Control Of Aquaculture Bird Pests

The northeastern aquaculture industry, centered in Pennsylvania and New York, consists primarily of trout production, but local facilities also produce a number of warm-water species, including bait fish, catfish, and goldfish. There are several different culturing methods for producing these fish.

Although trout culturing occurs primarily in concrete and earthen raceways, it also occurs in ponds that are sometimes used for pay-to-fish sites. Narrow concrete raceways are usually set in rows over a small rectangular area, whereas earthen raceways are more linear and resemble a series of interconnected ponds that can meander over a large geographic area. Warm-water fish species are almost always produced in ponds at facilities varying in size from farms with a single 0.1-acre pond to several acres of concrete or earthen raceways to more than 100 acres of ponds.

The diversity of northeastern aquaculture and the adaptability of bird species to exploit this resource has led to correspondingly diverse bird-predation problems. Several integrated approaches are needed to alleviate these problems.

The following information will help producers identify and assess predation losses caused by primary bird predators as well as suggest species-specific control measures that are also cost effective for reducing these losses.

Table of Contents

Assessing Predation Losses

It is important to assess fish losses to birds before implementing control measures. Several approaches to assessing predation losses exist, but they vary with the number of birds, foraging rates, and size classes of the fish consumed. The formula in table 1 shows how to estimate yearly fish consumption by a particular bird predator by using species-specific information on bird feeding rates.

By using information on the sizes of fish consumed by each species of bird and on the value of these fish, the annual economic loss to bird predation can be estimated. For accurate estimates, the information used in this formula must be carefully obtained and applied. With most species, bird numbers vary considerably both throughout the day and throughout a given season. Therefore, several counts must be taken daily during the damage season.

Primary Bird Predators at Northeast Aquaculture Facilities

Great Blue Heron

The great blue heron, one of the most common and most numerous species at northeastern aquaculture facilities, is considered to inflict the most damage to the industry. It is readily distinguished from other species of heron by its larger size (4-foot body length) and its slate blue coloration, which is more mottled in juvenile birds. Adult birds have a white head; juveniles do not have this coloration. The blue heron occurs in varying numbers throughout the year and forages at aquaculture facilities primarily at dawn and dusk. Night foraging is rare.

While present at trout-rearing facilities, each heron consumes on average 2.2 live trout per hour. Average prey is 9 inches long, but trout up to 14 inches in length may be consumed. At warm-water facilities, great blue herons consume smaller, but proportionally more, fish. At these facilities, herons are thought to consume about 0.5 pound of fish per day.

Other Herons

The black-crowned night heron is not widely occurring but can be numerous at northeastern aquaculture facilities. These herons have a chunky body and a short neck; they are about 2 feet long, with black back and head and white stomach. The black-crowned night heron typically arrives at dusk to start feeding and may continue to feed through the night. At western aquaculture facilities, where they are a more common problem, it has been estimated that black-crowned night herons consume 1.2 live trout per hour apiece, and they consume trout averaging 7.5 inches in length. However, more limited data from the Northeast suggest that trout averaging 6 inches in length are the preferred prey size of the black-crowned night heron. Because of their nocturnal foraging habits, it may be difficult to determine the presence and the extent of predation of this species.

The green-backed heron is the only other commonly occurring heron species that frequents northeastern aquaculture facilities. One of the smallest herons, it has a chunky body, and short neck and averages 1.5 feet in length. It has a dark-olive back and streaked breast with relatively short bright-orange or yellow legs. Because it is primarily a solitary bird, it usually occurs at very low densities and is typically seen from dawn to dusk. Observations at northeastern aquaculture facilities suggest that the green-backed heron eats about 3 live fish per hour, and they average 4 inches in length.

Common Grackle

The common grackle, a frequent predator at northeastern aquaculture facilities, exerts an impact primarily in the spring months, when large breeding colonies form and small trout fingerlings initially are placed in outside raceways. After fingerlings exceed 5 inches in length, grackles shift their diet to invertebrates and grains and sometimes leave the hatchery altogether.

The common grackle is one of the largest members of the blackbird family and measures approximately 16 inches long. Males are slightly larger than females and are iridescent black. Females have brownish plumage. During daylight hours, grackles sometimes forage in large flocks. At northeastern trout-rearing facilities during the spring, grackles consume about 3 live trout per hour at an average length of 3 inches. At warm-water facilities, grackles are sometimes present but have not been observed to consume fish.

Mallard

The mallard is a common species of waterfowl that frequents northeastern aquaculture facilities. However, the predation problems it poses occur in isolated situations. Where predation situations do occur, mallards achieve extremely high densities throughout the day and have adapted to feed in trout raceways stocked with high densities of smaller fish. In these situations, mallards generally consume 4 fish per hour, with the fish averaging 4, but sometimes reaching 6, inches in length. In other situations, mallards may feed on only aquatic vegetation or fish feed, so careful observations are essential to determine whether losses of fish are occurring.

Belted Kingfisher

The belted kingfisher is seen at many northeastern aquaculture facilities throughout the year. However, the typically low densities at which it occurs and the smaller fish it consumes lessen its potential impact on the industry. Approximately 1 foot long, the belted kingfisher is easily recognized by the white band separating the dark head from the gray body. An efficient predator of small fish, the belted kingfisher forages throughout the day by plunging directly into the water to capture fish near the surface and then rapidly flies off with its prey. It consumes fish averaging 3 inches in length at a rate of almost 2 fish per hour.

Osprey

An efficient predator of larger, more valuable fish, the osprey achieves only low to moderate densities during the spring and fall migratory periods. Commonly called the fish hawk, the osprey resembles a large hawk with a 2-foot-long body and a wingspan of 4.5 to 6 feet. However, white on breast and head distinguishes it from other birds of prey. Although it may appear for only a few weeks of the year, its impact can be substantial in terms of the size of the fish it consumes. Ospreys take about 2 fish per hour that average 12 inches long and can consume fish up to 24 inches long.