Krill biomass estimations from moored upward looking echosounders

Autonomous platforms such as moorings with upward looking echosounders are becoming increasingly used to monitor Antarctic krill (Euphausia superba) abundance, behaviour and distribution. Such platforms have the advantage of collecting data in areas and seasons inaccessible to traditional ship-based acoustic surveys (such as at depth or beneath the sea ice in winter). However, as stationary platforms, biomass from moored echosounders cannot be estimated in the same manner as moving vessels or gliders. Here, we present a method of estimating krill biomass and temporal uncertainty by using integration intervals derived from one nautical mile of water current flow above a moored echosounder and ADCP in East Antarctica. We interpolated historical records of krill length frequencies from ship surveys and predator gut contents to convert autonomously collected backscatter to krill biomass year-round. Biomass densities integrated across the 387m water column varied seasonally and exhibited high variance between nautical mile integration intervals. Variance is sensitive to the temporal integration range and is reduced when integrated across the length of a traditional ship transect (250 nautical miles). Arrayed deployments are required to scale mooring echosounder data to survey area biomass estimates, and consideration is needed to determine appropriate methods to derive spatial variance from these stationary platforms.