Fish are important biotic controls in many stream systems, and may be crucial in arid land streams. This study aimed to assess the importance of fish in Southwestern streams by investigating the influence on ecosystem function of an abundant sucker species, Catostomus insignis, in the upper Gila River in southwest New Mexico. Suckers are the most abundant large bodied fish in the upper Gila, but are patchily distributed throughout the river over space and time due to frequent movements between habitat patches. C. insignis creates feeding depressions in mobile sediments (e.g. sand, gravel, silt) as it feeds on invertebrates and detritus; these depressions act as depositional zones for benthic organic matter, and can cover up to 100% of the surface area of the benthos within 24-48 hours. C. insignis potentially act as ecosystem engineers, in that they physically modify the structure and topography of the stream bottom, and at rates (hours to days) which are relevant to the growth and habitat use of other organisms. The timing and intensity of feeding depression formation is highly variable, and is likely directly related to the number of fish present in the habitat. To help understand the dynamic nature of patch formation, we tracked individual movements of PIT-tagged (passive integrated transponder) C. insignis using a portable PIT telemetry system and measured the rate and intensity of grazing scar formation to assess the influence of C. insignis on ecosystem structure and function.
Approximately half of the individual fish were considered “sedentary”, as they were only detected in the pool where they were originally captured; the distance traveled by “mobile” individuals was variable, from 15m to more than 3000m. In addition, the population of fish found within a focal pool was highly variable over the 2 months of the study, with estimates ranging from 10 to 28 large (>250mm total length) individuals. Fluctuations in the number of fish within a habitat are likely to impact the resources available to other organisms, and appear to influence the amount of sediment (8-70g sand m-2 hr-1) moved within the habitat.