Towards a model of a floodplain fish population and its fishery

Synopsis A model is developed which describes the way in which the fish populations of African rivers and their fisheries are influenced by the different types of flood Regime. Ichthyomass and fish catch are dependent on both the extent of flooding during high water and the amount of water remaining in the system during the dry season. The relative number and individual weight of fish are determined by the intensity of flooding, whereas the total number surviving to the next year depends more on the low water regime. Catch per unit effort falls with increasing difference between areas flooded at high and low water. A negative log-log relationship exists between catch and the ratio of maximum area flooded to minimum area of water remaining in the system. This relationship of catch to flood ratio may form the basis for a general index for the evaluation of both year-to-year variations within a floodplain, and differences between floodplains. Lines of equal catch are also derived for various combinations of high and low water areas; these might be used as guidelines for the hydrological management of tropical flood-plains.     

Winter fish community structures across floodplain backwaters in a drought year

We examined fish community structures in floodplain backwaters in the Kiso River, Central Japan, in an uncommon drought year. Nine floodplain backwaters with varying levels of flood inundation frequency were surveyed for winter fish communities, and measured for local environmental variables in summer and winter periods. The winter fish community was better represented, with less diverse and physiologically tolerant species in backwaters with a decreasing flood pulse frequency. The temporal frequency of hypoxic conditions (dissolved oxygen of <2 mg l^?1) and chlorophyll a in summer explained the heterogeneity of the fish community along the gradient of inundation frequency, while no variables measured in winter had an explanatory power. These results suggest that drought-induced, infrequent flood pulses in the autumn–winter period limited the dispersal opportunity of fishes, and thus maintained community structures selected by summer limiting factors until the survey took place in winter. Future comparisons of our data with those in non-drought years would provide important implications to the sustainable management of floodplains for wintering fishes against the increasing probability of future flow regime changes.     

A comparison of the composition of the juvenile fish catch from the Sokoto–Rima floodplain, Nigeria in years preceding and immediately after upstream dam completion

The species composition of the juvenile fish catch from part of the Sokoto–Rima floodplain was monitored over four seasons. In three of the years, 1981–1984, the flood pattern of the river was normal. During the 1984–1985 season the cycle was disrupted by completion of the Goronyo Dam, approximately 100 km upstream from the sampling area. The composition of the juvenile fish catch was markedly altered during this season and the diversity greatly reduced. Ways in which fish species could be affected by the change in flood regime are discussed.     

Comparing the fish assemblages and food‐web structures of large floodplain rivers

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Ecological fish production in the inland delta of the Middle Danube, a floodplain river

Synopsis Ecological fish production in one parapotamal arm of the Middle Danube inland delta in Slovakia, based on 10 samplings during 5 consecutive years, ranged from 349 to 3272 (bar|x = 1066) kg ha^–1 in total production (P_T) and from 39 to 662 (bar|x = 204) kg ha^–1 in available production (P_A). Young-of-the-year fish made up an estimated 49–62% (bar|x = 54) of the P_T. According to the food production/food consumption budget calculated by three different methods, less than half of the P_T is of autochthonous origin, while most is from other sources. Floodplain fish production includes two components: fish originating within the floodplain and fish that are temporary immigrants. The latter component varies considerably in response to the hydrological regime. To assess the P_T in riverine ecosystems, different seasons and years are needed to give realistic values.This paper is dedicated to my friend and sometime co-worker, Eugene K. Balon, at the occasion of his 65th birthday and transition to University Professor Emeritus. He was one of the first ichthyologists to undertake quantitative studies in large rivers in the late 1950's, when most fish biologists thought that fishes inhabit only the sea, lakes, reservoirs and small streams.     

Linking Flow Regime,Floodplain Lake Connectivity and Fish Catch in a Large River-Floodplain System,the Volga–Akhtuba Floodplain (Russian Federation)

River-floodplain systems are amongst the most productive—but often severely impacted—aquatic systems worldwide. We explored the ecological response of fish to flow regime in a large river-floodplain system by studying the relationships between (1) discharge and inundated floodplain area, with a focus on spatial and temporal patterns in floodplain lake connectivity, and (2) flood volume and fisheries catch. Our results demonstrate a non-linear relationship between discharge and floodplain inundation with considerable hysteresis due to differences in inundation and drainage rate. Inundation extent was mostly determined by flood volume, not peak discharge. We found that the more isolated lakes (that is, lakes with a shorter connection duration to the river) are located at higher local elevation and at larger hydrological distance from the main rivers: geographical distance to the river appears a poor predictor of lake isolation. Although year-to-year fish catches in the floodplain were significantly larger with larger flood volumes in the floodplain, they were not in the main river, suggesting that mechanisms that increase catch, such as increased floodplain access or increased somatic growth, are stimulated by flooding in the floodplain, but not in the river. Fish species that profit from flooding belong to different feeding guilds, suggesting that all trophic levels may benefit from flooding. We found indications that the ecological functioning of floodplains is not limited to its temporary availability as habitat. Refugia can be present within the floodplain itself, which should be considered in the management of large rivers and their floodplain.     

The dependence of fish community structure and dynamics on floodplain and riparian ecotone zone in Parana River,Brazil

Flood intensity was a main factor determining access to the riparian/floodplain ecotonal resources of the upper Parana River, Brazil, and in consequence determining yield of the dominant trophic groups, which are fish feeding on flooded grasslands and on riparian fruits and leaves. Prey and predator density both declined in years of low floods, and predators did not recover until the next year of high flood, more slowly than in the case prey fish, most probably due to differences in life cycle length. The life cycle of one of the most important species, Curimba (Prochilodus lineatus — Characidae) depends on access to the floodplain lagoons and return to the reservoir after maturation for intensive growth. The riparian/floodplain habitat complexity and connectivity has great significance for fish community dynamics and fisheries yield, and may serve a reason to create a national park in the last floodplain section of the upper Parana River.     

Effects of decadal changes in the hydrological regime of the middle reach of the Paraná River (Argentina) on fish densities

Changes in fish densities recorded over 14 years (1996–2009) were studied for effects of long-term variation of the hydrologic regime. We collected field data with an echo sounder in scour holes of minor channels draining an area of floodplain in the middle reach of the Paraná River. Fish densities in 2000–2009 were significantly lower than in the previous decade. The decrease was associated with a marked reduction of water levels, flood magnitudes and connectivity of channels with the nearby floodplain lakes. This distortion of the flood pulse likely had an effect on the life history strategies of the fishes. The effects of damming in the upstream basin and other man-made perturbations are minor in the middle reach. However, the decadal alterations of regime are intimately linked to climate fluctuations in the Paraná River basin during the past century. Tendencies of observed fish densities are similar to results reported in literature on the influence on fishes for similar long-term alterations of the flood regime in river flood plain systems.     

Vegetation,invertebrate, and fish community response to past and current flow regulation in floodplains of the Savannah River,Southeastern USA

In 2005 and 2006, the United States Army Corps of Engineers released experimental flows to mimic natural spring flooding on the highly regulated lower Savannah River in the Southeastern US. We used vegetation, invertebrate, and fish communities to assess how past regulation and current experimental releases were affecting the ecological conditions on floodplains. The nearby Altamaha River, of similar size but retaining near-natural spring pulses, served as a reference. We did not find that past flow regulation of the Savannah River had significantly altered floodplain forest structure. However, numbers of tree seedlings in bottomland hardwood forests were higher during a year with an artificial pulse than one without. Analyses of invertebrate and fish communities suggested some limited differences in floodplain community structure between rivers, with differences being most pronounced for predaceous Dytiscidae beetles and Esox spp. fishes. Artificial re-creation of small spring flood pulses down the Savannah River in 2005 and 2006 elicited some responses from fish and invertebrates, but did not shift species assemblages towards those occurring on Altamaha River floodplains.     

Effects of flooding on recruitment and abundance of Golden Perch (Macquaria ambigua ambigua) in the lower River Murray

Flooding is often considered a stimulus for production of fish in floodplain rivers. In the southern Murray–Darling Basin (MDB), Australia, however, few native fish species have been shown to use the floodplain for spawning, and recruitment has been positively and negatively associated with flooding. In 2010/11, extensive flooding in the lower River Murray provided an opportunity to investigate the recruitment response of Golden Perch (Macquaria ambigua ambigua) following 10 years of drought and floodplain isolation. Annual variation in Golden Perch abundance and recruitment were investigated in anabranch and main channel habitats at Chowilla in the floodplain geomorphic region of the lower River Murray over a 7‐year period incorporating the flood and 6 years of in‐channel flow. Spatial variation in recruitment in the lower River Murray was also investigated by comparing the age structure of Golden Perch in the swamplands/lakes, gorge and floodplain geomorphic regions. Golden Perch abundance in the Chowilla region increased significantly postflooding compared with drought years. Age structures indicated that increased abundance was due predominantly to fish spawned during the flood (2010/11) and the previous year (2009/10), which was characterised by in‐channel flows. Age structure was similar in the nearby Katarapko Anabranch system indicating a uniform postflood recruitment response in the floodplain geomorphic region. Juvenile Golden Perch from the 2010/11 and 2009/10 cohorts were less apparent in the gorge and swamplands/lakes regions. Golden Perch have flexible life histories and will spawn and recruit in association with in‐channel rises in flow and overbank flows, but significant increases in abundance in the lower River Murray may result from overbank flooding. Contemporary approaches to flow restoration in the MDB emphasise overbank flows and floodplain processes. We suggest, however, that environmental flow management that incorporates floodplain and in‐channel processes, at appropriate spatio‐temporal scales, will result in more robust populations of Golden Perch.     

Biology of the kingsoldier bream (Argyrops spinifer, Forsskål 1775; Sparidae), from the Arabian Sea, Oman

It is recognized by the FAO that management of fish resources in the western Indian Ocean is complicated by the lack of data on the basic biology and landing statistics for exploited fish species. In this paper data are presented on the population biology of kingsoldier bream, Argyrops spinifer (Forsskål 1775), in the Arabian Sea with a view to contributing towards the development of management plans for its sustainable exploitation. Samples were obtained from the Omani artisanal hand‐line fishery catch, January 2001–December 2002. Total length (L_T) of fish sampled ranged from 20 to 68 cm (males) and 25 to 70 cm (females). The male : female ratio was similar up to 55 cm L_T but the larger size‐classes were composed predominately of male fish. L_T/carcass weight (W_C) relations for male and female fish were similar and the combined data produced an equation W_C = 0.00005 × L_T^2.67. Marginal increment analysis indicated an annual cycle of opaque and translucent ring formation in otoliths that was related to the change in surface water temperature during the annual monsoon period. The opaque zone was completed in May and the translucent zone began to form in June. Female and male fish age ranged between 2 and 25 years. Both males and females exhibited similar asymptotic growth patterns; the combined von Bertalanffy growth function was L_t = 64.6(1 ? e^{[?0.142(t + 0.489)]}). Gonadosomatic indices and gonad condition of male and female fish indicated that the spawning season occurred between September and January. Size (L₅₀) and age at first maturity (A₅₀) were estimated to be 36.5 cm L_T and 5.0 years for males and 37.2 cm L_T and 5.6 years females, respectively. The results of this study provide fundamental information on the biology and population dynamics of A. spinifer in the Arabian Sea that can be used in management models for the continued sustainable exploitation of this species in the Omani demersal coastal fisheries.     

The potential trade‐off between artisanal fisheries production and hydroelectricity generation on the Kafue River,Zambia

1. Freshwater resource managers are increasingly obligated to consider the impacts of large river engineering projects on ecosystem services. We evaluated the effect of altered water regime from the operation of a large dam on the production of the downstream tropical floodplain fishery of the Kafue River, Zambia. We compared the benefits of increased hydropower relative to potentially lost fishery production. 2. We compiled a long‐term data set consisting of experimental gillnet catches, artisanal harvesting effort and monthly river flows for 25 years prior to and 29 years after the 1977 completion of the upstream Itezhi‐Tezhi Dam. As a metric of the flood regime, we calculated a canonical correlation score for each hydrological year before and after dam closure. For the period following dam construction, we used the Muskingum method of flood routing to estimate ‘no‐dam’ flows through the fishery area and downstream hydroelectric turbines at the Kafue Gorge Dam. 3. We compared 16 alternative models of catch per unit effort (CPUE) with and without an effect of water regime on fish population growth rate. Using the two best fitting models, we estimated the total observed fishery harvest and simulated ‘no‐dam’ fisheries harvest and found no significant effect of altered water regime on fishery production. 4. We estimate that the large upstream dam increases downstream hydropower production by about $18 million USD per annum. The reduction in fishery production caused by the altered water regime is not significantly different than zero, although the average reduction amounts to about $2.3 million annually. The total estimated value of harvest ranges from $1.3 million to $56 million annually. 5. Large observed declines in fish abundance over the 54‐year study period are attributed primarily with similarly large increases in total fishing effort in this mostly open‐access artisanal fishery. 6. These results contrast with other examples of the effects of flow alteration on fish, probably because levels of fisheries exploitation on the Kafue River are very high relative to better studied regions on other continents; our focus on the whole fish community; and the unprecedented length of the time series we considered. If the goal is to sustain fishery production, investments in altering flow regime are likely to be less effective than investments to decrease fishing effort.     

Effects of macrophyte complexity and hydrometric level on fish assemblages in a Neotropical floodplain

Habitat heterogeneity provided by aquatic macrophytes and water level variations (flood pulse) are essential factors in structuring fish assemblages. This study aimed to describe the fish species that prefers macrophytes covered areas and to evaluate how macrophytes complexities and a flood pulse influence the structure of fish assemblages and selected attributes (viz. fish species density, species richness and evenness). Sampling was performed with seining nets in five floodplain lakes associated with the Baía River, before (November to December 2011) and after (February to July 2012) a flood, considering different degrees of macrophyte complexity (Absent, intermediate, and high). A total of 48 fish species was recorded, with Characiformes the most dominant. Eight species were indicators of high complexities (seven before the flood and only one after). Significant differences among the different degrees of macrophyte complexity and before and after the flood were found for assemblage structure, species density and richness. Fish assemblage attributes were high in higher habitat heterogeneity provided by macrophytes, but all before the flood. Nevertheless, macrophyte stands with high and intermediate complexity were less affected by the flood, suggesting that the structure propitiated by macrophytes favors the persistence of the fish assemblage in floodplain lakes. Therefore, any action towards conservation of fish assemblages in macrophytes should consider dam operation upstream, to ensure seasonality of flood pulses.     

Floodplain lake fish assemblages in the Amazon River: directions in conservation biology

The Neotropical region is renowned for its high biodiversity, and the Amazon River basin contains the highest number of fish species of any river system in the world. In recent years, habitat fragmentation and exploitation of biotic resources have threatened biological integrity and provoked to need for sustainable management and conservation of the Amazon River system. We studied 36 floodplain lakes along 2000 km of the Amazon River. The fish assemblages associated with flood forests are moderately diverse, with low species dominance and reduced populations. To detect nestedness of fish assemblage composition in floodplain lakes, a nested subset analysis was performed on species presence–absence. The incidence matrix (species × lakes) was maximally packed using the Nestedness Temperature Calculator software. The results of ranking lakes and species allow us to establish targets for conservation. Such strategy for sustainable management should be focused on maintaining the Amazonian biodiversity.     

Early fish growth varies in response to components of the flow regime in a temperate floodplain river

1. The biological productivity of floodplain rivers is intimately related to their flow regimes and it has been proposed that fish production should be linked to components of the flow regime in productivity models. To assess applicability of existing models of productivity in floodplain rivers, we tested predictions about growth during the early life stages of a common, short‐lived fish (Australian smelt Retropinna semoni) in a non‐flow‐altered, temperate Australian floodplain river. 2. The morphometric condition of larval and juvenile fish measured over a five‐year period was positively related to annual discharge, but the highest average seasonal growth rates occurred in two years of contrasting hydrology, one with early spring flooding and the other with predominantly low flows and a late season (within channel) flow pulse. 3. Analysis of daily growth measures indicated that timing, river height, the duration of in‐channel flow events and antecedent flood events are all significant factors influencing the early growth of Australian smelt. The flexible manner in which fish growth responds to these factors appears to be an effective early life history strategy for a short‐lived species occupying a highly variable environment. 4. Growth rates conformed to some predictions of the Flood Pulse Concept (in particular the Extended Flood Pulse Concept), but specific growth responses suggest that the Riverine Productivity Model and tenets of the Low Flow Recruitment Hypothesis best describe the production of Australian smelt in this system. We suggest that none of the existing conceptual models adequately describes fish productivity in temperate Australian floodplain rivers but that aspects of each are likely to be relevant under different flow conditions.     

Age, growth and mortality of Clarias gariepinus (Siluriformes: Clariidae) in the Mid-Cross River-Floodplain ecosystem, Nigeria

Clarias gariepinus is a threatened highly prized species used for some elite ceremonies by the local communities. Artisanal fishers take advantage of this species annual breeding migration from the lower Cross River to the floodplain lakes in Mid-Cross River during the rainy season, and some migrant stocks are not able to spawn. Since there is a lack of information on this species population dynamics in the Mid-Cross area, this study aimed to evaluate the age, growth and mortality to support the development of effective management plans. For this, monthly overnight gill net catches (from 6 to 72mm mesh sizes) were developed between March 2005 and February 2007. Growth parameters were determined using the FiSAT II length-frequency distribution. A total of 1 421 fish were collected during the survey. The asymptotic growth (L(infinity)) was 80.24cm, growth rate (K) was 0.49/year while the longevity was 6.12 years. The annual instantaneous rate of total mortality (Z) was 2.54/year and the natural mortality (M) was 0.88. Fishing mortality (1.66/year) was higher than the biological reference points (F(opt) = 0.83 and F(limit) = 1.11) and the exploitation rate (0.66) was higher than the predicted value (E(max) = 0.64) indicating that C. gariepinus was over exploited in the Mid-Cross River-Floodplain ecosystem. Some recommended immediate management actions are to strengthen the ban of ichthyocide fishing, closure of the floodplain lakes for most of the year, restricted access to the migratory path of the fish during the flood period and vocational training to the fishers. In order to recover and maintain a sustainable harvest, I suggest that a multi-sector stakeholder group should be formed with governmental agents, community leaders, fishers, fisheries scientists and non-governmental organizations. These short and long term measures, if carefully applied, will facilitate recovery of the fishery.     

Relationships between fish assemblages, macrophytes and environmental gradients in the Amazon River floodplain

During the flood season of 1992–1993, 139 species of fishes were collected from a floodplain lake system in the central Amazon Basin. Fish species distribution was examined relative to abiotic variables in seven vegetation strata on Marchantaria Island, Solimões River. Both environmental variables and species distributions were influenced by a river channel to floodplain-interior gradient. Species diversity was significantly higher in vegetated areas than in unvegetated areas, with deeper water Paspalum repens stands harbouring the highest diversity. As a result, species richness and catches were positively related to habitat complexity, while catch was also negatively related to dissolved oxygen (DO) and water depth. Low DO and shallow waters appeared to act as a refuge from predation. Fish assemblages were related to water chemistry, but species richness was not. Canonical correspondence analysis provided evidence that floodplain fish assemblages formed by the 76 most common species were influenced by physical variables, macrophyte coverage and habitat complexity, which jointly accounted for 67% of the variance of fish species assemblages. Omnivores showed no pattern relative to the river channel to floodplain-interior gradient while detritivores were more likely to be found at interior floodplain sites and piscivores closer to the river. Piscivores could be further separated into three groups, one with seven species associated with free-floating macrophytes in deep water, a second with five species found in shallow waters with rooted grasses and a third with six open water orientated species. The results suggest that fish assemblages in the Amazon floodplain are not random associations of species.     

Asian aquatic plant moth Parapoynx diminutalis,accidentally introduced earlier,contributes to control of an aquatic weed Hydrilla verticillata in South Africa

The Shire River drains from Lake Malawi to the Lower Zambezi River. Annual flow is dependent mainly on lake level, partially controlled by the operation of a barrage at Liwonde to regulate flows for hydroelectricity generation in the escarpment reaches of the river. Downstream of the escarpment, the floodplains of the Lower Shire River support important subsistence fisheries that yielded on average 5 100 t y^?1 between 1977 and 1993. Catches from the major fishery area, the Elephant Marsh, were positively correlated with mean annual flow for the same year. Since the target fish species (mainly clariid catfishes) were mostly caught in their second and third year, improved recruitment through higher breeding success at higher flow levels is less likely to explain interannual variation in catches than is greater upstream migration of fish from the Lower Zambezi River in high-flow years. In 1978 a sudden closure of Liwonde Barrage resulted in rapid drainage of stagnant water from the Elephant Marsh floodplain vegetation into the river, causing a mass fish mortality and thus a marked decline in catches from 1979, followed by gradual recovery by 1981. The delay in recovery contrasts with the annual direct catch/flow relationship, and raises questions about the most important influences on Lower Shire catch rates.     

Fish recruitment in a large,temperate floodplain: the importance of annual flooding,temperature and habitat complexity

1. Large river floodplains are considered key nursery habitats for many species of riverine fish. The lower Volga River floodplains (Russian Federation) are still relatively undisturbed, serving as a suitable model for studying the influence of flooding and temperature on fish recruitment in floodplain rivers. 2. We examined the interannual variability in recruitment success of young‐of‐the‐year (YOY) fish in the lower Volga floodplain in relation to flood pulse characteristics and rising water temperatures in the spring. We sampled four areas with different flooding regimes, in three consecutive years (2006–2008). 3. Extensive areas with a long duration of flooding accommodated high densities of young fish. This suggests that extended inundation improves the recruitment success of river fish. In areas with extensive flooding, the biomass of YOY of most fish species was about three times higher in 2006 and 2007 than in 2008. We hypothesise that low spring temperatures in 2008 may have caused this reduced recruitment and that a flood synchronised with rising temperature enhances recruitment success. 4. Extensive flooding was particularly favourable for species characterised by large body size, delayed maturation, high fecundity and low parental investment, such as pike Esox lucius, roach Rutilus rutilus and ide Leuciscus idus. Gibel carp Carassius gibelio, a species tolerant of high temperature and hypoxia, did particularly well in small waterbodies in the driest parts of the floodplain. 5. Structural characteristics of floodplain waterbodies explained much of YOY fish density. These species–environment associations varied from year to year, but some species such as common bream Abramis brama, roach and gibel carp showed consistent relationships with structural habitat characteristics in all years, despite large interannual fluctuations in flood pulse and spring temperature.     

Adaptive management of an environmental watering event to enhance native fish spawning and recruitment

1. A common goal of many environmental flow regimes is to maintain and/or enhance the river's native fish community by increasing the occurrence of successful spawning and recruitment events. However, our understanding of the flow requirements of the early life history of fish is often limited, and hence predicting their response to specific managed flow events is difficult. To overcome this uncertainty requires the use of adaptive management principles in the design, implementation, monitoring and adjustment of environmental flow regimes.
2. The Barmah-Millewa Forest, a large river red gum forest on the Murray River floodplain, south-east Australia, contains a wide variety of ephemeral and permanent aquatic habitats suitable for fish. Flow regulation of the Murray River has significantly altered the natural flood regime of the Forest. In an attempt to alleviate some of the effects of river regulation, the Forest's water regime is highly managed using a variety of flow control structures and also receives targeted Environmental Water Allocations (EWA). In 2005, the largest environmental flow allocated to date in Australia was delivered at the Forest.
3. This study describes the adaptive management approach employed during the delivery of the 2005 EWA, which successfully achieved multiple ecological goals including enhanced native fish spawning and recruitment. Intensive monitoring of fish spawning and recruitment provided invaluable real-time and ongoing management input for optimising the delivery of environmental water to maximise ecological benefits at Barmah-Millewa Forest and other similar wetlands in the Murray-Darling Basin.
4. We discuss possible scenarios for the future application of environmental water and the need for environmental flow events and regimes to be conducted as rigorous, large-scale experiments within an adaptive management framework.