Design of a Compound Inclined Screen Trap for Anadromous Salmonid Smolts

Abstract

AbstractIn the late 1980s, Minnesota Department of Natural Resources fish managers became compelled to address the decline of wild steelhead Oncorhynchus mykiss in Lake Superior. Requisite to the development of stock–recruitment models, permanent smolt traps were designed and constructed on two Lake Superior tributaries. The design was engineered to function in a broad range of discharges with complete structural integrity, to keep itself free of debris, and to sample a large portion of the emigrating salmonid population. The first of these traps was built on the French River (discharge, from 3 to >1,840 ft3/s (cfs)) and began operation in 1994. After several years of very successful operation a second trap was built on the Knife River (discharge, from 5 to >9,000 cfs) that began operation in 1997. These structures employ fixed stainless steel dewatering screens permanently fixed at opposed compound inclines on each side of a channel that carries water to a catch–retention box, which is also fixed and above the level of the stream. Fish falling onto the screen are directed by gravity to the center channel, where they travel with the flowing water to the catch box. A stainless steel, overshot, rotating drum screen in the catch box has effectively retained fish with low mortality while expelling water‐borne debris. Both traps have operated continuously during open‐water periods since 1997 to sample emigrating salmonid smolts (primarily wild steelhead). Smolt capture efficiency has been estimated to be 94% with the French River trap and 85% with the Knife River trap. Fish managers may wish to examine and consider this design as a possibility for addressing their smolt monitoring needs.