State biologists advise sampling, local removals to tackle Duck Lake’s grass carp problem

Washington Department of Fish and Wildlife staff told the Ocean Shores Freshwater Advisory Board that long‑standing plant‑grazing grass carp and an illegal introduction of yellow perch have combined to collapse Duck Lake’s fishery and drive summer algal blooms. Ken Behan, WDFW warm‑water program manager, said the next concrete steps are sampling fish to determine whether stocked grass carp are sterile and enabling community removal efforts.

Behan traced the problem to two grass carp stockings—one in 1994–95 and a second later—combined with herbicide treatments that reduced vegetation to “less than 5%” of target cover. That loss of nursery habitat, he said, enabled yellow perch to “explode,” shifting the lake’s biomass and leaving predominately small, low‑value perch. "We caught 4 yellow perch one year, then the next year we caught 80, and then it was thousands," Behan said, describing how overcrowding has driven early maturation and reduced angling value.

A central technical concern is whether the stocked grass carp are triploid (sterile). Behan explained that triploidy is imposed in hatcheries so fish do not reproduce; records for carp stocked before and around 1995 show lower triploidy rates than previously reported. "Grass carp are supposed to be sterile," he said, noting USGS and WDFW protocols can test fin plugs to establish whether the current population is fertile or sterile. If carp are diploid (fertile), Behan said, the management urgency increases because the population can reproduce and quickly rebound after removals.

WDFW recommended several practical steps: rescind the local ordinance that limits bowfishing and spearfishing for grass carp so organized removal (including tournaments and volunteer events) can proceed, collect and freeze samples of removed fish for triploidy testing, and map vegetation seasonally to plan habitat recovery and predator stocking. Behan said community angling groups sometimes run successful night bowfishing tournaments and that such events can help suppress carp if legal barriers are removed.

Behan outlined treatments that are technically possible but impractical here: whole‑lake chemical eradication with rotenone—followed by restocking—has been used elsewhere but is constrained by regulatory requirements, a salt‑water outlet that prohibits rotenone use under label instructions, and very high costs. "One barrel of rotenone costs about $1,100 right now," he said, and a lake this size would require orders of magnitude more product and lengthy regulatory compliance, making rotenone unlikely.

Speakers suggested other tactics including alum treatments to bind nutrients (a city with similar problems, Camas, recently tried alum on Lacamas Lake), mechanical removal, targeted predator stocking (WDFW mentioned sterile tiger muskie as a control option for abundant small fish), and planting native vegetation while suppressing noxious species. Behan emphasized the work is incremental: "Removing the carp is an ongoing active effort," he said, and monitoring of vegetation density and fish catch rates would guide further action.

Residents raised health and food‑safety questions about eating fish caught in treated canals. A resident asked whether analyses will check for herbicide residues in carp that are sampled; the board and WDFW agreed testing for contaminants can be part of sample handling where feasible.

The board did not take formal action on rescinding the ordinance during the meeting but said staff are working with city officials on rewriting or rescinding it to allow community removal efforts. Next steps recorded in the meeting included collecting carp samples, scheduling vegetation mapping in spring, and pursuing legal/regulatory work required for predator stocking or nutrient treatments.