On some catfish farms, for example, great blue herons have been found to be feeding primarily on wild noncommercial fish. Spending considerable amounts of money on controlling herons may not be jus-tified in those situations, unless there is evidence that the birds are responsible for other problems such as disease transmission or spreading unwanted wild fish. Cormorants have been observed feeding exclusively on shad in some catfish ponds and appear to prefer shad over catfish when shad are abundant. In other situations, or on farms where wild fish are not abundant, herons, cormorants and other bird predators may be causing significant losses and control costs may be justified. Species and numbers of birds using the facility should also be considered.
Potential damage is greater for some species than others.
Additionally, birds often wound fish that they do not eat so potential losses may be higher than one would estimate by food requirements alone. If only 10 or 20 birds are using a facility, an aggres-sive scaring program may not be economically justified unless there isa risk that more birds will be attracted to the facility by the ones already using it, or unless a particularly highvalue crop worth several dollars per pound is damaged. No. 400 contains information on potential losses producers can expect from a number of bird species. Physical characteristics of farms or production facilities are also important in determining appropriate control strategies. Size of ponds or raceways is an important factor in determining whether exclusion or frightening techniques would be more cost effective.