Population biology of fishes

 

  1. Populations
    1. Definition
      1. individuals in a species in an area
      2. shared gene pool
      3. stock: terminology used by resource managers: exploited population
    2. Characteristics
      1. size
      2. age/size structure
      3. Schedules of reproduction and death
      4. growth rate
  2. Population size
    1. function of area and density
    2. area: where is boundary?
    3. density or abundance
      1. #/m2
      2. #/m3
      3. estimating population size
        1. mark-recapture
          1. catch individuals, mark them, release and recapture.
          2. proportion of marked individuals in second catch = total marked individuals as prop. of entire population
          3. stringent assumptions
          4. time consuming, expensive
        2. catch per unit effort
          1. fewer fish, harder to catch them; CPUE declines
          2. theoretically possible to estimate the total number in population this way
        3. spawning stock estimate
          1. given knowledge of size-fecundity relations, estimate average fecundity
          2. sample eggs and estimate total egg abundance
          3. then can estimate total numbers of females
    4. carrying capacity: number of individuals that can be sustained
  3. Population structure
    1. Size structure
      1. Population is comprised of individuals of different size
      2. Size structure describes the sizes present and their relative abundance
      3. Different sizes may have very different ecological roles: eat different stuff, eaten by different predators, use different habitats
    2. Age structure
      1. Ages present and their relative abundance
      2. Year-class: individuals born in a particular year.
        1. Their age changes but they are always members of same yearclass
      3. example of an age structure: cui-ui (fig 23.2)
        1. Long-lived fish, but only a few yearclasses in population, due to frequent reproductive failure
        2. Bypass channel to river improved viability of population, more yearclasses present in '86.
    3. Importance of knowing size/age structure
      1. larger/older individuals tend to have higher fecundity
      2. more valuable to production of future generations
    4. Open v. closed
      1. How connected are local groups of individuals?
      2. Do offspring return to natal population or go elsewhere?
      3. Closed populations: self-seeding. Most lakes, salmon
      4. Open populations: larvae likely to disperse widely.
  4. Population dynamics
    1. Populations increase with
      1. birth (b: per capita birth rate)
      2. immigration
      3. recruitment: entering fishable population (exploited population) or reaching demersal habitat (e.g. coral reef fishes). Recruitment rate: rate at which new individuals added
      4. also, biomass of population increases with individual growth
    2. Populations decrease with
      1. death (natural and fishing mortality; d: per capita death rate)
      2. emigration
    3. factors that control population growth and population size
      1. abiotic
      2. biotic: different kinds of 2-species interactions
        1. competition
        2. predation, parasitism
        3. commensalism
        4. mutualism
      3. density dependent (fig. 8.1 diana)
        1. def: intensifies as population increases in size
        2. increases in mortality, reduced reproduction with higher population density
        3. per capita birth rate highest at lowest pop size
        4. per capita death rate lowest
        5. where equal, population growth rate constant, population is at K
        6. implicitly, there is a limiting resource that determines K
      4. density independent
        1. Magnitude of effect on reproduction or survival does not vary with population size
        2. Climate often cited; bad weather kills same proportion of individuals regardless of number.
      5. most likely birth and death rates do not change over some range of densities, and are density-dependent at higher densities
    4. Stock-recruitment relations
      1. if b and d are density-dependent, then (fig 8-2, diana)
        1. per capita increase in numbers (b-d) decreases with N
        2. total increase in numbers, (b-d)*N, is highest below carrying capacity
      2. stock-recruitment relations are important tool in fisheries science
        1. the idea is to exploit populations at the hump: the population level at which the most individuals are produced every year; surplus production
      3. do fish populations really behave this way? (fig 6.1, Pitcher and Hart)
        1. there is in the long run some evidence that recruitment is lower at high and low population levels
        2. but look at all that scatter
        3. which relationship would be stronger: stock-recruitment (predicting future recruitment from current stock) or recruitment-stock (predicting future stock from current recruitment)?
  5. Stochastic processes in marine fishes
    1. The emphasis so far has been on deterministic, compensatory processes
      1. systems should be predictable
      2. this may be relatively accurate for some closed pops
    2. Marine fishes: inherent variability
      1. scientists realized a long time ago that fisheries were unpredictable
      2. Haddock in north sea (Pitcher and Hart, fig 3.4)
        1. yearclasses can vary in strength 1-2 orders of mag
      3. Bay anchovy: (Newberger and Houde fig 3) subsequent yearclasses more than 10X different in abundance
      4. explanation:
        1. larval survival is variable, based on food availability and water flow
        2. variation in larval survival explains most of variation in subsequent adult abundance
      5. marine fisheries management has been based on a model of predictability, and it just ain't so