A note on non-random error structure in trawl survey abundance indices

Trawl survey time-series are routinely used in stock assessments to provide indices of relative abundance. There is the general assumption, for each year y, that the expected value of the trawl survey index (X-y) is related to the biomass (B,) by a single proportionality constant q: E(X-y) = qB(y). In reality, the constant q varies due to many factors. An important factor, which is almost always ignored, is the role that non-trawlable ground can play in the variation of q. A general formula is derived for the q of a stratified random trawl survey. When the survey area contains non-trawlable ground, strata-specific data on the proportion of non-trawlable ground and on the preference of the species for trawlable/non-trawlable ground is required to weight stratum estimates correctly. In the absence of the correct stratum weights, a shift in the spatial distribution of the stock can combine with differing proportions of non-trawlable ground to introduce non-random error and possibly spurious trends into biomass indices. Each survey and species/stock should be considered on a case-by-case basis. A stratum-specific assessment of the proportion of non-trawlable ground is a pre-requisite for the production of trawl survey biomass indices.