sources of fish decline habitat disruption
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Sources of Fish Decline Habitat disruption. Breeding areas Larval development areas Bottom structure. Not all patterns are negative. Population processes: aid to intelligent management?. 1830’s – concerns about fluctuations in catch in North Sea Disbelief that humans could cause this - PowerPoint PPT PresentationTRANSCRIPT
Sources of Fish DeclineHabitat disruption
• Breeding areas
• Larval development areas
• Bottom structure
Not all patterns are negative
Population processes: aid to intelligent management?
• 1830’s – concerns about fluctuations in catch in North Sea
• Disbelief that humans could cause this• C. D. J. Petersen (Denmark) applied science
– mark-recapture to estimate population size– collected data on age-dependent reproduction– applied population model to predict connection
between fishing mortality & fish populations
• Criticized as “irrational” (not “profitable”)• Tested during WW I and WW II
What do we need to know?
• Demography: the study of processes affecting populations
• Processes adding to populations:– births, immigration
• Processes subtracting from populations:– death, emigration
• Base number of individuals • Whether the processes are constant through time
– could vary with season or other scales of time
– could vary with the density of individuals, which change over time
What if processes are constant?
• Population size in the next generation will depend on the base and the difference between births and deaths– assuming we have an isolated group of
individuals
Nt+1 = Nt + b – d
Exponential growth
What if processes vary with density?
• If populations get larger, what do you predict will happen to birth rates?
• If populations get larger, what do you predict will happen to death rates?
• How many individuals are added to the population when birth rates and death rates are equal?
“Logistic” growth pattern
Summary of population models
• The Logistic model of density dependence predicts maximal sustainable yield at ½ K
• “S-shape” curve of population growth may not be seen when– The response to density lags changes in the
environment– For populations with large excesses of births
(r>2) and where generations are distinct
Added realism: individuals vary in “b” and “d”
• Size (or age) influences– Reproductive capacity (# of offspring likely)– The risk of being eaten by a predator– The probability of being captured in a net
• Age-specific demographic processes– Fecundity– Survivorship
Age-specific parameters
• Start with a bunch of ♀ individuals newly born (= a cohort)
• Determine the number of individuals that survive to each successive age (“x”)
– Sx
– The probability of survival from birth to age, “x”: lx
• The number of ♀ offspring produced per
♀ individual of age “x”: mx
Life Table = collection of data on Sx, lx, mx
• We can then project how each cohort will contribute to the population through its lifetime
• Some values derived from a life table:– Net Reproductive Rate, R0 = the number of ♀
progeny expected to accumulate during the entire lifetime of an average ♀
– Intrinsic growth rate, r
– Reproductive Value (Vx) = the expected number of future ♀ progeny for a ♀ of age “x” (relative to that of a newborn, = R0)
The real world is not a set of simple equations
• Randomness is a factor– “Deterministic” models always follow the same
path given the same conditions– “Stochastic” models include chance
• How is this done?– Use an average value for a parameter– But for any generation, the value used can deviate
somewhat from that average– “Coefficient of Variation” and “distribution”
define the limits of deviation
Success of species-based management
What are the connections between food web and demographic approaches?
• What demographic parameters are influenced?
• Are models still useful and how?
An alternative to capture fisheries