Dealing with Lake Lowell's Carp

By Joe Kozfkay - Idaho Department of Fish and Game It's December of 2010, well below freezing, and 20 mph winds out of the west have pushed the waves on Lake Lowell to more than two feet. Not big waves by any means, but as we - a rag-tag group of Fish and Game "fish squeezers," Deer Flat Refuge personnel and volunteers - haul in the purse seine, attempting to catch common carp, the wind and waves certainly don't help. Shortly after completing our second haul, our youngest technician slips on the icy deck and nearly falls into the frigid water. I mentally prepare a story for my boss and the technician's parents detailing why hypothermia really isn't a big deal. Can't be that bad. After all, people polar bear swim all the time and say it's refreshing. Luckily, somebody grabs him, averting this small disaster, and I get to keep my job. After a couple more mediocre hauls with the purse seine, my mind wanders. Why did someone bring carp to this country in the first place? Why are there so many in Lake Lowell? Is there anything we can do about it? Common carp are native to Eurasia and were introduced to the United States in 1877. The then-newly formed U.S. Fish Commission, under pressure from recent immigrants, thought carp would be a beneficial addition to the fish fauna of the United States. Their reasons were relatively sound: Carp had high growth and reproductive rates and could sustain high harvest, could adapt to most aquatic environments including polluted waters, would be popular as table fare, and would be benign to other fishes. With the best of intentions, carp were spread throughout the country mostly via railcars and quickly established populations in many waters. Carp cuisine never gained a wide following, most likely due to the fish's many free-floating bones. Consequently carp were largely ignored by most anglers. Years later, evidence mounted that the proliferation of common carp had hidden and unintended consequences. Carp damaged other fish communities, even entire aquatic systems. Most of the damage is caused by the carp's feeding behavior which includes ingesting insects and sediment from the bottom, consuming the insects, and then spitting out the remaining mouthful of sediment. This "mud rooting" behavior destroys the roots of beneficial aquatic plants that provide food for ducks and cover for fish, and muddies the water column. Many clear natural lakes, rivers and wetlands have been converted to turbid water systems by the sloppy feeding habits of carp. Nobody knows for sure when carp first arrived at Lake Lowell. The earliest records indicate their presence in the system since at least the 1950s. As early as 1955, Fish and Game biologists began studying carp populations with the hopes of developing cost-effective methods for reducing their abundance. During 1957, personnel caught several thousand carp from Lake Lowell, marked them, released them, and then re-sampled thousands more. Estimates indicated that the reservoir contained 1.24 million individuals weighing a total of 4.1 million pounds. Lake Lowell is only 9,000 acres when full, so densities exceeded 450 pounds of carp/acre. In the following decade, large traps and seines were deployed with little effect. Only applications of rotenone, a fish poison, produced any meaningful results. Temporary carp reductions then allowed game and panfish populations to rebuild. Over the following two decades, carp populations increased gradually, yet panfish, bass and bullhead fisheries still persisted. Then during the early 1990s, a protracted drought hit southwest Idaho. Low spring reservoir levels prevented panfish reproduction for several years, which then reduced predation on carp eggs and young. Carp populations exploded, and fishing suffered. By 2006, fishing for panfish had become poor, and though bass were still readily caught, large bass had disappeared. Anglers and fisheries managers were frustrated. Fish and Game staff members began a series of investigations aimed at determining what caused these declines. Several survey methods were employed including electro-fishing, trap and gill netting, as well as larval fish trawling. Regardless of the method, the results were the same; very few panfish, especially young ones, and carp were abundant and everywhere. In an attempt to rebuild panfish abundance, Fish and Game staff members and local volunteers transferred more than 9,000 crappie as well as some perch and bluegill to Lake Lowell during 2009 and 2010. These fish spawned successfully in both years, but survival of small fish was inadequate to rebuild populations. So it was back to the drawing board with a new slogan: Know your enemy. More detailed investigations of Lake Lowell's carp were undertaken with the hope of finding a practical control option. Studies focused on gaining a better understanding of carp biology and carp movement patterns using telemetry, and estimating carp population size with a mark-recapture effort. During the fall of 2010, we marked and released more than 6,000 adult-sized carp with the help of corn-baited traps. Afterward, we used purse seine hauls and drag seines, with help from a local commercial fishermen, to recapture 6,000 more carp. Few marked carp were recaptured, indicating a large population - 1.28 million carp and 5 million pounds - almost identical to five decades earlier. At these carp densities, it will be impossible to improve desirable fish populations, restore aquatic plant communities and improve water quality in Lake Lowell without first resolving the underlying problem - overly abundant carp. Controlling nuisance fish populations is a difficult task, especially in a large water body like Lake Lowell. The only practical solution would be to apply rotenone. Rotenone is a natural occurring substance extracted from the roots of several tropical plants in the bean family. It blocks a fish's ability to process oxygen at the cellular level, killing fish when applied at adequate concentrations. Rotenone remains lethal for several days until being broken down by sunlight. Unfortunately, there are drawbacks. Rotenone doesn't discriminate between desirable and undesirable fish species. It kills both. If Lake Lowell were treated, it would take several years and intensive stocking to rebuild fishable game fish populations. Secondly, rotenone is expensive. At average minimum lake levels, treatment costs would run into the millions of dollars, an amount far out of reach of state and federal agencies at this time. Alternatively, cost could be reduced substantially if the reservoir reached low levels during an extended drought period or through a planned drawdown. Fish and Game, in cooperation with refuge staff and other groups, are in the initial stages of determining whether a rotenone treatment is desirable or achievable. The issues s urrounding Lake Lowell's carp population are major, but it's not all doom and gloom out there. Quality largemouth and smallmouth bass fishing opportunities still exist, especially in early summer. Bluegill are numerous around flooded brush and weed beds surrounding the reservoir. Also, channel catfish - stocked annually as fingerlings - survive at high rates and grow rapidly. Many of Lake Lowell's catfish now exceed 10 pounds. Though it appears that there is plenty of room for improvement, catching a few fish, having a shot at a big one, watching wildlife, and getting some fresh air all beat the heck out of mowing the lawn. Joe Kozfkay is a fisheries biologist in the Southwest Region.