INDEXES OF RELATIVE ABUNDANCE FROM FISH SPOTTER DATA BASED ON DELTA-LOGNORMAL MODELS

Fish spotters are pilots in small aircraft employed by commercial fishermen to locate, identify, estimate the size of, and direct boats toward pelagic fish schools. Data describing species, location, and size of schools can be inexpensively obtained from fish spotters but are difficult to interpret statistically. We developed an index of relative abundance from fish spotter data based on extended delta-lognormal models and applied the method to data for northern anchovy (Engraulis mordax). In contrast with previous approaches, our method used all available data, provided an index for northern anchovy that was proportional to abundance, and explicitly modeled factors (pilots, regions, seasons, and time of day) that affected observations by fish spotters. We also included information about mixed layer depth and sea surface temperature in models for a reduced study area and found that environmental data, where available, can be used to improve estimates of relative abundance from fish spotter data. Simulation results indicated that our approach is a cost-effective way to improve biomass estimates for pelagic species like northern anchovy.