The Carp of Malheur Lake

“The rooting up of the bottom and the constant mumbling of the mud has rendered the water almost opaque…”

“The native species evidently existed as a diversified group, while the introduced carp is capable of changing the entire community composition, bring about a change as marked as that produced by civilized man on the original vegetation.”

            -Alvin R. Cahn, 1929

"Gil" informs visitors to Malheur National Wildlife Refuge of the carp problems.

I recently visited Malheur National Wildlife Refuge in Oregon to get a first-hand look at their carp problem.  It’s a big problem.  Several years ago I attended a workshop at the refuge to learn about these issues (read previous blog post).  Malheur Lake proper is approximately 78 square miles (almost 50,000 acres) although this area fluctuates greatly from year to year depending on water inflows.  The lake is a remnant of an ancient lake that was once drained by the Malheur River (Snake River tributary) and the entire basin in larger than several U.S. states.  The basin is now closed and contains many wetlands and playas (Harney Lake) and is mainly supplied by the Blitzen, Donner, and Silvies rivers.   The lake once supported hundreds of thousands of nesting birds and providing a resting area for millions more – arguably one of the most important resting areas in the Pacific Flyway.  At least is used to be.  The lake is shallow and contained an abundance of submergent and emergent vegetation that provided a refuge and food source for invertebrates that birds like to eat.  No more.

Malheur Lake is 7 miles by 9 miles and once had abundant vegetation and millions of birds using it annually.  Now the lake is mostly devoid of vegetation.

The challenge of controlling the prolific common carp is daunting anywhere – much less on a lake the size of Malheur.  But actions are underway and all hope is not lost.  The entire basin is highly modified and numerous irrigation diversions are used to manage water to maximize bird production on the refuge and provide water for agricultural needs.  Portions of the refuge were added over time, as different ranch units.  This manipulation of waters results in a labyrinth of canals, diversions, and water control structures.  Even recently it was hard to sort all of this complexity out – and going to the “old timers” might have been your most reliable source of knowledge.  But things are changing – almost every inch of the water system has now been mapped and made available in a database.  A technician spent the better part of a summer armed with a 4-wheeler, GPS, and camera and travelled nearly every mile of every diversion and canal to locate and map every water control structure.

 

Malheur NWR has a labyrinth of diversions and water control structures.

Divide and Conquer     

So…the basin in highly manipulated and fragmented…often something biologists lament about.  This creates a perfect opportunity to start to partition off portions of the refuge to at least stop the carp from moving freely and better yet to kill them off in opportunistic areas.  Go for the low-hanging fruit.  In recent years many new and modern diversion screens have been installed to prevent common carp, as well as other fish like the sensitive redband trout from entering the diversions.  The fish are sent back to the river and the screens are meant to keep even the smallest fish and eggs from entering the diversions.  A brush periodically travels the length of the screen to keep debris from fouling and clogging the system.  For carp this means access denied to potential good spawning areas.

Fish screen on a  diversion from the Blitzen River.  The automatic brush that keeps the screen clear of debris is circled.

In addition, several new fish ladders have been installed at diversion dams on the Blitzen River.  These fish ladders provide passage for fish.  But we don’t want the carp to move around, right?  The ladders are equipped with fish traps.  Refuge staff check these traps daily – the native fish, redband trout, mountain whitefish, and Tui chub, among others are released upstream of the dam.  All common carp are removed and euthanized.  This is the beginning of a strategic approach to “divide and conquer” the common carp.  Conquer may be the incorrect term as past experience tells us that complete eradication of this prolific species is impossible – but suppression is the goal.  Recent research has indicated that carp levels below 100 lbs/acre may be a potential management target that minimizes the detrimental effects on a system and that the ecosystem may still function properly at this level.  Levels are definitely much higher than this in Malheur Lake.

Fish ladder on the Blitzen River leading to a fish trap.

Lifting the fish trap from the ladder.

Small male common carp captured in the trap.

Efforts are underway to explore the feasibility of commercially harvesting carp from the system.  Heck, humans have been pretty good at fishing other fish into submission, why not common carp?  Pulsed commercial harvest might improve conditions but likely needs to be conducted often, maybe every 2-3 years as populations can double quickly.  We are currently developing monitoring options to track the effects of carp suppression.  Hopefully the lake and refuge can once again be a great place for birds.

White-faced ibis on the Double-O Unit of the Malheur NWR.

Submitted by Jeff Jolley