J. Schweigert & R. Emmett DFO, Pacific Biological Station, Nanaimo, BC NMFS, Astoria

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Workshop on Enhancing Stock Assessments of Pacific Sardine in the California Current through Comparative Co-operative Surveys May 23-24, 2011 La Jolla, California. J. Schweigert & R. Emmett DFO, Pacific Biological Station, Nanaimo, BC NMFS, Astoria. Swept Area Trawl Surveys. Concept:
Workshop on Enhancing Stock Assessments of Pacific Sardine in the California Current through Comparative Co-operative SurveysMay 23-24, 2011La Jolla, CaliforniaJ. Schweigert & R. EmmettDFO, Pacific Biological Station, Nanaimo, BCNMFS, AstoriaSwept Area Trawl SurveysConcept:Abundance = Density * AreaStatistical Considerations:Assumes entire distribution of population is knownAssumes random distribution of fish schools within survey areaUnbiased sampling – no vessel or gear avoidanceSampling Considerations Implicitly, always trade-off between sampling intensity (cost) and survey precision (variability in estimates)Survey costs rarely considered explicitlyTheoretically, more samples = more accurate and precise estimateIn practice, generally more samples = more precise estimateBUT, more samples = greater accuracy only if sampling is unbiasedCritical to know what you are sampling and that it is representative and unbiasedSardine DistributionResearch trawl surveys: transect linesDetermining a core region:
  • Plotted survey catch densities 1997-2010
  • 4x4km sized grid cells
  • 428 of a total 875 trawls had presence of sardine
  • 86% of positive catches are within core region boundaries
  • 16, 740.08km²
  • Pacific Sardine Trawl SurveysNormally about 10-12 days of vessel timeSurveys conducted north to southLimited tows within inlets~8 tows per day/nightTow speed ~ 5 knotsNet depth – headrope surface to 5 mNet width – 30-32 mNet height – 10-12 mTrawl Survey Data
  • Data from research cruises
  • 1997-2010
  • Biological Data Total Catch: Numerated by species and weight Sardines: Length, weight, sex Otolith collection for age Oceanographic dataBiomass Estimation MethodFive regions along the Vancouver Island coast were included in the sardine abundance estimation calculation.Maximum net depth 30 meters.Method:Weight of sardines caught determined by tow.Volume swept by the net calculated (swept volume). Sardine Biomass calculated: Weight of sardines/ swept volume * total volume of the region.Ocean Ecosystem Indicators of Salmon Marine Survival in the Northern California Current – May, June, September surveyshttp://www.nwfsc.noaa.gov/research/divisions/fed/oeip/a-ecinhome.cfmCoastwide Pacific Sardine Trawl Survey
  • Surveys should cover the entire distribution of the population
  • Surveys should occur at the time of peak population concentration
  • Surveys should occur during last half of July through first weeks of August when northward migration has peaked
  • Surveys should be co-ordinated and begin in both north and south with separate vessels meeting at the borderOutstanding IssuesNortherly distribution of sardine is not well understood. Need to sample the northern edge of the population migration north of Vancouver Is.
  • Inter-annual and in-season variation in sardine northward migration will affect sampling design and some accommodation for environmental conditions is needed
  • Inlets on Vancouver Island and Mainland need to be surveyed.
  • Maintaining access to suitable trawl vessels is critical. Intercalibration of vessels.
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