Empirically Speaking

Floating Islands: Clearer Water and More Fish?

By Dr. Wes Neal

 Floating islands are increasing in popularity as a way to provide an attractive centerpiece in ponds as well as help improve water quality. These islands are hydroponic systems that, when fully covered by growing plants, are essentially wetlands that float on the water’s surface and remove nutrients from the water. They maintain better water quality, increase water clarity, and may improve oxygen availability in deeper water. Some manufacturers have even claimed that these islands can increase the fish production in ponds. While there has been a lot of research on floating island effects on water quality, especially nutrients, claims of increased fish production had not been independently verified. In this month’s Empirically Speaking, we will explore the assertion that floating islands can increase fish production for pond owners.

Floating islands consist of a polymer fiber platform that floats on the water surface and in which living plants grow. The plant roots penetrate through and hang beneath the floating mat into the water, and can greatly expand the island’s surface area. These roots become an attachment surface for microscopic aquatic plants (periphyton) and other slimy forms of life (biofilms) that are important to the pond. The periphyton community acts like a natural wetland to trap fine suspended particles and to remove nutrients. Because the plants and periphyton are attached to the island, nutrients are stored away from free-floating phytoplankton and the water becomes less green as a result. This also helps keep the nutrients in the pond instead of being lost downstream when the pond overflows.

Reported benefits of floating islands, including improvements in water clarity, nutrient reduction, and increases available habitat for fish, have been verified scientifically. Benefits to fish production are hypothetically possible, as improvements in fish habitat or increases in productivity at lower levels in the food chain should allow the pond to support a greater weight of fish. To test this hypothesis, we designed a simple experiment at Mississippi State University’s experimental pond facility to measure how floating islands affect fish production.

This research used small ponds of the same size and depth. Four of eight ponds received floating islands. More specifically, we use floating streambed wetlands, an advanced form of floating island that actively moves water from below the island up and across a channel in the center of the island using compressed air. The size of each island was about 2% of pond area based on previous research recommendations. The remaining four ponds did not get an island and were used for comparison.

Ponds were filled with water just before adding the islands, which were then covered with a mixture of planter’s soil, peat, and wetland plant seed. All ponds were stocked with bluegill and Largemouth bass following standard stocking recommendations. Ponds were allowed to develop naturally until draining and fish collection 20 months after final stocking.

The response of fish populations to the floating islands was noticeable. Ponds with islands had about 20% more fish by weight than those without. Growth rates were similar between treatments, and the difference in fish production appeared to be caused by more juvenile bluegill being produced in ponds with floating islands. Bass did not have time during the experiment to reproduce, and there was no difference in bass size or total weight between ponds with and without islands.

In other words, it appears that the manufacturer claims are true. Floating islands, at least the floating streambed wetlands we used in our study, do indeed increase fish production. The lack of differences in growth were not surprising, as the ponds in this study were not managed as fisheries, and no harvest occurred of either fish species. Under normal pond management conditions, harvest of smaller fish is used to reduce competition for food resources, thus channeling the increased productivity into fewer, larger, faster-growing fish. It would be very interesting to repeat this study in managed ponds to see if this theory holds true for ponds with floating islands.     

There are a number of reasons why we may have seen this increase in fish production. First, the root systems below the islands from plants growing on the surface act to expand surface area for periphyton and biofilms in the water column. These tiny plants and critters create a concentrated wetland effect. More specifically, bacteria, algae, cyanobacteria, heterotrophs, and other microbes develop into three-dimensional communities that remove nutrients from the water and form the foundation for diverse food chains. These communities provide a nutrient-rich food source for grazing snails, insects, and other creatures, which in turn support larger predators including small fish.

This surface area effect has been shown in other research studies. For example, bluegill were held in tanks with various amounts of structure, and fish production increased consistently as surface area of cover increased. This increase was due to increasing periphyton and other biofilm abundance with surface area to which they could attach. Insects, in turn, fed on the periphyton, which in turn fed the bluegill and supported more weight of fish.

Another possibility is that the active circulation created by the floating streambed wetlands improved habitat. This active circulation results from compressed air being released below the island, where it rises to the surface and through an opening in the island. The air carries water with it, which deflects off of an aluminum plate and moves horizontally across the island (Figure 1). This creates a stream in a channel on the island’s surface. This action helps to circulate and aerate water in the pond, which has also been shown to increase productivity.

The increase in fish production in ponds with floating islands occurred despite a number of mechanical failures and issues with plant establishment. Maintenance of airlines was increasingly required during the study, as the plastic air lines which floated at the water’s surface cracked due to exposure to the southern sun. This interrupted stream flow periodically, but was easily repaired. Alternative materials for airlines could eliminate this issue in the future, especially the use of weighted lines that sink and are not exposed to UV radiation.

We had a few other concerns worth mentioning. First, it was difficult to establish plants in the hot Mississippi sun. Seedlings would grow when there was rain, but then bake in the sun before their roots could reach the waterline. A combination of planted species and volunteer species did eventually establish, but future planting should consider laying sod on the island and irrigating plants until established. Grazing was also an issue, as muskrats were observed consuming plants on the island.

Finally, our ponds were not very similar to traditional fishing ponds. They were much smaller (about a quarter acre) and shallower (four feet maximum) than the typical pondmeister’s pond. It would be interesting to try this experiment again in larger, deeper ponds that experience more significant temperature and oxygen stratification during warm weather.

In conclusion, the floating islands we used in this study effectively increased fish production. However, you probably should not run out and buy one just to grow more fish. These systems are rather expensive, and the modest increase in fish production would likely not be worth the investment. Similar or greater increase in fish production can be much more economically achieved using water quality management, including fertilization. However, if you are thinking about a floating island for improving water quality or simply adding a new feature to your pond, they may come with the added benefit of increasing the poundage of fish your pond can produce.