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 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?

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 migration First 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 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) [N 1 N 2 N 3 …] t  [N 1 N 2 N 3 …] t+1 (easy in spreadsheets) X,Y positions and fates of large sample of individuals