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15 Cards in this Set
- Front
- Back
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Why is surplus production important? |
- how surplus production models can be used to determine appropriate harvest levels in fisheries - to manage populations, need to know about population dynamics - problem is how to produce biggest crop without endangering the fish |
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Yield definition? important unit of measure |
similar to crop size - expressed in biomass - goal for species, and $$ is optimal yield - optimal yield is need to be less than maximal sustainability |
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2 factors that decrease biomass of a fish stock |
- Natural Mortality (M) - Fishing Mortality (F) |
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2 factors that increase biomass of fish each year |
- growth (G) - recruitment (R) |
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Equation that takes 2 factors of increase biomass and 2 factors of decrease |
S2= S1+R+G-M-F S2= weight of stock at end of year S1= weight of stock at start of year G= growth in weight of fish remaining alive M= weight of fish removed by |
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According to S2= formula, when is population at equilibrium? |
when S1= S2 and R+G = M+F - equilibrium means the population stays at a constant state |
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formula when population is being exploited, but stable |
recruitment + growth = natural losses + fishing yield |
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3 factors that influence population size |
1. greater recruitment 2. greater growth rate 3. reduced natural mortality |
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Sigmoid growth curve explanation |
- initially, population will grow slowly, and then it will reach a maximum rate of increase in the middle of the curve (if we want max population growth, we should keep population grown around the mid point) ** highest productivity is at a lower density then the top of the curve** - then it will hit carrying capacity - rate of increase formula at any point of of the curve (dN/dt) = rN(K-N)/K - use old bio info to understand variables |
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rule of exploitation definition? |
maximum yield obtained from populations at less than maximum density |
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Surplus production models |
- sigmoid growth curves with logistic equation models are the basis for simplest surplus production models - in these models growth, recruitment, and natural mortality are combined into a single variable |
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Surplus production model formula |
(dN/dt) = rN(K-N)/K - qXN - qXN = mortality rate from fishing - q = catchability constant - x = fishing effort - fishing effort qX = mortality rate from fishing |
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Figure displaying response of exploited populations to different fishing episodes (points A-E) |
A = fishing is intensive, but fishing population recovers B= fishing is much less intense and population recovers completely in a short time C = moderate fishing intensity, but time for recovery creates equilibrium where population returns to that of each episode D= more frequent fishing episodes cause stock to collapse E = excessive fishing drives stock to extinction |
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Figure showing catch rate (yield) and biomass of stock depending on fishing mortality rate |
- optimum yield occurs at a point with a lower fishing mortality rate, lower catch rate and high population size (biomass) than the point where MSY occurs |
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Limitation of surplus production model |
- that recruitment is assumed to be solely dependent on population size (# of spawners) - fluctuation in environment affecting recruitment are not included in formula - can't set exact quotas in recreational fisheries - hard to count population size of some fish species |
