lecture 2 review compensatory rate change is the ecological basis for sustainable populations and...
TRANSCRIPT
Lecture 2 review
• Compensatory rate change is the ecological basis for sustainable populations and harvesting
• Compensatory change may involve– Increases in adult survival rate at low N– Increases in juvenile survival rate at low N– Increases in growth and mean fecunity at low N
• Generally mean fecundity decreases dramatically in harvested populations, so compensation is mainly in juvenile survival
• A good measure of compensation in juvenile survival is the “Goodyear compensation ratio” K=(maximum survival rate)/(survival rate in unharvested population)
Limits to compensatory responses• Most populations exhibit high juvenile
survival at very low densities
• But occasionally (5-10%?) compensation fails at low densities, leading to low equilibrium or extinction
N
SJ
SJ
N
-Allee effect (eggs don’t get fertilized, eg scallops); rare
-Cultivation/depensation (competitors/predators of juveniles increase when N is low, eg bass-bluegill)
-Trophic cascades (green water/clear water states)
-Botsford’s effect (size dependent cannibalism)
(Invasive species have to exhibit this ability)
Is the Beverton-Holt invariant M/K=1.6 a valid generalization based on your
analysis of the data in Fishbase?
y = 2.1161x
R2 = 0.5872
y = 1.6372x
R2 = 0.3182
y = 2.0041x
R2 = 0.8567
y = 1.1363x
R2 = 0.2195
0.01
0.1
1
10
100
0.01 0.1 1 10
vonBertalanffy K
Na
tura
l mo
rta
lity
ra
te M Age structure data
Length converted catchcurve
Tagging
Z vs E plot
Life history trajectories
• Whenever you handle a fish, ALWAYS ask yourself these questions:– How old is it?– Where was it spawned?– Where will it spawn?
Life history stanzas (partitions of the life history trajectory)
The eggie
Larval drift, density-independent mortality
Juvenile migrationFirst juvenile nursery area: small, strong density-dependence in mortality
Spread into larger juvenile nursery area(s), mortality much lower
Adult foraging areas, most often with complex seasonal migration patterns
Spawning migration
Fractal, complex diurnal movement
Characteristics of LHT
• There is typically very strong selection for behaviors that take fish back to spawn in the places where they were successfully produced (this is not just a salmon thing)
• Seasonal migrations become more pronounced as fish grow
Time
Random model
Distance from tagging site
Migration model
Distance from tagging site
Time
Characteristics of LHT
• Natural mortality rates vary as M=k/(body length), starting at a few percent per day and often falling to a few percent per year
• Body growth typically follows a vonBertalanffy length curve of the form
length=L[1-e-K(a-ao)]• Sometimes there is a “kink” in the growth curve,
with small juveniles either showing extra fast growth (if they seek warm microhabitats) or extra slow growth (if they face very high predation risk).
Characteristics of LHT• Maturation typically occurs at 50%-70% of maximum
body length, with fecundity then being proportional to body weight
0
10
20
30
40
50
60
70
0 2 4 6 8 10 12
Age (years)
Len
gth
(cm
)
0
500
1000
1500
2000
2500
3000
3500
4000
Wei
gh
t (g
)
Length (cm)
Hayes model Length
wt (kg)
Hayes Model weight
But some fish like these New Zealand brown trout practically stop growing at maturity, and make massive (45%) investments in eggs (Hayes et al TAFS 2000)
Representing LHT in models
• Age structure accounting (block trajectory by even age intervals)
• Stanza structure accounting (Ecosim)
• Individual-based models (track movement)
[N1 N2 N3 …]t [N1 N2 N3…]t+1 (easy in spreadsheets)
Log Numbers at age
Age (months)
Weight at age
Log Numbers at age
Age (months)
Weight at age
X,Y positions and fates of large sample of individuals