Lecture 5: Ecology and Evolution of Harvested Populations

Question 1

Harvesting

Answer 1

• Humans harvest plants, animals, and other products from natural world
• Harvesting is inevitable for living organisms to survive
  
  • Energy, biomass, and nutrients

Question 2

General harvesting rule

Answer 2

• Harvest out recruitment and leave stock alone
• Recruitment: new additions to population
• Stock: Current population

Question 3

dN/dt = r N [(K-N) / K ]

Answer 3

• dN: change in abundance
• dt: change in time
  
  • dN/dt: change in abundance over time
• r: intrinsic rate of population growth of species
• K: carrying capacity

Question 4

Use of logistic curves to predict harvesting

Answer 4

• Used to know when to harvest food

Question 5

When is population growth maximized

Answer 5

• Harvest population from K to K/2 so population is at max growth rate

Question 6

How harvesting affects demographic of harvested species

Answer 6

• Population always in region of max growth: highest chance of replenishment
• “Have our fish and eat them too” : allows harvesting and self-sustaining populations

Question 7

Fish harvesting curve

Answer 7

• MEY: Maximum Economic Yield where benefit - cost maximized
• MSY: Maximum Sustained Yield, point of max population growth
• MEY where higher population size, decrease in fishing effort
• Economic pressure to harvest more

Question 8

Consequences

Answer 8

• Constant financial pressure to harvest more because B > C even when yield is decreasing
• Populations are harvested to point where they have low population growth rates
• Improvements in technology make cost curve flatter

Question 9

Three options

Answer 9

• Fixed quota
  
  • Set quota at MSY(allow fishing until MSY reached, keep population at K/2 point)
• Fixed effort
  
  • Define effort stringently(regulate specific seasons, regions to fish)
  • For fixed amount of effort. When stocks small, effort used up quickly without much fish. When stocks large, effort used up but with more fish caught
• Fixed escapement

Question 10

What happens in a bad year

Answer 10

• Gradually fishing down population
• Fixed quota doesn’t work in bad years

Question 11

Fixed effort to rescue

Answer 11

• Need to define stringently
• Political pressure to change effort leads to ratchet effect
• Ideally set effort so that H = R at MSY
• Set intersection at K/2
• In good years, stretched out, more fish available

Question 12

Pressure for increased harvesting ratchets up harvest

Answer 12

• Only works during good year
• More effort to fish
• When back to normal year, hard to take away effort

Question 13

Back to normal year

Answer 13

• MSY below K/2

Question 14

Fixed escape strategies

Answer 14

• Works best with salmon and migratory fisheries
  -Allow some % of population to migrate up river before allowing fishing to start
• Ensures fraction of population escapes(fish able to undergo natural lifestyle)
• Expensive and complicated
  
  • Only able to monitor limited number

Question 15

Until fishery collapses

Answer 15

• Net profits will always lead to more pressure to fish and get more fish
• Tight regulation, enforcement, and cautious catch quotas below estimated MSY help mitigate negative effects on the population
  
  • Catch quotas < MSY provide natural insurance by creating ecological stability
• Persistent economic incentive to continue fishing even when industry is falling