Migration Delays at Head-of-Tide Weirs are a Function of Tidal Cycles and River Flows in Anadromous Twaite Shad Alosa fallax

Riverine barriers can have significant negative impacts on the spawning migrations of anadromous fishes, with barriers of low passage efficiency limiting access to upstream spawning areas. Even fish that pass a barrier can be negatively affected via migration delays and energetic costs. The spawning migrations of 74 twaite shad Alosa fallax were tracked at two head-of-tide weirs on a bifurcated channel in the River Severn’s upper estuary, western England, to test how barriers in tidal areas can affect river entry and incur migration delays. Although the weirs had a combined passage efficiency of 98.9%, median passage time was 19.8 h, with the longest passage time being 16.8 days, and those passing fastest being relatively large fish that approached during spring tides and higher river flows. A higher proportion of fish approached and passed the weir on the larger channel that generally had the dominant flow, yet the weir on the other channel had the higher probability of passage, with the effect of elevated river levels being less important, and individuals approaching this weir were less delayed. With river level being a function of the cyclical nature of the lunar and daily tidal cycles, temporal variation in passage efficiencies was predictable. As the environmental conditions that stimulated the entry of fish into the upper estuary were largely unrelated to the conditions facilitating weir passage, there was a high potential for migration delays at these barriers that potentially incurred considerable energetic costs.