Report No FR0300

W Parr

Oct 1992



The biological approach to reservoir management which offers the greatest potential for controlling algal levels, viz. the manipulation of fish populations has been identified.

The report allows an informed judgement to be made over further funding of a study to investigate the practicability of algal control by manipulating fish communities in UK reservoirs.

If it is decided to proceed with such a study, guidance is given on different approaches and appropriate fish stocking densities.


A recent review (Parr and Clarke 1992) identified manipulation of fish populations as a potentially useful method of controlling phytoplankton in reservoirs. This report examines the different approaches available to control algal levels using fish, and identifies which of these methods offers the greatest chance of success in terms of reducing total phytoplankton levels and, more specifically, levels of blue-green algae.


Phytoplankton may cause numerous problems for water treatment, including filter blockage, filter penetration, production of tastes and odours, disruption of coagulation processes and associated in-reservoir problems (rapid hypolimnetic deoxygenation with resultant increases in manganese levels, etc.). More recently, toxin production by blue-green algae has caused concern over the use of reservoirs for recreational purposes, as well treatment problems. A cost-effective method of controlling algal bloom development in general, and blue-green algae in particular, will reduce these concerns and help reduce the cost of treating water from some reservoirs.


Manipulation of fish populations can be used to control algal levels in reservoirs. Of the different approaches available, the use of phytoplankton-eating (phytophagous) fish appears to be the most suitable, followed by pike stocking. The use of other native fish offers little benefit in terms of lower algal abundance, and may make matters worse. Potential problems are noted regarding biomanipulation techniques, which will prevent their use in many reservoirs, but the use of silver carp to control algal levels is worthy of further study.


A biomanipulation experiment should be carried out in a small lake or lakes (ca. 5-20 ha) to test the practicability of using silver carp to control algal levels in UK reservoirs.


Methods of manipulating fish populations to reduce the levels of phytoplankton standing crops in UK reservoirs are reviewed. Use of silver carp appears to be the most appropriate method but further research on silver carp stocking is recommended before any definite conclusions on cost-effectiveness can be made. Practical problems also need to be identified.

The evidence surrounding the contribution of blue-green algae to the diet of silver carp often appears to be contradictory. Most authors acknowledge that phytoplankton usually forms a large part of the food of this fish, but whilst some workers have shown that the fish actively select against blue-greens by feeding predominantly on other phyto- and zooplankton and/or by increasing the contribution of sediment to the diet, other authors have noted that blue-green algae may be almost the sole constituent of the diet during intensive blooms. Despite this, some blue-green algae are not as readily digested as other phytoplankters such as diatoms, which in turn tend not to be as easily digested as zooplankton.

Nevertheless, phytoplankton populations can be controlled by silver carp. However, stocking density is crucial to the success of this programme. At high stocking densities predation on zooplankton is so heavy that grazing (by zooplankton) on smaller algae is reduced to such an extent that these smaller algae are able to grow to higher densities than if no silver carp were present. Refuges in which the zooplankton can grow without being grazed by fish have reversed the stimulatory effect of high silver carp stocking density on phytoplankton levels in small-scale experiments, but not in full-scale reservoir studies.

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