Book: Waterways & Wetlands Chapter: 3 Management for conservation and amenity Section: Fish Metadata: Details Buy this book

Fish are a good indicator of the water quality and general health of rivers and other watercourses. Fish in ponds and lakes need careful management if populations are going to remain in balance, and not cause deterioration of the ecosystem. Small wildlife or educational ponds should not be stocked with fish, and any stocking of large ponds, lakes or watercourses should only be done after consultation with the Environment Agency and English Nature (or equivalent).

In many cases, the management of water for angling or fisheries can combine well with conservation management. Both are aimed at maintaining a rich and varied ecosystem with a good variety of plant and invertebrate species, although conflicts may arise where predators such as herons need control. For further reading see Alabaster, J S (1985) and Muus and Dahlstrom (1971).

General requirements

The two basic requirements for fish, which together determine the character of any fishery, are:

• Adequate oxygen in the water. Game fish (salmon, trout and grayling) require much higher concentrations of oxygen than do coarse fish (carp, tench, roach, rudd, bream etc). Since cold water holds more oxygen than warm water, game fish are limited to cool waterways and, in general, to swift streams where turbulence increases aeration. Trout survive in ponds but only where the temperature never exceeds 20°C (68°F) and where summer warming plus bacterial decomposition does not fatally lower the dissolved oxygen content. Coarse fish, by contrast, tolerate fairly wide temperature fluctuations and a few species, such as carp, persist even in highly eutrophic or polluted waters with a low oxygen content.
• Adequate food, which in turn is largely determined by the water's pH. Acid waters never support as much life as alkaline waters, and trout in mountain tarns, for example, may grow to no more than one sixth the weight of trout in productive chalk streams. Coarse fish prefer a pH between 7 and 9 with pH 7.5- 8.5 being ideal. Fish feed principally on smaller creatures which live on or among water plants or which fall into the water from overhanging trees and shrubs. It is therefore always important to allow aquatic and bankside plants to flourish in some areas even if they are kept within limits elsewhere.

Further requirements of game fish, but not coarse fish, include:

• Clear flowing water for breeding. Whether migratory or not, all game species reproduce only in shallow gravel-bottomed streams which do not become silted due to slow flow.
• Water free from pollution. High concentrations of toxic substances or extremes of deoxygenation kill all aquatic life. In general game fish are affected far more quickly than coarse fish by water quality, and thrive only in water which is free of pollutants.

Management should always be in stages or restricted to limited areas at any one time, to avoid disturbing or harming fish. Too much stirring up of bottom ooze can lead to increased decay and deoxygenation. Extensive weed clearance can do the same if the cuttings are not removed, while food supplies are inevitably disrupted and temporarily reduced by this work

Stocking

On rivers, stocking of fish for angling has only short-term benefits. An improvement in habitat requirements and water quality is the only way to make long-term improvements.

On new ponds where the aim is angling, it is best to prevent fish entering the pond as it fills so that the population balance can be fully controlled. Do not stock until the pond has had time to develop a healthy aquatic vegetation and to achieve a measure of stability. If possible, wait at least one full year. Meanwhile, you can encourage pond life by planting marginal and aquatic vegetation and by introducing invertebrates. These may be collected from nearby ponds of similar water type, transported in water-filled containers and dumped into the shallows. When collecting, net free-swimming creatures in shallows, pick over the bottom stones to scrape off animals clinging to them and scoop out some of the surface mud from the bottom. Do not delay too long before releasing the animals since predators are at an advantage in confined conditions. Snails and freshwater shrimps may be collected or bought from fish farms. Snails should be introduced in stony shallows where they can feed on algae, while shrimps should be scattered in sandy shallows and among the beds of water plants. Suitable sanctuary areas surrounded by fine-mesh wire netting help snail populations establish themselves free from predation by fish.

Initially fish should be stocked in new ponds at a rate of no more than 30kg per hectare (150lbs per acre) of water surface, even where food and plants have been provided. Otherwise the pond's resources may be depleted and the programme set back for some time.

Suitable species for introduction to new ponds include :

• Carp (Cyprinus carpio). These are bottom feeders which do not spawn in most waters in this country. This is an advantage in new ponds because it gives other aquatic organisms a better chance to develop.
• Tench (Tinca tinca). These are bottom feeders which, like carp, sift the mud, but take different food and so do not compete with carp.
• Roach (Rutilus rutilus), which feed on algae, plant fragments and insects among the beds of aquatic plants. Roach often over-breed but the surplus can be netted.
• Rudd (Scardinius erythrophthalmus) which feed from the surface and in mid-water and so do not compete with bottom feeders. Rudd tend to hybridise with roach in small pools where spawning places are limited, so it is best to choose between the species in this case.
• Bream (Abramis brama). These are bottom feeders which should only be introduced after the pond has had time to mature. They tend to stir up the mud, causing turbidity which inhibits aquatic plant growth.

Species which should not be introduced, where angling is the main concern, include small fish such as gudgeon, ruffe, minnows and sticklebacks and the piciverous perch and pike. A few large perch may be useful to keep down the number of excess fry, but they tend to over-breed and small perch deplete the insect life needed by other fish. They do, however, provide amusement for young anglers and winter fishing when other species are semi-dormant.

Resting and feeding

In still waters, resting and feeding habitat is generally assured, provided there are extensive shallows which are well-vegetated but not choked with weed growth. The situation is a little more complicated in streams.

Fish in streams seek spots where they can get as much food as possible with the least effort, at the same time fending off competitors and protecting themselves from predators. In swift streams and small rivers, fish settle in pools between which lie stretches of 'thin' water where the current is swift and clear over boulders or pebbles. Thin water is unattractive to fish because it is too shallow and exposed unless waterside trees and shrubs provide cover. Also, thin water areas supply little food although they do support a specialised insect life adapted to them.

Typically, each suitable pool holds a single dominant fish, quietly positioned just off the current where virtually no energy is needed to stay in place while it waits for choice morsels to float past. Other fish occupy progressively less satisfactory positions down to the smallest, which find the least food and use most of their energy fighting the current. If the dominant fish is caught, the sub-dominant fish moves into place almost immediately and so on down the line. In order to increase the overall population, yet at the same time avoid a diminution in the size of individual fish, it is necessary to create new pools in less suitable stretches. It does no good to simply dump more fish into the stream, since they just drift down, or less often work their way up, until they find a favourable location. New pools and resting places can be created in 'thin' water by:

• Building stone or cement-bag weirs (see dams, weirs and sluices) or stone-filled gabion 'croys' (see banks, revetments and access) part way across the stream so that scouring occurs below each weir to form a new small pool. If the weirs are extended farther they deflect the current against the opposite bank. This may be desirable if the margin is badly silted but should otherwise be avoided.
• Installing temporary log or concrete slab dams held in place with steel spikes or lengths of angle iron driven into the bed (see dams, weirs and sluices). These are most suitable for the fairly sedate chalk streams of the South, where it is necessary to maintain summer water levels. Before the waters rise in winter, the stakes should be removed and the pieces pulled onto the bank for reuse next year.

Spawning

Game fish

Trout (Salmo trutta) and Salmon (Salmo salar) spawn in autumn and winter, in fast-flowing water about 300mm (1') deep over beds of small friable gravel. The ova take a long time to develop: from six to twelve weeks for trout and from seven to twenty-eight weeks for salmon, depending on the water temperature. Grayling (Thymallus thymallus) spawn March-May in slightly deeper water over sand or gravel. Their eggs hatch in three to four weeks, depending on temperature.

There are several threats to successful spawning of these species, aside from predation:

• In calcareous streams, calcium carbonate deposits may lock the gravel into a hard mass, preventing fish cutting the 'redd' or bowl-shaped hollow which holds the developing ova.
• Certain areas, particularly the inner bends of meanders, may be attractively gravel-bottomed in the spawning season but later become silted, smothering the ova.
• Where spawning areas are inadequate, successive mating pairs of fish may use the same spot to dig their redds, exposing the ova of previous pairs, which cannot then survive.

To prevent these problems:

• Rake over the spawning beds with a sharp steel rake in early autumn, before the spawning season when water levels are low for easy working. This breaks up solidified gravel and dislodges mud and vegetation from the previous summer. Inspect the beds in the winter and rake them out again if they get muddy. You need to know what water levels to expect in the spawning season, since in early autumn some of them may be out of water.
• Do not clean out areas likely to suffer siltation, such as the inner bends of meanders. These should not be made attractive to spawning fish. Instead, concentrate on straight lengths of shallows.
• Extend spawning areas, if they are inadequate, by cleaning out side streams or carriers about 600mm (2') wide, removing mud down to the gravelly bottom. This also disturbs any hibernating eels, which prey on small fish, and sends them downstream.

Coarse fish

Coarse fish depend on higher water temperatures for spawning than do game fish, and so spawn later, mostly in May and June. The ova, which in most species are deposited on underwater plants, wood or among stones, only take from several days to a fortnight to develop, depending on the species and water temperature.

Predation

Fish spawn from several thousand to half a million eggs each season, depending on the species and the size of the individual. Of these, only a few are destined to develop into adults. Predators take a heavy toll at all stages, but especially among very young fish which are important in the intermediate links of the aquatic food chain. Predation becomes a problem only where angling is involved or in restricted conditions such as small ponds or feeder streams where predators are at an unusual advantage.

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