Gene's Corner: Carp

Written March 2026, by Gene Welch, Professor Emeritus, Civil and Environmental Engineering, University of Washington

The European carp has been around since its introduction in the 1890s. Cyprinus carpio is really a minnow, that is, it’s in the minnow family (Cyprinidae). Fish sampling in Moses Lake by a State Fish and Wildlife (WA DFW) crew reported an average size carp of 8 pounds, which does not seem like a “minnow".

Catching a big carp with hook and line can be fun – they are strong fighters. Also, they are a

very edible source of protein. Carp are an import food fish in Asian and eastern European

countries. I ate carp at community fish fries growing up in Illinois.

So why are carp considered bad actors? The USEPA reported in 1993 that carp “feed by sucking up large volumes of soft organic bottom sediment, expelling all but edible particles, recycling the masses of nutrients. They are extremely prolific and difficult to control except by extreme measures, such as poisoning or extreme drawdown.” They were reported to mix bottom sediment to a depth of 10 inches, and to excrete soluble phosphorus, which is the usable form to grown algae.

There are no estimates on how many tons of carp are in Moses Lake. However, there was

commercial fishing for carp in the 1970s that reportedly removed about 20-30 tons/year (2.9-

4.4 lbs/acre), which apparently did not affect catch rate (WA DFW). The carp population was

determined in Green Lake, Seattle, in 2004-2005 by WA DFW, using electro-fishing and mark

and recapture, at 107 lbs/acre. Moses Lake is more productive than Green Lake, judging from its phosphorus concentration. The total P (TP) concentration in Green Lake was 52 ppb (parts per billion) in 1981, and 22 ppb during 1992-2000 after an alum treatment. Average May- September TP in Moses Lake has averaged 43 ppb in the Lower Lake and Rocky Ford Arm the past 8 years, not including 2024. So, judging from phosphorus, the Moses Lake carp population may be about double that determined in Green Lake, or around 200 pounds/acre.

What would happen to lake phosphorus if carp were removed? Carp were removed by

poisoning a 600-acre lake in Minnesota with a population at 1300 lbs/acre. Lake phosphorus

decreased from 350 to 100 ppb, showing the P recycling effect of carp. The electro-fished

Green Lake carp population was reduced by 39%, resulting in an estimated decrease in internal loading of 10%.

There were probably more carp in Moses Lake in the 1970s when total P averaged 85 ppb in the lower lake and treated wastewater continued to flow into the lake. So, the lake is much less productive now with much less algae and detrital food for carp, which are known to recruit poorly in low-productive (low-P) lakes. So, if TP were 30 ppb or less, as in 4 of the last 5 years, there should be less nuisance algae and detrital food for carp and a smaller carp population.

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