1. 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

2. No advisory
0.3 – 0.5 ppm
Advisory
Data pending

3. 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)

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

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

6. 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).

7. Baseline conditions Walleye Hg (ppm) Walleye age (years)
Walleye age (years)

8. Altered growth Walleye Hg (ppm) Walleye age (years)
Current
Double weight gain at age
Half weight gain at age
Walleye Hg (ppm)
Walleye age (years)

9. RBT diet and doubling weight gain at age
Walleye Hg (ppm)
Walleye age (years)

10. Increasing system productivity
Walleye Hg (ppm)
Walleye age (years)

11. Decreased water level fluctuation
Walleye Hg (ppm)
Walleye age (years)

12. 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.

13. Mid-life span simulation (gizzard shad diet)
Walleye Hg (ppm)
Walleye age (years)

14. 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.

15. Rainbow trout stocking
0.1 ha each
College
Lake
25 ha

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

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

18. 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

19. Northern pike weight change at length
Total length (mm)

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

21. 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)
Total length (inches)

22. 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)
T-Hg pre-manipulation (ppm)

23. 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.

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

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

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

27. 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.

28. Thank you Hg 80
200.59