I should preface this article by stating that there are never any absolutes when it comes to nature and critters. This means that there are always exceptions to every “rule,” so I can only write about them in general terms. Still, the tips below will offer a great deal of help to ponderers who want to protect their fish.
Ornamental fish have many predators, and of all these, mink are the most efficient. A single mink can kill an entire collection of very expensive koi — or even inexpensive but still important pet fish — in just a few short nights without leaving much evidence. In most situations, the owner of the pond does not even know there is a problem until most of the fish are gone. And when the damage is discovered, it can be a very, very sad day. (“Devastating” is a word I hear a lot.)
Horror stories of lost fish are very common when mink (or “water wolves”) show up. Mink are so efficient at catching their prey that they remind me of wolves in their predation … but while wolves hunt on land, mink do it swimming in water. Since my last article on mink in 2007, there is new information available that’s critical to protecting our fish, and I want to share it with you. I also want to revisit this subject due to the serious damage mink predation can cause.
Mink are found across most of the continental U.S. Just about the only place they do not live is in the Southwest, where it is much drier. This is because mink are almost always associated with water. If your pond is near a naturally occurring body of water such as a lake, river or creek, then the appearance of mink is a distinct possibility. How close is near? Well, a pond could be as far as a mile from a water body and this would still be within a mink’s territory. Ponds in more urban situations have less of a mink problem, but if there are “wild” park areas close by, mink can be present.
Otters are also a potential problem, but they are not as prevalent, will not venture as far from naturally occurring big water and need more wild areas. Small rivulets of water are not normally otter hangouts, but mink will follow even these small drainages. Otters and mink are in the same family, Mustelidae, so they do have some similar habits. One big difference between the two: otters leave a lot more evidence.
Evidence of Mink Predation
Perhaps the best evidence of mink predation is that there is very little evidence. The fish will act spooked and not act normally if a mink has been in the pond. They can take out all the large koi from a pond in a just a few short nights without the owner ever knowing they were there. Mink are normally active at night or at twilight, so most people have never seen a mink even though they are everywhere.
Sometimes a koi can be much larger than the mink that’s trying to catch it, but the mink can still capture the fish and drag it out of the pond. This is where you may find some evidence. The process of dragging out the fish usually leaves a few scales behind. By contrast, if a raccoon takes a koi, there are normally a lot of scales and blood left behind. The raccoons almost always tear plants up and tip over pots. I call ’coons “bull in the china shop” critters. Mink, on the other hand, are very delicate in their habits, carefully poking their noses in and out of holes in the rocks and then diving underwater for minutes at a time. They are carnivores, which means they like meat. Frogs, birds, muskrats and fish are their main diet.
The majority of mink predation on ornamental fish occurs during the colder months, when the water is cold. In the Midwest, the months from October to April — when water temperatures are around 50 degrees or lower — are typically the periods of highest predation. Fish metabolism is so slow in these conditions that they can barely flick a fin, let alone outswim a mink.
Mink predation can still occur when the water is warmer, but this usually occurs in smaller ponds where the fish can be cornered.
If the water is warmer you may see more fish that are damaged but alive. You might see fish that have torn fins, bite marks and missing scales — but are still swimming around. In warmer water it is possible that the fish are better able to escape being killed, but they can still sustain serious damage. Also, damaged and injured (but still living) fish are often left behind by female mink, which are smaller, or young mink, which are not yet efficient predators.
Our ornamental fish are so colorful that they are like a flashing beacon under the water, saying, “Come eat me!” Native game fish are much safer from mink predation due to their natural camouflage. Being drab in color, they can hardly be seen from above. Earthen basin ponds containing our game fish have cloudier water, thereby providing even more protection. Most of our ponds, on the other hand, have very clear water, and this also allows brightly colored fish to be seen from a distance.
Mink will prey on large fish as well as smaller ones. The really small fish will be eaten on the spot, but the larger ones will most likely be carted off to a safer spot to eat. Mink will also take fish back to a den to feed young in the spring. Mink are normally loners in the spring, but they occasionally live in pairs. In the fall females are often training their kits, so they may be in a group at that time. There are not many predators other than mink and otters that are so proficient at catching every last fish in a pond. It is also amazing how a mink can drag off a very large fish that’s so much bigger than itself. As I mentioned earlier, there will be scales left behind on the rocks and along the trail from the fish being dragged.
If you suspect the presence of a mink (or any nocturnal predator), look for fresh, wet tracks on the rocks around the pond in the early morning. A good habit to get into is inspecting your pond every morning as early as possible. Wet tracks are the most obvious at that time but will dry quickly when the sun comes up. Mink will also leave a trail between the pond and the closest connection to tall vegetation that may lead to more native habitat. This trail could have tracks and scales scattered along it. Tracks in snow and mud are easy to see. However, most times of the year, we do not have these easy tracking conditions. Mink tracks (at right) are similar to squirrels which are small and have very visible, easy-to-spot toes and claws. What Attracts Mink
Of course, water is a mink’s main attractant. Mink can hear the sound of water from a long distance away. Water also attracts all kinds of critters that mink like to prey on, like frogs and birds.
After a pond cleanout there is always the smell of muck. Mink know this smell, and it will attract them from a distance. After a waterfall shutdown, there again is the smell of algae and muck. Many times I have seen predation the day after a shutdown or cleanout.
Once the mink have found your water and have been successful at catching a fish or two, they will be back — guaranteed! It may be the same night, or perhaps the next night … but either way, count on them becoming your new neighbors. After your pond is completely empty of fish they may take a break from visiting for a few weeks, but it will still be a regular spot that they check in their territory. Once this has occurred, perhaps the only things that can change the scenario are if the mink are killed by another predator (including man) or the habitat is changed.
Solutions to eliminating mink predation
Once the evidence of mink is observed, a fish keeper has to act fast. Otherwise, a lot of damage can occur in just one more night. The fastest and most secure technique is to dye the pond water black, which simulates a more natural, earthen basin pond. There are dyes that are very eco-friendly and dissipate over time. Warmer water temperature and sunlight degrades the dye, so the effect can last from a couple weeks to a couple months, depending on these variables. The only problem with dye is that you cannot see your fish unless they are at the surface. Obviously, most pond owners want to be able to see their fish and do not want black water. But the dye is used only in an emergency situation, and it’s better than losing your fish. Have black dye on hand so that the pond can be dyed immediately. I like black dye better than blue because it creates darker conditions and looks more natural to me. The aquatic dyes will not harm an ecosystem, which includes fish and plants that grow to the surface. If fish are still being fed, they will still be able to find floating foods. Any submerged plants may die due to the light not being able to get to them.
Once the fish are protected by the darkened water, you can then focus on taking care of the mink problem. I always recommend finding a local trapper to capture the mink. Call the local Conservation Officer for permission to trap mink and to find the name of a local trapper that may be able to assist you. There are also animal control businesses that you can call. Mink are more difficult to trap than most critters, so it takes a trapper with experience. Make sure that the trapper you hire specifically has experience with mink.
Caging your fish in your pond is also a good option. Set the cage up in early fall. Locate as large a cage as possible in the deepest part of the pond. Make the cage out of netting that’s one inch by one inch, which is small enough that a mink cannot get through. Add aeration near the cage, but not in it. The cage protects the fish during the winter, but when the fish are uncaged during the summer they are still vulnerable. This is especially true if the pond is small; the fish are still at risk then because they can more easily be cornered.
A good strategy to help fish elude mink (as well as other predators) is to add structures to the pond. The fish can hide in and around the structures. Some good options are black barrels with large holes cut in them, water lilies in pots or Koi Kastles®. Providing structures does not necessarily save your fish from future mink predation, but it does give them a better chance to elude these water wolves. Structures also give the fish something to which they can orient themselves, which is calming for the fish. For these reasons it is healthier for the fish to have structures in the pond, so add some structure even if mink predation is not a problem at the moment!
Fish can also be brought indoors, or a mink-proof structure can be built over an existing pond. Both are a lot of work, but they may be the best options for you. A mink-proof structure cannot have any gap larger than an inch in it — the same is true along the bottom of the structure, at the interface of the structure and the pond edge.
The final option is to simply keep inexpensive fish and expect the loss of fish occasionally. This may not be the best option for you, but it is something to think about.
To say it is devastating to lose all your fish in a few short nights is an understatement. Mink can do this without much evidence left behind. In a lot of situations the fish keeper did not know the fish were gone until weeks later when the ice thawed. When mink have found your pond as a source of food, they will continually check it out for years. This means it will be risky to keep quality fish in the future … unless you stay vigilant and use the solutions listed above. With a little bit of knowledge and some quick proactivity, you can protect your beloved fish from these deadly water wolves.
by Jaimie Beyer, Midwest Waterlandscapes;
POND Trade Magazine Sep/Oct 2013
Several years ago I built a 20,000 gallon koi pond at my house. I researched it thoroughly. I had it all, bottom drains, settling tanks, skimmers, UV, all kinds of filtration. I was going to have the best pond with the best water quality. I seeded the filter with ammonia chloride, and waited for the ammonia and nitrite to zero out. Now I was finally ready for fish. Boy, this was going to be GREAT! The perfect pond with the perfect water. To seal the deal in regards to perfect water quality I plumbed in fresh well water from my house and adjusted the flow so that the pond would have a 10 percent water change every day. So we harvested some three year olds, and I painstakingly picked out five of the best fish I could find. Since I'm a "Koi Professional" I knew what to do. Start slowly, let the filter mature, don't get anxious and start piling fish in there. So in the fish go. They're going to love this. Just five fish in 20,000 gallons in a koi pond with awesome filtration and constant water changes. 
