Statistical problems in krill stock hydroacoustic assessments

Two primary issues are at question for hydroacoustic assessments of krill, The first is the methods applied to establish biomass in a survey area and the second is the improvement in accuracy of target-strength measurements. In the case of statistical methods, there are no clear guidelines for deciding what method is most appropriate, this is made even more difficult by the fact that most survey methods assume the population is fixed in space, relative to the sampling interval. There remain several unsatisfied needs for improvements in sampling design and tests for systematic trends in survey data collected from non-stationary populations, which have not been well addressed by present techniques. However, this does not invalidate the use of available methods to conduct surveys and analyze results. In the case of target-strength accuracy, even if the present values were very accurate, the issue of interest would seem to be not the absolute amount of biomass present in an area, but rather how it is distributed. The issue of patchy years vs more even distribution would seem to have more impact on ecosystem management than absolute accuracy of biomass estimates.