A COMPARISON OF TWO ELECTROFISHING PROGRAMMES USED TO MONITOR FISH ON THE ILLINOIS RIVER,ILLINOIS, USA

The Illinois River Biological Station monitors fish communities in the Illinois River using two different electrofishing programmes, one using three‐phase alternating current [AC; The Long Term Illinois Fish Population Monitoring Program or long‐term electrofishing (LTEF)] and the other pulsed direct current [DC; The Long Term Resource Monitoring Program (LTRMP)]. In 2001, we replicated three‐phase AC sampling sites with pulsed‐DC electrofishing gear to test for differences between the two collection methods and programmes. Electrofishing runs at each site were standardized by length and time, with 48 samples collected for each gear. Our objective was to test for differences in fish catch rates using total catch, species richness, fish size ranges, and sample composition and structure. Total catch was significantly greater for LTRMP electrofishing (4368 total fish, mean = 91.0 fish per sample) compared with LTEF electrofishing (1423 total fish, mean = 29.6 fish per sample). Species richness also was significantly greater for LTRMP electrofishing (50 total species, mean = 12.9 species per site) compared with LTEF electrofishing (38 total species, mean = 7.9 species per site). Size ranges of fishes, composed of 100‐mm‐length groups, showed higher total catches for the LTRMP within all length groups. Although our analyses suggest that collections from the LTRMP were significantly greater in most instances, a consistent pattern of species composition between the two programmes was not evident. Our results suggest that caution must be taken when attempting to compare fish community composition and structure information between these programmes. Copyright © 2011 John Wiley & Sons, Ltd.     

DIEL DIFFERENCES IN 0+ FISH SAMPLES: EFFECT OF RIVER SIZE AND HABITAT

Studies of diel patterns in 0+ fish assemblages in rivers are scarce and produce contradictory results. We compared day and night electrofishing samples at 29 sites on rivers of different sizes in the Czech Republic. Diel patterns of 0+ fish differed among sites, but were consistent within each river stretch. River size was negatively correlated with both the day : night ratio of abundance (day samples produced more 0+ fish in smaller streams, night samples produced more 0+ fish in larger rivers) and similarity between day and night assemblage. A higher day : night ratio of abundance and lower similarity between day and night samples were observed at shelter‐lacking habitats (beach) compared with shelter‐providing habitats (boulder bank and flooded terrestrial vegetation). Differences in 0+ fish size were not dependant on diel period, river size, habitat or species. Day‐only or night‐only samples provided only two‐third of the information provided by a combination of day and night samples. The effect of habitat type and river size on diel differences in electrofishing catches should be considered when deciding on 0+ fish sampling protocol. Copyright © 2011 John Wiley & Sons, Ltd.     

A Comparison of Electrofishing and Visual Surveying Methods for Estimating Fish Community Structure in Temperate Rivers

Studies attempting to describe fish community structure in shallow riverine environments typically rely on electrofishing and/or visual (snorkelling) surveys, but few have addressed the relative efficiencies of these two methods at estimating fish density and biomass across wide ranges of geography, taxonomy and life history stages. Multiple paired electrofishing and visual surveys were conducted in 18 temperate Canadian rivers in order to obtain community‐wide density and biomass estimates from both methods. Partial canonical multivariate analyses were applied to the paired fish community matrices comparing the results of both surveying methods at the taxonomic levels of family, genus and species, as well as size classes within families and species, to assess the particular effectiveness of each sampling method. Although electrofishing estimates of family and species richness were generally greater, snorkelling surveys tended to generate higher density and biomass estimates for different size classes of many salmonid and cyprinid species. Moreover, mean river biomass estimates derived from visual surveying matched those obtained from our best mean river biomass estimates arising from the two methods combined. This study provides empirical evidence that electrofishing and visual survey methods generate different types of information when assessing fish community structure at the family level or by size classes. Our results provide ample background information for determining the most accurate sampling method for a particular fish community assemblage, which is fundamental to fisheries management and research. Copyright © 2014 John Wiley & Sons, Ltd.     

Relationship between river surface level and fish assemblage in the Ocklawaha River,Florida

Understanding the relationship between river flows and levels and fish communities is important to setting minimum flow and level regulations designed to prevent ecological harm. We reviewed a long‐term Florida Fish and Wildlife Conservation Commission data set from the Ocklawaha River, Florida, to evaluate trends in fish abundance and fish communities relative to historical river levels. Fish data were collected using electrofishing at the Ocklawaha River, Florida, during 1983–1994. Electrofishing data were used to estimate catch per minute and biomass per minute of selected species, fish diversity, richness and evenness. Catch rate variables were compared among years and significant differences in abundance and community indices were related to historical water levels using multiple regression models. Results indicated that differences in abundance and community indices among years were more common at the site with increased river level variability. Regression models indicated that fish abundance and species richness were positively related to river levels, and species diversity was negatively related to variability in river level during the two years prior to sampling. Spotted sunfish Lepomis punctatus exhibited the most variability in species‐specific catch per effort and spotted sunfish abundance was positively related to river levels. Low river levels negatively influenced fish abundance and fish communities, and minimum flows in Florida should manage for the periodicity of low flow events to prevent sequential years of adverse effects on fish populations. Copyright © 2005 John Wiley & Sons, Ltd.     

Fish community structure in natural and engineered habitats in the Kansas River

We investigated fish assemblage structure in engineered (rip‐rap) and natural habitats (log jams and mud banks) in the Kansas River USA to determine if natural structures had higher abundance and diversity of fishes at a local spatial scale. A total of 439 randomly selected sites were boat electrofished from May to August 2005 and 2006. Mean species diversity and richness were significantly higher in rip‐rap than log jams and mud banks. Mean relative abundance (CPUE; number of fish collected per hour electrofishing) of six of the 15 most common fishes (>1% of total catch) were most abundant in rip‐rap, two were most abundant in log jams, and none in mud banks. Rip‐rap had the highest relative abundance of fluvial specialist and macrohabitat generalists, whereas mean CPUE of fluvial dependents was highest in log jams. Although a discriminant function analysis indicated that nine size classes (eight species) discriminated among three habitat types, the high misclassification rate (38%) suggested a high degree of fish assemblage overlap among the habitats. Although previous work has suggested that engineered structures (rip‐rap) and urbanization are linked to reduced biotic diversity or reduced growth of fish species, our results suggest that at a local scale rip‐rap may not have the same negative impacts on fish assemblages. Published in 2009 by John Wiley & Sons, Ltd.     

Studies on the fish catch efficiency of different types of fishing gear in Kaptai Reservoir, Bangladesh

A comprehensive analysis of the efficiency of different fishing gear in capturing Cyprinidae (major carp) species in Kaptai Reservoir, Bangladesh, was conducted. The catch per unit effort (CPUE) data were collected during a 1‐year survey, covering a total of 493 independent fishing units spread across the four major fishing grounds of the reservoir. The study examines the CPUE and catch composition between months and study sites, and the effects of reservoir water level fluctuations, gear diversity and fish distribution patterns on the fish catches. The mean CPUE for all gear ranged between 2.91 and 30.86 kg unit^?1 day^?1. Analysis of variance showed significant differences among the fishing gear types. The CPUE differed significantly between months for lift nets, large‐meshed (LM) gill nets, cluster hooks and hand lines. None of the fishing gear exhibited significant differences between sites except for cluster hooks. The CPUE of small‐meshed (SM) seine nets was significantly higher than that of other gear. The reasons for such differences in the fish catches were identified as the choice of fishing places, the net sizes, the number of hooks, lures and baits, water turbidity, wind, waves and rainfall. The CPUE indices for lift nets showed significant variation between months for the Cyprinidae species. A significant variation in catch at the different sites was found for all nets, except SM seine nets. The contribution of the Cyprinidae species showed significant differences between sites only for hand lines. The coefficient of variation of CPUE for fishing nets did not vary much more than 55% (lift net), whereas the highest variation of 49% was recorded for hand lines among the hooks and line. Higher reservoir water levels were followed by reduced CPUEs for the Cyprinidae species and vice versa, whereas the highest water levels corresponded to the higher CPUE for the Clupeidae species.     

Evaluation of artificial PVC pipe structures as fish habitat in Spring Lake,Western Illinois,USA

Three types of artificial fish habitat constructed of polyvinyl chloride (PVC) pipe were placed at three sites in Spring Lake, a drinking water reservoir in McDonough County, Illinois. Direct current (DC) electrofishing was used on a monthly basis from May through October 2008 at each site, and a corresponding control area. The fish catch per unit effort (CPUE) and mean size of fish captured were compared between the artificial structures and the control areas. The CPUE and mean size of fish captured also were compared to the number of ‘branches’ on the artificial habitat. The fish catch rates for the various structures were evaluated among the three sites on the lake. The mean CPUE was significantly higher for the PVC structures, and compared to control sites, for bluegill (Lepomis macrochirus), white crappie (Pomoxis annularis) and largemouth bass (Micropterus salmoides). This was not performed for gizzard shad (Dorosoma cepedianum). No significant differences were detected in mean size for all four species between the PVC and control sites. A linear regression indicated no significant relationship between the number or size of fish, and the number of branches on the PVC structures. Bluegills used the PVC structures significantly more in the dam area than in other areas of the lake. In contrast, gizzard shad used the PVC structures significantly more in the west end of the lake. These results suggest that PVC structures, which can be constructed of common, inexpensive materials, can be effective in attracting fish in drinking water reservoirs. Another advantage of the PVC structures is that they are non‐organic and, therefore, will not degrade water quality.     

Catch per unit effort estimation and factors influencing it from recreational angling of sockeye salmon (Oncorhynchus nerka) and management implications for Lake Toya,Japan

The present study examined the factors influencing catch per unit effort (CPUE) and standardized the CPUE of sockeye salmon (Oncorhynchus nerka) from offshore angling in Lake Toya, northern Japan. A generalized linear model (GLM), based on a negative binomial error distribution, was used to standardize the catch‐and‐effort data collected from anglers, using questionnaires and interview surveys during the fishing season (June) in 1998, 1999 and 2001–2012. Year, week, fishing area, number of fishing rods, fishing duration and Year × Week were the factors that significantly (P < 0.05) influenced CPUE. Anglers' fishing experience had no significant effect (P = 0.06) on CPUE. Limiting fishing duration, number of anglers and fishing rods may reduce fishing pressure and ensure sustainable management of the fishery. The results of the present study on standardized CPUE can also be useful in fine‐tuning age‐based models, such as Virtual Population Analysis (VPA–ADAPT), for the fish species in the lake, studies that are currently lacking. Regular and interdisciplinary studies that include biophysical factors are required to provide more understanding of the variations in the abundance of the fish species in the lake and in the ecosystem at large.     

Catch per Unit Effort and yields as indicators of exploited fish communities: application to two West African reservoirs

Two indicators of fish exploitation, catch per unit effort (CPUE) and yield per unit area, were used with surveys of small‐scale fishery landings to compare two African reservoirs that differ in their level of fish exploitation, being high for Selingue Reservoir and low for Manantali Reservoir, both located in West Africa. The heavy exploitation in Selingue Reservoir induced a decline in catches per fishing trip (9.5 kg trip^?1), whereas the catch remained high at Manantali Reservoir (17.0 kg trip^?1). Moreover, the annual fish yield per unit area was lower for Manantali Reservoir (19 kg ha^?1) than for Selingue Reservoir (79 kg ha^?1). There was no relationship between the Morphoedaphic Index and catches for the two reservoirs. Rather, only the fishing effort and annual water‐level variations defined the reservoir fish yields. Based on the results of the present study, it appears that CPUE and fish yield per unit area represent simple and robust indicators for describing the impacts of fishing intensity in aquatic ecosystems.     

Two‐dimensional modelling of habitat suitability as a function of discharge on two Colorado rivers

The quantity of water that should be retained in streams and rivers for the benefit of fish during periods of water scarcity is a question of considerable interest to river managers and biologists. Although instream flow methodologies have existed since the 1970s, no single method has been widely accepted for use on large warm‐water rivers because of their high species richness and generalized fish habitat use patterns. In this paper, we present an approach similar to instream flow incremental methodology, but which uses two‐dimensional flow models and biomass estimates derived from multiple sites on two Colorado rivers for predicting the effect of discharge on adult standing stocks of two native fish species. Suitability criteria are developed for bluehead and flannelmouth sucker (Catostomus discobolus and C. latipinnis) by comparing adult biomass in individual meso‐habitat units with modelled depths and velocities. We find that roundtail chub (Gila robusta) biomass is not correlated with depth and velocity, but appears to be positively associated with indices of habitat heterogeneity. Species biomass and total usable habitat area are predicted as a function of discharge for each site and data show good correlation between predicted and measured biomass. Results suggest that the Colorado and Yampa Rivers have similar potential for native fish biomass, but low summer discharges limit native fish biomass on the Yampa River. Copyright © 2005 John Wiley & Sons, Ltd.     

Optimizing a Standardized Electrofishing Sampling Protocol for Warmwater Resident Sportfish in the Tallapoosa River,Alabama, USA

A two‐year electrofishing study was initiated in the Tallapoosa River, Alabama, to identify an optimal standardized sampling program for three principal resident sportfish: Alabama bass Micropterus henshalli, redbreast sunfish Lepomis auritus, and redeye bass Micropterus coosae. Samples were conducted in spring (May), summer (July), and fall (October) in 2010 and 2011 from seven 1‐h transects. Spring samples of Alabama bass had lower catch per effort (CPE) and were more skewed towards fish between 200 and 300 mm total length (TL) than samples in other seasons; whereas, fall samples collected more redeye bass >200 mm TL but CPE was similar among seasons. Fewer, but larger, redbreast sunfish were sampled during fall compared with other seasons. Mean CPE of all three species was independent of transect duration. The total time spent electrofishing and processing fish in order to estimate a mean CPE with a specified precision was a function of transect duration and CPE. More effort was needed as CPE decreased for most species, but the relations between transect duration and total effort were parabolic. A precision of within 10% of the mean CPE was unattainable for most species as a result of logistic considerations. Based on the results of this study, it appears that fall is the optimal sampling time for these species in the Tallapoosa River and the optimal transect duration is likely 10 min. At a precision level of 20% of the mean, the number of 10‐min transects required ranged from 5 to 40, with a total sample time for each individual species of 0.82–7.16 h. Copyright © 2014 John Wiley & Sons, Ltd.     

Effects of dam removal on brook trout in a Wisconsin stream

Dams create barriers to fish migration and dispersal in drainage basins, and the removal of dams is often viewed as a means of increasing habitat availability and restoring migratory routes of several fish species. However, these barriers can also isolate and protect native taxa from aggressive downstream invaders. We examined fish community composition two years prior to and two years after the removal of a pair of low‐head dams from Boulder Creek, Wisconsin, U.S.A. in 2003 to determine if removal of these potential barriers affected the resident population of native brook trout (Salvelinus fontinalis). Despite the presence of other taxa in the downstream reaches, and in other similar streams adjacent to the Boulder Creek (including the brown trout, Salmo trutta), no new species had colonized the Boulder Creek in the two years following dam removal. The adults catch per unit effort (CPUE) was lower and the young‐of‐the‐year catch per unit effort (YOY CPUE) was higher in 2005 than in 2001 in all reaches, but the magnitude of these changes was substantially larger in the two dam‐affected sample reaches relative to an upstream reference reach, indicating a localized effect of the removal. Total length of the adults and the YOY and the adult body condition did not vary between years or among reaches. Thus, despite changes in numbers of adults and the YOYs in some sections of the stream, the lack of new fish species invading Boulder Creek and the limited extent of population change in brook trout indicate that dam removal had a minor effect on these native salmonids in the first two years of the post‐removal. Copyright © 2007 John Wiley & Sons, Ltd.     

Temporal distribution and taxonomic composition differences of larval fish in a large navigable river: A comparison of artificial dike structures and natural habitat

Distributions of larval fish in a regulated river were determined by sampling 15 sites in 2003 using larval fish light traps. Typical of large river systems in the Mississippi River drainage, collections in the Kanawha River were dominated (>93%) by Cyprinidae and Percidae species. Catch per unit effort (CPUE) along artificial dike structures were higher than CPUE along other shoreline sites, indicating the importance of these structures as nurseries. Taxonomic richness within artificial dike structures was greater than or equal to normal shoreline reference sites. Cyprinidae (Pimphales type and Notropis type) were collected significantly more on artificial structures than reference sites (p < 0.01). We collected higher numbers and observed greater CPUE of some sport fishes (Percidae) at the artificial dike structures. Percidae CPUE was significantly higher (p < 0.01) on artificial dike structures than reference sites. Percidae distribution appears to be positively influenced by the introduction of artificial dike structures in comparison to other abundant fish groups. Artificial dike structures encouraged taxonomic and larval fish diversity, and positively influenced some game fish species. The results of this study suggest that artificial dike structures are habitats that increase larval fish diversity in large, navigable rivers. Copyright © 2010 John Wiley & Sons, Ltd.     

Larval fish catches in the lower Milk River,Montana in relation to timing and magnitude of spring discharge

Larval fishes were sampled in the Milk River, Missouri River drainage, Montana from May to August 2002, 2003 and 2004 to describe temporal spawning distribution in relation to spring discharge. Total larval catch‐per‐unit‐effort (CPUE) in 2002 (28.9 fish/100 m³) was an estimated 29 times greater than in 2003 (0.99 fish/100 m³) and 16 times greater than in 2004 (1.78 fish/100 m³). In 2003 and 2004 more than one third of the total catch occurred before 12 June whereas in 2002, only 5% of the total catch occurred before 12 June. Marked differences in larval species composition were also observed between years, suggesting that a later peak in discharge may benefit some species and an earlier peak others. In 2002, when flows peaked later (at 77 m³ s^?1), common carp Cyprinus carpio represented 37% of the total larval catch. Common carp were proportionally less abundant in 2003 (7.2%) and 2004 (1.4%) than in 2002. In 2004, when flows peaked (at 163 m³ s^?1) 32 days earlier than in 2002 but only 15 days earlier than in 2003 (at 73 m³ s^?1), shorthead redhorse Moxostoma macrolepidotum and suckers Catostomus sp. were the numerically dominant taxa. These results indicate that the timing, not necessarily the magnitude, of peak spring discharge may influence spawning success in the lower Milk River, as indicated by larval fish catches. Copyright © 2008 John Wiley & Sons, Ltd.     

Comparison of a Habitat Productivity Index (HPI) and an Index of Biotic Integrity (IBI) for Measuring the Productive Capacity of Fish Habitat in Nearshore Areas of the Great Lakes

A Habitat Productivity Index (HPI) and an Index of Biotic Integrity (IBI) were compared as measures of habitat productive capacity for fish assemblages in nearshore areas of Lake Erie and Lake Ontario. Forty-three species of fishes were captured by boat electrofishing at three areas with contrasting habitats—coastal wetlands, harbor breakwalls, and exposed shorelines. HPI and IBI were correlated among samples as expected, but HPI was most closely correlated with fish community biomass, whereas IBI was correlated with fish species richness. The HPI and IBI indices differed significantly among samples from the three habitat areas in both lakes, reflecting the differences in the abundance and composition of fish catches. The ranking of habitat productive capacity depended on the index: species richness and IBI were highest at the coastal wetlands, and biomass and HPI were highest at the harbors. Results support the contention that to effectively determine habitat productive capacity, both the production and diversity characteristics of the fish community need to be evaluated.     

Fish species richness is a key predictor of community biomass and productivity in littoral regions of Severn Sound,an embayment of southern Georgian Bay,Lake Huron

Catches from standardized littoral electrofishing (EF) surveys (1990–2016) at various locales in Severn Sound, Georgian Bay, were analyzed. Catches were adjusted for size-selectivity using published results of laboratory immobilization experiments. Adjusted abundance-mean size relationships were consistent with the metabolic theory of ecology where body size has a central role in structuring community features. Relationships among species richness and areal estimates of biomass and annual production of the adjusted catches were assessed. Richness was the key variable driving other community metrics and was, in turn, largely driven by habitat fetch metrics and sampling time of day. Richness, biomass, and production decreased with increasing maximum effective fetch. Richness increased with time after sunset. Year and locale had smaller roles in variation of community metrics with more vegetated/ less urbanized areas having higher values. Higher fish production is expected in sheltered areas where the contributions of allochthonous and benthic primary production are expected to be greater. Further analyses using additional EF datasets from Great Lakes’ littoral zones should increase understanding of the differences between littoral and offshore fish productivity and help guide management of littoral habitats to ensure healthy fish communities.     

Performance of a Vertical‐Slot Fish Pass for the Sea Lamprey Petromyzon marinus L. and Habitat Recolonization

In 2011, a vertical‐slot fish pass was built at the Coimbra Açude‐Ponte dam (Mondego River, Portugal), approximately 45 km upstream from the river mouth. The performance of this infrastructure for sea lamprey passage was evaluated between 2011 and 2015 using several complementary methodologies, namely radio telemetry [conventional and electromyogram (EMG)], passive integrated transponder (PIT) telemetry and electrofishing surveys. During the study period, the electrofishing revealed a 29‐fold increase in the abundance of larval sea lamprey upstream of the fish pass. Of the 20 radio‐tagged individuals released downstream from the dam, 33% managed to find and successfully surpass the obstacle in less than 2 weeks, reaching the spawning areas located in the upstream stretch of the main river and in one important tributary. Fish pass efficiency was assessed with a PIT antenna installed in the last upstream pool and revealed a 31% efficiency, with differences between and within migratory seasons. Time of day and river flow significantly influenced the attraction efficiency of the fish pass, with lampreys negotiating it mainly during the night period and when discharge was below 50 m³ s^?1. Sea lampreys tagged with EMG transmitters took 3 h to negotiate the fish pass, during which high muscular effort was only registered during passage, or passage attempts, of the vertical slots. The use of complementary methodologies provided a comprehensive passage evaluation for sea lamprey, a species for which there is a considerable paucity of valuable data concerning behavioural, physiological and environmental influences on obstacle negotiation. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.     

Diversity and Community Structure of Fishes in the Regulated Versus Unregulated Tributaries of the Mekong River

Diversity and community structure of fishes were studied in three neighbouring tributaries of the Mekong River in Thailand, namely the Mun, Songkhram and Gam Rivers. The rivers are located in the same ecoregion but have contrasting levels of both hydrological regulations and mitigation measures; the Mun River has a hydropower dam with a fish ladder and sluice gates that are opened during the wet season each year, the Gam River has several irrigation dams with a fish ladder at each dam site, and the Songkhram River has no dams along its river course. A total of 124 freshwater fish species were sampled in these rivers from August 2009 to June 2010. Overall species richness was highest in the Songkhram River (112), followed by the Mun (97) and Gam (54) Rivers. Average per site species richness was also significantly different among rivers but not among sampling months. Abundance–biomass comparison plots revealed considerably overlapping distributions of these two metrics from the dry to early rainy seasons in the Songkhram River and, to a lesser extent, in the Mun River. Fish assemblage data were classified into six clusters with similar community structure. Fish assemblages in the Gam River constituted a single cluster, while those in the other two rivers formed multiple clusters depending on the sampling season. The results of the cluster analysis are discussed in relation to the dominance of the three migration guilds (white, black, and grey fishes) of the Mekong River fishes. The effectiveness of the mitigation measures was determined to be limited in alleviating adverse impacts of dams in these tributaries of the Mekong River. Copyright © 2014 John Wiley & Sons, Ltd.     

Variations in fish assemblages in a tributary of the upper Paraná River,Brazil: A comparison between pre and post‐closure phases of dams

This paper purposed to evaluate alterations in fish assemblage (structure and species richness) and variations in the abundance of the most abundant species captured in the Piquiri River, due to the closure of Porto Primavera (in 1998; located upstream the Piquiri River mouth) and the Itaipu (in 1982; located downstream) dams in the upper Paraná River. The effects of selected environmental variables were also determined. Fishes were sampled during four distinct periods. Six samplings were conducted in each period, at two different sites (Campina and Apertado). For sampling, gill and trammel nets were used. Species abundances were indexed by catch per unit of effort. Spatial and temporal changes in environmental variables were summarized by a principal components analysis (PCA). To summarize structure of the assemblage, a nonmetric multidimensional scaling (NMS) was used. To test the significance of between‐group differences at each periods and sites we used a multiresponse permutation procedure (MRPP). To test the association between fish assemblage and the environmental variables matrices, we used procrustean superimposition approach. A total of 2693 individuals were caught, belonging to 69 species, 4 orders and 19 families. Species richness differed spatially; the Campina site had greater mean richness (13) per sampling than the Apertado site (10). The structure of the fish assemblages showed significant temporal and spatial variations, which were more evident in the last period at the Apertado site (after the closure of Porto Primavera Dam). Spatial and temporal changes in structure of fish assemblages were not associated to environmental variables. Temporal changes in abundance, both in number and in weight, of the most abundant species were verified in both sites. The lack of migratory species was highlighted, which sharply decreased in the last studied period, when they were replaced by sedentary species. These findings appear to be related to regional factors, such as the decrease of floods registered in the upper Paraná basin, exacerbated by the control prompted by dams. Copyright © 2009 John Wiley & Sons, Ltd.     

The last snapshot of natural pelagic fish assemblage in Lake Turkana,Kenya: A hydroacoustic study

A hydroacoustic survey and supplementary gillnet investigation were carried out in the open water of the central part of Lake Turkana in September 2009. Overall acoustic fish density and biomass were assessed as 1381 ind./ha and 30 kg/ha, respectively. The fish density estimate was lower than the results from two previous investigations in the 1970s and 1980s (long-term average 3739 ind./ha), but the biomass remained relatively unchanged (long-term average 25.4 kg/ha). A decreasing gradient in pelagic fish density from the western to eastern shore of the lake was observed. Fish were distributed unevenly within the water column. During the day, a majority of fish aggregated in the mid-water layers (10–12 m below the water surface), creating on echograms the so-called Midwater Scattering Layer. This feature dissipated completely during dusk and the majority of fish occurred in the surface layers at night. These diel vertical fish migrations influenced day and night hydroacoustic estimates of the total fish abundance. Synodontis spp., Lates spp. and Schilbe uranoscopus dominated the catch of both mid-water and bottom gillnets installed in open water areas. Hydrocynus forskalii and Brycinus spp. contributed significantly to the catch of mid-water gillnets while Bagrus bayad and the endemic Haplochromis macconneli occurred only in the catch of the bottom gillnets.