Freshwater Sponge growing under BioHaven Islands attracts fish and keeps Fish Fry Lake crystal clear

Let us show you rather than tell you! Here are three videos showing the abundance of freshwater sponge growing in Fish Fry Lake. The sponge absorbs nutrients, and is in turn grazed on by fish. Despite warm summer water temperatures, algal levels remain reduced year-round.


The following is Dr. Mark Reinsel’s curated background info on freshwater sponge:

  • They are the simplest of multi-cellular animals.
  • Freshwater sponges are exposed to more adverse and variable environmental conditions than marine sponges, so they have developed gemmules (like seeds) as a means of dormancy. Gemmules can live in a dormant state and then “germinate” when conditions are again favorable.
  • Freshwater sponges are only found in clean lake waters and slow streams. They live in flowing water to bring in food and oxygen, and excrete wastes. They evidently cannot tolerate polluted waters–I did not find what nutrient concentrations they can tolerate and I doubt if it is known, since they have not been highly studied. I found a few abstracts on chemical composition but they focused on organics. I would assume they have a P concentration similar to other animals.
  • Many freshwater sponges appear green because they contain algae, which live on sponges in a symbiotic relationship.
  • Freshwater sponges serve as food for a variety of other aquatic invertebrates, including caddisflies and midges. No mention of fish feeding on them.
  • Twenty to thirty species of freshwater sponge live in North America. Several live in Montana.
  • Sponges have an annual cycle with growth in the summer, die-back in the winter and growth beginning again in the spring.
  • The spicules that give sponges their shape, and allow their identification by species, are composed of silica.

Fish Fry Lake

Fish Fry Lake is 6.5 acres, about 54 acre feet of water, and experiences nutrient inflow from groundwater that perks into the lake. Over the last nine years we have experimented with how to cycle those nutrients, particularly phosphorus, into beneficial life forms instead of algae and cyano bacteria, or massive beds of aquatic vegetation. Many waterways in North America fight nutrient pollution, so it is with a hopeful outlook based on recent developments that I pen these words.

We have been focussed on finding ways to intercept nutrients before they cycle into algae. Here in the north, grass carp, manatees, thread fin shad, and a few other critters that happen in the south are disallowed here, or winter temperatures prevent them.

Biofilm-generating microbes are one life form that works for us. But surface area and circulation limits their growth. They are however, a terrific way to cycle nutrients into biofilm/periphyton/fish. So we do a lot of islands here, because they (along with the plant roots that grow through them) provide massive surface area. We also have two floating stream beds on Fish Fry, to enhance circulation. This has been a winning combination!

Fish numbers, and harvest, reflect this. Fish Fry Lake is very probably the most productive wild fishery in Montana. In practical terms what this means is that a fisher person can catch twenty fish in twenty minutes, of various age classes. A few years ago we averaged a fish every two minutes…now it’s a fish a minute. And there’s a really good likelihood that big fish, in their species class, will be part of the mix.

Nature as Facilitator

Now, nature seems to have taken a hand in our effort. Over the last two years a native form of freshwater sponge has colonized our floating stream beds and nearby floating islands, where circulation happens. This sponge is a filter feeder. It eats the suspended solids present in the water. And our bluegill and sunfish eat the sponge. This is a particularly direct path between inflow nutrients, a component of suspended solids, and fish.

The sponge is tan in color, suggesting a diatom relationship. Diatoms are a form of phytoplankton, but they are not associated with the boom/bust blue green algae cycle that results in oxygen depletion.

What is the nutrient composition of living sponge? Is it helping us in our battle to cycle phosphorus into fish? More to come! In the meantime, check out the videos above to find info about bluegill and sunfish relating to sponge, and a short technical overview of freshwater sponge.