Cruise report on northeast Atlantic mackerel (Scomber scombrus) abundance and distribution in the North Sea and west of the British Isles from 1st to 20th of October 2012
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This report summarizes the findings of the Northeast Atlantic (NEA) mackerel research cruise from 1st to 20th of October 2012. Two chartered fishing vessels, M/V “Brennholm” and M/V “Christina E” covered areas in the northern part of the North Sea and west of the British Isles with the overall aim to estimate spatial distribution and abundance of mackerel by means of echo sounder and pelagic trawling. In addition, herring distribution and abundance were also estimated by means of acoustics and pelagic trawl sampling. Systematic CTD casts were conducted by both vessels during the survey period, as well as collection of sea surface temperatures (SST) from weekly satellite information. The SST during the survey period was close to the long-term average for the survey area. Temperature at 10 m depth ranged from 8.5 to 13.5°C, with coldest water in the north, while temperature at 50 and 100 m depth showed cold water stretching from the south in the areas surrounding the Norwegian trench. The highest mackerel concentrations were found south-east of Shetland and in the northernmost areas of the North Sea. Vertical and horizontal distribution, behavior and abundance of mackerel were to a large extent different compared to what we expected and compared to previous established knowledge from both the fishing fleet and scientific surveys in similar areas and periods. In contrast to a general deep distribution in dense schools observed previously at this time of the year, mackerel were also found swimming close to the surface in small and loose aggregations. Total abundance of NEA mackerel within the covered area was estimated to 1.144 million tons. Total abundance of herring was estimated acoustically to be 2.934 million tons. The estimated mackerel abundance was significantly lower than expected, probably due to low availability to echosounder and sonar detection as well as considerable concentrations of mackerel distributed outside the covered area during the survey period. Most of the pelagic trawl samples with the new Multpelt 832 sampling trawl were taken in surface waters. Trawl samples west of 2°W were dominated with 0-group mackerel and had a wide distribution.