Netting

Currently, netting is the most popular method for controlling bird mortality at toxic ponds.

Effectiveness:

Covering of a small pond with the appropriate net, properly maintained, is quite effective in keeping birds away from the covered pond. However for larger ponds netting has been shown to be a less than ideal solution. There are many anecdotal accounts of birds becoming entwined in the nets, and human rescuers working desperately from under the net (in a boat) trying to untangle the bird and convince the bird to fly away. Sometimes the effort is successful, often it is not. Nets covering ponds larger than about 1/2 of an acre have a history of collapsing under heavy snow loads, and during high winds. It has been well established that major migration events of large numbers of birds occur at night following episodes of the lowering of barometric pressure. On the BIRDAVERT research project, for instance, large numbers of birds were counted on ponds after a sharp drop in barometric pressure. In one instance over 900 Grebes along with several hundred ducks and geese arrived early in the morning following the passage of a weather front and the associated pressure drop. Examination of radar flight tracks as well as before and after pond counts also showed this same effect for the BIRDAVERT research as well as many other studies (pers. Comm. Brian Cooper, pers. Com. Sid Gauthreiaux)

Unfortunately accompanying these drops in pressure and associated large migration events are high winds and severe weather conditions. This leads to a very high likelihood that the net will be damaged or downed at the precise time when it is needed the most. The time expended in repairing or replacing the net can often exceed I week and history shows many landings by birds during the repair period. The health risks to the persons doing either the boat rescues or the repairs is unknown. Certainly a person falling into or coming into repeated contact with the water at some ponds would be disastrous.

Cost:

While netting is quite economical on small ponds (less than 1/2 acre) it is quite expensive to cover a large pond. And because of the logistics involved, the price per acre increases geometrically as the pond gets larger. For instance ponds of about 1/2 acre may cost 10 or 20 thousand dollars to net, while bids to net a 40 acre pond were 1.8 million and 2.4 million dollars respectively. Besides the high initial cost of netting for ponds larger than I acre, maintenance on larger ponds is extremely expensive. Netting suppliers typically tell their customers to expect between 25% - 35% of the original installation cost PER YEAR for maintenance. Even if a net does not suffer catastrophic failure they typically need to be replaced every 4 or 5 years due to the degradation of the netting material caused by UV radiation as well as other weather related factors. This applies to nets with the UV protection built in. Those without UV protection typically last about 2 to 3 years.

Summary of Netting:

In essence netting seems to be quite effective in keeping birds off of small ponds but is limited by it's inability to cover large ponds economically. Additionally, netting high maintenance costs and propensity for catastrophic failure due to high winds and or snow loading often lead to episodic events of high bird mortality (exact figures are unavailable). These realities often preclude netting as a viable alternative for defending large ponds.

An additional consideration is the effect of netting on the industrial process. Netting often interferes with the evaporation of water from the pond, typically the pond is designed to work as an evaporation concentrator. Slowing the evaporation process may lead to expansion of the ponds surface and volume.