Download Lake Trout Fishery Management

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
Kokanee abundance
8
Drawdowns to 2051 ft became
routine beginning in 1966
7
6
5
Shrimp established in 1975
4
3
2
1
Albeni Falls Dam Built
96
92
88
84
2000
Year
80
76
72
68
64
60
56
52
0
KOKANEE SURVIVAL
• Declined in 2007; low for all ages
• Survival less than 50% is a concern
• Predation limiting survival to spawning
Indices of LT Abundance, Lake Pend Oreille
19
53
19
85
19
92
19
94
19
96
19
98
20
00
20
02
20
04
20
06
20000
18000
16000
14000
12000
10000
8000
6000
4000
2000
0
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 Trout
Fishery Management
• Lake trout are late maturing and long lived,
so are vulnerable to over-fishing.
• The largest lake trout populations were overfished 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 overfishing of lake trout!
Lake Trout
Size Structure
15%
Gillnets
Trapnets
Angling
Percent
10%
5%
0%
20
IDFG 2007
40
60
Length (cm)
80
100
Lake Trout
Growth
100
Superior, MI
Michigan, WI
Pend Oreille, ID
Length (cm)
80
60
40
20
0
0
McKee et al. 2004
Burnham-Curtis and Bronte 1996
5
10
15
Age (years)
20
25
Lake Trout
Maturity
100%
Maturity (%)
80%
60%
Males
(6.5 years)
Females
(7.3 years)
40%
20%
0%
450
500
550
600
650
700
Length Class (mm)
IDFG 2007
750
800
Lake Trout
Future Prognosis?
100,000
Observed
Predicted
90,000
This?
80,000
Abundance
70,000
60,000
Which is better for
Lake Pend Oreille?
50,000
40,000
30,000
20,000
Or,
this?
10,000
0
1999
IDFG 2007
2001
2003
2005
2007
Year
2009
2011
2013
2015
400
350
Biomass
Kokanee Biomass,
Production and Yield
Production
Yield
 This
250
200
150
100
50
0
2000 2020 2040 2060 2080 2100 2120 2140 2160 2180 2200
Year
400
350
Biomass
Production
Yield
300
Or This 
Kokanee (mt)
Kokanee (mt)
300
250
200
150
100
50
0
2000 2020 2040 2060 2080 2100 2120 2140 2160 2180 2200
Year
Lake Trout
Fishery Management
3,500
40,000
3,000
35,000
30,000
25,000
Angling
2,000
Trap Nets
20,000
Gill Nets
1,500
Natural
15,000
Abundance
1,000
10,000
500
5,000
0
0
Jan Feb Mar Apr May Jun
IDFG 2007
Jul
Month
Aug Sep Oct Nov Dec
Abundance
Number Killed
2,500
Lake Trout
Mortality
• 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?
Kilograms (millions)
• Lake trout exploitation (angling &
commercial nets) ~50%
– Exploitation rates of ~ 40% collapsed
Great Lakes fisheries
5
• Rainbow trout
exploitation still 4
3
low
2
• Predation still
Lake Michigan lake trout
too high
1
0
1879
1889
1899
1909
1919
Year
1929
1939
1949
1959
Lake Trout
Distribution
Great Bear Lake
(31,153 km2)
(452 m)
Lake Pend Oreille
(383 km2)
(351 m)
Great Slave Lake
(27,195 km2)
(625 m)
Lake Superior
(82,414 km2)
(405 m)
Lake Michigan
(58,016 km2)
(285 m)
Lake Huron
(59,596 km2)
(220 m)
Lake Ontario
(19,529 km2)
(237 m)
Lake Erie
(25,745 km2)
(64 m)
Probability of Kokanee Collapse
With Different Mortality Rates for RBT and LT
Likelihood of Kokanee Collapse
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
-30% -20% -10% 0% 10% 20% 30% 40% 50%
Change in Mortality
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.