Influence of Drought Conditions on Brown Trout Biomass and Size Structure in the Black Hills, South Dakota

Abstract

We evaluated the influence of drought conditions on the biomass of brown trout Salmo trutta in Spearfish Creek, upper Rapid Creek, and lower Rapid Creek in the Black Hills of western South Dakota. Stream discharge, mean summer water temperature, the biomass of juvenile and adult brown trout, and brown trout size structure were compared between two time periods: early (2000)(2001)(2002) and late drought (2005)(2006)(2007). Mean summer water temperatures were similar between the early-and late-drought periods in Spearfish Creek (12.48C versus 11.58C), lower Rapid Creek (19.28C versus 19.38C), and upper Rapid Creek (9.88C in both periods). In contrast, mean annual discharge differed significantly between the two time periods in Spearfish Creek (1.95 versus 1.50 m 3 /s), lower Rapid Creek (2.01 versus 0.94 m 3 /s), and upper Rapid Creek (1.41 versus 0.84 m 3 /s). The mean biomass of adult brown trout in all three stream sections was significantly higher in the early-drought than in the late-drought period (238 versus 69 kg/ha in Spearfish Creek, 272 versus 91 kg/ha in lower Rapid Creek, and 159 versus 32 kg/ha in upper Rapid Creek). The biomass of juvenile brown trout was similar (43 versus 23 kg/ha) in Spearfish Creek in the two periods, declined from 136 to 45 kg/ha in lower Rapid Creek, and increased from 14 to 73 kg/ha in upper Rapid Creek. Size structure did not differ between the early-and late-drought periods in lower Rapid and Spearfish creeks, but it did in upper Rapid Creek. In addition to drought conditions, factors such as angler harvest, fish movements, and the nuisance algal species Didymosphenia geminata are discussed as possible contributors to the observed changes in brown trout biomass and size structure in Black Hills streams.

Influence of Didymosphenia geminata Blooms on Prey Composition and Associated Diet and Growth of Brown Trout

“…In a previous study on Brown Trout populations in Black Hills streams, James et al (2010a) found that biomass of Brown Trout declined significantly in Spearfish Creek following a drought. In Rapid Creek, biomass of large Brown Trout also declined following the drought; however, mean biomass of small Brown Trout increased appreciably from 14 to 73 kg/ha (James et al 2010a). The increase in small Brown Trout biomass was coincident with the occurrence of D. geminata blooms in Rapid Creek, and since then has remained relatively high (James et al 2010a).…”

“…The selected sampling reaches from Castle and Spearfish creeks have not experienced D. geminata blooms. In addition, the three sampling reaches were chosen because they have been previously researched (James et al 2010a(James et al , 2010b and because these locations have historical composition and abundance data for salmonids (Bucholz and Wilhite 2009). Each of these sampling reaches was surveyed for D. geminata coverage, Brown Trout diet and body condition, and invertebrate composition and abundance.…”

“…A smaller size-group, hereafter referred to as small Brown Trout, consisted of fish that were 100-199 mm TL. This size-group was selected because of the observed increase in biomass of small Brown Trout (100-199 mm) in Rapid Creek coincident with D. geminata blooms (James et al 2010a). The larger size-group was composed of fish that were 200-300 mm TL and are hereafter referred to as large Brown Trout.…”

“…Blooms of D. geminata were first observed in the Black Hills, South Dakota, in Rapid Creek in 2002. Recent work with Brown Trout in Rapid Creek showed that biomass of small trout (<200 mm) increased in Rapid Creek after D. geminata blooms were observed, a pattern not observed in other Black Hills streams where D. geminata was absent (James et al 2010a). At the same time, changes in invertebrate composition (i.e., higher dipteran abundance, lower EPT abundance) associated with D. geminata blooms were documented (James et al 2010b).…”

“…At the same time, changes in invertebrate composition (i.e., higher dipteran abundance, lower EPT abundance) associated with D. geminata blooms were documented (James et al 2010b). These observations prompted James et al (2010a) to postulate that increased recruitment of small trout may be linked to the establishment of D. geminata in Rapid Creek. The objective of this study was to determine whether changes in prey availability associated with D. geminata blooms influence diet composition, growth, or body condition of Brown Trout in a stream affected by D. geminata, especially for smaller fish (<200 mm).…”