2 MATERIALS AND METHODS

2.1 Fish sampling and tagging

During the spring spawning seasons in 2016, 2017, and 2018, we captured a total of 123 Sakhalin taimen spawners (67 females and 56 males) in the Karibetsu River (watershed area: 83 km2), a tributary of the Sarufutsu River in northern Hokkaido, Japan (Fig. 1). The fish were sampled daily with a cast net as they ascended a fish ladder adjacent to a weir (approx. 1 m high × 12 m wide) located approximately 22 river kilometers (rkm) from the ocean (hereafter, referred to as Site L). Note that this site marks an approximate downstream boundary of the Sakhalin taimen spawning ground in the Karibetsu River. Each fish captured was anesthetized with eugenol solution (approx. 30 ppm; FA100, Pharma Animal Health Co., Ltd., Osaka, Japan), sexed, measured for fork length (FL), marked with a 23-mm passive integrated transponder (PIT) tag, and released above the weir. The number of tagged fish represented 29–37% of the annual run of the species in this river according to the estimates obtained by using underwater imaging sonar (Rand & Fukushima, 2014). FL was 772 ± 92 mm (average ± SD) for females and 720 ± 139 mm for males upon capture, but for statistical analyses fish size refers to FL standardized to values in spring 2018 when most fish (90) were measured. The standardized FLs of individuals measured only before 2018 were estimated based on their original measurements and an estimated annual growth rate of 24.7 mm year–1 (Fukushima & Rand, 2021). Fish were collected under Hokkaido Government permits No. 168 (2016), 201 (2017), and 191 (2018). The fish sampling protocol used in this study was approved by the Animal Care Committee of the National Institute for Environmental Studies.
In 2018 and 2019, antenna arrays (Oregon RFID, Portland, OR, USA) were installed to detect PIT-tagged fish at Site L and four tributary sites (T1–T4) located 0.79–3.53 rkm upstream from Site L (Fig. 1). One pass-through antenna was built across the stream at each tributary site, whereas one pass-through antenna and an additional pass-over antenna were built side-by-side at Site L at the upstream exit of the fish ladder and across the spillway, respectively, to detect both ascending spawners and descending post-spawners. Arrival timings at Site L of the 90 fish captured there in 2018 are represented by the time of their capture for tagging, as the fish were handled only briefly before release upstream and were captured roughly in the order they arrived at this site.
The swimming speed of Sakhalin taimen during upstream and downstream migrations was estimated by dividing the distances traversed between Site L and tributary sites by differences in the respective detection times in 2019, when there was no fish sampling (i.e. no effects of tagging on fish migration). The upstream migration speed, which was log-normally distributed, was then used to calculate the mean and 95% confidence interval of the total travel time of the spawners from a presumed starting point at the river mouth to a point at 24.2 rkm, the average distance to the tributary sites. Further details on the PIT tagging and detection are described by Fukushima and Rand (2021).
Water temperature and water level were monitored at Site L throughout the 2018 and 2019 spawning seasons at 10-min intervals using temperature and pressure loggers (UA-001-64 and U20-001-01, respectively; Onset Computer Corporation, Bourne, MA, USA). Two pressure loggers—one underwater and the other on the riverbank—were deployed to correct for changes in atmospheric pressure.