Improving the Collection, Management, and Use of Marine Fisheries Data (2000) / Chapter Skim
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Appendix C: Evaluation of Summer Flounder Surveys
Appendix C: Evaluation of Summer Flounder Surveys
Pages 181-196
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From page 181... ...
The winter survey includes offshore strata on the eastern side of ¢.> Georges Bank and all but the very deepest strata (deeper than 183 m or 100 fathoms) from Hudson Canyon to Cape Hatteras.
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From page 182... ...
The spring and fall NEFSC surveys currently use a standardized #36 Yankee otter trawl rigged with 16-inch rollers on the sweep, 5-fathom trawl legs, and 1,000-pound Polyvalent doors (Azarovitz, 1994)
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From page 183... ...
A 03 37 2 0.015 0.50 0.71 modified version ofthe#36Yankeeis used for 03 38 2 0.011 2.50 2.12 the winter survey. The main modifications of the 03 39 1 0.002 0.00 N/A winter gear include a rubber disk (4 inch)
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From page 184... ...
All of the NEFSC surveys use a stratified random design with bathymetric limits as the orimary stratifying variable (<9 m, 9-18 m, 18.1-27 m, 27.1-55 m, 55.1-110 m, 110.1-185 m, and 185.1-365 m) , with additional subdivisions intro , , is .
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From page 185... ...
Depths range from O m to 27 m. 185 39 38 ~ 36 FIGURE C-2 Strata sampled on NEFSC inshore bottom trawl surveys from Buzzards Bay, Massachusetts, to Delaware Bay, Delaware.
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From page 186... ...
State Surveys The Rhode Island Department of Fish and Wildlife has conducted a trawl survey of its coastal waters since 1979 (Lynch, 1985~. Originally, a stratified random design with 11 depth strata was used to monitor the waters in Rhode Island Sound, Block Island Sound, and Narragansett Bay.
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From page 187... ...
1 980 1 990 Year - 30 - 20 - 10 - O 1 980 1 990 FIGURE C-5 Mean number of summer flounder per tow for estimates of age 1+ fish from surveys used in summer flounder assessment. NOTE: CTDEP = Connecticut Department of Environmental Protection; MADMF = Massachusetts Department of Marine Fisheries; NEFSC = Northeast Fisheries Science Center; NJBMF = New Jersey Bureau of Marine Fisheries; RIDFW = Rhode Island Department of Fish and Wildlife.
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From page 188... ...
NOTE: CTDEP = Connecticut Department of Environmental Protection; MADMF = Massachusetts Department of Marine Fisheries; MDNR = Maryland Department of Natural Resources survey; NC = North Carolina Pamlico Sound survey; NEFSC = Northeast Fisheries Science Center; NJBMF = New Jersey Bureau of Marine Fisheries; RIDFW = Rhode Island Department of Fish and Wildlife; and VIMS = Virginia Institute of Marine Sciences survey. s = spring survey; f = fall survey; w = winter survey; and all = all surveys.
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From page 189... ...
189 Cat o Cot a' Cut Cat a' Cat Cat ¢ o a' V)
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From page 190... ...
As of 1994 each river was APPENDIX C divided into two equal distance strata for calculating abundance estimates. A pilot study using a stratified random design was initiated in 1991.
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From page 191... ...
Both types of areas are subject to "bad tows": inshore areas usually because of untrawlable bottom, gear damage, and conflicts with inshore fixed gear; offshore usually because of the difficulty of towing at greater depth in stronger currents, which some191 TABLE C-5 Stratified Estimates for Summer Flounder from the 1995 NEFSC Surveys Winter Spring Fall Stratified mean Standard error Relative error (percent) Number of stations 95 percent confidence intervals Lower Upper 10.93 1.31 12 116 1.09 0.23 21 14 94 122 2.40 0.33 8.36 0.63 1.76 13.50 1.54 3.04 times results in crossed trawl doors/tow wires, gear damage due to unseen uncharted bottom structure, and conflicts with offshore fixed gear.
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From page 192... ...
One of the advantages offered by stratified random designs over such designs as simple random sampling is that the former designs provide a more precise estimate of the mean if the strata boundaries encompass similar densities of animals and higher sampling rates are used in the strata with the larger variances. How well each design achieves this goal of increased precision can be evaluated after the fact by comparing the variance of the estimate achieved to the variance that would have been obtained if a simple random sample design had been used.
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From page 193... ...
, the resultant estimated maximum efficiency along with the minimum standard error this would yield is provided in the table. In terms of standard error, the optimal allocation would result in substantial reductions of around 62 to 70 percent over simple random sampling.
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From page 194... ...
The last column in this table gives the number of stations per stratum averaged over the optimal allocations for each species. This allocation is presented as a simple compromise over all three species to determine if a single allocation pattern could be devised that
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From page 195... ...
The actual rankings of strata by catches of yellowtail and winter flounder in the variability are not, however, exactly the same in northern group of strata and greater catches of the two years. summer flounder in the south group of strata, 195 TABLE C-9 Percentage of Total Number of Stations to be Allocated to Each Stratum Based on Optimal Allocation (Summer Flounder, 1995 and 1996 NEFSC Winter Surveys)
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From page 196... ...
If spatial patterns are not very persistent, an adaptive allocation scheme (Thompson and Seber, 1996) might be beneficial, again using some combination of catches of the flounder species in the current survey to allocate additional stations to the more variable strata.
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