Colorado Parks and Wildlife Fisheries Management to Remediate Mercury Contamination in Sport Fish Dr. Jesse M. Lepak Colorado Parks and Wildlife 26 September 2013 Hg 80 200.59

No advisory 0.3 – 0.5 ppm Advisory Data pending

Bioenergetics QR (respiration/metabolism) QS (standard) QA (active) QH + + QR (respiration/metabolism) QS (standard) QA (active) QH (SDA) QG C (consumption) (Growth) QU (excretion) QF (egestion) Wisconsin Fish Bioenergetics Model 3.0 (Hanson et al. 1997)

Bioenergetics Daily predator Hg concentration = Original Hg content + (Consumption * Prey Hg * α) - elimination Predator weight

Example: Brush Hollow Reservoir Immature walleye Male walleye Female walleye Walleye Hg (ppm) Walleye weight (g)

Simulate management to reduce ♂ mercury? Establish male “baseline” model; Double and half weight gain at age (changes in density due to harvest); Switch to rainbow trout diet and double growth; Increase system productivity (bloom or biomass dilution: lower prey Hg, WI); Decrease water level fluctuation (decreased methylation rates: lower prey Hg, SD).

Baseline conditions Walleye Hg (ppm) Walleye age (years) 3 4 5 6 7 8 9 10 11 12 Walleye age (years)

Altered growth Walleye Hg (ppm) Walleye age (years) Current Double weight gain at age Half weight gain at age Walleye Hg (ppm) 3 4 5 6 7 8 9 10 11 12 Walleye age (years)

RBT diet and doubling weight gain at age Walleye Hg (ppm) 3 4 5 6 7 8 9 10 11 12 Walleye age (years)

Increasing system productivity Walleye Hg (ppm) 3 4 5 6 7 8 9 10 11 12 Walleye age (years)

Decreased water level fluctuation Walleye Hg (ppm) 3 4 5 6 7 8 9 10 11 12 Walleye age (years)

Summary Female walleye had lower than expected Hg due to RBT consumption (growth dilution); Simulating increased walleye growth alone decreased Hg, but not below 0.5 ppm; Simulations with reduced prey Hg showed the largest reductions in walleye Hg; These are only simulations, results may vary (e.g., methylation increase with productivity); Simulated responses mid-life span rapidly reached equilibrium.

Mid-life span simulation (gizzard shad diet) Walleye Hg (ppm) 3 4 5 6 7 8 9 10 11 12 Walleye age (years)

Do responses really happen this quickly? We tested a whole-lake management action experimentally; 25 ha College Lake, dense, slow growing northern pike population (crayfish diet); Provided rainbow trout as higher quality forage to quantify growth dilution; Repeated measures on individuals (2 months; 30 fish and 1 year; 15 fish); A pond component (~40 days) was conducted to verify results.

Rainbow trout stocking 0.1 ha each College Lake 25 ha

Initial sampling Length, gastric lavage (diet), weight Biopsy Release into College Lake or ponds Floy Tag 50:50 denture cream/neosporin

Stocking and recapture ~1 kg of RBT per pike (27,000, 150 mm);

Northern pike diet response At pond draining: 1 RBT remained Ponds Other inverts Warbler ▼ ▼ Crayfish Diet composition (% wet biomass) Rainbow trout Northern cricket frogs Other fish n = 254 n = 30 n = 12 n = 8 n = 15 Sample period/treatment

Northern pike weight change at length Total length (mm)

Northern pike Hg response Δ T-Hg concentration (%) College Lake Experimental ponds Δ weight (%)

Northern pike Hg response 50% of the pike exceeded advisory levels before; We removed 35% of those from the advisory. 1.25 1.00 0.75 0.50 0.25 0.00 Pre-manipulation Post-manipulation T-Hg concentration (ppm) 400 500 600 700 800 Total length (inches)

One year later… 1.25 1.0 0.75 0.50 0.25 0.0 Recapture (2010) 1:1 line T-Hg post-manipulation (ppm) 0.0 0.25 0.50 0.75 1.0 1.25 T-Hg pre-manipulation (ppm)

Implications Providing quality (low Hg, high calorie) prey has potential to reduce sport fish Hg rapidly; More cost effective and ecologically “safe” to manage for prey species already present or increase productivity/decrease methylation; Even if low Hg, high quality prey are present, individual sport fish may have elevated Hg; The general patterns observed were present across the landscape in Colorado.

N,P N,P No advisory 0.3 – 0.5 ppm Advisory Data pending

Given that you caught a piscivore…. Probability of an advisory Chlorophyll a (mg/L)

Given that you caught a piscivore…. PCA including chlorophyll a, secchi depth and total phosphorus Probability of an advisory PCA (unitless)

Conclusions Food web structure (from productivity to predators) is extremely important; Food web structure can change rapidly through management and stochasticity; Caution, there are reasons why some of these actions may increase bioaccumulation (e.g., increasing productivity); Implications for monitoring and potentially remediating mercury contamination in sport fish.

Thank you Hg 80 200.59