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

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:

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

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  1. 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

  2. Swept Area Trawl Surveys Concept: Abundance = Density * Area Statistical Considerations: Assumes entire distribution of population is known Assumes random distribution of fish schools within survey area Unbiased sampling – no vessel or gear avoidance

  3. Sampling Considerations Implicitly, always trade-off between sampling intensity (cost) and survey precision (variability in estimates) Survey costs rarely considered explicitly Theoretically, more samples = more accurate and precise estimate In practice, generally more samples = more precise estimate BUT, more samples = greater accuracy only if sampling is unbiased Critical to know what you are sampling and that it is representative and unbiased

  4. Sardine Distribution Research trawl surveys: transect lines

  5. Determining 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

  6. Pacific Sardine Trawl Surveys Normally about 10-12 days of vessel time Surveys conducted north to south Limited tows within inlets ~8 tows per day/night Tow speed ~ 5 knots Net depth – headrope surface to 5 m Net width – 30-32 m Net height – 10-12 m

  7. Trawl 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 data

  8. Biomass Estimation Method Five 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.

  9. Ocean Ecosystem Indicators of Salmon Marine Survival in the Northern California Current – May, June, September surveys http://www.nwfsc.noaa.gov/research/divisions/fed/oeip/a-ecinhome.cfm

  10. Coastwide 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 border

  11. Outstanding Issues Northerly 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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