High Cost of Bird Predation

Bird predation on fish farms can be a major cause of mortality of the farmed fish stock (including crustaceans such as marron and yabbies). The major predator in W.A. is the cormorant (shag) but a number of other birds, such as herons, egrets, kookaburras, kingfishers, ibis and grebes (dabchicks) also predate upon fish stocks. Recently butcher birds have been reportedly responsible for marron deaths during harvest operations. Of course all these birds are protected species under the Wildlife Conservation Act. the Department of Conservation and Land Management will issue licences to trap and remove offending birds (which usually return) and if necessary a shooting permit.

It may not be readily apparent that you have a costly problem if only a few of these birds are taking a few fish every day or so, but over the year these add up. And, unfortunately, cormorants are social birds which communicate new feeding grounds to others in their flock. Also most predation by cormorants occurs very early in the morning so that you may not be aware of the losses until harvest. One visual indication of cormorant predation in marron or yabby ponds is the scattering of gastroliths (the calcium store) around posts and other suitable perches. A recent paper by Rowland highlights this problem of predator increase for silver perch farming. Late one month 6 black cormorants discovered a pond but by early the next month there were 41 visiting the same pond. It was calculated that this flock of 41 black cormorants could have consumed all the fish in that pond in 25 days. Rowland reported that the value of fish consumed in the experimental ponds was $A107 000 against a cost of overhead bird netting of $A25 000. From these values it is obvious that the netting more than pays for itself.

A marron farmer who had both covered and uncovered ponds recently discovered the benefits of covering with anti-bird netting. The ponds concerned contained 0+ (about 6 months old ) which had been stocked at the same rate and when drained the uncovered ponds yielded around 500 0+ while the covered yielded in excess of 8000 0+ - a considerable difference.

Using a combination of Rowlands paper and papers on marron production (Morrissy) it can easily be worked out what the potential losses could be for marron farms. If a 1000m² pond was stocked at 3/m² (3000) and the average size was 10 g, one black cormorant would consume 50 - 100 marron in one day, an immediate dollar loss of around $25-$50 (commercial rate for 0+ is approx. 50 cents each), however the expected harvest of an average size of (100g) means that the marron eaten would have weighed 5-10kg with a value of around $150 - $300 (at $30/kg) which represents a considerable loss. This does not take into account stock that is damaged but not consumed and later dies. It is interesting to note that one black cormorant could consume all the marron in this pond in 30 - 60 days. The expected harvest from this pond would be around 250kg with a dollar value of around $7 500. Compare this potential loss with covering the pond which would cost approximately $2000 (variable depending on the situation).

There are a large number of other methods of stopping bird predation that have been researched or promoted in the past: electronic bird scarers, gas guns, strung parallel lines, falconry and strungup plastic birds of prey (scarecrows) but only complete enclosure with barrier netting, properly maintained, is 100% effective 100% of the time.

Anti-bird mesh is usually installed over the whole pond site of a fish farm at a height suitable to allow easy access of farm vehicles and machinery that are used in the day to day operation of the farm as well as for occasional pond maintenance. It is not recommended to cover ponds individually as it is generally more expensive as well as making access to ponds more difficult.

References

Conservation and Land Management. Hayman Road, Como

Morrissy N.M., Walker P., Moore W. Predictive equations for managing semi-intensive grow-out of a freshwater crayfish (marron), Cherax tenuimanus (Smith 1912) (Decapoda: Parastacidae), on a commercial farm. Aquaculture Research, 1995, 26, 71-80.

Rowland, Stuart J. 1995. Predation of Bidyanus bidyanus (Teraponidae) in Ponds by Cormorants. The Progressive Fish-Culturist 57:248-249, 1995.