Sulfur stable isotope indicators of residency in estuarine fish

Abstract

Estuarine fish may remain as residents in areas with favorable conditions for feeding and refuge, but becomemobile and transient where conditions are less favorable. We developed a new approach involving sulfur stableisotope (δ 34S) distributions in fish muscle tissue to track residents and transients across estuarine salinity zones.Salinity tracking was based on δ 34S contrasts between freshwater and marine waters. This 5-yr study of twoLouisiana estuaries showed that riverine and upper-estuarine fish consistently had low δ 34S values (-5‰ to +5‰)in contrast to fish from the lower, more marine portion of estuaries that had higher δ 34S values (11-17%).Residents were identified using tests of normality within community-level δ 34S distributions, and conceptuallywere considered animals permanently residing at the sampling station but also any animals present from nearbyareas with similar salinities. Transients had δ 34S values atypical of both the location of capture and the localsalinity 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 estuaryat 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 δ 34S approach may be a general technique for evaluating fish residency and movement across salinity zones inestuaries. © 2011, by the American Society of Limnology and Oceanography, Inc.