Influence of habitat dynamics on the distribution and abundance of the federally threatened Santa Ana Sucker, Catostomus santaanae, in the Santa Ana River

Habitat degradation affects native stream fish populations worldwide. We examined the impact of fluctuation in environmental variables on the population dynamics of the federally threatened Santa Ana sucker, Catostomus santaanae, in the Santa Ana River, California through: 1) annual quantitative surveys of C. santaanae abundance and habitat at three 100-m sites between 2001 and 2008 and 2) annual surveys of habitat composition within a 30-km stretch of the Santa Ana River between 2006 and 2008. We used Akaike’s Information Criterion (AIC) to evaluate competing models that used environmental variables to explain variation in C. santaanae abundance among sites and years. The most plausible model identified a positive relationship between C. santaanae abundance and both the amount of coarse substrate (i.e., gravel and cobble) and rate of discharge among site-years. Surveys at the 30-km scale indicated that the prevalence of coarse substrate declined in a downstream direction in each year and that the total amount of this habitat type varied annually. Specifically, cobble/gravel habitat was mostly confined to the upstream 4 km, 9.6 km, and 5.1 km of the survey area in 2006, 2007, and 2008, respectively. Fine sediment comprised the bulk of downstream habitat every year. This large-scale flux in the distribution of coarse sediment likely has a large effect on the population dynamics of C. santaanae in the Santa Ana River. Our results underscore the need to maintain and enhance suitable C. santaanae habitat to ensure its long-term persistence in the Santa Ana River.