Effects of variable and sizeselective gill-net fishing on lifehistory evolution in grayling
Thrond O Haugen
&
Leif Asbjørn Vøllestad
What to expect from
size-selective fishing?
a
La
In other words: An open question!
— ?
—
—
When considered
trait by trait
? Growth
+
Adult survival
—
+
Size-selective fishing
Growth
—
What to expect from sizeselective fishing—a reaction
norm perspective
Theoretical studies show that variation
in growth-dependent survival affects the
shape and posision of maturation
reaction norms (e.g. Stearns & Koella 1986)
 For any combination of mortality responses to
growth, rapid growers are predicted to mature
earlier than slow growers
 Expect a smaller age-size maturation space
with increasing mortality
Size at maturity
Maturation reaction norms and
survival
0.5
0.1
0.9
c
b
a
|slope|= |-a/b|
width = c
Age at maturity
Increasing mortality
The study species
Lake/river
Tributary
Mature
Egg
3-8 years
Max age: 28 years
Larvae
130-140 °D
Swim-up larvae
2–3 weeks
Juvenile
Fry
During September
Gravel
130-140 °D
The study system
Osbumagasinet
Lake
Mesh size
(mm)
Efforta
Fishery
since
Les
30b
200–250
e.p.c
ØM
29
>>250
1970s
Ht
29
0–100
1950s
Aur
35
80–100
e.p.
Os
30
120–150
e.p.
Number of gill nets per km2 per year
b The present mesh-size. It has changed
Norway
during the 1900s
c e.p. = entire period
N
Os
1954
Aursjøen
Hårrtjønn
Ht
a
1920
Aur
ØM
Øvre
Mærrabottvatn
1910
Lesjaskogsvatnet
Les
1880
10 km
Characteristics of the lakes
and the grayling populations
Les
ØM
Ht
Aur
Os
Altitude (m)
611
1156
1172
856
848
Area (km2)
4.52
0.28
0.35
33.67
9.33
Mean depth (m)
10
2
7
20
25
Growth season (days) 160–180 125–165 100–140 145–165 125–145
Grayling:trout ratio 1:(0.5–3) 1:4
10:1
1:1
1:8
CPUE (#ind. 100 m-2) 4.9–9.2 0.2–5.1 6.1–13.4 0.8–4.3 0.2–2.9
N e (msat based)
>1000 36–170 12–42
25–110
—
Microsatellite FST = 0.05–0.21
Juvenile trait QST = 0.00–0.92
Koskinen, Haugen & Primmer (2002), Nature
Lesjaskogsvatnet 1903–2000
1900
1950
2000
Monofilament nylon nets
Multiple mesh-size survey
Relaxed size-selective fishery
SIntensive size-selective fishery
The Objectives
Do grayling (and co-occuring trout)
decrease age and size at maturity in
systems with intensive size-selective
fisheries?
Among-lake level (synchronic data)
Within-lake level (allochronic data)
If so, is this solely due to growth-rate
changes/differences resulting from
changed/differential fishing pressure?
Methods
Multiple mesh-size gill-nets
12–52 mm for the synchronic data
6–10 surveys in the 1995–1999 period per lake
19–52 mm for the allochronic data
7 surveys
Ageing and back calculation of growth
using otoliths
Maturation pattern estimated from
multiple logistic regression
Life-table simulations
The synchronic data
Relative size (%)
Differential maturation pattern
25
20
15
10
5
0
-5
-10
-15
-20
-25
-30
30
25
20
15
10
5
0
-5
-10
-15
-20
-25
-30
Øvre Mærrabottvatn
Hårrtjønn
0.1
0.9
0.9
0.5
0.1
Aursjøen
Osbumagasinet
Lesjaskogsvatnet
0.9
0.9
0.5
0.1
0.5
0.9
0.5
0.1
0.1
1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12
Age
Life-history evolution in response
to differential survival
7.0
0.22
6.5
0.20
5.5
0.18
5.0
GSI
Age at maturity
6.0
4.5
0.16
4.0
3.5
0.14
3.0
2.5
0.4
0.12
0.5
0.6
0.7
0.8
Annual survival rate
Haugen (2000), Oikos; Haugen & Vøllestad (2001), Genetica
Convergent evolution
Age at maturity
7.0
6.0
5.0
4.0
3.0
2.0
0
100
200
300
Gill-nets per km2 per year
Haugen (2002), Submitted
Standardized trait value (sd unit)
Responses to differential fishing
intensities
2.8
2
1
0
-1
-2
Egg size
GSI
Fecundity
Age at maturity
La
Survival
G1
G2
G3
0 100 200 300
Number of gill nets per km2 per year
Haugen (2002), Submitted
2.5
1000
b
2.0
a
1.5
100
|slope|= 1.0
|a/b|
width = 0.5
|slope|
width
10
Age at maturity
0
width of reaction norm
|slope| of reaction norm
Size at maturity
Also affects maturation
reaction norms
0.0
200
100
300
Effort
Haugen (2000), Oikos
Life-table simulations
Relative fitness
Predictions for Lesjaskogsvatnet
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
32 mm
28 mm
No fishing
3
4
5
6
Age at maturity
Haugen & Vøllestad (2001), Genetica
Responses to changed fishing regime
340
32 mm
28/30+22 mm
30 mm
28 mm
Length at maturity
320
1927: 28 mm
1923
32 mm
300
1903
280
1951
1975: 28+22 mm
1998
260
1992: 30+22 mm
1991
240
1985: 30 mm
1981
220
1982: 30+22 mm
200
3.5
4.0
4.5
5.0
5.5
6.0
6.5
Age at maturity
Haugen & Vøllestad (2001), Genetica
Has growth changed during the 1900s?
Back calculated length (mm)
400
L1
L2
L3
L4
Yes, but not as expected according
to
L5
changes in density
100
Selection for lowered growth under high
fishing pressure
30
1920 1930 1940 1950 1960 1970 1980 1990 2000
Haugen & Vøllestad (2001), Genetica
Conclusions
All three data sets supported that the
maturation pattern evolved in response
to size-selective fisheries
In an adaptive manner
The response was not due to changed or
differential growth pattern
Not a plasticity response
In Lesjaskogsvatnet growth changed
directly in response to gill-net selection
and not indirectly due to density effects
That’s all folks!