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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

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lecture 2 review
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

SJ

SJ

N

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

(Invasive species have to exhibit this ability)

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)

slide3
Is the Beverton-Holt invariant M/K=1.6 a valid generalization based on your analysis of the data in Fishbase?
life history trajectories
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
Life history stanzas (partitions of the life history trajectory)

The eggie

First juvenile nursery area: small, strong density-dependence in mortality

Juvenile migration

Larval drift, density-independent mortality

Spread into larger juvenile nursery area(s), mortality much lower

Spawning migration

Fractal, complex diurnal movement

Adult foraging areas, most often with complex seasonal migration patterns

characteristics of lht

Random model

Distance from tagging site

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

Migration model

Distance from tagging site

Time

Time

characteristics of lht1
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 lht2
Characteristics of LHT
  • Maturation typically occurs at 50%-70% of maximum body length, with fecundity then being proportional to body 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
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)

X,Y positions and fates of large sample of individuals