Additional evidence for fisheries acoustics: small cameras and angling gear provide tilt angle distributions and other relevant data for mackerel surveys

Fisheries acoustics surveys are effective tools in marine resource assessment and marine ecology. Significant advances have occurred in recent years with the application of multiple and broadband frequencies to enable remote species identification. There is, however, still the need to obtain additional evidence for identification, and the estimation of the size and tilt angle distribution of fish, which influences their acoustic target strength. The former two requirements are usually met by obtaining simultaneous net samples: there are limited, if any, recognised successful techniques for the latter. Here, two alternative tools for obtaining evidence for all three requirements are examined: angling gear and small video cameras. These tools were deployed during surveys of Atlantic mackerel (Scomber scombrus). In 2014 angling was actually more efficient than pelagic trawling (the standard technique) and over two survey periods (2012 and 2014) provided length frequency distributions that were not significantly different. A small video camera was deployed into mackerel schools, providing species identification and fish orientation. Image analysis was then applied, producing tilt-angle distributions of free swimming wild mackerel for the first time. Mean tilt angles from three deployments were very variable with 95% of observations falling between -70° and 39° with evidence of a multinomial frequency distribution. A video equipped lander was also deployed onto the type of rocky seabed where deployment of a trawl would be impossible: this confirmed the presence of Norway pout and suggested it was the dominant scatterer on this type of seabed. These techniques are complementary to traditional trawling methods, but provide additional insights into fish behaviour whilst satisfying standard requirements of identification and supplying biological samples. Crucially, the small cameras deployed approximate the size of the animals under observation and allow for measurement of behaviour (specifically tilt) that are more likely to represent those conditions encountered during surveying.