Just 20 years ago we had fewer than 100 otters in Missouri. Most of them lived in the Bootheel. Now thousands of river otters inhabit streams, rivers and reservoirs throughout the state. River otters weigh 20 to 30 pounds and consume about 2.5 pounds of food a day. That adds up to a lot of vittles, prompting many people to ask, “Just what are all these otters eating?”
To answer that question, the University of Missouri and the Missouri Department of Conservation's Resource Science Center conducted an extensive study of otter diets. Over a two-year period, we collected 4,750 river otter scats (droppings) from 80 miles of Ozark streams. In addition to finding out what otters mainly eat, we also wanted to see if their food preferences changed with the seasons.
Otters really eat everything. We found remains of crayfish, fish of all kinds, frogs, ducks, muskrats, snakes, and even a turtle in the scat we collected.
By far the most important food item for otters in the Ozarks, however, is crayfish. During the summer season, we found crayfish remains in more than 98 percent of otter scats. Even during the winter, crayfish remain an important prey item. On the Big Piney River, crayfish remains were found in 75 percent of scat samples. On the Osage Fork, crayfish were found in 92 percent of winter scats.
No other study has found crayfish to be as important in the diet of stream otters. The reliance on crayfish during the winter was especially surprising because crayfish are less active then.
Otters, however, eat whatever is available, and Missouri Ozark streams contain lots of crayfish. They have among the highest production and population densities recorded anywhere. Although most crayfish burrow into mud to wait out the winter months, the two main species of crayfish that inhabit the Big Piney and Osage Forks rivers hide under rocks and debris where otters can easily find them.
Our Ozark otters also eat fish, but their consumption of fish varies a great deal through the year. During the summer, only 14 percent of scats collected contained fish remains. However, during the winter season 84 percent of scats from the Osage Fork River and 88 percent of scats from the Big Piney River contained fish remains.
Otters in the Ozarks eat several types of fish, including sunfish, suckers, carp, minnows, topminnows, catfish, darters, mosquito-fish, drum, shad and bass. Anglers have reported otters eating bass for years, but until recently it has been impossible to confirm this by the remains in otter scat. Recently, however, researchers have developed a new technique that can identify fish species by examining the position, shape and patterns of near-microscopic formations on scale surfaces.
Using this new technique, we found that 9 percent of scats from the Osage Fork River and 12 percent of scats from the Big Piney River contained bass remains. The remains of rock bass (goggle-eye) appeared in 7 percent of scats from the Osage Fork River and 13 percent of scats from the Big Piney River. Suckers occurred in 4 percent and 7 percent of scats from the Osage Fork and Big Piney Rivers respectively.
Although the study confirms that otters eat bass and rock bass, it could not determine the impact that otters have on bass populations. That would take a much more comprehensive study of bass populations.
Like many Missourians, river otters seem to enjoy the taste of frogs. We found frog remains in 10 to 14 percent of scats, regardless of season. Apparently, even in winter, the relatively warm ground and spring water that feeds these Ozark streams keeps frogs active and available to otters.
Otters also seem to enjoy a little variety in their diet. Small percentages of the scat samples examined included remains of ducks, muskrats, snakes and even a turtle.
Ducks, muskrats, and snakes have been reported in other studies across North America. Only once before, in Florida, have turtles been reported in otter diets.
The turtle we found proves the rule that otters are opportunistic and will take advantage of almost any kind of food they can find. That makes it difficult to say with certainty exactly what otters eat. However, after collecting and examining 4,750 scat samples, we have a better understanding of otter diets. The data we collected will help wildlife managers and biologists manage populations of these furry feeders in the future.