Analysis of Spanish acoustic surveys for sardine, 1991–1993: abundance estimates and inter-annual variability

Systematic acoustic surveys to estimate the Spanish fraction of the Atlantic Sardine Stock were begun in 1983. Since 1991, with the use ofthe new Sirnrad EK-500 echosounder-echointegrator, the covered area has been extended to 1000 m isobath to observe the distribution of the main pelagic species in the area. The last three surveys, from spring 1991 to 1993, had a range of 20 to 1000 m isobath. They were first analysed using the traditional methodology proposed in Pastor el al (1986) which do es not give any estimation ofthe variance. Therefore, the data was then analysed using geostatistic techniques, and the resulting relative abundance estimates were compared. Sardine showed high variability between years and zones in both distribution area and density, expressed as number of fish per square nautical mile. The two methods of analysis gave different biomass estimates by zone, especially in 1993. Variograms computed over areas of fish presence did not show, in general, a clear spatial structure and sills were reached at 3 n.m. of range. The precision ofthese variograms was low, ranging from 23% to 40 %, expressed as relative standard error. In order to improve precision it seems necessary to increase the sampling intensity and change the survey grid. For sardine, a systematic parallel with random start survey design, with 6 n.m. between transects, would be more appropriate