Intercalibration of survey methods using paired fishing operations and log-Gaussian Cox processes

25 We present a statistical method for intercalibration of fishery surveys methods, i.e. 26 determining the difference in catchability and size selectivity of two methods, such 27 as trawl gears or vessels, based on data from paired fishing operations. The model 28 estimates the selectivity ratios in each length class by modelling the size distribution 29 of the underlying population at each station and the size-structured clustering of 30 fish at small temporal and spatial scales. The model allows for overdispersion 31 and correlation between catch counts in neighboring size classes. This is obtained 32 by assuming Poisson distributed catch numbers conditional on unobserved log33 Gaussian variables, i.e. the catch is modelled using log-Gaussian Cox processes. 34 We apply the method to catches of hake (Merluccius Paradoxus and M. Capensis) 35 in 341 paired trawl hauls performed by two different vessels, viz. the RV Dr. 36 Fridtjof Nansen and the FV Blue Sea, operating off the coast of Namibia. The 37 results demonstrate that it is feasible to estimate the selectivity ratio in each size 38 class, and to test statistically the hypothesis that the selectivity is independent of 39 size or of species. For the specific case, we find that differences between size classes 40 and between species are statistically significant. 41