Characteristics and distribution of coastal fish schools around Robinson Crusoe island (Juan Fernandez Archipelago, Chile)

The Juan Fernandez archipelago is located along an east-west ridge in the southeastern Pacific Ocean, off the coast of central Chile. It consists of three oceanic islands organized into two subsystems: Alejandro Selkirk (AS), Robinson Crusoe and Santa Clara (RC-SC). The marine ecosystem of this island complex is considered a high priority of national and international conservation, due to the high endemism of species and importance for the local economy. Likewise, biological and fisheries studies of coastal fish resources have been a focus of interest during the last decade. The present work aims at characterizing fish schools assemblages through of a hydroacoustic survey developed during the austral spring of 2015 around the RC-SC. Acoustic observations were collected using SIMRAD EK-60 scientific echo sounders and stereo video cameras mounted on a ROV. Surface (S), medium (M), bottom (F) and columnar (C) schools were identified, with S-type schools being the scarcest. The horizontal distribution of the schools was mostly coastal, comprised by the 20 and 180 m isobaths, organized mostly around rocky reefs. We observed multispecies fish assemblages organize mostly in columnar schools off the rocky reefs (Juan Fernandez morwong, pink maomao, calantias, bigeye, Juan Fernandez trevally, sea sweep, amberjack, rosefish and redbanded perch). The diurnal-nocturnal observations indicated that the schools are highly aggregated during daytime, therefore future acoustic surveys shall be conducted during the day, considering factors as availability, accessibility and logistics. The multispecies nature of the schools requires a careful sampling design of optical system deployment, to avoid biases in quantitative species proportion estimates. Regarding the above, a distribution model is proposed for fishing grounds and distribution of schools, which also contributes to laying the foundations of population assessment programs that require further exploration, supporting the sustainable and responsible exploitation of resources of the RC-SC subsystem.