Sulfur stable isotope indicators of residency in estuarine fish
Limnol. Oceanogr., 56(5), 2011, 1563-1576 | DOI: 10.4319/lo.2011.56.5.1563
ABSTRACT: 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 (δ34S) distributions in fish muscle tissue to track residents and transients across estuarine salinity zones. Salinity tracking was based on δ34S contrasts between freshwater and marine waters. This 5-yr study of two Louisiana estuaries showed that riverine and upper-estuarine fish consistently had low δ34S values (−5‰ to +5‰) in contrast to fish from the lower, more marine portion of estuaries that had higher δ34S values (11–17‰). Residents were identified using tests of normality within community-level δ34S 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 δ34S 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 δ34S approach may be a general technique for evaluating fish residency and movement across salinity zones in estuaries.