Lecture 5 - Ecology and Evolution of Harvested Populations

what is the general rule when it comes to harvesting?

harvest out the recruitment and leave the stock alone

recruitment

new additions to the population due to birth processes not immigration

stock

the current population

dN

change in abundance

dt

change in time

dN/dt

change in abundance over time

r

intrinsic rate of population growth of the species

K

carrying capacity

what happens as N approaches K according to the logistic growth model?

dN/dt equals zero so growth rate is zero

what is the drawback associated with density dependent growth?

it can limit population size

ex: in Tern, carrying capacity on islands is limited by nesting space

when will population growth be maximized?

harvest population from K to K/2 so population can be self sustained

maximum economic yield

where benefit-cost is maximized

maximum sustained yield

the point of maximum population growth or in other words when K/2 is harvested out

fixed quota strategy

set quota at MSY so allow fishing until max point of growth is reached then stop fishery

effort when stocks are small

gets used up quickly without catching much fish

effort when stocks are large

effort is used up and catch way more fish

ratchet effect

force that moves easily in one direction but not in the other

what happens when you used fixed quota during a bad year?

bad because if you fish the same amount you will drive the population down

fixed effort strategy constraints

needs to be defined stringently and there can be political effort to change the effort leading to the ratchet effect

what happens in a good year when recruitment is equivalent to harvest at MSY?

recruitment is higher and pressure for increased harvesting ratchets up the harvest

what happens in a normal year?

Harvest is equivalent to recruitment before MSY is reached

what are the benefits of fixed escapement strategies?

• works well with salmon and migratory fisheries

• allows some of the population to migrate up the river before allowing fishing to start

• ensures that a fraction of the population escapes

what are the constraints of fixed escapement strategies?

expensive and complicated

what are the benefits of tight regulation, enforcement and careful catch quotas below MSY?

it can mitigate negative effects on the populations and negative effects on society like losing jobs also it created ecological stability or in other word insurance for fisherman