Upstream migration without complete osmoregulatory switching in an amphidromous goby: Estimated by body condition changes in different salinity environment

Preference for saline water of an amphidromous goby maintained during migration to upstream freshwater areas

“…Thus, G. petschiliensis consistently expresses a preference for saline water over freshwater from juvenile to adult stages. In Japan, many adult and juvenile G. petschiliensis live in freshwater areas (Oto, 2019;Sato et al, 2010;Yamazaki et al, 2006), although there are a few exceptions (Nakanishi, 1978). Katano et al (2006) reported that juvenile G. petschiliensis sometimes migrate to upper stream reaches jumping over dams, even when G. urotaenia and G. opperiens could not move over them in the same streams.…”

“…The standard length of the species becomes 50 mm or more within a year after entering streams (Oto, unpublished data). In addition, all fish were captured from the upper tidal area in July, when the migration to stream areas from the Sea of Japan was observed in central Japan in previous studies (Harada, 2005; Oto, 2019). Considering that many juvenile G. petschiliensis could also be found in a freshwater area of a tributary around the sampling point, the majority of the fish used in the experiments had likely been migrating to a freshwater area.…”

“…This is consistent with the idea that osmoregulatory adaptation to hypertonic environments evolved with the preference for saline water (even salinities higher than the body fluids in G. petschiliensis ). Considering that adults retain the ability to hypo‐osmoregulate even after entering a freshwater area (Oto, 2019), the osmoregulatory system is not changed irreversibly before and after migration. This physiological trait, which contrasts with other diadromous fishes, might prevent G. petschiliensis from acquiring a preference for freshwater when migrating upstream.…”

“…It is known that salinity affects the activity levels (Swanson, 1998) or swimming performance of some euryhaline fishes (Kolok & Sharkey, 1997;Serrano et al, 2010;Wakeman & Wohlschlag, 1977). Excessive physiological stress resulting from freshwater acclimation might decrease the behavioural activity of juvenile G. petschiliensis, considering that adult G. petschiliensis incur a higher energetic cost in freshwater compared to brackish water at a salinity of 20, as reported by Oto (2019). Moreover, it is possible that the combination of short-term and freshwater acclimation prevented experimental fish from completely changing the osmoregulatory systems into freshwater types because they seemed to be captured immediately after entering the hypotonic environment (i.e., the sampling site).…”

“…Because the juveniles migrate upstream in certain seasons, forming large schools (Dotu, 1955;Oto, 2019), their physiological preference seems to control their behaviour intuitively, as documented in other diadromous fishes. On the contrary, if juveniles migrate to freshwater areas against salinity preference, this implies that G. petschiliensis has a behavioural trait that has never been reported in any other diadromous fish.…”