Lake level to be lowered to aid harvest of endangered fish

September 4, 2018

BULLHEAD CITY — Bureau of Reclamation Lower Colorado Region is lowering water levels in Lake Mohave to aid in harvesting razorback suckers, an endangered species of fish, from lakeside rearing ponds.

Lake Mohave is above Davis Dam on the Colorado River near Bullhead City and Laughlin.

Water levels in Lake Mohave will lower steadily from its Aug. 31 elevation of 642 feet above mean sea level to an elevation of about 634 feet by the week of Oct. 8 and will remain at that level for about two weeks, according to a release from the agency. Water levels will begin to rise in late October.

Boaters may experience decreased access to ramps and should be extra cautious on the lake, especially in shallow areas, officials said.

The work is part of annual river operations timed to coincide with conservation activities for the fish.

Each year, Reclamation’s Lower Colorado River Multi-Species Conservation Program gathers tens of thousands of newly hatched razorback sucker larvae from Lake Mohave and transfers them to state and federal hatcheries throughout the Southwest. After an initial growth period in these hatcheries, many of the fish are placed in lakeside rearing ponds around Lake Mohave, where they continue to grow and learn how to forage for food. In the fall, these fish are harvested from the lakeside ponds, tagged with microchips, and released into Lake Mohave.

Numerous studies have identified the major factors contributing to the decline of razorback suckers and other large-river fish as the construction of main stem dams, which resulted in cool tailwaters and reservoir habitats replacing the once warm and dynamic river environment, and allowing an increase in competition and predation from non-native fish also established in the Colorado River basin. Dam construction reduced spring flows and eliminated important nursery areas for the fish, which prefer calm, low-flow environments — such as backwater habitats — for larval growth. At the same time, the introduction of many non-native fish species, which prey heavily on juveniles, also is a primary reason for population recovery failure.

Populations of adult razorbacks — which have a lifespan estimated at more than 40 years — persisted in several of the reservoirs created in the Lower Colorado River Basin. The largest reservoir population, estimated at 75,000 in the 1980s, occurred in Lake Mohave, but had declined to less than 3,000 by 2001.

Razorback suckers can grow to up to 3 feet in length, according to the U.S. Fish and Wildlife Service, and weigh up to 13 pounds. Typical adult fish are 12 to 18 inches in length.

Populations are sustained by stocking efforts. Since 2006, about 70,000 razorback suckers have been stocked from Davis to Parker dams, according to Bureau of Reclamation 2016-2017 monitoring and research updates, with the population estimated in 2016 to be slightly more than 5,300.

In a report funded by the Lower Colorado River Multi-Species Conservation Program released in January at a Colorado River Aquatic Biologist meeting by Marsh & Associates Native Fish Lab, 1,890 razorback sucker released from 2009 to 2011 were contacted in 2017 and 50.5 percent of fish with passive integrated transponder tags have been contacted every year from 2013-2017. Lake-wide population estimate based on PIT scanning data are the highest since scanning began.

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