1. Potential strategies for: “best use” of sterile fish, optimized stocking densities, and future direction….

2. Current Situation Over half of our rainbow produced are sterile
Sterile rainbow tend to have higher mortalities at the egg and fry stage so production costs are higher
The higher mortalities and increased mortalities has constrained FFSBC’s egg collection capacities for both Pennask and BW.
FFSBC will be setting a production capacity until the situation can be resolved for both BW and Pennask sterile rainbow
We need to identify the best use for sterile fish

3. What is best use? Preventing spawn bound problems
Creating trophy fisheries
Co-stocking with harvestable 2n rainbow trout (mandatory catch and release for sterile fish)
Conservation
Decreased cost/benefit?
Increased effort?

4. Figure 1. Hypothetical growth trajectory (Upper panel) and corresponding Ford-Walford plot (Lower panel) comparing growth of diploid and triploid rainbow trout. The parameter values corresponding to equation 2 are; Diploid-a = 16, b = 0.71 and Triploid-a = 15, b = 0.81 (Paul Askey, 2007).

5. Figure 2. Relative growth of 3N rainbow trout in length (left panels) and weight (right panels) for first two seasons post stocking. Grey, open circles represent lakes with little or no interspecific competition and both BW and FV strains. The dark closed circles are from Buchanan Lake which contained three other species of fish. Dashed line is 1:1 line and solid lines represent most parsimonious model (if different from 1:1 line) (Paul Askey, 2007).

6. Figure 3. Relative survival of 3N rainbow trout in the first (left panels) and second year (right panels) post stocking. Top panels are lakes not accessible to loons and bottom panels are lakes with loon access. The most parsimonious model is represented by solid lines, unless 1:1 line is more parsimonious. Black triangles and black lines represent FV strain and grey circles and grey lines are BW strain. Note: Scale on x and y axis are not constant across panels (Paul Askey, 2007).

7. Figure 4. Inferred probability distribution used for Monte Carlo analysis based on maturation mortality for males from three data sources (Paul Askey, 2007).

8. Top panel: Frequency distribution of expected angler effort given stocking of 2N or 3N rainbow trout. The variation in diploid effort is due to the uncertainty in maturation mortality. Bottom panel: The expected percent change in effort given a switch from 2N to 3N stock. The light grey bar indicates the no-change value of 0. Both panels represent 1000 Monte Carlo samples, and parameter values averaged over strain effects.

9. Conclusions (Askey 2007)
Triploids may not be suitable for habitats with stressful conditions (e.g. competition, predation, marginal habitats)
Fishery response of stocking triploids is a direct function of the harvest rate (older age classes not available)
Lakes managed for trophy fisheries will gain the most benefit from stocking triploids

10. Opatcho Lake, 2006

11. Moss Lakes, 2004-2007 FV2n showed significantly higher growth rates
Mortality at age 1 was equal for Moss lake #1 and #2, but lower for FV3n in Moss #3 (FV2n=122, FV3n n=178)
2n survival past age 1 was significantly higher for Moss Lake #2 and Moss Lake #3, but not Moss Lake #1

12. Black Lake, 2004-2007 2000 stocked/year for each strain
Catch composition mainly 1+ individuals
Highest survival shown for BW2n
Growth differences between groups are not biologically meaningful
High effort lake

13. Whale Lake, 2006-2007 1300 stocked/year for each strain
Catch composition mainly 1+ individuals
Again, highest survival shown for BW2n
Growth differences between groups are not biologically meaningful

14. Roche Lake, 2007 2 3 4 Length 210-406 365-540 520-550 n 151 25 stocked
15000 51 9 10
7500

15. Roche Lake Angling Survival

16. Pooled Analysis
Mean (SE) size at age (a) and relative growth rate (b) for Fraser Valley 1 year old and Blackwater 1.5 year old ploidy groups in winterkill (wk), occasional winterkill (owk) and non-winterkill (nwk) lakes. Black circles represent mean Blackwater ploidy group values, open circles represent mean Fraser Valley ploidy group values. Error bars represent standard error of the means.

17. Stocking Rates
Askey 2007, developed a stocking optimization model for small lakes in British Columbia
Effort response is low if too few fish are stocked or, too many fish are stocked
Need to link stocking rates to lake productivity and effort (Can we use BEC zones or growth as a surrogate to TDS?)
Need to understand the required rate of AF3n stocking, or management adjustments necessary to ensure older age classes of these specialty fish are available to anglers
Further work required……..

18. Proposed Project Multi-region
Utilize lakes on current flight-lines (need to maintain these flights for at least 4 more years)
A set of control lakes and a set of manipulated lakes (density) containing 2n and AF3n stocks (mono and co-stock situations)
Fall 2008 stock assessment of all lakes
Follow up assessment 2010, 2011, and 2012, with ongoing effort monitoring
Goals: refine stocking rates and understand how effort responds to AF3n stocking regimes

19. Other questions?
Are there lakes in BC that receive too little effort to justify stocking? (e.g. ~170 lakes stocked with rainbow in Region 3)
How do we address the lack of older age-classes? Is this a Regional or Provincial issue?
Are sterile rainbow trout an effective tool for increasing angler effort?
Should stocking rates be adjusted for sterile fish due to their apparent initial lower survival rate? Is supplementation with 3n a better alternative where a lake is stocked predominantly with 2n individuals?
What does the cost/benefit equation look like for sterile fish?