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In many temperate and sub-tropical regions the flood coincides with rising temperatures of spring and summer thermal coupling. This favours the growth of young fish by increasing the amount of food available and the rate at which it can be metabolised.

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Delay of flooding until late summer or early autumn in most rivers would result either in failure of the fish to spawn or in poor growth and low survival of the young fish due to the lower floodplain productivity in the cooler season. In some rivers such as the Murray- Darling in Australia and the Okavango in Botswana the downstream flood occurs during winter thermally decoupled and floodplain dependent production is rel- atively low in both these cases.

There is little evidence for floodplain use by any life history stage of any fish species in the Murray-Darling system Humphries, King and Koehn , which may be due to the limited advantage of occupation of the floodplain during the cooler time of year.

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In some temperate systems occupation of the floodplain or the anabranches and backwaters of the main channel in winter appears more as a refuge from high flow than a feeding and breeding migration Holcik In natural systems floods may be interrupted by one or more drought periods. Discontinuities are also induced in regulated systems when the primary user places demands on the water that interrupt the smooth progression of flooding. Such discontinuities may be particularly damaging to white and grey fish total spawners, which may spawn during the first flooding but whose eggs and larvae are then unable to colonise the floodplains because of the temporary recession of the waters.

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Black and grey fish multiple spawners are less likely to be affected by such discontinuities but may lose one or more broods when the floodplain dries during the recession. The smoothness of the flood is a measure of the steadiness of the rise and fall of the waters. It is the inverse of flashiness, which is the rapidity with which the river responds to local flood events. As smaller streams respond only to rainfall on their immediate basin they are extremely flashy. As the basin area increases the river tends to average out the rainfall over its surface and thus becomes less and less conditioned by local events Figure 3.

Fish faunas of smaller rivers and low order streams must have reproductive and shelter seeking behaviours that are adapted to sudden changes in the discharge if they are to survive. However, species living in higher order systems are usually better adapted to smoother flood curves. The smoothness of the flood curve is particularly critical for total spawning white fish, as temporary recessions can interfere with larval drift in the same way as discontinuities in flooding.

For example Nikonorov, Maltsev and Morgunov found that there are no important spawning grounds for sturgeons left downstream of the Volgograd reservoir in the Volga River due to the sharp fluctuations in water level resulting from the operation of the power station. The fluctuations cause mass destruction of sturgeon eggs and oocytes were resorbed in 30 percent of female sturgeons. Severe fluctuations in level also pose potential difficulties for marginal spawners and some classes of nest builders such as T. Excessive, rapid variation in level can strand attached egg masses of the marginal spawning phytophils resulting in the failure of that batch of spawn.

Equally retreating waters could expose nests leaving the eggs and fry to desiccate. Similar arguments apply to many of the invertebrates that serve as one of the major food sources for the growing fish. Figure 3. The rate of the rise and fall of the water level is potentially critically important for many organisms.

Overly rapid changes in level can affect fish more directly. During the rising waters rapid increases in level can submerge nests of bottom breeding species to too great a depth.

Tilapias Oreochromis, Sarotherodon and Tilapia species , for example, will tolerate only a narrow range of depths and substrate types for their nests. If the water is too deep, turbidity and low light levels do not permit them to complete their breeding. The rapid currents associated with such transitions in water current can sweep larvae and eggs of phytophilous species that deposit their eggs on the margins of floodplain and species with pelagic and semi-pelagic larvae in the main channel past their appropriate destination. During falling waters an overly rapid retreat of the flood is commonly assumed to increase the risks of stranding of fish in the temporary pools and channels of the floodplain resulting in unduly high mortality at this critical season.

The amplitude of the flood reflects the difference between the water level at low water and the maximum level reached during the flood. The higher the flood the greater the area of floodplain submerged. This means that the area available for nutrient recycling according to the flood pulse concept is greater Junk et al. Deeper higher amplitude floods produce greater flooded areas that can provide spawning sites, food and shelter for the fish.

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The influence of amplitude on fish with drifting larvae is less easy to speculate upon, as the factors affecting survival and growth during the earliest drifting phases is generally unknown. In some species such a Prochilodus and Semaprochilodus adult fish may be stranded in floodplain lagoons that are isolated from the river.

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Those closest to the river are connected yearly during flooding but lagoons at greater distances are connected less frequently and only during floods of greater amplitude. This accounts for the correlation between Prochilodus abundance and flood intensity in the Orinoco found by Novoa and in the La Plata system by Quiros and Cuch Periodic higher floods would therefore renew the fish and other faunas of lagoons that are more separated from the main channel.

The duration of flooding measured from bankfull on the rising flood to bankfull at drawdown influences the time available for fish to grow and for them to shelter from predators. As such, longer duration of flooding extends the growing season resulting in heavier fish that have a greater potential to survive the following dry season and an improved reproductive potential.

Duration of flooding may also affect the floodplain vegetation. This in turn contributes to de-oxygenated conditions in the system.

Similarly changes in flooding patterns can alter the viability and composition of floodplain forests as with the disappearance of the red gum in parts of Australia Bren and Acacia species in the Pongolo system in South Africa Furness Because both amplitude and duration can have positive and negative effects on the dynamics of the various fish species, the optimal flood for any group of species probably lies in a compromise between the two.

Any volume of water available for environmental flows or constructed floods can have a number of ratios depending on the way in which the water is released Figure 4. Because different species respond differently to different types of flood regime, a correct balance between these various factors for all fish species may be difficult to achieve through a standardised flood repeated annually and a range of flood types over a number of years may be more suitable.

Figure 4. Environmental flows and the constructed floods associated with them call for a manipulation of the amounts of water in the river. Very often this will be a determined volume negotiated with other users of the resource as reserved for the needs of the living aquatic organism.

It is then essential to make the best use of this water. Given that some degree of flooding of the floodplain is needed to secure the survival of many of the species comprising river fish communities, the relationship between the amplitude and duration of the inundation is critical. In these circumstances a long flood of low amplitude will produce a smaller flooded area for a greater duration, which means that reproductive success and fry survival may be lessened but that growth may be enhanced.

However, if the flood is of high amplitude but too short a duration reproductive success may be higher but the young fish may not have sufficient time to grow and store sufficient fat. This would increase later losses through predation, as the smaller fish are more vulnerable and would also lower survival through the prolonged dry season as energy reserves may prove insufficient.

Density-dependent mortality might also rise as the larger fish compete for reduced trophic resources. Baran et al. On the other hand the model of Halls et al. Unfortunately there are very few analyses of catches by floodplain fisheries that have been carried to the level of detail needed to resolve this question. At intervals flood patterns can deliver extreme events that may challenge the capacity of the physical and living components of the ecosystem.

Such extreme floods have tragic consequences for human populations whose occupation of the riparian zone of the river is adapted to more normal events. Living aquatic organisms can be severely affected by both abnormally high and low discharges. High discharges can wash away adult and juvenile fish, especially in rivers that have been hard engineered to contain flow in the main channel.

Similarly, drifting eggs and larvae can be washed past suitable floodplain nurseries and lost to the population. Extremely low flows may operate mainly on water quality. They can lead to deoxygenation of the water through natural processes or through the failure of self-purifying mechanisms to correct human induced euthrophication see articles by Szmes and Leibman and Riechenbach Klinke in Leipolt In extreme circumstances low flows can lead to desiccation of much of the riverbed and of an increased percentage of floodplain water bodies.

The dry season is a period of great stress to the majority of river fish species. At this time most species are confined to the main channels of the river although some specialists can survive in permanent floodplain waterbodies. Variations in water level at this time can have a great impact on the extent and nature of various habitats for a range of organisms including fish, Puckridge et al. Flow may cease in the main channel and deoxygenated conditions may appear both in parts of the river channels and in the floodplain waterbodies.

The numerous individuals generated during the flood have to find space in the much-reduced environment on the floodplain itself the water volume during the dry season may be less than 5 percent of the volume during the flood. Many species seek refuge in tributaries and in deep pools within the main channel, thus conservation efforts have to be directed at maintaining adequate water in such habitats.

On the floodplain, insufficient channel flooding can result in the permanent waterbodies becoming desiccated and their fish populations defunct. Many species feed little during the dry season, an effect that Lowe-McConnell termed the physiological winter. Conservation measures should seek to ensure that adequate water is provided so that a number of floodplain water bodies and the refuge areas within them are maintained with adequate water in them throughout the dry season.

Fish are at their most vulnerable to the fishery and other predators during the low water period, so both main channel and floodplain refuges should be protected by law against illegal and excessive fishing. The models of Welcomme and Hagborg and Halls et al. Stabilizing river flows to an almost constant discharge throughout the year may appear more efficient than retaining a pronounced flood pulse in that it would avoid much of the drawdown mortality and apparently lead to more stable fish stocks.

It would favour fish species that are repeat spawners and are able to survive in the main channel alone Lae The alternation between dry and wet phases confers an advantage in terms of overall aquatic productivity in fluctuating systems, such as flood rivers and lakes, as compared to more stable systems Junk et al. The advantage of the flood cycle to activities other than fishing, such as drawdown and irrigated agriculture, cattle grazing, wildlife is such that it cannot be ignored in planning for sustainable use of such land-water interface zones.

The question of what comprises the optimal relationship between the duration of the flood and the period when the river is separated from the floodplain during the drawdown remains unresolved. Models provide information on the dynamics of fish populations under different regimes of low and high water Figure 5 but assume the flood as a feature of the model.

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Generally the longer the flood-phase the shorter the period of low water with its attendant high mortality. Figure 5. Contour plots of equilibrium yield t for P.

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Rivers are used for a number of human functions other than fisheries and the needs of high economic profile activities, such as power generation, frequently cause conflicts between abstractive industries and the water requirements of the fish and of fisher communities.

Many of the current provisions for flow and flood regime control are inappropriate to fisheries in that the flood is managed for the conflicting objectives of fisheries, animal grazing and agriculture particularly drawdown agriculture and rice culture. In such conflicts the agricultural interest invariably prevails. One reason for this, apart from the greater financial and political power of the agricultural lobbies, is that the flood conditions required for agriculture are relatively well understood, whereas the requirements of fisheries are less clearly defined. It is part of the purpose of this paper to draw attention to the need to better refine fisheries models in order to represent fishery interests more effectively in negotiations for the allocation of water to fish.

Four main tools exist for predicting the responses of fish species to differences in flooding in large rivers produced by human agencies.

Knowledge of the biology of individual species can be used to predict the reaction of the species to some characteristics of the flood curve such as timing, smoothness and rapidity of change. Modelling of fish community responses to differences in flood regime are more appropriate when looking at dynamic issues such as amplitude, duration and the relationship between them.