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How useful is surplus production analysis in assessing the fishery for New Zealand red rock lobsters (Jasus edwardsii)?

By: Breen, P.A.
Contributor(s): Kendrick, T.H. (NIWA. Wellington) | NIWA Wellington (N.Z.).
Material type: materialTypeLabelBookSeries: New Zealand fisheries assessment research document ; 95/24.Publisher: Wellington : NIWA, 1995Subject(s): RED ROCK LOBSTERS | NEW ZEALAND | FISHERY RESOURCES | STOCK ASSESSMENT | PRODUCTION MODELS | FISHING EFFORTOnline resources: Click here to access online In: New Zealand fisheries assessment research document In: New Zealand fisheries assessment research documentSummary: This paper examines potential biases in stock assessment conclusions drawn from surplus production modelling in an important New Zealand fishery with a long time series of catch and effort data. We describe the assessment procedure, discuss what might be the most serious potential problems with the model and data series, then describe an age structured data simulator with biological inputs from one area. We used the simulator to generate catch and effort time series with various kinds of systematic error. Five indicators were estimated with the assessment model and compared with the 'true' values from the simulator. The assessment model performed reasonably well when the data were simulated without systematic error, despite several potentially serious structural problems with the model. The choice of model 'shape' influenced the results, but not seriously and not consistently. The various indicators were estimated with different accuracy: the ratios of current to optimum biomass (B/sub curr// B/sub MSY) and effort (E/sub curr//E/sub MSY/) were estimated less well than virgin biomass (B/sub 0), maximum sustainable yield (MSY) and current surplus production (CSP). Errors in early catch data and systematic error in CPUE caused the greatest bias in results. MSY and CSP were highly robust to these errors, especially in the stressed-fishery cases. Although the ratio B/sub curr//B/sub MSY) was sensitive to these errors, the indicator remains useful in the stressed-fishery cases. The ratio was over-estimated when systematic errors were present, so conclusions based on this ratio in stressed-fishery cases are credible. The ratio E/sub curr//E/sub MSY/ was generally badly estimated, sensitive to the errors simulated, and performed better in the lightly-fished cases. We suggest this indicator should not be used as a basis for stock assessment advice. (auths.)
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Item type Current location Call number Copy number Status Date due Barcode
JOURNAL JOURNAL WELLINGTON
JOURNALS
NO. 95/24 1 Available J09949
JOURNAL JOURNAL WELLINGTON
JOURNALS
NO. 95/24 2 Available J09950

29 refs; 2 tables; 32 figs; 1 appendix

Not to be cited without permission of the author(s)

This paper examines potential biases in stock assessment conclusions drawn from surplus production modelling in an important New Zealand fishery with a long time series of catch and effort data. We describe the assessment procedure, discuss what might be the most serious potential problems with the model and data series, then describe an age structured data simulator with biological inputs from one area. We used the simulator to generate catch and effort time series with various kinds of systematic error. Five indicators were estimated with the assessment model and compared with the 'true' values from the simulator. The assessment model performed reasonably well when the data were simulated without systematic error, despite several potentially serious structural problems with the model. The choice of model 'shape' influenced the results, but not seriously and not consistently. The various indicators were estimated with different accuracy: the ratios of current to optimum biomass (B/sub curr// B/sub MSY) and effort (E/sub curr//E/sub MSY/) were estimated less well than virgin biomass (B/sub 0), maximum sustainable yield (MSY) and current surplus production (CSP). Errors in early catch data and systematic error in CPUE caused the greatest bias in results. MSY and CSP were highly robust to these errors, especially in the stressed-fishery cases. Although the ratio B/sub curr//B/sub MSY) was sensitive to these errors, the indicator remains useful in the stressed-fishery cases. The ratio was over-estimated when systematic errors were present, so conclusions based on this ratio in stressed-fishery cases are credible. The ratio E/sub curr//E/sub MSY/ was generally badly estimated, sensitive to the errors simulated, and performed better in the lightly-fished cases. We suggest this indicator should not be used as a basis for stock assessment advice. (auths.)

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