Seasonal inter‐calibration between acoustic and multi‐mesh gillnets sampling for fish biomass assessment in reservoirs

This paper presents the results of an inter‐calibration between acoustic and gillnets sampling in two North African reservoirs according to seasons. Gillnets with multi‐mesh were designed for sampling fish in lakes while acoustic surveys were performed with a split beam Simrad EK60 echosounder. Sampling events were carried out during summer (September 2015), autumn (December 2015), winter (March 2016) and spring (June 2016) in two Tunisian reservoirs (Kasseb and Siliana) with different depths and shapes. Gillnet catches showed a high proportion of barbell whatever the seasons in Kasseb Reservoir, while at Siliana Reservoir, significant seasonal changes in relative abundances have been evidenced. The highest fish biomass of the entire water column was observed in winter daytime (103 kg/ha) in Kasseb Reservoir and in summer daytime (283 kg/ha) in Siliana Reservoir. Average biomass observed in autumn (December) for the two reservoirs were lower than the other seasons. During spring, density daytime values in Kasseb were higher than during nighttime while it was the opposite in summer and autumn. Fish densities detected in Kasseb Reservoir in vertical beaming was higher than those detected in horizontal beaming (p < .05). The longitudinal distribution of fish in the reservoirs showed that there is no clear trend in fish densities according to strata. High numbers of fish were detected in deep strata and big fish were located in the surface water near the dam of Kasseb Reservoir. A significant linear correlation was showed between acoustic density/NPUE and acoustic biomass/BPUE but the perfect correlation with the 1:1 fit was showed only between acoustic biomass/BPUE.     

Impact of Juvenile Stocking Size on the Major Carp Production in a Minor Reservoir,Bibinagar, India

The present study deals with the impact of juvenile stocking sizes of the major carp on production in a minor reservoir, Bibinagar, Nalgonda, India. Reservoir surface area equaled over 23.8 ha and water from the reservoir was used for irrigation and fisheries. Four experiments were conducted for 4 years from 2000–2001 to 2003–2004. The experiments were planned in such a way that every year juvenile stocking size was held constant and subsequent fish production analyzed. During the first year’s experiment (2000–2001) a stocking size of 25–30 mm (fry) was maintained. Similarly 50–55 (advanced fry), 75–80 mm (fingerling) and 100–105 mm (advanced fingerling) were stocked during 2001–2002, 2002–2003 and 2003–2004 respectively. Uniform yearly stocking densities (2000/ha) were established in each experiment in the month of July and fish were harvested in June of the subsequent year. Major carp production was enhanced with larger stocking size. Yearly productions equaled 144.00, 231.48, 632.91 and 1005.03 kg/ha/year with the above stocking sizes. Variation in stocking sizes had significant (P<0.05) effects on fish production. The number of fish per kg production decreased gradually with increases in stocking size with an average value of 3.97. Reservoir production of catla was the most abundant species, followed by rohu, common carp, mrigal and grass carp. These results show that stocking size has a great impact on fish production and the stocking of advanced fingerlings will provide maximum carp production in minor reservoirs in India.     

Postimpoundment Changes in Water Quality and Fish Stocks in Two Large West African Reservoirs (Manantali and Sélingué, Mali)

The recently established large reservoirs Manantali on the river Bafing, tributary to the Senegal River and Sélingué, located on the river Sankarani, tributary to the Niger River, have followed similar changes in limnology and fish populations as those established in West Africa 20 to 30 years ago. Basic water parameters, such as oxygen content, pH, electrical conductivity and temperature show seasonal variations largely determined by the changes in wet and dry seasons. Both waterbodies are monomictic, with water mixing taking place in January, the coldest period of the year. Pre-impoundment forest clearence of the future reservoir bottom in the shallow Sélingué reservoir was carried out only in the proximity of the dam site. In the deeper reservoir Manantali the savanna forest was cleared from an area of about 25 percent, and the tree gallery was cleared along the river. The initial filling of Manantali reservoir was followed by a nearly complete drainage of the reservoir. After refilling some months later, the dissolved oxygen content stayed reasonably high and therefore there was no fish mortality. Also no hydrogen sulphide or a large-scale spread of noxious aquatic plants developed. At present the fish fauna is dominated by cichlids and pelagic species, i.e. at Sélingué by clupeids. and at Manantali by cyprinids (Leptocypris niloticus). In both reservoirs was the genus Citharinus considerably reduced. At Sélingué reservoir disappeared Protopterus annectens. The fish yield estimate is 27 kg/ha/year for Manantali, and 40 kg/ha/year for Sélingué. Both rerservoirs have recorded a slight increase in electrical conductivity. They both serve as a valuable water resource and fish supply for the Sahel region.     

Distribution patterns of fish biomass by acoustic survey in three Tunisian man‐made lakes

In order to test the hypothesis that fish distribution is related to water depth, three exploited reservoirs were sampled at day and at night using a Simrad EK60 echosounder. Acoustic surveys were carried out between February and June 2008, which focused on the fish size, species density (fish per ha) and biomass (kg ha^?1) along vertical and longitudinal gradients. It was evident in the surface layer (0–3 m) during daytime that the larger fish sizes occupy waters near the dam or the middle of Lakhmess and Sejnane reservoirs. In the upper layer at nighttime, a gradient of fish size is shown proportional to the depth at Lakhmess Reservoir and inversely proportional at the man‐made Sejnane Lake. At Lakhmess and Laabid reservoirs, fish density was significantly higher in deep water (3926 ± 1409 and 624 ± 258 fish per ha) rather than in the upper layer (988 ± 2 and 8 ± 2 fish per ha in daytime, respectively), while at Sejnane Reservoir the number of fish per area was similar along the vertical gradient. The biomass was significantly higher in waters deeper than 3 m at Sejnane Reservoir whereas at Lakhmess it was higher in the surface layer. Fish biomass increased from upstream (0.16 ± 0.05 kg ha^?1) to middle (3.07 ± 2.96 kg ha^?1) and downstream (3.82 ± 2.30 kg ha^?1 at night) areas in the man‐made Laabid Lake while a similar longitudinal gradient occurred in the deepest Sejnane Reservoir. The vertical gradient in fish distribution is discussed. From the dam to the tributary of the entire water column, it was concluded that fish biomass distribution was governed by depth and was most abundant in areas with deep waters.     

三峡水库175 m试验性蓄水期库区及其上游江段鱼类群落结构时空分布格局

根据2010—2012年在三峡水库及其上游江段5个江段的商业性渔获物调查结果,对该区域鱼类群落结构的时空分布格局进行了分析。调查中共收集到鱼类87种,隶属于8目18科63属。沿坝前江段溯河而上至库尾以上流水江段,鱼类种类数逐渐增加。采用聚类分析(CA)和非度量多维标度(NMDS)方法对三峡水库175 m试验性蓄水期间各江段的鱼类种类组成进行分析,发现这5个调查江段的鱼类种类组成可以分为两个类群:类群Ⅰ包括秭归、巫山、云阳3个库区的江段,其鱼类种类组成在各年间的差异程度较大;而类群Ⅱ包括库尾的涪陵江段以及上游的江津江段,其鱼类种类组成在各年间差异程度较小。采用CA和NMDS方法对鱼类群落结构时空分布格局的分析结果表明,5个调查江段的鱼类群落结构在各年间可以分为不同类群;云阳与涪陵江段的鱼类群落结构的相似性逐年增加,而与秭归、巫山江段的相似性逐年减少;涪陵江段与江津江段的鱼类群落结构相似性也呈逐年减少趋势。总体而言,175m试验性蓄水对库中和库尾江段的鱼类群落结构的影响较大,但对库首以及库尾以上流水江段的影响均较小。     

Indirect effect of different fish communities on nutrient chlorophyll relationship in shallow hypertrophic water quality reservoirs

Effects of different fish communities on the proportion of different nitrogen and phosphorous forms and the amount of phytoplankton (chlorophyll a) were examined in two consecutive years (1992–1993) in three Hungarian shallow water reservoirs (Cassette and outer reservoir of the Kis–Balaton Water Protection System, and Marcali reservoir). Possible interactions between nutrient concentrations and the amount of phytoplankton in these reservoirs were also examined. Considerable differences in the proportions of different nutrient forms were observed between the three test sites, which could be explained by the presence of different fish stocks in these reservoirs. In the Cassette, the fish biomass necessary for a water quality improvement was around 50 kg ha⁻¹. Phytoplankton biomass was controlled by the zooplankton, consequently chlorophyll a concentrations decreased considerably, while those of dissolved nutrients significantly increased. In the outer reservoir, phytoplankton was controlled bottom-up, since the 250 kg ha⁻¹ fish biomass was larger than the critical value due to the high proportion of planktivorous species. Chlorophyll a concentrations were high, and nutrients were mainly in particulate form (in algal cells). In the Marcali reservoir, the recently introduced silver carp population could not control fully the phytoplankton. The biomass of phytoplankton decreased only slightly, while its composition changed considerably. Although biomanipulation with silver carp is suitable for ceasing cyanobacterial blooms, reduction of the amount of planktivorous fish seems to be a more adequate method for increasing water transparency, rather than introduction of phytoplankton feeding fish.

     

Localization of somatostatin-like immunoreactivity in the pancreatic islets of the hagfish,Myxine glutinosa and the lamprey Lampetra fluviatilis

Summary Somatostatin-like immunofluorescence has been found by immunostaining in cells of the bile duct mucosa and pancreatic islet parenchyma of the hagfish, Myxine glutinosa, and the islet lobules of the lamprey, Lampetra fluviatilis.We are grateful to Dr. Alan Thorpe, Queen Mary College, London University for the gift of antiteleost insulin serum     

Reservoirs promote the taxonomic homogenization of fish communities within river basins

Most studies analyzing patterns in biotic homogenization of fish communities have used large-scale approaches, while the community-level effects of species introductions and local extinctions within river basins have been sparsely analyzed. In this article, we examine patterns in freshwater fish α- and β-diversity in relation to the presence of reservoirs in a Mediterranean river (Guadiana river; Iberian Peninsula). We used fish samples from 182 river localities and 59 reservoir ones to address two main questions: (i) do reservoirs favor the establishment of invasive fish species?; and (ii) do reservoirs bear taxonomically homogenized fish communities? Although total species richness was not different between rivers and reservoirs, the latter had more invasive species and less native ones. Fish species found in reservoirs tended to be larger ones, but invasive species of any size showed higher preferences for reservoirs. Native species that were rare or absent in reservoirs were those that showed higher sensitivity to invasive species in rivers. Reservoir fish communities were taxonomically homogenized in relation to river ones, both when considering all fish species and using only natives or only invasive ones. Our results suggest that invasive species occupying reservoirs constitute an ecological filter excluding most native species from such systems. Invasive species in the study area are often widely introduced elsewhere, while native species found in reservoirs are congeneric and ecologically similar to those found in other Iberian studies. Thus, we conclude that reservoirs promote taxonomic homogenization at multiple spatial scales, while could also be promoting the functional homogenization of Iberian fish communities.     

Fisheries in Semi-Arid Northeast Brazil with Special Reference to the Role of Tilapias

The reservoir fisheries in the semi-arid Northeast Brazil have been studied based on the data collected by the Department of Action Against Droughts and placed against the background of what is known of similar fisheries in the tropics elsewhere. Of about 60,000 reservoirs in the region, DNOCS controls about 100 reservoirs since 1970. Physical characteristics are provided together with total areas of the reservoirs. Unfortunately very little basic limnological data are available. In all 42 species of fish and crustaceans were introduced into these reservoirs of which only fourteen fish species and one shrimp were successful in establishing self-propagating populations, others being stocked on a regular basis. The recent stocking of Colossoma macropomum and Cyprinus carpio are however unsuccessful in reservoirs. The fish yield of DNOCS controlled reservoirs for 1950–1990 was 112 kg.ha⁻¹ (Davies, 1972; Gurgel and Oliveira, 1987). It amounts now to about 20,000 tons annually taking into account incomplete and scaled down reporting of catches. This indicates a total fish catch from 60,000 reservoirs in the Northeast of Brazil of very substantial size. The CPUE and catches of all species and that of Tilapia rendalli and Oreochromis niloticus are given together with a discussion of the fisheries and recommendations for management. The total CPUE varied from 18–1,100 kg.ha⁻¹. However, most fishermen are only part-timers. It appears likely that very substantial yields of fish are being harvested in Northeast Brazil but are under-reported. Management strategies now in operation in reservoir fisheries are still very pragmatic. With the accumulation of data and generalizations, more viable strategies could be devised.     

Ecology and fishery management of reservoirs in India

India has 19 370 small reservoirs with a total water surface area of 3 153 366 ha. At least 100 of them have been subjected to scientific studies. Habitat variables responsible for a reservoir's productivity can be summed up into climatic, morphometric and hydro-edaphic factors. The peninsular reservoirs are characterized by a narrow range of fluctuations in water and air temperature across seasons, a phenomenon which prevents the formation of thermal stratification. Many reservoirs in the upper peninsula show thermal stratification during summer. Wind-induced turbulence facilitates the return of nutrients to the trophogenic zone. Most reservoirs on the mountain slopes of Western Ghats, Himalayas and the other highlands are deeper, with steeper basin walls, compared to irrigation impoundments. Mean depth does not show any direct correlation with productivity, either at primary or fish level. A high shoreline development index gives a better indication of productivity. Plankton, benthos and periphyton pulses of Indian reservoirs coincide with the months of least level fluctuations. Oligotrophic tendencies shown by some reservoirs are mainly due to poor nutrient status and other chemical deficiencies. In most cases, poor water quality is accountable to poor catchment soil. Low levels of phosphate and nitrate are not indicative of low productivity due to quick recycling of these nutrients. Specific conductivity reflects the production propensities of reservoirs satisfactorily. Almost all productive reservoirs have a klinograde oxygen curve and a vertical stratification of chemical variables such as pH, carbon dioxide, total alkalinity and specific conductivity. High seasonal rainfall and discharge of water during monsoon result in high flushing rates, which do not favour colonization by macrophytic communities. Similarly, inadequate availability of suitable substrata retards the growth of periphyton. Plankton constitutes the major link in the trophic structure. A rich plankton community with well-marked succession is the hallmark of Indian reservoirs with blue-green algae as the major component. The main factors that retard the growth of benthos are a rocky bottom, frequent water level fluctuation and rapid deposition of silt and other suspended particles. Large reservoirs, on average, harbour 60 species of fishes, of which at least 40 contribute to the commercial fisheries. Fast-growing Indian major carps are the prominent commercial fishes. Dam construction has adversely affected populations of many other species such as Tenualosa ilisha, Torspp. and Cirrhinus spp. Formulae for estimating fish yield potential and stocking density are described. While culture-based fisheries have been successfully practiced in many small reservoirs, the management norm followed in medium and large reservoirs is primarily on capture fishery. In large and medium reservoirs, stocking was successful only when stocked fishes bred. Indian experience on species enhancement and introductions is described. Environmental enhancement of small reservoirs has been attempted in some reservoirs of Tamil Nadu. Modeling, using standard population parameters, such as the density-dependent growth, size dependent mortalityand weight–length relationshipis discussed. Two exotic fishes viz., Oreochromis mossambicus and Cyprinus carpio have been introduced into Indian reservoir with discouraging results. Hypophthalmichthys molitrix, after an accidental introduction, has performed well in Gobindsagar, a reservoir with a distinct cold water regime. Reservoir fisheries in India are well poised for development, provided scientific management norms are adopted.     

Similarities in correlates of native and introduced fish species richness distribution in Brazilian reservoirs

We assessed the relationships among native and introduced fish species richness and a set of explanatory variables, including area, altitude, reservoir age, temperature, human influence index and fish abundance. We expected to find different relationships based on species origin (i.e., native or introduced). Based on compiled data from Brazilian reservoirs, we modeled the variation in the number of native and introduced fish species by generalized linear mixed models with different spatial autocorrelation structures. Reservoirs located in warmer regions and with higher fish abundance showed higher species richness of both native and introduced fish. Reservoir age tended to be negatively correlated with native species richness. Our results suggest that reservoir communities in warmer regions may be more resistant to local extinctions caused by impoundments, but they are more susceptible to species introductions. Our results also highlight that an extinction debt can be expected for native species because older reservoirs showed lower native species richness.     

Trophic structure of nine Czech reservoirs regularly stocked with piscivorous fish

For nearly 20 years, most Czech reservoirs supplying drinking water have been under statutory protection which permitted reservoir managers to manipulate fish stocks in order to maintain a sustainable water quality. The most common biomanipulative measure adopted was stocking with piscivorous fish (mostly 5 cm fry) using an annual stocking level of approx. 25 000 fish per reservoir. Nine reservoirs were studied for signs of top-down food web effects, as predicted by the trophic cascade hypothesis based on levels of total phosphorus (TP), chlorophyll a (Chl a), zooplankton biomass (ZB) and zooplankton community structure. In all nine reservoirs, only small Daphnia species were recorded, such as D. galeata and D. cucullata. The proportion of large-bodied daphnids retained on a 0.71 mm sieve was less than 10% of the total crustacean biomass in all reservoirs. The relationship of Chl a level – TP, and of ZB – Chl a, was positive under enhanced piscivory and did not differ statistically from the relationships in other reservoirs with natural fish stocks. This implies that bottom-up forces remained stronger than top-down ones in the studied reservoirs, despite the stocking of piscivorous fish. The failure of this attempt at biomanipulation may be due to an insufficient stocking rate of predatory fish and/or inadequate data on the resident planktivorous fish levels.     

The relevance of size parameters as indicators of fishery exploitation in two West African reservoirs

Fish communities living within the artificial reservoirs of Manantali and Selingue in Mali are compared after the monitoring of small-scale fishery landings over 13 months (May 2002–June 2003). The fishing pressure constitutes the main difference of these two similar reservoirs. The effects of fishing on the structure of the fish communities are analyzed by using fish size spectra and derived indicators. In Selingue Reservoir, the fishing gears mostly target smaller species (mean length of 165 mm) than in Manantali Reservoir (mean length of 210 mm). Unlike in Selingue, the size structure does not change between seasons in Manantali. In Selingue, fishing practices that target the fish recruitment of the year constitute a structuring factor of the fish sizes observed in the catches. In spite of similar values of the slopes of the global fish communities’ size spectra between the two reservoirs, they clearly display an intense exploitation at Selingue. Then, size-based spectra indicators represent a potential tool for assessing the impact of fishing on fish communities in small-scale fisheries.     

Perch (Perca fluviatilis L.) parasites reflect ecosystem conditions: a comparison of a natural lake and two acidic reservoirs in Finland

Parasite communities of perch were studied in a natural lake (Vetämäjärvi) and two reservoirs (the Kyrkösjärvi and Liikapuro reservoirs) located in Western Finland. All water bodies studied are small, shallow and humic. However, the reservoirs are more acidic than the lake (pH 5.9 and 5.3 vs. 6.4). Altogether, 18 parasite species were found, but the component communities were reduced in the harsh conditions of the reservoirs (12 and six species) as compared with the lake (17 species). In addition, the mean number of metazoan parasite specimens per fish was markedly lower in the reservoirs (64.3 and 14.3 specimens) than in the lake (116.1 specimens). Our prediction on the depauperation of parasite communities in reservoirs as compared with the lake was confirmed. Many ectoparasite groups were rare and most of the digenean parasites using molluscs as their intermediate host were absent from the reservoirs due to the inability of molluscs to tolerate the acidic water. The only digenean parasite found in the less acidic reservoir was Bunodera luciopercae. There were also species that preferred reservoirs as was expected. The protozoan ciliate Capriniana piscium benefiting acidic water and Acanthocephalus lucii, due to the abundance of its intermediate host, were most prevalent in the reservoir conditions.     

鲢鳙鱼养殖小型水库底栖动物群落季节动态

底栖动物是鱼类重要的天然饵料,评估水体中底栖动物的现存量可以指导渔业生产中鱼类的放养数量。为了探究淡水生态养殖水库中底栖动物群落的季节动态,于2013年4月、7月、10月和2014年1月对三河水库的底栖动物群落进行了调查分析。研究共采集到7个属的底栖动物,隶属于颤蚓科、摇蚊科和蠓科,未采集到软体动物。相对重要性指数(IRI)计算结果表明,菱跗摇蚊属Clinotanypus(IRI=7136)、颤蚓属Tubifex(IRI=6734)和尾鳃蚓属Branchiura(IRI=1384)是优势类群,分别占总捕获数量的34.26%、50.38%和10.96%。不同季节之间底栖动物的总密度和生物量差异显著(P0.05),均为冬季春季夏季秋季。冬季总密度(4100个/m~2)和总生物量(10.14 g/m~2)最高,春季(1446个/m~2;1.07 g/m~2)次之,夏季(579个/m~2;0.66 g/m~2)较低,秋季(492个/m~2;0.64 g/m~2)最低。非度量多维尺度分析(MDS)和群落相似性分析表明底栖动物群落结构季节差异显著(P=0.001),2013年三河水库的底栖动物群落可明显划分为3个:春季群落、夏秋季群落和冬季群落。皮尔森相关分析表明,底栖动物总密度与溶氧和营养盐呈正相关关系,与其他水理化因子呈显著负相关关系(P0.05)。冗余分析表明,氨氮、盐度、pH和浊度是三河水库底栖动物群落季节差异的显著影响因子(P0.05),总氮对底栖动物群落的季节差异具有边缘显著影响(P=0.08)。     

香溪河流域梯级水库底栖动物群落比较

对香溪河流域3座梯级水库（古洞口一级水库、古洞口二级水库和三峡水库香溪河库湾）的底栖动物进行了比较生态学研究。运用主成分分析（PCA）对梯级水库的水质进行研究,表明沿着水库的梯度水质逐渐恶化。运用非度量多维标度法（NMS）对梯级水库的底栖动物群落结构进行相似性比较,表明选择水库湖泊区的样点进行梯级水库底栖动物生态学研究具有典型代表性。在物种组成和密度方面,古洞口一级水库和香溪河库湾都是寡毛类占优势,古洞口二级水库是摇蚊幼虫占优势。但3个水库都是耐污种占优势。采用典型对应分析（CCA）解析底栖动物群落结构与环境因子的关系,显示3个水库群落结构的差异主要由浊度造成。用TOC／Turb来表征有机物对浊度的贡献,结果显示古洞口二级水库有机物对浊度的贡献相对较小,因而表明泥沙等无机物对浊度的贡献相对较大。     

A comparison of phytoplankton community assemblages in artificially and naturally mixed subtropical water reservoirs

1. The effect of mechanical and natural mixing of water on phytoplankton community assemblages was compared in one reservoir with a destratification unit operating during the wet summer months (North Pine Reservoir) with two adjacent reservoirs without artificial mixing (Wivenhoe and Somerset Reservoirs) over 6 years in subtropical Australia. 2. All three reservoirs were dominated by cyanobacteria, with the same three genera representing 75–80% of the abundance, i.e. Aphanocapsa/Merismopedia/Cyanodictyon group, Cylindrospermopsis raciborskii (Wo?oszyńska) Seenayya et Subba Raju and Planktolyngbya. Associations between the dominant genera were consistent across all three reservoirs although there were reduced seasonal differences in abundance in North Pine Reservoir compared with the other reservoirs. 3. Peaks in abundance of the solitary filamentous species C. raciborskii and Planktolyngbya occurred earlier and the densities of the colonial species Aphanocapsa/Merismopedia/Cyanodictyon were lower in the reservoir where the destratification unit was switched on in spring compared with the naturally mixed reservoirs, possibly reflecting the differential effect of artificial mixing on colonial versus solitary filamentous species. 4. Phosphate concentrations were positively correlated with algal densities in the two naturally mixed reservoirs but not the artificially mixed reservoir where phosphate concentrations at the surface were near the limit of detection (0.06 μm P). Artificial mixing may, therefore, promote the growth of species able to utilise and store low concentrations of phosphate, such as C. raciborskii.     

Culture-based fisheries in minor perennial reservoirs in Sri Lanka: variability in production, stocked species and yield implications

Sri Lanka is blessed with a large number of irrigation reservoirs. Depending on their hydrological regimes they are broadly categorized into perennial and seasonal reservoirs, and are secondarily used for inland fisheries. The Ministry of Fisheries and Aquatic Resources in Sri Lanka initiated a programme to introduce culture‐based fisheries in minor perennial reservoirs (< 250 ha) whereby 15 minor perennial reservoirs situated in eight administrative districts were selected for introduction of culture‐based fisheries in its initial year of implementation. These reservoirs were stocked with fish fingerlings of catla Catla catla, rohu Labeo rohita, common carp Cyprinus carpio, Nile tilapia Oreochromis niloticus and post‐larvae of freshwater prawn Macrobrachium rosenbergii. Stocking commenced in 2004; stocking data from 2004, 2005 and 2006 are considered in this study. Stocking densities varied from 217 to 870 ha^?1 year^?1, with an average density of 425 ha^?1 year^?1. Fish catch statistics for 2005, 2006 and 2007 were considered; high variability in fish yield among reservoirs as well as in some reservoirs between the years were observed. Mean annual fish yield from all 15 reservoirs increased from 57.3 to 208.1 kg ha^?1 year^?1 after the introduction of culture‐based fisheries. Overall fish production in all 15 reservoirs increased by 263%, ranging from 42.8 to 1344%. Overall contributions of Nile tilapia, catla, rohu, common carp and freshwater prawn to fish harvest were 47.4, 27.2, 16.9, 4.3 and 0.7%, respectively. Biomass gain per fingerling stocked with respect to catla, rohu, common carp and freshwater prawn are 0.51, 0.42, 0.26 and 0.04 kg, respectively, indicating attractive monetary gains through stocking of these species. Results indicate the viability of culture‐based fisheries in minor perennial reservoirs. Possible reasons for high variability in fish yield and areas for follow‐up studies are discussed.     

Perch (Perca fluviatilis L.) parasites reflect ecosystem conditions: a comparison of a natural lake and two acidic reservoirs in Finland.

Parasite communities of perch were studied in a natural lake (Vetämäjärvi) and two reservoirs (the Kyrkösjärvi and Liikapuro reservoirs) located in Western Finland. All water bodies studied are small, shallow and humic. However, the reservoirs are more acidic than the lake (pH 5.9 and 5.3 vs. 6.4). Altogether, 18 parasite species were found, but the component communities were reduced in the harsh conditions of the reservoirs (12 and six species) as compared with the lake (17 species). In addition, the mean number of metazoan parasite specimens per fish was markedly lower in the reservoirs (64.3 and 14.3 specimens) than in the lake (116.1 specimens). Our prediction on the depauperation of parasite communities in reservoirs as compared with the lake was confirmed. Many ectoparasite groups were rare and most of the digenean parasites using molluscs as their intermediate host were absent from the reservoirs due to the inability of molluscs to tolerate the acidic water. The only digenean parasite found in the less acidic reservoir was Bunodera luciopercae. There were also species that preferred reservoirs as was expected. The protozoan ciliate Capriniana piscium benefiting acidic water and Acanthocephalus lucii, due to the abundance of its intermediate host, were most prevalent in the reservoir conditions.     

Convergence of fish communities from the littoral zone of reservoirs

1. Understanding factors that regulate the assembly of communities is a main focus of ecology. Human‐engineered habitats, such as reservoirs, may provide insight into these assembly processes because they represent novel habitats that are subjected to colonization by fishes from the surrounding river basin or transported by humans. By contrasting community similarity within and among reservoirs from different drainage basins to nearby stream communities, we can test the relative constraints of reservoir habitats and regional species pools in determining species composition of reservoirs. 2. We used a large spatial database that included intensive collections from 143 stream and 28 reservoir sites within three major river basins in the Great Plains, U.S.A., to compare patterns of species diversity and community structure between streams and reservoirs and to characterize variation in fish community structure within and among major drainage basins. We expected reservoir fish faunas to reflect the regional species pool, but would be more homogeneous that stream communities because similar species are stocked and thrive in reservoirs (e.g. planktivores and piscivores), and they lack obligate stream organisms that are not shared among regional species pools. 3. We found that fish communities from reservoirs were a subset of fishes collected from streams and dominant taxa had ecological traits that would be favoured in lentic environments. Although there were regional differences in reservoir fish communities, species richness, patterns of rank abundance and community structure in reservoir communities were more homogonous across three major drainage basins than for stream communities. 4. The general pattern of convergence of reservoir fish community structure suggests their assembly is constrained by local factors such as habitat and biotic interactions, and facilitated by the introduction of species among basins. Because there is a reciprocal transfer of biota between reservoirs and streams, understanding factors structuring both habitats is necessary to evaluate the long‐term dynamics of impounded river networks.