Zebra mussels are also commonly found in the recessed areas around some keel coolers. Without some control measure in place, zebra mussel populations can become quite dense and may reduce the efficiency of heat exchange through the channel. A dense build-up of zebra mussels in this area can also lead to accelerated damage due to pitting.

Operational Considerations: Vessels operating primarily on the Illinois, Ohio, and Upper Mississippi rivers tend to have the largest infestations of zebra mussels. The water on these rivers is relatively cool, slow and contains only a moderate amount of suspended sediment - very good conditions for zebra mussel growth and survival. In addition, the lock and dam system and extensive backwater areas may offer greater opportunities for the development and retention of zebra mussel larvae. Vessels operating primarily on the lower Mississippi will also suffer some infestation, but maximum infestation levels are likely to be much lower. The lower Mississippi has reduced opportunity for settlement, and the high temperatures and suspended sediment cause periodic zebra mussel die-offs during the summer.

Vessels that come in frequently for maintenance can be checked and treated for zebra mussels at the same time. Vessels which are in the water for more extended periods of time - such as excursion vessels, face a higher risk of zebra mussel build up.

Structural Considerations: Zebra mussels will only affect systems that have contact with raw river water. When operating in a medium or high risk area, you will have at least some zebra mussel infestation. You must answer the following question: is it critical that these areas remain free of zebra mussel infestation?

Zebra mussels will attach to the walls and grating covering the sea chest. Zebra mussel infestation may reduce the size of the sea chest, reduce suction and increase sedimentation by blocking the grating. Infestation in the sea chest may also lead to infestation in the piping system. Narrow diameter piping will become blocked more quickly than larger diameter piping. Copper is toxic to zebra mussels and copper piping will not be vulnerable to infestation.

Several commonly used alternatives for zebra mussel control are listed below. Heat treatment and anti-foulant paint have been recommended by the US Army Corps of Engineers for zebra mussel control on Corps vessels. Despite some potential problems, some have chosen to use chlorine to treat for zebra mussels on towboats. An effective treatment using chlorine is suggested below. We have only listed non-specific treatments for zebra mussel control. There are many companies that carry other products. Some of these products or treatments may be more effective for your operation.

Mussels may also build up in the internal piping system exposed to untreated raw water. Some shipyards clean this area by pigging. This procedure is akin to that used for removing obstructions from a sewer line. This procedure is fine if there is only minimal mussel build-up, but problems develop if the infestation is more severe. At least one shipyard has had to remove a section of piping that became completely blocked during pigging.

The disadvantages of reactive treatment are:

1. Some build-up is allowed and there is therefore some risk of reduced efficiency
2. The zebra mussel uses byssal thread fibers to attach to the hull. When the mussel is removed, these fibers remain attached to the surface. This creates a rough surface and can encourage new settlement.

If dry-docking is infrequent or there are critical areas on the boat that must be maintained free of zebra mussel buildup, there are several alternatives for preventative control:

Several vessels have had the sea chest and the covering on the sea chest painted with anti-foulant paint. The most effective anti-foulant paints contain either (1) copper or zinc which are toxic to zebra mussels, or (2) contain silicone that discourages attachment. If a vessel has had zebra mussel build up in the recesses around the keel cooler, it is advisable to paint the area with anti-foulant paint as well. The results from this treatment have been very encouraging, and there have been very few or no mussels found attached to areas treated with anti-foulant paint. It is unclear whether zebra mussels are still able to attach to untreated piping leading from a treated sea chest.

Another alternative is treatment with heat. This method is advocated by the US Army Corps of Engineers for zebra mussel control on Corps vessels. REF CORPS WEB SITE If the temperature of all the water in the sea chest is raised to 100⁰F for at least one continuous hour, all zebra mussels directly affected by this water should be killed. In order for this treatment to be effective, it is important that the heated water affect the covering on the sea chest as well as the area inside the sea chest. This treatment has not been installed on any vessels yet, but has been recommended by several experts on zebra mussel fouling. The advantage to this type of control is that the piping system can also be treated without risk of increased corrosion.

It has been noted that zebra mussels are able to attach to areas around the skin keel coolers despite the high temperature of the engine water. It is possible that the raw river water moving past may offer the mussels enough relief from the high temperatures of the coolers. This area, however, needs further study.

Recent experiences have shown that regular applications of chlorine can be effective in controlling zebra mussels on vessels operating on the upper rivers. Vessels using this method of control have reported no mussel infestations in the sea chests although mussels are still found attached to keel coolers if they are not treated with antifoulant paint. To get effective control, one chlorine puck (like those used in pool strainers) is introduced into the sea chest once a month. This form of chlorine is easy to handle and requires minimal retrofit. The disadvantage of using chlorine is the increased risk of corrosion to the sea chest and piping.

Regardless of whether heat or chlorine treatment is used, it is not necessary to treat every month of the year. Abundances of larvae are very low from 1 November to 30 April and treatment during this time is therefore unnecessary. From 1 May to 30 October, treatment should be done monthly to keep new larvae from becoming established. In this way, zebra mussels are eliminated from the system when they are very small, and no problems are caused by the release of larger shells from dead animals. We strongly recommend that the treatment water be discharged while the vessel is underway to minimize the effects of very hot water on the animals living at dockside.

Although zebra mussels have become the focus of attention, it is possible that domestic towboat and barge activities can also transport other unwanted species.