERMANOVA (permutational multivariate analysis of variance) was used to test the association between species and regions and temporal shifts. Cluster analysis revealed major recipient areas and composition of the introduced species. Finally, changes in distances between donor and recipient sites were tested with PERMANOVA.

Results

The number of introductions between biogeographic regions mirrored the European and North American dominance before World War II (WWII) and the trends in recreational fishing, biocontrol programmes and food production, especially in the Sino-Oriental region, which has a long tradition of aquaculture and fishkeeping. Over the years, the origins and composition of introduced species changed uniquely in each biogeographic region, although the most introduced species are common to every region. Salmonids and other cold-water species were frequently introduced before the 1950s, whereas tropical ornamental and aquaculture species currently prevail. Distances between donor and recipient sites did not vary over the time. After WWII, the Sino-Oriental region consolidated its dominance and the Ethiopian and Neotropical regions emerged as new global donor and recipient regions.

Main Conclusions

Global policy should focus on tropical ornamental and aquaculture species, which could benefit from global warming, especially in the Sino-Oriental region, because it currently dominates freshwater fish species flows, and the Ethiopian and Neotropical regions, because they recently emerged as important global donor and recipient regions of freshwater fish introductions.     

Spatial distribution and seasonality of ichthyoplankton and anthropogenic debris in a river delta in the Caribbean Sea

Temporal changes in larval fish species composition and abundance compared with other components of the seston are described in four estuarine habitats in the Atrato Delta, Colombia. In comparison with zooplankton, fish larvae and egg density and anthropogenic debris abundance were low in the South Atrato Delta. Transparency, water temperature and chlorophyll a were the major factors influencing the spatiotemporal distribution of ichthyoplankton in the delta. The most abundant fish larvae were Astyanax sp. 1, Anchovia clupeoides, Cetengraulis edentulus, Anchoa sp., Bathygbius curacao, Dormitator maculatus, Hyporhamphus sp., Atherinella blackburni, Gobiosoma sp. 1 and Menticirrhus americanus (92·8% of total abundance). Spatial temporal analysis shows that in this delta, shrub (arracachal) and grass (eneal) habitats are important for freshwater and estuarine species, whilst mudflat and mangrove are important for estuarine species and estuarine–marine species, since most flexion and post‐flexion stages of these species were found there. Anthropogenic debris density never surpassed the total ichthyoplankton density, but was ubiquitous. Shrub and mangrove habitats had higher densities of anthropogenic debris, since these are flood‐stem habitats that trap solids.     

Fishes and salinities in the St Lucia estuarine system—a review

The recorded salinity ranges of freshwater, estuarine and marine fish species in Lake St Lucia, a Ramsar and World Heritage Site, are documented. The freshwater group is most diverse and abundant under oligohaline conditions, although the Mozambique tilapia (Oreochromis mossambicus) was common under all salinity regimes. Estuary resident species also favoured oligohaline conditions but, in contrast to the freshwater taxa, were well represented in salinities up to 40 ‰. The marine group was most diverse and abundant within the salinity range 10–40 ‰, but a large number of species could also be found in salinities up to 70 ‰. Very few fish species were able to tolerate salinities between 70 ‰ and 110 ‰, with only O. mossambicus surviving for extended periods in salinities above 110 ‰. All the aquatic macrophytes and most of the zoobenthos within the lake appear to die out within the salinity range of 50–60 ‰, thus creating additional stress to those fish present under such conditions. The food resources least affected by extreme hypersalinity are the microphytobenthos and detritus food chains, with detritivorous fishes being dominant when the lake is in this state. Mass mortalities of fishes in Lake St Lucia have been recorded under both low (<5 ‰) and high salinity (>70 ‰) conditions. The fish kills are often triggered by exceptionally low or high water temperatures which affect the osmoregulatory abilities of these species. Hypersaline conditions and fish mortalities under the most recent closed estuary mouth conditions (2002–2005) are reviewed. If the surface area of St Lucia (35,000 ha) is compared to the total surface area of all South African estuaries (approximately 70,000 ha), then the possibility exists that the loss of the Lake St Lucia nursery area for estuary-associated marine fish species over the past few years may cause significant short-term declines in the future abundance of these taxa on both a local and regional scale.     

Assessment of a vertical slot fishway in south‐eastern Australia designed to pass numerous species and size classes of fish

In the Barwon River, Australia, a tidal barrage formed a major impediment to fish movement so in 2013 a vertical slot fishway was installed. The assessment of fishways on tidal barriers is rare in Australia so to ensure the fishway was achieving its ecological objective (i.e. successfully passing the target size range of fish of 20–400 mm total length), fish were trapped at the entrance and exit on 12 occasions and the species composition, abundance and length of fish at the two locations were compared. Additionally, a section of the river downstream of the fishway was sampled to ensure fishway trapping accurately reflected the species composition wanting to use the fishway to move upstream. Eighteen species and 69,246 individual fish were caught in the fishway traps. Catch rates between locations did not differ for Common Galaxias (Galaxias maculatus) or Australian Smelt (Retropinna semoni), although species‐specific catch rates were lower at the exit for Flat‐headed Gudgeon (Philypnodon grandiceps), Tupong (Pseudaphritis urvillii) and Yellow‐eye Mullet (Aldrichetta forsteri). Length distribution between locations only differed for Australian Smelt with small fish under‐represented at the exit location (<25 mm total length). Eight species of fish were collected downstream of the fishway that were not collected in it; however, all of these were estuarine dependent except the non‐native Common Carp (Cyprinus carpio). Our results indicate that vertical slot fishways are a suitable design for improving river connectivity at a low head, tidal barrages in south‐eastern Australia. The study reiterates the importance of reinstating connectivity for species with obligate marine/freshwater migratory life history traits, and the indirect benefits of increased productivity made available to upstream areas.     

Distribution and abundance of fishes in the San Francisco Bay estuary between 1980 and 1982

In 1980 a long-term study of the fishery resources of the San Francisco Bay estuary was initiated in an effort to delineate the importance of freshwater inflow to fish and invertebrate abundance and distribution in the bay. An analysis of the trawl data collected between January 1980 and December 1982 illustrates the influence of the timing and magnitude of freshwater inflows on fish fistribution and abundance in this estuary from the perspective of monthly, seasonal and annual time scales. Normally found in the delta, Suisun Bay and San Pablo Bay during periods of increased salinity, pelagic species moved downstream after the two peak flows studied, while demersal species usually found in Central San Francisco Bay moved upstream. Such upstream movements may be due in part to transport by strong density-driven currents.Timing and magnitude of monthly catches of some species varied on a seasonal cycle coincident with variations of freshwater inflow. Most species, especially the marine species, showed no consistent cycle of monthly catches. In the wet years of 1980 and 1982 the distributions of freshwater, estuarine and anadromous species were extended downstream into San Pablo, Central and South San Francisco Bays and some marine species, including the flatfish, were more abundant in the upstream areas. In the dry year of 1981 when bay salinities were higher, few marine species extended their distributions upstream into San Pablo and Suisun Bays. Jacksmelt was the only fish of the 15 most abundant species with its peak abundance in 1981. Most marine species were more abundant in the San Francisco Bay estuary in the wet years.     

Fish Collections Taken from a Small Agricultural Water Withdrawal Site on the Groot River,Gamtoos River System,South Africa

Summary

Periodic water releases from Beervlei Dam on the Groot River of the Gamtoos River System were planned to flush the riverine pools of brack water and replace the pools with better quality water which was used to flood irrigate lucerne lands. Fish collections were made in a small irrigation canal situated on the banks of the Groot River. Three fish species were collected. Results indicated that up to a total of 131 smallscale redfin minnows, Pseudobarbus asper, are removed from the system every five minutes at this one small water withdrawal site. Larger minnow specimens and carp, Cyprinus carpio, were entrained during high river flows. When the river flow was reduced there were higher catches of young of the year fish. The fish are carried with the pumped water, which is flood irrigated onto lucerne fields, resulting in stranding and eventual mortality. It is recommended that the use of suitable screens, such as wedge-wire screens, could reduce the number of fish removed from the system during these irrigation periods.     

Whole body‐element composition of Atlantic salmon Salmo salar influenced by migration direction and life stage in three distinct populations

Body‐element content was measured for three life stages of wild Atlantic salmon Salmo salar from three distinct Newfoundland populations as individuals crossed between freshwater and marine ecosystems. Life stage explained most of the variation in observed body‐element concentration whereas river of capture explained very little variation. Element composition of downstream migrating post‐spawn adults (i.e. kelts) and juvenile smolts were similar and the composition of these two life stages strongly differed from adults migrating upstream to spawn. Low variation within life stages and across populations suggests that S. salar may exert rheostatic control of their body‐element composition. Additionally, observed differences in trace element concentration between adults and other life stages were probably driven by the high carbon concentration in adults because abundant elements, such as carbon, can strongly influence the observed concentrations of less abundant elements. Thus, understanding variation among individuals in trace elements composition requires the measurement of more abundant elements. Changes in element concentration with ontogeny have important consequences the role of fishes in ecosystem nutrient cycling and should receive further attention.     

Feeding ecology of fish in a coastal river of the Atlantic Rain Forest

Feeding ecology of fish assemblage in a costal river of the Atlantic Rain Forest in Southern Brazil was investigated. Monthly samples using different fishing techniques were completed on a section of the river characterized as a freshwater environment, and an adjacent tide-influenced estuarine section. Species were grouped into three major Feeding Mode Functional Groups: Zoobenthivore, Omnivore and Detritivore. The river section with the greatest occurrence was the site where the most abundant species are likely to feed most intensively. Freshwater strangler species (Cyphocharax santacatarinae - detritivorous, Rhamdia quelen and Oligosarcus hepsetus - zoobenthivorous) presented higher stomach repletion in the freshwater environment while marine migrants (Genidens genidens and Etropus crossotus - zoobenthivorous) and estuarine residents (Bairdiella ronchus - zoobenthivorous) presented higher repletion in the tide-influenced estuarine stretch. Use of food resources was influenced by spatial and temporal variations linked to environmental characteristics such as salinity, habitat heterogeneity, life history evolution, feeding specialization and resource partitioning. Hypotheses concerning potential keystone species, ecosystem engineers or strong interactors were also discussed.     

Distribution of fishes in the Río Guayalejo-Río Tamesí system and relationships with environmental factors in northeastern Mexico

A priority issue in ecology and biogeography is understanding the patterns in species diversity and the causal factors of their distribution, which allows the generation of information for conservation strategies. The longitudinal distribution of fishes and their relationships with environmental variables were studied in the Guayalejo-Río Tamesí system (northeastern Mexico) from February 2000 to July 2001. A total of 5918 fish were caught in 27 collections along an altitudinal gradient in the main river course, from high mountain (1500 masl) to coastal plain near Tampico. Forty-three native and five exotic species, belonging to 35 genera in 23 families, were identified. Cluster analyses identified four major fish habitats in the river system. A distinctive euryhaline marine fish habitat (1) occurs near the mouth with native and two exotic species. Two other habitats consist essentially of freshwater fish species that are distributed along the longitudinal gradient. One of these habitats (habitat 4) shows greater diversity, as per the Shannon index value, and also includes amphidromous fish, in addition to two exotic freshwater fish; the other (habitat 2) includes freshwater, euryhaline and three exotic species. The changes in the frequency of occurrence and the abundance of Gambusia vittata, Astyanax mexicanus, and Xiphophorus variatus contribute to explaining differences between these habitats. Another habitat (3) is represented by two sampling sites located near the mouth and consist of freshwater and euryhaline fish and three exotic cyprinids with broad salinity tolerance. The low abundance and richness of exotic species suggest little impact on native fish fauna in this river. The fish assemblage of the Guayalejo-Tamesí river system species changes along a longitudinal gradient with the addition, replacement and presence of indicator species. Upstream fish fauna is mostly composed of freshwater species, some of them generalists that inhabit the entire longitudinal gradient, others that are restricted to certain sites, and the remainder of species is an assemblage composed of a mixture of euryhaline freshwater and marine species near the mouth.     

Effects of past and projected river discharge variability on freshwater production in an anadromous fish

相似文献   

Comparative population genetics of <Emphasis Type="Italic">Basilichthys microlepidotus</Emphasis> (Atheriniformes: Atherinopsidae) and <Emphasis Type="Italic">Trichomycterus areolatus</Emphasis> (Siluriformes: Trichomycteridae) in north central Chile

To describe comparative population genetic structure of the Chilean silverside Basilichthys microlepidotus and the catfish Trichomycterus areolatus, four rivers and three sites within each river were investigated by the analysis of haplotype polymorphisms of the mitochondrial Control Region. For both species, analyses revealed significant differentiation among rivers and low differences within rivers. However, the species differ in haplotype composition; individuals of B. microlepidotus shared some haplotypes in all four rivers, while individuals of T. areolatus showed a different haplotype composition in most rivers. This difference may be explained by the different ecological features of the species. Assuming that both silversides and catfish were present before the separation of the rivers, B. microlepidotus migrated after river isolation, probably using coastal water, while T. areolatus has probably never migrated between these rivers. The long times that the studied rivers have been separated should be taken into account in future conservation plans for the freshwater fish of Chile.