Local spatial heterogeneity in blue whiting length structure

We have used "MultiSampler", a multiple opening-closing device that allows obtaining several samples from a single trawl haul, during the trawl-acoustic survey targeting blue whiting (Micromesistius poutassou) on their spawning areas west of the British Isles in spring 2005 and 2006. Typically, two consecutive samples were obtained with the vessel towing at same direction and at similar depth all the time, as if when fishing without the multisampler. Typically, cod-end was opened for 10-20 minutes for each sample. During standard survey operation without the multisampler, total towing time would have been similar but only one sample could have been obtained. Multiple samples taken within a distance of just 1-2 nautical miles reveal substantial variation in mean length of blue whiting in the samples originating from the same haul. Within-haul, between-sample variability is not much less than between haul variation, and may even be larger. Our findings highlight that spatial heterogeneity can be combated (1) by taking more trawl samples and (2) by keeping tow duration sufficiently long. The first option is preferable because it allows estimating uncertainty, whereas long tows hide small-scale variability.
KEYWORDS: Spatial heterogeneity; Sampling design; Trawl surveys

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