YIELD CURVES.

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Presentation transcript:

YIELD CURVES

YIELD CURVES The information needed to draw yield curves with usage of analytic models is growth rate and natural mortality of fish. ‘S’ shaped curve results growth in weight overtime. An exponential decline occurs with survival over time.

YIELD CURVES From the biomass recruit model, biomass per recruit curve will be obtained. This curve will always decrease by increasing the effort. In any fishery with decreasing in CPUE, the biomass will increase when effort increases. From these two curves, it is possible to derive yield per recruit curve for a fixed age of fish capture Tc. The Tc is the age at which the fish becomes vulnerable.

YIELD CURVES In yield / recruit curve, F is taken as independent variable and Y/R is dependent variable. Knowing the yield, keeping Tc constant, for a given value of F, the number of recruits could be ascertained by dividing the total yield by yield in gms of recruit The curve drawn in the above figure is called as yield per recruit curve, and the peak of the curve is MSY. Yield isopleths could be drawn for various combinations of F and Tc.

YIELD CURVES The yield per recruit curve depicts the MSY. The MSY depends on age at first capture Tc, and in turn Tc depends on mesh size used for a fishery. The following curves show how the yield is affected with Tc. MSY is highest at highest value of Tc with higher value of fishing effort.

YIELD CURVES The Tc and F could be managed by stock assessment scientist/fishery mangers to have highest MSY. ‘F’ is proportional to effort. Tc is the function of gear selectivity. Thus combining a range of values of Tc with a range values of F, sustainable yield could be achieved, for a certain level of Tc and F.

YIELD CURVES In the above graph, curve B is higher compared to curve A. But it has lower value of FMSY but with higher MSY/R. The main difference between two curves is the natural motality rate 0.2 for the curve B and 4.8 for the curve A. Several parameters can influence yield curve. Among them ‘M’ affects the yield curve. The variations in M can influence on the shape of the curves Y/R.

YIELD CURVES Several parameters can influence yield curve. Among them ‘M’ affects the yield curve. The variations in M can influence on the shape of the curves Y/R. Thus when M is lower, lower FMSY is produced and a higher MSY/R. Increasing the fishing effort above FMSY leads to decrease in total yield. When M is high, FMSY in difficult to estimate in the Y/R curve.

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