Reservoir Fish Research Articles

Fish invading deserts: non‐native species in arid Moroccan rivers

Abstract Desert rivers are highly dynamic systems in which dry periods, frequently involving the cessation of superficial flow, alternate with violent flash floods. In spite of their territorial importance and high functional and biological diversity, desert rivers have received little attention in the scientific literature, especially in Northern Africa. This study investigated the distribution of non‐native fish, their relationship with river damming and their potential impacts on native biodiversity in the main river basins draining arid areas of Morocco (Oued Draa, Oued Ziz and Oued Ghir), based on field sampling covering 84 stream reaches. Thirteen fish species were recorded, eight of which are non‐native. Two species (stone moroko, Pseudorasbora parva, and bleak, Alburnus alburnus) had not been previously recorded in Morocco, while the native sandsmelt (Atherina boyeri) had not been cited as an invader. Pumpkinseed (Lepomis gibbosus) was the most widely distributed non‐native fish. Non‐native fish species were associated with reservoirs, colonizing the regulated downstream reaches. In contrast, unregulated upstream reaches tended to be free of non‐natives. The low abundance of native fish in reservoirs seems to be caused by the impacts of introduced fish species. Management options for environmental flow are limited, owing to reduced water availability. Non‐native fish management should thus focus on limiting the spread of species already introduced and avoiding new introductions, especially into reservoirs. Copyright © 2014 John Wiley & Sons, Ltd.

계절적 몬순에 의한 댐 인공호 및 농업용 저수지에서의 영양상태지수(TSI), 경험적 수질 모델 및 어류 트로픽 구조

The key objective of this study was to evaluate trophic state and empirical water quality models along with analysis of fish trophic guilds in relation to water chemistry (N, P). Trophic state index (TSI), based on total phosphorus (TP) and chlorophyll-a (CHL), ranged between oligotrophic and hypereutrophic state, by the criteria of Nurnberg(1996), and was lower than the trophic state of total nitrogen (TN). Trophic relations of Secchi depth (SD), TN, TP, and CHL were compared using an empirical models of premonsoon (Pr), monsoon (Mo), and postmonsoon (Po). The model analysis indicated that the variation in water transparency of Secchi depth (SD) was largely accounted (p < 0.001, range of R2 : 0.76.0.80) by TP during the seasons of Mo and Po and that the variation of CHL was accounted (p < 0.001, R2 = 0.70) up to 70% by TP during the Po season. The eutrophication tendency, based on the TSITP vs. TSIN:P were predictable (R2 ranged 0.85.0.90, p < 0.001), slope and y intercept indicated low seasonal variability. In the mean time, TSIN:P vs. TSICHL had a monsoon seasonality in relation to values of TSIN:P during the monsoon season due to a dilution of reservoir waters by strong monsoon rainfall. Trophic compositions of reservoir fish reflected ambient contents of TN, TP, and CHL in the reservoir waters. Thus, the proportions of omnivore fish increased with greater trophic conditions of TP, TN and CHL and the proportions of insectivore fish decreased with greater trophic conditions.

Sources of heavy metals in surface sediments and an ecological risk assessment from two adjacent Plateau reservoirs.

The concentrations of heavy metals (mercury (Hg), cadmium (Cd), lead (Pb), chromium (Cr), copper (Cu) and arsenic (As)) in surface water and sediments were investigated in two adjacent drinking water reservoirs (Hongfeng and Baihua Reservoirs) on the Yunnan-Guizhou Plateau in Southwest China. Possible pollution sources were identified by spatial and statistical analyses. For both reservoirs, Cd was most likely from industrial activities, and As was from lithogenic sources. For the Hongfeng Reservoir, Pb, Cr and Cu might have originated from mixed sources (traffic pollution and residual effect of former industrial practices), and the sources of Hg included the inflows, which were different for the North (industrial activities) and South (lithogenic origin) Lakes, and atmospheric deposition resulting from coal combustion. For the Baihua Reservoir, the Hg, Cr and Cu were primarily derived from industrial activities, and the Pb originated from traffic pollution. The Hg in the Baihua Reservoir might also have been associated with coal combustion pollution. An analysis of ecological risk using sediment quality guidelines showed that there were moderate toxicological risks for sediment-dwelling organisms in both reservoirs, mainly from Hg and Cr. Ecological risk analysis using the Hakanson index suggested that there was a potential moderate to very high ecological risk to humans from fish in both reservoirs, mainly because of elevated levels of Hg and Cd. The upstream Hongfeng Reservoir acts as a buffer, but remains an important source of Cd, Cu and Pb and a moderately important source of Cr, for the downstream Baihua Reservoir. This study provides a replicable method for assessing aquatic ecosystem health in adjacent plateau reservoirs.

DNA barcoding of freshwater fishes and the development of a quantitative qPCR assay for the species-specific detection and quantification of fish larvae from plankton samples.

The barcoding of mitochondrial cytochrome c oxidase subunit 1 (coI) gene was amplified and sequenced from 16 species of freshwater fishes found in Lake Wivenhoe (south-eastern Queensland, Australia) to support monitoring of reservoir fish populations, ecosystem function and water health. In this study, 630-650 bp sequences of the coI barcoding gene from 100 specimens representing 15 genera, 13 families and two subclasses of fishes allowed 14 of the 16 species to be identified and differentiated. The mean ± s.e. Kimura 2 parameter divergence within and between species was 0.52 ± 0.10 and 23.8 ± 2.20% respectively, indicating that barcodes can be used to discriminate most of the fish species accurately. The two terapontids, Amniataba percoides and Leiopotherapon unicolor, however, shared coI DNA sequences and could not be differentiated using this gene. A barcoding database was established and a qPCR assay was developed using coI sequences to identify and quantify proportional abundances of fish species in ichthyoplankton samples from Lake Wivenhoe. These methods provide a viable alternative to the time-consuming process of manually enumerating and identifying ichthyoplankton samples.

Defining a fish bio-assessment tool to monitoring the biological condition of a cascade reservoirs system in tropical area

We studied five tropical reservoirs in the cascade system of the Tietê River (State of São Paulo, Brazil) in order to evaluate the suitability of a fish multimetric index (Reservoir Fish Assemblage Index—RFAI) as a bio-assessment tool. For this purpose, we investigated the spatial and temporal variability of the RFAI with the objectives of testing the effects of the unit measure (catch in numbers or individual weight) of abundance and trophic metrics categories on the index response and, to identify the best sampling season for a bio-assessment program. We located seventy-two sampling sites in three different reservoir zones (fluvial, transition, and lacustrine) and collected samples by habitats – lateral (L), tributary mouth (M) and central (C) – inside each reservoir zone. We recorded the fish assemblages and several physicochemical and habitat variables in the dry and rainy seasons. We tested candidate metrics for range, responsiveness, and redundancy and selected nine for inclusion in the final indices (RFAIN, the index calculated when we expressed fish in number of individuals, per species, and RFAIW, calculated when we expressed fishes in weight, per species). We used a discrete scoring criterion (1,3,5), and the final indices varied from 9 to 45. RFAIN and RFAIW were highly correlated and able to detect a clear spatial variability among the five reservoirs. RFAIN showed a higher temporal variability especially in the smaller reservoirs of the system (Bariri, Ibitinga and Nova Avanhandava), allowing us to consider the dry season as the preferable sampling period for the development of bio-assessment programs. RFAIW appeared to be more responsive than RFAIN, and was in agreement with the environmental gradients observed when analyzing the physicochemical and habitat variables collected in the present study. These findings indicate that the fish multimetric approach is a suitable tool for the assessment of the biological conditions of these reservoirs.

Behavioral attributes of turbine entrainment risk for adult resident fish revealed by acoustic telemetry and state-space modeling

Background: Fish entrainment through turbine intakes is one of the major issues for operators of hydropower facilities because it causes injury and/or mortality and adversely affects population abundance. Entrainment reduction strategies have been developed based on the behavior of downstream migrating fishes, particularly diadromous species. However, knowledge of the behavior of migratory fishes has very limited application for reducing the entrainment of resident fishes, including several species that represent important recreational and aboriginal fishery resources in reservoirs. In this study, we used fine-scale acoustic telemetry and state-space modeling to investigate behavioral attributes associated with entrainment risk of resident adult bull trout (Salvelinus confluentus) in a large hydropower reservoir in British Columbia, Canada. Results: We found that adult bull trout resided longer in the vicinity of the powerhouse and moved closer to the turbine intakes in the fall and particularly in the winter. Bull trout were more likely to engage in exploratory behavior (characteristic of foraging or reduced activity) during periods when their body temperature was lower or higher than 6°C. We also detected diel changes in behavioral attributes, with bull trout distance to intakes and probability of exploratory behavior slightly increasing at night. Conclusions: We hypothesize that the exploratory behavior in the forebay is associated with foraging for kokanee (nonanadromous form of Oncorhynchus nerka), which have been shown to congregate near the dams of hydropower reservoirs in the winter. Our study findings should be applicable to bull trout populations residing in other reservoirs and indicate that entrainment mitigation (for example, use of deterrent devices) should be focused on the fall and winter. This work also provides a framework for combining acoustic telemetry and state-space models to understand and categorize movement behavior of fish in reservoirs and, more generally, in any environment with fluctuating water levels.

A Review on the Fish Communities in the Indian Reservoirs and Enhancement of Fisheries and Aquatic Environment

The reservoirs play an important role in the developmental process of a Nation and also have an integral role in fisheries and livelihood security of the local community. With the increase in population growth reservoirs are becoming important provider of animal protein and for generation of employment in particular to poorer sectors of the people. Reservoirs have many uses from generation of electricity to irrigation purpose and also providing habitat to fishes and other aquatic life and in turn also help to provide feed and create revenue for fish communities. In India, reservoirs are playing a crucial role in the fisheries. Fish communities are often used as indicators of environmental quality. In terms of fish diversity altogether 117 fish species were recorded from Indian reservoirs exhibiting rich fish diversity. These reservoirs have both positive and negative impacts on fishes and other aquatic environment. Therefore, present study is emphasized on synthesizing the available information on fish diversity and community structure of the potential Indian reservoirs and its effects on fisheries and other aquatic environment in reservoirs in India. Some strategies have been suggested for sustaining river and reservoir fish biodiversity.

Participatory Evaluation and Verification of Improved Post Harvest Fishery Technologies on Selected Sites of Oromia water bodies

The study was conducted in the year 2009 to 2013 in different major sites of Oromia water bodies. These sites include lakes and reservoirs found in Abaya, Finchawa, Gilgalgibe, Koka, Langano, Malkawakana and Zeway. This research study focuses on fish smoking technology, fish drying using simple tent dryer, fish meal technology and fish cage retaining. Participatory Rural Appraisal (PRA) was conducted to assess the capture and culture fisheries post-harvest technologies information and communication support for resource exploitation, demand for information, opportunities and constraints of fishermen’s needs in the study area. The result indicated that simple drying using tent dryer at Abaya Lake, Langano, Finchawa and Gilgelgibe reservoirs are suitable and productive for the local fishermen. On the other hand, fish smoking at Gilgelgibe reservoir was not successful due to the unavailability of bricks made from mud in the area. At Koka reservoir fish meal technology was effective and well accepted by the locals. At Langano Lake fish smoking and fish meal technology were also suitable and successful. Moreover, fish smoking at Melkawakana reservoir and Zeway Lake, including fish drying using simple tent dryer were successful and in addition fish cage retaining were successful and recognized by the beneficiaries as possible source of income thus, ready for advance scaling at Melkawakana reservoir.

Mercury biomagnification in subtropical reservoir fishes of eastern China

Little is known about mercury (Hg) biomagnification in the subtropics, aquatic systems with high species diversity resulting in complex food webs. High atmospheric Hg emissions and ubiquitous reservoir fisheries may lead to elevated Hg bioaccumulation in Chinese freshwater fishes. However, stocking practices using fast-growing species can result in low fish total Hg (THg) concentrations. Here, we describe Hg transfer within the fish food web of a large subtropical reservoir, Qiandao Hu (Xin'anjiang reservoir) situated in eastern China. We measured food web Hg biomagnification and THg concentrations in 33 species of stocked and wild fishes. Mercury concentrations in most fishes were low, though we also found high Hg concentrations in wild top predators. The food web structure, assessed using stable isotopes of carbon (δ(13)C) and nitrogen (δ(15)N), demonstrated a high degree of omnivory and a long food chain. THg concentrations were highly correlated with fish δ(15)N values. The regression of log10THg against δ(15)N revealed the overall Hg biomagnification rate was low. This study shows that where long food chains exist in subtropical reservoirs, elevated Hg accumulation in top predators can occur despite a low Hg biomagnification rate.

Characterization of genetic variance within and among five populations of Sperata seenghala (Skyes, 1839) revealed by random amplified polymorphic DNA markers

The genetic diversity among five populations (Bhadbada reservoir, Mohinisagar reservoir, Bansagar reservoir, Bargi reservoir and Gandhisagar reservoir) was revealed using random amplified polymorphic DNA markers. 10 random primers screened, 5 primers revealed various banding patterns and yielded 71 total loci as an average of which 39.60 (55.77%) were polymorphic between the population and 86.84% within the population of Sperata seenghala . Population wise the highest genetic polymorphism was obtained in Bhadbada reservoir as 67.61% whereas the lowest was in Gandhisagar reservoir as 49.30%. However, Analysis of Molecular Variance indicated low genetic diversity (H pop = 0.0921 ± 0.1249; I = 0.1584 ± 0.1942) in Bansagar reservoir. Relative genetic differentiation (G ST = 0.3993) and restricted gene flow ( N m = 0.7523) as an average indicated low gene diversity among the fish populations. The un-weighted pair group method with averages (UPGMA) dendrogram showed 05 major clusters, each cluster representing a population. Fish population of Mohinisagar reservoir showed high genetic distance (0.3981) with respective Bargi reservoir population and highest genetic identity (0.8846) reflected between Bansagar and Gandhisagar reservoir. Highest genetic distance between Mohinisagar and Bargi reservoir fish populations shows no significant correlation between genetic and geographical distance of the genotypes collected from different lentic and geographical isolated water bodies. This investigation indicated that lowest genetic diversity existed in different geographic populations of S. seenghala . All the five populations were found to be low in genetic variation, which is useful information for future conservation measures of S. seenghala confined in natural water bodies of Madhya Pradesh.

Food-web structure in the hypertrophic Rietvlei Dam based on stable isotope analysis: Specific and general implications for reservoir biomanipulation

Fish predation on zooplankton is the basic foundation for top-down biomanipulation of lacustrine ecosystems. To test this premise, we determined stable isotope (SI) values (δC and δN) of representative samples of major planktonic (phytoplankton, zooplankton), benthic (submerged macrophytes and associated epiphytes, benthic macro-invertebrates) and nektonic (fish) food-web components, collected from 3 to 7 shallow inshore locations (with additional plankton samples at 1 or 2 deep offshore sites) in Rietvlei Dam over a period of 30 months. The resulting δC values did not indicate significant consumption of zooplankton by fish, while the δN values for fish confirmed their wide trophic separation from zooplankton. Instead, SI values indicated that fish relied mostly on food resources of benthic origin (through direct consumption or piscivory). The SI signatures of individual fish species were consistent with their known feeding habits. The lack of trophic couplings between zooplankton and fish accords with previous gut content analyses of fish and analyses of zooplankton abundance and size structure in hypertrophic reservoirs. Marginal utilisation of zooplankton by indigenous reservoir fish is attributable to their native origin as riverine species unaccustomed to feeding on zooplankton. These findings indicate that top-down biomanipulation is unlikely to be effective as a management tool in eutrophic South African reservoirs. Primary producer components exhibited surprisingly wide and unsystematic temporal fluctuations in both δC and δN values; some potential contributory factors are considered. Changes in phytoplankton δC values were broadly tracked by zooplankton – their nominal consumers. Some questions arising from the study, and some apparently anomalous findings are identified and discussed. Keywords : δC, δN, trophic structure, plankton, benthos, fish, warm-water ecosystems, South Africa

Life Cycle Assessment for environmentally sustainable aquaculture management: a case study of combined aquaculture systems for carp and tilapia

Life Cycle Assessment (LCA) was applied to evaluate the potential environmental impacts associated with two-net cage aquaculture systems of common carp (Cyprinus carpio carpio) and tilapia (Oreochromis niloticus) in the Cirata reservoir, Indonesia. The studied system included fingerling production in hatcheries, fish rearing in cages, and transport of fry and feed as well as that of harvested fish to markets. The environmental impact indicators were calculated based on the annual production in 2006–2007 using the CML2 Baseline 2000 method, and expressed per tonne of fresh fish delivered to the market. The rearing performances and the environmental efficiency of the system were highly dependent on the lake water quality. Therefore the location of the cages and associated practices influenced the environmental impacts. Feed was identified as the major contributor to land occupation, primary production use, acidification, climate change, energy use and water dependence. Those impacts were mainly linked to the production of fishmeal followed by the production of crop-based feed materials and the production of electricity for feed processing. Eutrophication was mainly the consequence of the fish growing stage and linked closely to nutrient loading from cages. Better feeding practices to reduce feed conversion ratio (FCR), as well as improvement of feed composition by using less fishmeal and more local plant-based materials along with improving energy efficiency of feed production processing should be implemented to improve the environmental profiles of carp and tilapia production. The reduction of FCR from 2.1 to 1.7 could decrease eutrophication by about 22%. However, it is of first priority to reduce the number of cages in order to improve the water quality of the reservoir. The comparison of Cirata reservoir fish culture to other sources of animal protein revealed that it generated average energy use but high eutrophication level. LCA was demonstrated to be a useful tool for decision-making when targeting improved environmental sustainability of cage aquaculture.

Estimating and Evaluating Mechanisms Related to Walleye Escapement from Rathbun Lake, Iowa

AbstractReservoir fisheries present managers a unique set of challenges. One of the most obvious, yet overlooked, challenges to maintaining sustainable reservoir fisheries is escapement. Yet, little is known about the escapement of reservoir fishes or factors influencing escapement. From June 2009 through March 2011, 4,137 Walleyes Sander vitreus &gt;300 mm were collected in the tailrace of Rathbun Lake, Iowa, individually tagged with visual implant tags, and returned to the reservoir to estimate escapement rates and identify influential factors. During this time, escapement was estimated as (1) the percentage of tagged Walleyes at large that were recaptured in the tailrace and (2) the proportion of tagged to untagged Walleyes present in Rathbun Lake. Tailrace recapture rates during individual sampling events ranged between 0.0% and 2.1% and minimum annual tailrace escapement was estimated as 13.6% (SE, 3%). In spring 2010, 8% (SE, 1%) of the Rathbun Lake Walleye population had been tagged and by spring 2011, 26% (SE, 2%) of the Walleyes were tagged, indicating that they had previously escaped. To understand factors related to escapement, individual fish capture–recapture data were analyzed in Program MARK using a multistate model to estimate apparent survival, detection, and escapement probability. Probability of escapement increased with increasing mean daily discharge and decreased with increasing fish length and release distance from the dam. Variable weights indicated that discharge was the primary factor related to escapement. Escapement probability increased exponentially with daily discharge and doubled as discharge increased from 40 to 60 m3/s. Our results suggest a substantial proportion of the Rathbun Lake Walleye population has been lost recently, due in part to record water releases, making management of this reservoir fishery challenging.Received September 6, 2012; accepted March 12, 2013

Short-term impact of reservoir impoundment on the patterns of mercury distribution in a subtropical aquatic ecosystem, Wujiang River, southwest China

Surveys on pre- and post-impoundment total mercury concentrations in water, seston and fish were conducted during 2007-2008 in the Wujiang River, southwest China. Compared with the pre-impoundment, total mercury in the reservoir and downstream water rapidly increased by 164-303% during the first half year of flooding, and then decreased by 61.7-62.2% after 9months of flooding. However, total mercury in seston from the reservoir and downstream generally declined. There was no significant difference in the total mercury between reservoir fish with different habitat preference and feeding habit. We find that fish tissue mercury concentration significantly increased threefold after flooding, and the increasing rate is lower than those in subarctic and temperate reservoirs. The pre- and post-impoundment fish samples exceeding the Chinese hygienic standard for tolerances of mercury in foods increased from 1.3 to 17.4%. Long-term monitoring of fish mercury level in Wujiang River should be undertaken to ensure local food security.

ZOOPLANKTON DIVERSITY OF AMBADI RESERVOIR TALUKA KINWAT, MAHARASHTRA

Plankton is most essential for many fishes as food. The growth of plankton feeding fishes mostly depends on plankton dynamics of the water body.In the complex chain of community metabolism, the different trophic strata, and a fraction of which ends up as fish flesh. Therefore, structure of different fish food biotic communities assumes greater significance to reservoir fish management. Ambadi reservoir in Kinwat Taluka of Nanded District of Maharashtra. represents 17 species of zooplankton.These include 6 species of rotifera, 4 species of copepoda, 6 species of cladocera and 1 species of ostracoda.

FISH DEPTH DISTRIBUTIONS IN THE LOWER MISSISSIPPI RIVER

A substantial body of literature exists about depth distribution of fish in oceans, lakes and reservoirs, but less is known about fish depth distribution in large rivers. Most of the emphasis on fish distributions in rivers has focused on longitudinal and latitudinal spatial distributions. Knowledge on depth distribution is necessary to understand species and community habitat needs. Considering this void, our goal was to identify patterns in fish benthic distribution along depth gradients in the Lower Mississippi River. Fish were collected over 14 years in depths down to 27 m. Fish exhibited non-random depth distributions that varied seasonally and according to species. Species richness was highest in shallow water, with about 50% of the 62 species detected no longer collected in water deeper than 8 m and about 75% no longer collected in water deeper than 12 m. Although richness was highest in shallow water, most species were not restricted to shallow water. Rather, most species used a wide range of depths. A weak depth zonation occurred, not as strong as that reported for deep oceans and lakes. Larger fish tended to occur in deeper water during the high-water period of an annual cycle, but no correlation was evident during the low-water period. The advent of landscape ecology has guided river research to search for spatial patterns along the length of the river and associated floodplains. Our results suggest that fish assemblages in large rivers are also structured vertically. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.

Biodiversity in heavily modified waterbodies: native and introduced fish in Iberian reservoirs

Summary Introduced species often thrive in modified systems, although it is not clear whether they have negative effects on native biota that might also be present or, alternatively, occupy niches that have been left empty due to the original habitat modification. We used 247 surveys of fish assemblages (48 species in all) in riverine reservoirs across the Iberian Peninsula to analyse the relationships between the species richness of introduced and native fish. Most of the samples (195) were from areas with a Mediterranean climate, while the reminder (52) had a temperate climate. Mediterranean reservoirs had more introduced than native species, with an opposite pattern in the temperate areas. The richness of non‐natives was positively related to the area of reservoir and human pressures and negatively related to altitude. Native richness was higher in larger and deeper reservoirs and lower in older and more impacted reservoirs, as well as in those with more introduced piscivores. There was a negative relationship between the richness of native fish and that of non‐native piscivores in Mediterranean reservoirs, confirmed through multiple regression and structural equation modelling. Contrastingly, native fish richness was mainly controlled by variables related to water quality in temperate areas, implying that the negative impacts of introduced fishes in reservoirs can be context dependent. The presence of introduced piscivores thus seems an important factor excluding many native fish from reservoirs in Mediterranean environments in the Iberian Peninsula.

Mobile sailing robot for automatic estimation of fish density and monitoring water quality

IntroductionThe paper presents the methodology and the algorithm developed to analyze sonar images focused on fish detection in small water bodies and measurement of their parameters: volume, depth and the GPS location. The final results are stored in a table and can be exported to any numerical environment for further analysis.Material and methodThe measurement method for estimating the number of fish using the automatic robot is based on a sequential calculation of the number of occurrences of fish on the set trajectory. The data analysis from the sonar concerned automatic recognition of fish using the methods of image analysis and processing.ResultsImage analysis algorithm, a mobile robot together with its control in the 2.4 GHz band and full cryptographic communication with the data archiving station was developed as part of this study. For the three model fish ponds where verification of fish catches was carried out (548, 171 and 226 individuals), the measurement error for the described method was not exceeded 8%.SummaryCreated robot together with the developed software has features for remote work also in the variety of harsh weather and environmental conditions, is fully automated and can be remotely controlled using Internet. Designed system enables fish spatial location (GPS coordinates and the depth). The purpose of the robot is a non-invasive measurement of the number of fish in water reservoirs and a measurement of the quality of drinking water consumed by humans, especially in situations where local sources of pollution could have a significant impact on the quality of water collected for water treatment for people and when getting to these places is difficult. The systematically used robot equipped with the appropriate sensors, can be part of early warning system against the pollution of water used by humans (drinking water, natural swimming pools) which can be dangerous for their health.

Environmental factors regulating the recruitment of walleye Sander vitreus and white bass Morone chrysops in irrigation reservoirs

AbstractUnderstanding the environmental factors that regulate fish recruitment is essential for effective management of fisheries. Generally, first‐year survival, and therefore recruitment, is inherently less consistent in systems with high intra‐ and interannual variability. Irrigation reservoirs display sporadic patterns of annual drawdown, which can pose a substantial challenge to recruitment of fishes. We developed species‐specific models using an 18‐year data set compiled from state and federal agencies to investigate variables that regulate the recruitment of walleye Sander vitreus and white bass Morone chrysops in irrigation reservoirs in south‐west Nebraska, USA. The candidate model set for walleye included only abiotic variables (water‐level elevation, minimum daily air temperature during winter prior to hatching, annual precipitation, spring warming rate and May reservoir discharge), and the candidate model set for white bass included primarily biotic variables (catch per unit effort (CPUE) of black crappie Pomoxis nigromaculatus, CPUE of age‐0 walleye, CPUE of bluegill Lepomis macrochirus and CPUE of age‐3 and older white bass), each of which had a greater relative importance than the single abiotic variable (minimum daily air temperature during winter after hatching). Our findings improve the understanding of the recruitment of fishes in irrigation reservoirs and the relative roles of abiotic and biotic factors.

Predation on microcrustaceans in evidence: the role of chaoborid larvae and fish in two shallow and small Neotropical reservoirs

This study was focused on the predation upon microcrustaceans by an invertebrate predator (chaoborid larvae), and vertebrate predators (fish), in two small reservoirs in southeastern Brazil, with and without macrophytes, in two climatic periods (dry and rainy seasons). Chaoborus larvae were sampled in the limnetic zone, as they are scarce in the littoral, and fish in both limnetic and littoral zones. Their diets were evaluated by the analysis of the crop (chaoborid) or stomach contents (fish). Chaoborid larvae consumed the dinoflagellate Peridinium sp. or other algae, rotifers, and planktonic microcrustaceans. The fish species that included microcrustaceans in their diets were juveniles caught in the littoral. Aquatic insects, plant fragments, and detritus were their major dietary items, microcrustaceans representing a minor item. Planktonic copepods contributed more to the diet of chaoborid larvae than planktonic cladocerans. Fish preyed on planktonic microcrustaceans, as well as on benthic and macrophyte-associated species. Microcrustaceans were not heavily preyed on by chaoborid larvae and fish in both reservoirs.