Multistage cluster sampling design and optimal sample sizes for estimation of fish discards from commercial trawlers
The fish discards surveys, conducted by Northern Ireland, Spain and England under the auspices of EC project 95/094, are multistage in design with a structure of hauls within trips within vessels. The data collected by on-board observers from these surveys enabled investigation into the optimal numb...
| Published in: | Fisheries Research |
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| Main Authors: | , , , , , |
| Format: | Article in Journal/Newspaper |
| Language: | English |
| Published: |
2002
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| Subjects: | |
| Online Access: | http://hdl.handle.net/10508/8896 http://hdl.handle.net/10261/328743 https://doi.org/10.1016/S0165-7836(01)00308-3 |
| Summary: | The fish discards surveys, conducted by Northern Ireland, Spain and England under the auspices of EC project 95/094, are multistage in design with a structure of hauls within trips within vessels. The data collected by on-board observers from these surveys enabled investigation into the optimal number of hauls, trips and vessels that require sampling to achieve target levels of precision for each gear type and country. Residual maximum likelihood (REML) was used to estimate the variance components between vessels, between trips within vessels and between hauls within trips within vessels, while examining the significance of the fixed effects of area, quarter and their interaction. To meet the REML requirement that the data are normally distributed, the extreme skewness of the data was corrected by a natural log transformation. The estimated lognormal variance components were used within the multistage formulae to estimate the optimal average sampling levels at each stage. The results showed that more resources than were available during the project were required to achieve a coefficient of variation of 20% for the estimation of discard levels for Northern Ireland twin-rig and Spanish pair trawls. SI |
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