Practical Hatchery Management of Warmwater Fishes
Chapter 8: Extensive Pond Production: Pest Control
Weeds occur in hatchery ponds because ponds provide suitable habitats for them to grow. Weeds make it difficult to harvest a pond, they provide shelter for predators, and make it more difficult to establish populations of desirable food organisms. The best way to avoid weeds is to eliminate suitable habitats. Aquatic weeds need light to grow and limiting access to light can help control them. Proper pond construction is the first step, deepening the pond edges to a minimum of 25 cm and having an adequate slope (≤2:1) so that a minimum depth of 50 cm is quickly reached. This combined with quickly filling the pond and establishing a phytoplankton bloom soon thereafter will help to limit light penetration to the pond bottom. By adequately shading the pond bottom it makes it difficult for many types of aquatic weeds to become established. Dyes can be added hatchery ponds in an attempt to limit light penetration in turn limiting emergent aquatic weed growth. Dyes are not appropriate in ponds where the fish are highly dependent on natural food organisms (i.e. primary nursery ponds). They are more appropriate for use in ponds where the fish are being fed a formulated feed. Aquashade® and other similar commercial products are effective in shading the bottom enough to limit the establishment of aquatic weeds in areas of a pond that are 50 cm deep or more.
Herbicides can be effective in controlling aquatic weeds, however, there are several things that need to be considered. Safely using the most appropriate herbicide is fundamental. Herbicides are designed to be toxic to aquatic plants but they are also toxic to a variety of other non-target organisms including humans. The applicator must read the label on the herbicide container and know the toxicity issues, safe handling procedures, modes of application, proper disposal, and other safety concerns. What herbicide to use depends on the type of aquatic weed to be controlled, its impact on non-target organisms, and any other associated risks.