Some basic tools for using population ecology as a management tool –

A Primer

• GROWTH

• RECRUITMENT

• MORTALITY

• COMPENSATION

Lake Pend Oreille

An On-the-Ground Application of Population Ecology

LPO Fishery - Desired Outcomes

• Ross Hall photos

Adult Kokanee Abundance in Lake Pend Oreille

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1

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852 56 60 64 68 72 76 80 84 88 92 96

2000

Year

Kok

anee

abu

ndan

ce

Drawdowns to 2051 ft became routine beginning in 1966

Shrimp established in 1975

Albeni Falls Dam Built

KOKANEE SURVIVALKOKANEE SURVIVAL

• Declined in 2007; low for all agesDeclined in 2007; low for all ages

• Survival less than 50% is a concernSurvival less than 50% is a concern

• Predation limiting survival to spawningPredation limiting survival to spawning

Indices of LT Abundance, Lake Pend Oreille

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1953

1985

1992

1994

1996

1998

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2002

2004

2006

Harvest Est.

Pop. Estimate

LPO Kokanee

• Primarily lake spawners, some tributary spawners, hatchery supplementation

• Longevity – 4 to 5 years (die after spawning)• Relatively low fecundity, but high plasticity

(compensatory ability)• Planktivores• Key prey item for BLT, RBT, LT as well as avian and

terrestrial predators

• Principle threats:– Predation– Habitat impacts

LPO Lake trout

• Lake spawners (fall)• Longevity – 30+ years, mature at 7 years• Highly piscivorous, but can survive on Mysis• Slow growth• Relatively low fecundity

• Principle threat:– Directed harvest

Lake TroutFishery Management

• Lake trout are late maturing and long lived, so are vulnerable to over-fishing.

• The largest lake trout populations were over-fished in the Great Lakes before sea lamprey became a problem.

• Most lake trout populations in Ontario inland lakes are considered over-fished.

• Lake Pend Oreille is a field test of over-fishing of lake trout!

Lake TroutSize Structure

0%

5%

10%

15%

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Length (cm)

Pe

rce

nt

Gillnets

Trapnets

Angling

IDFG 2007

Lake TroutGrowth

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0 5 10 15 20 25

Age (years)

Le

ng

th (

cm)

Superior, MI

Michigan, WI

Pend Oreille, ID

McKee et al. 2004Burnham-Curtis and Bronte 1996

Lake TroutMaturity

0%

20%

40%

60%

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100%

450 500 550 600 650 700 750 800

Length Class (mm)

Mat

urity

(%

)

Males(6.5 years)

Females(7.3 years)

IDFG 2007

Lake TroutFuture Prognosis?

0

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1999 2001 2003 2005 2007 2009 2011 2013 2015

Year

Ab

un

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nce

Observed Predicted

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1999 2001 2003 2005 2007 2009 2011 2013 2015

Year

Ab

un

da

nce

Observed Predicted

This?

Or,this?

Which is better forLake Pend Oreille?

IDFG 2007

Kokanee Biomass,Production and Yield

This

Or This 0

50

100

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2000 2020 2040 2060 2080 2100 2120 2140 2160 2180 2200

Year

Kok

anee

(mt)

Biomass

Production

Yield

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2000 2020 2040 2060 2080 2100 2120 2140 2160 2180 2200

Year

Ko

ka

ne

e (

mt)

Biomass

Production

Yield

Lake TroutFishery Management

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1,000

1,500

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2,500

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3,500

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

Num

ber

Kill

ed

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5,000

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35,000

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Abundance

Angling

Trap Nets

Gill Nets

Natural

Abundance

IDFG 2007

Lake TroutMortality

• Natural (LPO2006 = 15.1% estimate):– Average = 18% (much higher if lampreys).– Range = 10–36% (much higher if lampreys).

• Fishing (LPO2006 > 44.5%; 0.58kg/ha):– Highest = 45% (Superior prior to lamprey).– Populations decline if harvest > 0.50 kg/ha.

• Total (LPO2006 > 59.6%):– Populations are sustainable if A < 50%.– Most populations decline if A > 50%!

Healey 1978

Can it work?

• Lake trout exploitation (angling & commercial nets) ~50%– Exploitation rates of ~ 40% collapsed Great

Lakes fisheries• Rainbow trout

exploitation still

low• Predation still

too high

1879 1889 1899 1909 1919 1929 1939 1949 1959

Year

0

1

2

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4

5K

ilogr

ams (

mill

ions

)

Lake Michigan lake trout

Lake TroutLake TroutDistributionDistribution

Great Slave Lake(27,195 km2)

(625 m)

Great Bear Lake(31,153 km2)

(452 m)

Lake Superior(82,414 km2)

(405 m)

Lake Michigan(58,016 km2)

(285 m)

Lake Huron(59,596 km2)

(220 m)

Lake Erie(25,745 km2)

(64 m)

Lake Ontario(19,529 km2)

(237 m)

Lake Pend Oreille(383 km2)(351 m)

Probability of Kokanee CollapseWith Different Mortality Rates for RBT and LT

0%

10%

20%

30%

40%

50%

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70%

80%

90%

100%

-30% -20% -10% 0% 10% 20% 30% 40% 50%

Change in Mortality

Lik

elih

oo

d o

f Ko

kan

ee

Co

llap

se

Bottom Line for Success on LPO:• We need to reduce predation on kokanee by

50 tons annually to begin kokanee recovery • RBT and LT are the two primary predators• Current needs – harvest LT and RBT to

reduce predation and slow growth of LT population

• LT will need to be managed at a suppressed level for the long term

• The public will need to support programs

Some Take-Away Points For F&W

Population Management: • If R>M populations grow, and vice versa• More fecund animal populations apt to grow more

rapidly in good, unoccupied habitat• Many species “compensate” at low densities by

increasing growth rates, lowering age of reproduction & increasing fecundity

• Late maturing, lower fecundity populations more susceptible to mortality agents

• “Predator pits” more likely to occur when predator species has abundant supply of alternative prey

• Habitat conditions affect R & M, but so also can external forces (Exploitation, Predation, Competition, Disease, etc)

Information and slides for this presentation were contributed by Melo Maiolie, Mike Hansen and Ned Horner.