Relationship between Head Difference among Pools and Upstream Success Rates of Gobies (Gymnogobius spp.) in Pool-and-Weir Fishway

Abstract

Many existing fishways in Japan are based on pool and weir designs. Whereas effective for species such as ayu and salmon, they are not suitable for small-bodied gobies (Gymnogobius spp.) during their upstream migration. Therefore, to acquire knowledge for enhancing fishway functionality, we conduct experiments using a full-scale fishway model and wild gobies with an average standard body length of 3.9 cm during their upstream migration period. The overfall section of the weir is inclined at 45° and features a thickness of 30 cm. When the head difference among pools (Δh) is 10 cm, more than 90% of the gobies successfully migrated upstream in the overfall section. However, beyond 15 cm (Δh), the flow velocity at the entry from the pool's tranquil zone to the overfall jet is about 160 cm/s or greater. Under these conditions, many gobies exhibit disrupted postures during upstream migration, thus resulting in lower success rates.