Echosounder derived metrics from small-scale coastal surveys

Echosounder surveys are appealing for monitoring benthic rocky reef fishes like rockfish (Sebastes spp.) as they cover large areas quickly and are non-destructive, but they are rarely used due to difficulties ensonifying benthic fish in rocky habitats.  We conducted echosounder and remote operated vehicle (ROV) surveys at 39 sites in Mowachaht/Muchalaht territory (Nootka Sound, BC, Canada) in August 2023.  We examined correlations between an echosounder biomass proxy (nautical area scattering coefficient (NASC)) and ROV benthic fish density using generalized linear models. Increases in the fish biomass proxy (NASC) within 10m of the bottom was correlated with increasing benthic fish density (deviance explained 52%), suggesting NASC helps predict benthic fish hotspot locations.  We compared echosounder and ROV habitat metrics using Spearman’s rank correlation tests and found rugosity and slope were comparable across methods with reduced analyst variability in echosounder metrics. This study provides a new and efficient method for identifying rocky reef fish hotspots in near-shore areas that can help fishers avoid non-target species and act as a guide for marine protected area (MPA) site selection.