Publication Abstract

The fuzzy relationship between acoustic and trawl surveys in the North Sea

S. Mackinson, J. van der Kooij and S. Neville

Adding extra information on the horizontal and vertical distribution of fish, both on and between trawl stations is reason enough to routinely perform acoustic surveys in tandem with annual groundfish trawl surveys. Ideally, acoustic and trawl density indices could be combined to maximise information on fish distribution and provide more reliable estimates of stock size. The core of the problem boils down to the question: "How does what we see on an echosounder relate to what we catch in a net?" Founded on the premise that there ought to be some kind of relationship, the fuzzy logic ‘model-free estimation’ approach presented here side-steps the need to understand specific mechanisms that determine the nature and variability of any relationship between acoustic and trawl catches. We show the development of fuzzy logic models that describe and predict the relationship linking acoustics and other environmental variables (inputs) with trawl catches (output), and further discuss the sensitivities and robustness of the approach. In the models examined, location and depth proved to be a better predictor of the trawl catches in the North Sea than the acoustic energy in the first 5 metres off the bottom. We suggest that finding the ‘hidden’ relationship will require closer attention to partitioning the acoustic data by species / assemblages and understanding the key gear and behavioural differences responsible for producing the high between gear variability.

Reference:

S. Mackinson, J. van der Kooij and S. Neville (2005) The fuzzy relationship between acoustic and trawl surveys. ICES Jounal of Marine Science, 62(8): 1556-1575