Size and condition of mackerel in research vessel trawl hauls versus commercial purse seine catches: implications for acoustic biomass estimation

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In September, after the summer feeding has ceased, the Northeast Atlantic mackerel (Scomber scombrus) stock tends to aggregate in ICES Area IVa before they start to migrate southwards in December-February towards the spawning grounds. It is during this autumn period that the Norwegian purse seine fleet catch the mackerel in the southern part of the Norwegian Sea and the northern part of the North Sea (IVa). Since 1999 IMR has also carried out annual acoustic surveys to estimate the biomass concurrently with this fishery. During these surveys the mackerel has been sampled with a small pelagic trawl (20 m opening) at a speed of 3-3.5 knots, and the age, length and weight has been measured for use in the biomass estimation. In the present study we demonstrate that the size, both in terms length (mean length and length at age) and condition (weight at length), of mackerel caught in the research vessel trawl hauls is significantly lower than that observed in the purse seine catches from nearby commercial vessels. By using data from purse seine caught mackerel instead of the trawl caught ones, the biomass estimates during 1999-2003 increased by 30% on average. These results also signify the importance of being careful by using research vessel trawl haul samples when estimating abundance and studying variations in growth and condition of high speed swimming species like mackerel.