Ocean gliders for acoustic monitoring: hydrography and distribution of anchovy in the south-eastern Bay of Biscay.
Supervisors: Ainhoa Caballero, Ivan Manso-Narvarte (AZTI)
Ocean gliders are employed in a variety of sensor configurations offering a great versatility. However, their utilisation in the assessment of fish distribution with a mounted echosounder in conjunction with hydrography is a nascent field that necessitates both validation and standardisation. During the latter part of the summer of 2022, the annual JUVENA survey was conducted in the south-eastern Bay of Biscay, incorporating a glider-mounted echosounder into the instrumental research equipment for the first time. The objective of this study is to capitalise on the data obtained from the glider mission for two distinct purposes: firstly, to validate and standardise the utilisation of gliders by comparing the data obtained with the data obtained from active acoustic and fishing vessel surveys, conducted in the same area and period; secondly, to assess and quantify the distribution of juvenile anchovy in relation to the storm that occurred during the glider mission. This latter objective is achieved through the analysis of glider-mounted CTD sensor data and SST, as well as CHL-A satellite data. This report presents a protocol for the pre-processing and subsequent processing of the acoustic data obtained from the glider-mounted echosounder. An analysis of the acoustic data in conjunction with the dates of the storm indicates that the anchovies are increasingly prevalent in coastal regions (compared to offshore areas) as the storm intensifies, persists and declines. Lastly, TS charts indicate that juvenile anchovies are distributed according to specific salinity-temperature-depth conditions. Additionally, several mesoscale events, including frontal systems and eddies, have been observed in 2D using L3 SST and CHL-A data. Some of the eddies align with the 2D description of previously described SWODDIES, but a comprehensive examination of the water column is required to confirm their classification. The results suggest that a glider-mounted echosounder is a valuable addition to vessel surveys, when an appropriate protocol is followed, as it is a more time-efficient, cost-effective and less resource-intensive method than conducting vessel surveys.