AFC#10 Farm Dam Management | african-fisherman

Drawing an analogy, the one end of the continuum would be equivalent to a game park or virgin forest while the other end of the scale would be marked by pen feeding or horticulture. Applying this continuum to the average stocked farm dam, the level of intensity roughly equates to something between a conservancy, where game has been introduced, and extensive cattle ranching. In other words these are not natural systems, by stocking with additional fish species the level of intensity has been unintentionally increased. Consequently an appropriate degree of intervention (management) is required to sustain the fish populations within the dams. Before we examine some management options we need to first decide what the aims and objectives are. Angling is an increasingly popular recreation and potential money earner on farms and ranches, the assumption is that the aim is to produce high quality angling. Exactly what ‘good fishing’ constitutes is highly debatable, but let's suppose for arguments sake that we need a 4lb average five fish bag, with the occasional 10 pounder to make it interesting. Our objective, therefore, would be to dedicate as much of the dam’s resources to achieving this aim as possible.

Some Principles and Effects

Any system, whether it be aquatic or land based, generally has finite resources and the basic currency of these resources are nutrients. Just as one controls the head of game or cattle according to the carrying capacity of the land, so too must we manage dams. Removal of excess animals is necessary for the others to survive and grow. In a natural system populations are controlled by natural cycles of starvation, disease and predation. However in dams stocked with exotic species, such as bass (Micropterus salmoides) and Nile bream (Oreochromis niloticus), the moderating effect of natural mortality from disease and predation is not felt as the responsible agents are not included in the stocking. Consequently these fish often multiply uncontrollably thus exceeding the carrying capacity of the dam. Under these conditions the fish display poor growth and begin breeding at a smaller and smaller size, a condition we all know as stunting. Unfortunately, stunting seldom remedies itself if the overcrowding continues as any available nutrients are channelled towards production of more offspring rather than growth.

Management Options

Do Nothing

This is the principle under which many dams are managed at the moment and in terms of fishing quality they more or less follow the same pattern of rise and fall. In the first years after stocking there is the ‘new dam syndrome’ during which the fish display excellent if not phenomenal growth. Under these conditions the introduced fish have abundant food and plenty of space, consequently there are no limitations to growth and they thrive. However as the dam matures free nutrients derived from the rotting of flooded vegetation and other organic matter become increasingly scarce as these resources are exhausted and become less available due to their assimilation by fish and aquatic plants. Tying up of nutrients obviously increases competition for food which means the fish are less able to acquire the quantities they need for maximum growth. This effect is compounded if there is successful reproduction, as not only is there competition for food but also space. Over time the carrying capacity of the dam is reached and stunting is inevitable. The rate at which this cycle progresses depends on many factors including the size of the dam, the species stocked, the amount of nutrient within the dam and the rate at which it is released. For example the nutrient released from flooded trees would be slower than that of flooded grass. The size of a dam is a factor which changes from season to season. Fluctuating dam levels correspond to phases of stunting and high growth depending on water level. Dropping dam levels concentrate the fish into less water which is nutrient poor. Furthermore, under these conditions fodder fish, such as bream, become highly susceptible to predation as areas of refuge, such as grassy shallows, are no longer available. If dams decrease substantially they often become muddy and dissolved oxygen levels decrease which stress the fish. This is further exacerbated by high concentrations of suspended silt which damages sensitive gill tissue of fish resulting in impaired oxygen uptake and a higher susceptibility to disease and predation. In visual predators, such as bass, muddy waters also affect their ability to feed. Consequently a fall in the water level and concomitant deterioration of the environment is often accompanied by high mortality of some fish species. This effect benefits more tolerant species, such as barbel (Clarias gariepinus), as nutrients are again freed in the form of dead fish. The pressure of overcrowding is also alleviated as the total fish population decreases. An increase in water level from low level results in much improved environmental conditions as space and nutrient levels increase as vegetation is again flooded. Surviving fish are able to grow and reproduce and the whole cycle begins again until the next drought. Consequently the fishing quality in dams that are not managed is highly erratic and often depends on the season’s rainfall. In dams which experience large fluctuations in the water level over time, there may be a total restructuring of the fish fauna as species which are more tolerant of poor environmental conditions, such as barbel, displace more sensitive species. So what may begin as a bass and bream dam might end up solely inhabited by barbel. Obviously dam management will never counteract the effect of drought but it can lessen the impact.

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Farm Dam Management

Compiled by Neil Deacon Bsc.Hons (Aquaculture & Fisheries Science), Ph.D. (Ichthyology), Rhodes University

An Introduction Into Dam and Fish Management

Before discussing specific points of dam and fish management it is important to first understand what is meant by dam management and what we mean to achieve by implementing any form of management. A dam in many respects can be seen as an aquatic farm. What can be grown in the dam, as with farming in the conventional sense, is determined by size, soil type, topography, nutrient level, pH and climatic conditions of the environment. The level of intensity at which a dam is 'farmed' depends on how it is managed and the expected return. Fisheries management is a continuum which at its most basic is a wild fishery, implying little or no management, extending to intensive fish farming in ponds where everything is controlled towards maximizing production.

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Surveying a dam to ascertain its Flora and Fauna make up is an important tool, and throw nets are very useful for this purpose.

Natural Cropping Through Predation

Our natural response to ‘fix’ a dam that has become over populated with bream is to add a predator or even two. Effectively this only tends to defer the problem to another species. By way of example, consider the potential impact of two commonly stocked predators, bass and robbies (Serranochromis robustus jallae), on a dam.

Micropterus salmoides commonly known as largemouth bass or black bass originate from North America and is represented in this country by two strains, the northern race and the Florida subspecies, M. salmoides floridianus. Superficially the subspecies are very similar, the only important difference is that the Florida strain grows faster and larger than the Northerns. As an exotic species, bass can be considered to be a super predator in our waters as the indigenous species, such as bream, have not really evolved strategies to cope with predation by this species. Consequently bream are a relatively easy target for bass and research in Lake Chicamba, Mozambique, has shown that during summer, bream comprise almost 100% of bass diets (O. Weyl & T. Hecht, Rhodes University).

Furthermore the spawning season of bass precedes that of bream by two months which means that young bass develop just in time to exploit newly hatched bream. This would be fine except that bass are extremely productive with a 1kg fish being able to produce approximately 6000 eggs at one spawning. Survival of young bass in our waters appears to be very high as initially there are abundant food resources in the form of bream, low predation and the relatively high water temperatures of our dams promote good survival and growth. It is not difficult to work out that under the right conditions even a few bass broodstock can result in the rapid population of a dam. Such rapid increase in the population can easily result in overpopulation with bass, which means the bream population is rapidly over-predated. As bream stocks diminish the bass run out of food and start stunting. Alternatively they eat everything, including each other, until all that is left is a couple of very large bass and nothing else.

The development of a robustus population shows a different pattern although can also end up stunted. From a management point of view robbies or Nembwe behave very much like other bream although their growth is not as good as mozzies (O. mossambicus), for example. Robbies, in common with other bream, have a high propensity for stunting as they become sexually mature at a small size. In robustus stocked in highveld waters this effect is compounded as they display relatively slow growth on account of a non-optimal temperature regime. Toots and Bowmaker (1976) recorded the growth of robustus to be only 75mm per annum in Savoury Dam (Henderson Research Station). No growth was recorded during winter months. In Lake Kyle (Muturikwe) in Zimbabwe growth is better with fish attaining 100mm in their first year (Richert, 1976). Female robustus of 200-250g produce + 250 eggs at a spawning and may spawn three to four times during a season, consequently population growth can also be incredibly rapid. Although robbies may quickly establish themselves in a dam, their initial impact on other bream species is negligible. Research in Kyle (Richert, 1976) indicates that at least until they are 250mm in length, robbies feed predominantly (80%) on invertebrates. As robbies become larger, 300-350mm, fish become more important in the diet (33%), however is still secondary to snails (55%). It has not been documented whether the importance of fish in their diet increases further as they get even larger. However, the knobbly gill rakers of robustus are an adaptation to an omnivorous diet, which is in contrast to the long spiky gill rakers of a specialised piscivore, such as bass. So in effect the stocking of robustus into a dam only serves to add an additional bream species which perhaps occupies an unfilled niche in the food chain, but does little to remedy overpopulation of other bream species. In fact may even add to the problem.

Managing Dams Stocked with Predators

From the previous discussion it is clear that a specialist fish predator, such as bass, has the potential to take over a dam which means fodder fish stocks, particularly bream, get heavily exploited and may even be eliminated. Accordingly dams have to be managed in a way that promotes survival of fodder fish so the populations of predators can be sustained. Therefore management of a dam stocked with a predator or predators requires a bipartite strategy which includes (i) control of predator populations and (ii) conservation of fodder fish. In effect the addition of a predatory fish has further increased the required intensity of management as now not only do the bream have to be managed but so does the predator population. One must bear in mind that any steps taken to conserve fodder fish will also protect the young of predatory fish, concomitantly increasing their survival. It should be noted that the addition of a predator, particularly bass, changes the character of a dam. Dams that produce good bream prior to the introduction of bass seldom continue to do so once bass are established. So before stocking your dam with bass you must decide whether you want bass or bream fishing as it is unlikely that will you have both. You should also bear in mind that the stocking of fish species falls under the jurisdiction of National Parks and Wildlife and therefore permission is required before undertaking any exercise of this nature.

Control of Predator Populations

How we manage populations of predatory fish in our dams is a question which requires research. For want of anything better a strategy (or philosophy) of ‘catch and release’ has been adopted for the management of bass in most Zimbabwean dams. The general indications are that this is not working as there is a tendency for bass to stunt and the fishing within the dams to go ‘off’ once the dam has matured. This phenomenon can be easily explained as a cycle of overpopulation followed by crash which leaves a few large surviving bass. Based on this we know that predator populations do have to be controlled (culled) but to what level has yet to be established. Using scientific methodology, some progress towards resolving this question was being made. Essentially this involved creating a database of growth in bass throughout the country from which it will be possible to calculate an expected growth rate for bass. Populations can then be managed so that growth rates are maintained at or preferably above the calculated average. This general strategy will aid in the identification and prevention of stunting, however, it will have to be refined so that a statistically normal population is maintained. Incorporation of slot limits, which is the catch and release of fish within a certain size range, is one of the tools used to achieve this. Generally these have to be determined for individual dams as other factors such as environmental conditions and fishing pressure need also be considered.

Conservation of Fodder Fish

In order for fodder fish (in particular bream) to survive they require refuges where they can reproduce, feed and rear young while safe from predation. The degree to which fodder fish require conservation will depend on the extent of natural refuges formed by the topography and structure within a dam. Breeding requirements of bream vary from species to species so it is important to ensure that there are suitable sites for successful reproduction of the species which are present. Protection of these sites from predatory fish is normally unnecessary as the relatively large size of bream broodstock generally excludes them from predation by bass or robustus. Certain species, such as niloticus, guard their nests very aggressively making them highly susceptible to capture on spinners and lures, therefore fishing pressure constitutes a major threat to breeding success.

Designating prime breeding areas as ‘no fishing’ areas during the breeding season should be considered. Large predators such as barbel can also threaten bream broodstock so numbers of this species should be controlled to reasonable levels. Bream are most susceptible to natural predation while they are young. Young bream will naturally congregate in shallow areas to avoid predation and to feed. Since bass and robustus will go into very shallow areas when feeding only a physical barrier will prevent predation of young bream by these species. The natural refuges provided by inundated grass and bushes is important to survival of very small fry, therefore complete removal of bank side vegetation, either mechanically or by grazing cattle is not a good idea. However a balance must be struck as in some dams the infringement of vegetation, particularly the red stemmed Chicamba, becomes a real problem, for although it affords protection to young bream it also smothers the shallows reducing the production of the microalgae on which they feed. Apart from anything else, uncontrolled bank vegetation also makes fishing on the dam difficult and unpleasant.

A well managed farm dam can support several sport fish, from tigerfish to bass and bream. Varied

species offer both bank and boat fishing, ideal for getting children involved.

As bream mature they seek deeper water so also require refuges in these areas. Submerged aquatic plants or weeds are natural refuges for small fish but can also choke a dam providing ‘less’ water for predators and have a high oxygen demand, so reduce the fish carrying capacity of a dam. The introduction of oxygen weed is not recommended as it spreads incredibly quickly and is difficult to control. Dams which already have aquatic vegetation, particularly oxygen weed, may require periodic physical removal of some of the weed if it develops large banks. There are herbicides which kill submerged water weed but these must be used with caution as the nutrients freed by the decay of the dead weed pollute the water. Dams with few deeper water refuges may require the addition of dead structure, but I will discuss this later under the section entitled ‘Increasing productivity’. Another possibility is the introduction of an indigenous eel grass, Vasneria spp. This plant seems to be ideal as apparently it stunts when excessively grazed which means when the bream population is too high the plant naturally exposes them to increased predation. The introduction of floating aquatic plants such as water hyacinth, Kariba weed (Salvinia molesta) and the small, red leafed, duck weed (Azolla nilotica) is not an option as they all have the potential to smother dams and therefore will eventually be detrimental to the fish. Transfer of these plants is also illegal. Unintentional transfer of all noxious weeds is a risk where boats are also used on other waters which may be infested, such as Chivero or Kariba. Cleaning and decontamination of non-resident boats should be considered as part of any management strategy.

Managing Dams Stocked with Bream

Most bream species suffer from the same problems outlined in the paragraph about robustus. The small size and the short time to reach sexual maturity predisposes all bream species to population explosions and consequently stunting. In wild populations and some dams predation by a host of natural predators counteracts this effect. Natural predation is usually focussed on smaller size classes which maintains a normally distributed population. In dams which experience selective fishing pressure in the form of gill netting (minimum size of fish targeted determined by mesh size) and angling, generally the larger size classes are eliminated. A normal structured population is often represented as a pyramid, the pinnacle corresponds to relatively few large fish while the base would represent early juveniles or fry. As the apex is removed the size of the breeding population is effectively increased because the presence of large fish prevents the smaller size classes from breeding successfully even if they are sexually mature. Large fish will always displace smaller fish from the best nesting sites and will often come into breeding condition before the smaller fish. This results in the disturbance of the attempts of smaller fish to breed and also exposes the offspring of smaller broodstock to higher predation pressure. It should not be forgotten that many of the larger bream species, including Oreochromis niloticus, O. mossambicus and Serranochromis robustus, will also prey on small fish if given the opportunity. If enough large fish are removed from the population, a situation arises where a disproportionate part of the population is reproducing successfully. As a result of their efforts the carrying capacity of the dam is exceeded and stunting gets progressively worse.

In order to prevent this we need to assume the role of natural predators and selectively harvest off excess offspring in the smaller size classes to allow the rest of the population to attain their maximum growth potential. In doing this the majority of the population channels nutrients into growth rather than reproduction. Selective removal of fish is achieved with relative ease by use of a fine meshed seine net. This method allows the unharmed release of large fish for conservation or capture by anglers. The quantity and size of fish that should be removed is assessed on the character of the dam (particularly nutrient content) and what is required in terms of angling quality. In other words the dam can be managed to produce fish of a size you want. However you cannot have your cake and eat it, the larger the average fish size the fewer there must be, unless additional steps are taken to increase the productivity within the dam.

Increasing Productivity

Earlier it was stated that any system has a finite nutrient resource which determines its carrying capacity, what use we make of this carrying capacity determines the productivity. We know from agriculture that productivity in the first instance can be increased by altering the environment to make better use of the land, such as clearing bush to grow crops. Non-arable land is not worth altering but we can make better use of it by grazing livestock or game on it and so increase productivity. We also know that generally productivity is increased or at least sustained by augmenting the nutrient resource with fertilizers. These basic principles also apply to increasing productivity in dams.