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Cormorants, Otters, and Herons..oh My!

The River Journal 2017 Holiday Newsletter is out. If you don't recieve it and would like to make sure and shoot me an emai and I'll add you to the list. This latest article is from the newsletter so I thought I would also post it here. Mike.

One of the things many of us who frequent the tail water rivers of East Tennessee have noticed over the past five years or so has been the increase in the number of predators who prey on trout, especially stocked trout who haven't yet developed the survival skills of holdover or wild fish. In particular avian predators like cormorants have increased their numbers dramatically, but even mammals like river otters,  also seem to be on the rise. Which of these predators have the most impact on our fisheries and our limited resources of available trout? I had my suspicions, but here's what I was able to find out from some of the available research. First up are our cranky old buddies, the Great Blue Heron.
In 2000-2001 a study was done below the Bull Shoals and Norfolk dams in Arkansas to determine the effect of Great Blue Heron predation on stocked trout. It was found that most captured live trout  by the resident heron population fell between 10.5 and 28 cm in length and that while trout did make up 68% of heron diet,  they only represented a 2.4% loss to the numbers of trout in these two rivers. Therefore they were not considered a major factor in trout loss when compared to other predators , human anglers,  and natural mortality rates. Next up,  and something I have particularly noticed on the Clinch River lately,  are river otters. I have personally watched a group of up to a dozen or more pursuing trout on the lower end of the Clinch on a number of recent trips. So how damaging are these fun loving animals to fisheries? There doesn't seem to be many studies on otters in river environments, but one  conducted in 1998-2000 on two Danish lowland rivers concluded that otters rapidly change their diet preferences when stocked trout are introduced,  and will prey disproportionately on the newly stocked fish which  were easier to catch. An adult otter eats between 15 and 25% of it"s body weight a day and can spend up to 5 hours a day foraging. So in rivers where their diet is primarily trout,  you can see how that adds up pretty quickly! Therefore it was determined that otter predation was significant,  and should be considered by fisheries managers when stocked trout are used, which brings us to our last predator on the list; the double crested cormorant. A study on the lower Platte River in Wyoming showed cormorant diet consisted of 85% suckers prior to the stocking of trout. After stocking,  trout made up 98% of their diet,  and that by the end of the study,  80% of the stocked trout had been consumed by the birds. Another study in 2001 tracked the numbers of stocked steel head consumed by cormorants living on an  island in the Columbia River. It was determined that this one colony of cormorants consumed  15% of the steel head stocked in the river.  Another study on the Minersville reservoir in Utah showed that cormorants consumed over 30% of the stocked trout within 2 weeks of stocking. 
With the numbers of these birds increasing year after year on Tennessee tail water rivers,  it's safe to assume that the cormorant poses a major threat to our limited resources. Unfortunately due to their protected status from the US Fish and Wildlife service as a migratory waterfowl, and no recognized hunting season like more desirable birds such as ducks and geese,  they enjoy complete protection from hunters and fisheries managers alike. It is possible however,  to get depredation permits from the Feds like  South Carolina did on the Santee Cooper reservoir system to reduce their numbers through managed hunts. Moreover,  several agencies have tried to reduce their consumption of stocked trout by changing stocking regimes and stocking larger trout that are less susceptible to being caught. Last year on the Holston,  when the numbers of cormorants were at their highest,  at least 1 in 5 of the trout we landed over 14 inches,  had evidence of injury from cormorants. So what can we do as anglers to combat these efficient predators? Directly, not a whole lot,  as outright shooting them would be illegal. Contacting TWRA and fishery managers and making them more aware of the problem will be good start. I plan to do just that,  and I'll be reporting on those conversations in subsequent posts.

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