Potential strategies for: “best use” of sterile fish,

optimized stocking densities, and future

direction….

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

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?

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0 2 4 6 8 10

Age (yr)

Len

gth

(cm

)

Diploid

triploid

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0 20 40 60 80

Length at t

Len

gth

at

t+1

Diploid

Triploid

1:1 line

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).

0.0 0.5 1.0 1.5 2.0

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1.0

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2.0

2N growth (mm/day)

3N g

row

th (

mm

/day

)

First fall sampling

Lengths

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2N growth (mm/day)

3N g

row

th (

mm

/day

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Second fall sampling

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2N growth (grams/day)

3N g

row

th (

gram

s/da

y)

First fall sampling

Weights

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2N growth (grams/day)

3N g

row

th (

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Second fall sampling

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).

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No loons

Pro

port

ion

3N r

etur

ns

First Year

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Loons present

Proportion 2N returns

Pro

port

ion

3N r

etur

ns

0.00 0.05 0.10 0.15

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No loons

Second Year

0.00 0.02 0.04 0.06 0.08 0.10

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Loons present

Proportion 2N returns

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).

0 0.2 0.4 0.6 0.8 1 1.2

Maturation mortality

Lik

eli

ho

od

Parameter likelihood

data points

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

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.

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Percent change in effort (Angler days/ha)

Fre

qu

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Expected effort (Angler days/ha)fr

eq

ue

nc

y

Diploid stock

Triploid stock

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

Fish Population Information BW2N BW3N BWAF3N

# stocked per year (2006) 800 800 800Stocking Formula Predicted (Stringer) 2522.7 2522.693 2522.693Stocking Formula Predicted (MEI) 2439.7 2439.748 2439.748Total fish re-captured 28 1 8Total net hours 99.82 99.82 99.82Average Gillnet CUE = 0.28 0.01 0.08

72 fish total including non-research 0.8

Opatcho Lake, 2006

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

AGEStrain 1.5 2.5 3.5

BW2n Length (mm) 230-333 390-426Weight (g) 148-457 725-976n 113 7

BW3n Length (mm) 219-312Weight (g) 125-379n 47

BWAF3n Length (mm) 200-323 444-459 451Weight (g) 117-430 984-1172 1186n 70 2 1

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

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

AGEStrain 1.5 2.5 3.5

BW2n Length (mm) 222-387 439-452 585-638Weight (g) 121-867 725-976 2711-3517n 70 2 3

BW3n Length (mm) 216-390 378Weight (g) 129-768 666n 18 1

BWAF3n Length (mm) 200-323 491 502-575Weight (g) 117-430 1597 1737-2876n 11 1 6

Roche Lake, 2007

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Age

Leng

th (m

m)

Pennask 2n

Pennask AF3n

Log. (Pennask 2n)

Log. (Pennask AF3n)

2 3 4

Length 210-406 365-540 520-550

n 151 25 4

stocked 15000 15000 15000

Length 220-338 370-451 376-569

n 51 9 10

stocked 7500 7500 7500

Roche Lake Angling Survival

Ploidy Caught Survived Relative Survival %

2n 180 167 933n 73 63 86

unknown 15 14 93

Pooled Analysis

Lake type-ploidy group

wk 2n wk 3n owk 2n owk 3n nwk 2n nwk 3n

Wei

ght (

gram

s)

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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.

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……..

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

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?