Aquatic Weed Management Control Methods

SRACPublicationNo. 3609

Aquatic Weed Management

Control Methods

JamesL. SheltonandTim R. Murphy*

Many different aquatic plants can be

found in, on and around fish culture

ponds. These plants range from

microscopic organisms known as

plankton algae which drift suspend-ed

in the water, to larger plants

rooted in the pond bottom. Certain

types of aquatic plants are essential

for fish production. However,

aquatic plants that interfere with

commercial fish production are con-sidered

to be weeds.

Intensive fish production often invol-ves

adding large amounts of commer-cial

feeds and inorganic fertilizers to

ponds. Nutrients introduced into the

water through feeds and fertilizers

often create an ideal habitat for

aquatic weed growth. Submersed

aquatic weeds are particularly un-desirable

because fish harvesting

seines will ride up over the weeds

and allow fish to escape. Ponds with

dense weed infestations can be im-possible

to harvest since the weight

of the weeds accumulating in the

seine can become too great to be

pulled. Additionally, separating fish

from weeds is a slow process and

can severely stress the fish.

*The University of Georgia.

.

 

Aquatic weeds

Aquatic plants that cause weed

problems may be placed into four

groups: algae, floating weeds, emer-sed

weeds (foliage above water) and

submersed weeds (majority of

foliage below water) (Figure 1).

Algae are the most common group

of weeds in aquaculture ponds. Shape

and size vary from microscopic

single- or multiple-celled plants to

branched plants that resemble sub-mersed

aquatic weeds. Unlike other

aquatic plants, algae do not produce

flowers or seeds. Algae are divided

into three groups: plankton algae,

filamentous algae (pond moss) and

the stoneworts (Charaspp. and

Nitellaspp.).

Plankton algae

produce the

majority of dis-solved

oxygen in

the pond and are

essential to fish

survival. In the

presence of sun-light,

green plants

release oxygen as

a by-product of

organisms consume oxygen. Because

of this diurnal cycle, oxygen con-centrations

are the lowest at dawn

and highest in the mid-afternoon.

Cycle imbalances can lead to oxygen

depletion and subsequent fish death.

In commercial fish ponds, excessive

plankton algae may result from the

high feeding rates necessary to pro-duce

large fish yields. In many cases,

fish production rates are limited by

the amount of feed that can be ap-plied

without plankton algae blooms

becoming so dense that dissolved

oxygenproblems cannot be managed.

The complexity of this cycle makes

attempts to treat ponds with algicides

to “thin out” excess algae growth very

risky. Although spot treatments of

plankton algae scums are effective,

photosynthesis

At night, plants

and other pond

problems with low dissolved oxygen

concentrations following algicide ap-plications

limit their use in fish culture

primarily for the control of filamen-tous

algae and stoneworts.

Certain types of algae produce com-pounds

which cause a musty flavor

or odor in fish flesh. These com-pounds

are absorbed by the fish and

can cause a highly offensive taste

known as “off-f lavor.” This condition

can be corrected within 3 to 10 days

if fish are moved to water that does

not contain these “off-flavor”com-pounds.

There is no definitive

evidence that thinning the plankton

algae bloom with algicides reduces

the incidence of “off-f lavor.”

Floating weeds float in or on the

surface of the water and obtain their

nutrients from water rather than soil.

Duckweed (Lemnaminorand

Spirodela polyrhiza) and watermeal

(Wolffiaspp.) are examples of com-mon

floating weeds.

Emersed weeds are rooted to the

bottom, but have stems, leaves and

flowers which extend above the

water surface. They primarily occur

on the shoreline and in shallow water

up to 10feet deep. Common emersed

weeds are waterlily (Nymphaeaspp.)

and alligatorweed (Alternanthera

philoxeroides).

Submersed aquatic weeds grow

under and up to the water surface.

Most submersed weeds have flowers

and seedheads that extend above the

surface of the water. Examples of

common submersed weeds include

hydrilla (Hydrillaverticillata) and

Brazilian elodea (Egeriadensa).

Management methods

Aquatic weed control is a manage-ment

plan that incorporates preven-tive

methods such as proper pond

construction and maintenance,

biological methods such as the grass

carp (Ctenopharyngodon idella)and

the use of labeled aquatic herbicides.

The development of an aquatic weed

management plan is dependent upon

correctly identifying the problem

weed(s) and selecting control

methods that are compatible with ef-ficient

fish culture procedures.

Basic methods used to control weeds

include preventive, mechanical,

biological and chemical techniques.

Determining which of these techni-ques

to use involves consideration of

the target weed species, fish produc-tion

objectives for the pond, secon-dary

water uses and the cost of

treatment options.

Preventive methods

It is easier and less costly to prevent

weed problems than it is to control

them once they develop. Careful

pond site selection and proper pond

construction practices are the first

steps in preventing aquatic weed

problems. Rooted aquatic weeds

and algae usually begin growing in

shallow water ( < 2 feet). Edges of

new and existing ponds should be

deepened so shallow water areas are

minimized. The USDA Soil Conser-vation

Service provides technical

assistance for pond construction and

renovation.

Farm ponds are commonly fertilized

t. increase the fish production .

capacity of the pond. Fertilization is

also an effective and economical way

to prevent the growth of many aqua-tic

weeds. Fertilization stimulates

the growth of plankton algae. This

algal growth is known as a bloom.

The bloom blocks sunlight from

reaching the pond bottom which

limits the establishment of rooted

aquatic weeds. The key to successful

control of aquatic weeds with fertili-zation

is to establish and maintain a

bloom before rooted weeds begin

spring growth.

Decreasing the pond water level

exposes shallow areas to freezing

temperatures and drying and can

effectively limit certain types of sub-mersed

weeds. For a drawdown to

be effective, the water level should

be lowered in the late fall and not -allowed

to refill until the early spring.

Some weeds, such as hydrilla and

cattail (Typhaspp.), are tolerant to

drawdown and cannot be controlled

by this method.

Biological control

The grass carp is a practical and eco-nomical

way to control certain types

of pond weeds. Grass carp effective-ly

control weeds with tender succu-lent

vegetation such as filamentous

algae and duckweed, but are ineffec-tive

in controlling weeds that have

tough, woody vegetation such as

waterlily and cattail. Many states

regulate the use of grass carp. Con-tact

your Department of Natural

Resources representative for state

regulations on the use of grass carp.

Mechanical methods

Various types of aquatic weed cut-ters

and harvesters have been devel-oped

for canals and large reservoirs.

Use of these machines is not practi-cal

in fish ponds. Early manual re-moval

of weeds by seining or raking

can prevent some weed problems.

Chemical control

Herbicides may be used to control

weeds in commercial fish ponds. The

first step in successful chemical con-trol

is accurate identification of the

problem weed. Weed identification

assistance is available through coun-ty

Extension and Department of

Natural Resources offices. After the

weed has been identified, a herbicide

that is labeled for commercial fish

ponds may be selected. The herbicide

label must be read and fully under-stood

by the user prior to application

to the pond. SRAC Publication No.

361, Aquatic Weed Management -Herbicides,

contains information on

commercial fish pond herbicides.

Integrated weed

management

Herbicides should be considered as

a temporary control method. Depend-ing

upon the herbicide selection and

the weed species, duration of control

can range from a few weeks to sev-eral

months. Long-term weed control

can be achieved by using a combina-tion

of recommended aquatic weed

methods. For example, use of the

proper herbicides followed by grass

carp stocking will effectively control

and prevent the reoccurrence of

most submersed weed problems