ANALYSIS OF SPANISH ACOUSTIC SURVEYS FOR SARDINE, 1991-1993 - ABUNDANCE ESTIMATES AND INTERANNUAL VARIABILITY

Systematic acoustic surveys to estimate the Spanish fraction of the At lantic sardine stock were begun in 1983. Since 1991, with the use of t he new SIMRAD EK500 echo-sounder-echo-integrator, the area covered has been extended to the 1000 m depth contour to observe the complete dis tribution of the main pelagic species. The last three surveys, from sp ring 1991 to spring 1993, had a range of 20-1000 m depth. They were fi rst analysed using traditional methodology, which does not give any es timation of the variance. The data were therefore analysed using geost atistical techniques and the resulting estimates of relative abundance were compared. Sardine showed high variability between years and zone s both in distribution and density, expressed as number of fish per sq uare 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 nmi of range. The precision of these vario grams was low, ranging from 23-40%, expressed as relative standard err or. In order to improve precision it is necessary to increase sampling intensity and change the survey grid. For sardine, a systematic paral lel survey design with a random start and 6 nmi between transects woul d be more appropriate. (C) 1996 International Council for the Explorat ion of the Sea.