Browsing Acoustic Surveys by Subject "Acoustics"
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Atlantic Herring in 6aS/7b, c Industry Acoustic Survey Cruise Report, 28 November – 07 December, 2016(Marine Institute, 2017)The individual stock assessments for Atlantic herring Clupea harengus in 6aN and 6aS/7b, c have been combined into one assessment encompassing both stocks following a benchmark in 2015 (ICES 2015a). ICES still considers that two separate stocks exist. The main reason for the merging has been that the catches of mixed aggregations in the commercial fishery and in the summer acoustic survey could not be separated into the different stock components for the purposes of stock assessment. The consequence of this has been a zero TAC for herring in these areas for 2016 and 2017. In its autumn 2015 plenary report, the Scientific, Technical and Economic Committee for Fisheries (STECF PLEN 15-03, 2015), noted that from a stock assessment perspective, it would be beneficial to allow small catches in both management areas in the form of a monitoring TAC, to maintain an uninterrupted time series of fishery-dependent catch data from both stocks. On the request of industry, part of the monitoring TAC was set aside in 2016 to conduct acoustic/trawl surveys in both 6aN and 6aS/7b, c. An industry/science collaboration survey was conducted on pre-spawning aggregations of 6aS/7, c herring during late November/early December in 2016. Herring were found in very dense aggregations in a few specific locations (Lough Swilly and Donegal Bay), but schools were smaller and in low numbers throughout much of the survey area offshore. This behaviour was evident in the monitoring fishery also during the time of the survey. The dense shoals were in shallow inshore areas, and in some cases very large (> 1nmi long). Three acoustic validation hauls were also carried out, only one containing herring. Fishing could not take place in shallow areas with the survey vessel. Samples from the monitoring fishery were used to augment samples from the survey and work up an estimate of abundance and biomass of herring. The survey estimated a minimum of 35,475 tonnes herring in the survey area. The hyper-aggregating behaviour and relatively low numbers of herring schools during the survey led to a high CV (~0.37) on the estimates of abundance and biomass. The vast majority of herring were mature, assumed to be of 6aS/7b, c origin due to their proximity geographically to the spawning beds and temporally to spawning time for this mainly winter spawning stock. Small horse mackerel (~20 - 24cm) were found throughout the area to the north and west of Tory Island in many large schools on the bottom