10:45 AM - 12:15 PM
[ACG40-P04] Automatic measurement of the number of wild salmon (Oncorhynchus keta) runs in the Yufutsu River, Hokkaido, Japan, where spawning run behavior differs among tributaries.
Keywords:resistivity fish counter, migration behavior, stream water temperature, long-term measurement, salmon resource conservation
Chum Salmon (Oncorhynchus keta) are an important fishery resource worldwide, as well as a carrier of marine derived nutrients (MDN) from the ocean to upstream rivers, riparian and terrestrial areas. However, there is concern that the recent rise in river water temperatures due to global warming may affect the timing of salmon runs and spawning periods. Even in the same river, chum salmon can be divided into two populations depending on the timing of their run: the early-season run (September to November) and the late-season run (December to January). The spawning beds of the early-season population are often filled with stream surface water and are considered to be susceptible to the influence of river water temperature. Therefore, it is important to accurately clarify the number of salmon runs for each population from the viewpoint of river ecosystem conservation as well as future fisheries resource conservation.
Generally, fish traps and other methods are used to measure the number of fish migrating upstream or downstream in fishways based on video images of their migratory behavior. However, these methods are labor-intensive, time-consuming, and costly. In the latter case, it is difficult to take measurements at night or when the water is turbid. On the other hand, the fish counter (resistivity fish counter) developed in the U.K. in the 1950s, is a device that measures the number of fish migrating upstream or downstream by installing three electrodes parallel across a stream channel for a counter and detecting the altering in electrical resistance between the electrodes due to the passage of fish bodies by a Wheat-stone bridge. In Japan, automatic measurement has not been conducted, probably because there were no commercially available fish counters. Recently, commercial fish counters have become available, and automatic measurement of salmon runs is now being conducted in Niigata and Hokkaido prefectures.
In this study, we used multiple fish counters in the Yufutsu River watershed, where early- and late-season salmon populations run, to determine the number of runs of each population and to understand the differences in population dynamics within the watershed.
Fish counters were used to measure run numbers from August or September to February in 2019 and 2020 at the Utonai and Tokisata weirs, which are movable weirs installed in the Yufutsu River. The Lake Utonai is located in the middle part of the river, and only the Yufutsu River and the Bibi River flow into Lake Utonai as salmon-run rivers, and the Yufutsu River flows out of the lake. In addition, the Yufutsu River has two weirs with a fish passage, the Utonai Weir about 1.5 km downstream from Lake Utonai and the Tokisata Weir about 3.0 km upstream from Lake Utonai.
Measurements showed that 605 and 402 individuals were measured at the downstream Utonai weir in the 2019 and 2020 runs, respectively. On the other hand, 101 and 123 individuals were measured at the Tokisata weir in each year. At Utonai-weir, the run was observed from November to January. However, the salmon run at Tokisata weir in the upper reaches of the Yufutsu River lasted until October in both years, and only one individual was confirmed after November in 2019. These results indicate that the early season population migrates up both the Yufutsu River and the Bibi River, while the late season population migrates only up the Bibi River.
By using multiple fish counters and placing them upstream and downstream of the confluence of the tributaries, we were able to determine the number and timing of returns to each of the tributaries in this study. As a result, populations with different spawning behavior were observed in each tributary within the watershed, a very rare case in Japan. The automatic measurement using the fish counter revealed previously unknown information, and the accumulation of such data was thought to contribute to the conservation of salmon populations.
Generally, fish traps and other methods are used to measure the number of fish migrating upstream or downstream in fishways based on video images of their migratory behavior. However, these methods are labor-intensive, time-consuming, and costly. In the latter case, it is difficult to take measurements at night or when the water is turbid. On the other hand, the fish counter (resistivity fish counter) developed in the U.K. in the 1950s, is a device that measures the number of fish migrating upstream or downstream by installing three electrodes parallel across a stream channel for a counter and detecting the altering in electrical resistance between the electrodes due to the passage of fish bodies by a Wheat-stone bridge. In Japan, automatic measurement has not been conducted, probably because there were no commercially available fish counters. Recently, commercial fish counters have become available, and automatic measurement of salmon runs is now being conducted in Niigata and Hokkaido prefectures.
In this study, we used multiple fish counters in the Yufutsu River watershed, where early- and late-season salmon populations run, to determine the number of runs of each population and to understand the differences in population dynamics within the watershed.
Fish counters were used to measure run numbers from August or September to February in 2019 and 2020 at the Utonai and Tokisata weirs, which are movable weirs installed in the Yufutsu River. The Lake Utonai is located in the middle part of the river, and only the Yufutsu River and the Bibi River flow into Lake Utonai as salmon-run rivers, and the Yufutsu River flows out of the lake. In addition, the Yufutsu River has two weirs with a fish passage, the Utonai Weir about 1.5 km downstream from Lake Utonai and the Tokisata Weir about 3.0 km upstream from Lake Utonai.
Measurements showed that 605 and 402 individuals were measured at the downstream Utonai weir in the 2019 and 2020 runs, respectively. On the other hand, 101 and 123 individuals were measured at the Tokisata weir in each year. At Utonai-weir, the run was observed from November to January. However, the salmon run at Tokisata weir in the upper reaches of the Yufutsu River lasted until October in both years, and only one individual was confirmed after November in 2019. These results indicate that the early season population migrates up both the Yufutsu River and the Bibi River, while the late season population migrates only up the Bibi River.
By using multiple fish counters and placing them upstream and downstream of the confluence of the tributaries, we were able to determine the number and timing of returns to each of the tributaries in this study. As a result, populations with different spawning behavior were observed in each tributary within the watershed, a very rare case in Japan. The automatic measurement using the fish counter revealed previously unknown information, and the accumulation of such data was thought to contribute to the conservation of salmon populations.