Where feasible, exclusion and barrier techniques are highly effective for controlling fish predation by birds. Netting exclosures generally are feasible only on small ponds or raceways (Figure 1).
Relatively inexpensive polypropylene netting is commercially available for this use, and simple supported netting systems have been constructed on trout raceways for only $ 0.14 per square foot.
However, these simple netting systems often interfere with routine fish maintenance and harvesting operations and are susceptible to ice and wind damage. For these reasons, they should be constructed for easy set-up and removal.
Total exclusion with netting is the only completely effective strategy for controlling all bird predation, but it is not cost-effective or practical in many situations. Overhead lines or wires generally deter most species of fish-eating birds if spacings are narrow enough.
Polypropylene line with spacings of 10 inches are used on a number of trout farms and are purported to exclude virtually all fish eating birds. Parallel lines are usually strung across ponds from steel cables that encircle the pond and are supported from 15-foot-high poles or treated wood posts.
Additional lines or netting can be suspended on the sides to form a complete exclosure and prevent predators from walking in from pond levees. If properly designed, such systems can provide a durable, all-weather exclosure that accommodates maintenance and harvesting operations for smaller scale operations such as trout-rearing facilities.
Logistics have yet to be worked out to accommodate harvesting and feeding operations at catfish farms that typically utilize ponds of 12 to 15 surface acres. Existing levees on many catfish farms are not wide enough to accommodate poles while still allowing vehicle access.
The cost of constructing such systems at large catfish farms also reduces the feasibility of this strategy. Based on 1997 costs of $0.22 per square foot for materials alone, plus the cost of labor, it would cost approximately $1 million to enclose a 100-acre farm.
Other barrier systems using wire, string or floating rope on or near pond surfaces have been partially effective in deterring cormorants . Wire, line, and string have been placed in parallel or grid patterns with spacings of 25 to 50 feet at a height of 8 to 14 inches above the water, while polyethylene rope with foam floats (floating rope) have been evaluated at 50- foot spacings.
The concept is meant to take advantage of the relatively long take-off distance that cormorants usually require to take flight from a pond. Parallel wires, lines, and floating ropes should be positioned perpendicular to the prevailing wind as cormorants generally take off and land into the wind.
Colored streamers and Mylar balloons have been used to increase effectiveness of these systems. In field trials, these systems appeared to prevent large flocks from landing, but singles and pairs of cormorants learned to land among wires and floating ropes.
Wire and string systems have been installed on small ponds up to 2.2 acres in size for as little as $15/acre and required about 3 working hours labor per acre. However, on a larger pond (9.1 acres), costs were $164/acre and required 15 working days to install.
The physical constraints of spanning large distances and the extensive support systems needed for wire systems limit their usefulness and, most cases, interfere with seining operations on large catfish ponds.
Most problems inherent to wire, line, or string grid systems are eliminated with floating ropes that do not require a support structure. However, ropes would probably have to be unfastened from at least one side of the pond to facilitate seining at harvest. Installing floating ropes is usually less expensive than constructing wire grids.
In 1992 it cost only $39 per acre to cover a 14-acre pond with floating ropes. Electric fencing around ponds and earthen raceways has prevented herons and egrets from preying fish. Most recent trials have involved a two-strand system set approximately 1 foot from the water’s edge.
Wires were spaced about 15 inches apart and connected to insulators attached to fiberglass posts spaced at 50-foot intervals. Costs varied with the type charger and fencing used. Solar powered chargers and polyester coated wire were slightly more expensive but less time consuming to set up and maintain than battery powered chargers and conventional wire fencing.
Encircling a 5-acre pond with the former type of fence cost approximately $400. Protecting 0.5 miles of earthen raceways at a trout hatchery cost $875. Fencing may not be effective where pond and raceway bottoms slope gradually from the bank, because wading birds can forage on the water side of the fence.
Even where the bottom drops off more steeply, herons have been observed flying over the fence and landing in deep water to take fish coming to the surface. Electric fencing will not deter cormorants,
gulls, and other birds that forage in the central part of the pond. Field tests of netting installed as a fence around the edges of catfish ponds to exclude herons and egrets have yielded mixed results.
In general, wading birds prefer to land on solid ground such as pond levees before wading into ponds. Perimeter netting would likely discourage the birds from walking into the pond to fish. This system may also be useful for deterring various birds from foraging along spawning mats at minnow farms.
However, in one field trial with horizontally-placed netting, great blue herons adapted by walking on and fishing from the net. Some birds even waded beneath the netting in bent-over body positions.
For best success with perimeter netting, water depth just beyond the net should be more than 2 feet deep to prevent birds from wading beyond the netting. Placing the net at a 45o angle over the water will discourage birds from standing on the net. Material costs for perimeter netting systems vary between $100 to $150 per acre (1997 prices).
Authors:
Gary A. Littauer , James F. Glahn , David S. Reinhold and Martin W. Brunson
