Understanding mackerel migration off Scotland: Tracking with echosounders and commercial data, and including environmental correlates and behaviour

The characteristics of mackerel schools are described from the results of an acoustic survey carried out in January 1994. Schools were typically found in midwater, over bottom depths of >100 m to just beyond the shelf break. They were of deep vertical spread (up to 105 m) with a mean estimated width of 200 m, with packing densities of up to 9 fish m−3 and a mean biomass of 640 tonnes. Schools were aggregated into discrete patches of high biomass (up to 12 000 tonnes) which were confined to relatively small areas and separated by distances of up to 50 miles. With this type of distribution it is possible to make repeated surveys over aggregations to determine direction and speed of migration. Spatial distribution from the acoustic survey was compared to that of the main commercial fleet to evaluate how well the latter reflected the distribution and migration of the stock. Using the fleet data, estimated migration rates were 13.0 cm s−1, 17.8 cm s−1 and 25.9 cm s−1 for the months of December, January, and February, respectively. The distribution of catches in January 1994 was compared with that of previous years and showed no reversal of the northward shift of recent years. The relationship between sea temperature, salinity conditions, shelf edge current measurements, and the distribution of schools is described for the first time.