Scottish Marine and Freshwater Science Volume 2 Number 7: Hydroacoustic surveys of five Scottish lochs

This report was published in May 2011.

“The results of hydroacoustic surveys conducted in the spring and summer of 2007 are presented. Two surveys were conducted on Loch Rannoch, while a single survey of each of Loch Garry (Drumochter), Loch Quoich, the Fionn Loch and Loch Maree was made. In general, excellent quality data were obtained with the hydroacoustic gear used and, as is often the case for hydroacoustics work, the most important challenges reside in interpretation. Differences in fish densities among lochs were relatively small (ranging from 8 fish ha -1 in the Fionn Loch to 18 ha -1 in Loch Rannoch for targets greater than -54dB, likely equating to fish greater than 4.5cm in length), though the densities of smaller, plankton-sized targets were more variable, perhaps reflecting seasonal effects and growth through the course of the survey sequence. By contrast, densities of both fish and plankton were highly variable with depth. There was a striking contrast between Rannoch, where fish were spread throughout the water column and were found in substantial numbers to great depth (>100m), and where there was no relationship between fish and plankton distributions, and the other lochs. In these, fish and plankton distributions were generally consonant and sharp density peaks were present. Our surveys indicate that valuable data can be obtained during daylight hours, though evidence here and elsewhere suggests that night-time surveys should also be conducted. More questions are raised than answered in this preliminary series of MSS surveys. Most of the technical issues for conventional downward scanning hydroacoustics have been resolved and a large number of biological issues have been raised that merit further investigation.”

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