Geostatistical comparison of two independent video surveys of sea scallop abundance in the Elephant Trunk Closed Area, USA

Abstract

Abstract Adams, C. F., Harris, B. P., and Stokesbury, K. D. E. 2008. Geostatistical comparison of two independent video surveys of sea scallop abundance in the Elephant Trunk Closed Area, USA. – ICES Journal of Marine Science, 65: 995–1003. Geostatistical prediction at unsampled locations is done by kriging, an interpolation technique that minimizes the error variance. Our goal was to verify the technique by comparing kriged abundance estimates with observed counts from an area containing the highest densities of sea scallop (Placopecten magellanicus) offshore of the northeastern USA. In 2006, two independent video surveys of scallop abundance were made in the Elephant Trunk Closed Area, one using a 5.6 × 5.6-km sampling grid and the other with a 2.2 × 2.2-km sampling grid. We generated kriged surfaces of scallop abundance with the 5.6-km grid data, using different combinations of variograms and theoretical models, then tested the null hypothesis of no difference between the predicted and assumed true values (i.e. the 2.2-km grid data). There were significant differences between predicted and true values for three out of four combinations of variogram–model fits to untransformed data, assuming isotropy. In contrast, there was no significant difference between kriged and true values for any combination of variogram–model fits to log-transformed, detrended, anisotropy-corrected data. Classical and robust variograms performed equally well. Kriging can be used to generate accurate maps of scallop abundance if the assumptions of geostatistics are met.