Salinity measurement on euryhaline fish ranging in brackish water using micro conductivity loggers

Abstract

Salinity measurement Euryhaline fishes living in brackish water have physiological mechanisms of osmoregulation and select environmental water with suitable salinity. Compared with the physiological mechanisms, there is limited information for behavioral acclimation for the salinity selection. One major reason why is that it is technically difficult to measure salinity experienced by fish. In this paper, we present the methods to measure salinity on free-ranging fish using with two conductivity loggers (ge-olocator and ORI400-DTC). The geolocator can discriminate whether a tagged fish migrated to freshwater region because it is energized only in salt water. ORI400-DTC can accurately record conductivity in addition to water depth and tempera-ture at one second intervals. One of the loggers was attached to a black seabream ( Acanthopagrus schlegelii ), one of major euryhaline fishes in coasts of Japan. The result of geolocator showed that the fish did not migrate to freshwater region for the entire recording of 4-day period. The result of ORI 400-DTC showed that the fish spent 48% of time for 3-day recording period in hypotonic water lower than the body fluid. In addition, the data suggested that the thermocline and halo-cline formed at near a depth of 1 m may affect the migrate behavior of the black seabream. We believe that application of ORI400-DTC is effective to understand behavioral acclimation of various euryhaline fishes migrating to brackish water. It is also expected that the geolocator can be applied to investigate migratory behavior of diadromous fishes entering freshwater regions.