Precision of acoustic fish abundance estimates: separating the number of schools from the biomass in the schools
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The gulf of Cariaco, Venezuela, is a rather isolated piece of coastal ocean where spatio-temporal variahility has bcen mcasured. In 1986, three repeated surveys have bccn camcd out in this gulf. The important differences in biomass cstimates are attnbuted to sampling error only. The biological nature of this variability is studied. It is due to a high concentration of the biomass in a few schools and correlatively to the hit or the miss of these large schools, during the sampling. Structural information for predicting them is sought. Also, the number of schools per elementary sampling distance unit (ESDU) and the biomass in each school wcre measured by echointegration. It is found that there is no clear relation between these two variables. Thus thcir spatial structures are studied separately, using geostatistics. The number of schools per ESDU is structured in spacc. Its histogram is not very dissymmetrical. The error variance of its mean estimate is reasonable. The biomass per school is not structured in space. Its histogram is highly skewed. The error variance of its mean estimate is high. The major error variance tenn on the abundance is due to the imprecision on the estimate of the mean biomass per school. The major quantitative problem is the indeterminancy at the sampling stage of the maximum biomass per school and correlatively the major problem is the characterization of the skewness of the biomass per school histogram. The possibility of using the number of schools as an index of biomass is discussed.
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