Sulfur stable isotope indicators of residency in estuarine fish

Estuarine fish may remain as residents in areas with favorable conditions for feeding and refuge, but become mobile and transient where conditions are less favorable. We developed a new approach involving sulfur stable isotope (delta S-34) distributions in fish muscle tissue to track residents and transients across estuarine salinity zones. Salinity tracking was based on delta S-34 contrasts between freshwater and marine waters. This 5-yr study of two Louisiana estuaries showed that riverine and upper-estuarine fish consistently had low delta S-34 values (-5 parts per thousand to + 5 parts per thousand) in contrast to fish from the lower, more marine portion of estuaries that had higher delta S-34 values (11-17%). Residents were identified using tests of normality within community-level delta S-34 distributions, and conceptually were considered animals permanently residing at the sampling station but also any animals present from nearby areas with similar salinities. Transients had delta S-34 values atypical of both the location of capture and the local salinity regime. Results showed good resolution of fish movement at small 0.1-4-km scales for low-salinity (< 2) upper-estuary stations, and good detection of long-range migrants from the upper estuary into the lower estuary at a coarser spatial scale of 10-30 km. On an average basis, 2/3 of the estuarine fish fauna was resident and 1/3 transient. Transients were a minority in most species but commonly included small as well as large fish. This novel delta S-34 approach may be a general technique for evaluating fish residency and movement across salinity zones in estuaries.