Owens Speckled Dace, Rhynicthys osculus sp., distribution and abundance in Round Valley, California

Authors: Nicholas* Buckmaster, Daniel Neilsen, Neil Kauffman, Christi Kruse, Elsbeth Otto, Lacey Greene

Year: 2015 (xlvii)

Abstract

Owens speckled dace are a formally undescribed subspecies of speckled dace, and are a California species of special concern. Owens speckled dace were historically common in the Owens River watershed, but suffered dramatic declines and local extirpations following water development projects and introduction of non-native predators (e.g. brown trout, Salmo trutta, and largemouth bass, Micropterus salmoides). Our study examined speckled dace distribution, stream habitat characteristics, and population structure within the Round Valley sub-basin of the Owens River watershed. Round Valley was identified by previous researchers (Sada, 1982, unpublished data; Becker, 1999, CA Dept. of Fish and Game surveys) as one of the last remaining locations within the Owens Valley where speckled dace persist. Our surveys indicate that speckled dace within Round Valley are locally abundant in seasonally flooded waters (e.g. irrigation ditches), and present in low-gradient perennial streams: notably Owens speckled dace co-occurred with piscivorous brown trout in several low-gradient streams. Owens speckled dace were absent entirely from high-gradient stream reaches. Our results indicate that Owens speckled dace are a batch-spawning species, and remain reproductive from April to September; however, all sampled populations indicated a single recruitment event in late spring. Preliminary results indicate that seasonally flooded habitat contained speckled dace populations with higher condition factors than perennial streams, and simple population structures, typified by unimodal length-frequency distributions. Perennial streams, where speckled dace occurred, supported speckled dace populations with bi-modal or tri-modal length frequency distributions, which is indicative of multiple recruitment events. Presumably, ubiquitous predation from brown trout has precluded speckled dace from high-gradient streams. Seasonal habitat and low-gradient stream reaches comprise the majority of occupied speckled dace habitat in Round Valley; the unimodal population structure of seasonally flooded habitats indicates that speckled dace are not persisting within these habitats year-to-year. Thus, speckled dace persistence appears to be contingent upon only a portion of the occupied habitat, such as perennial irrigation ditches and low-gradient stream reaches.