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 become mobile and transient where conditions are less favorable. We developed a new approach involving sulfur stable isotope (d 34 S) distributions in fish muscle tissue to track residents and transients across estuarine salinity zones. Salinity tracking was based on d 34 S contrasts between freshwater and marine waters. This 5-yr study of two Louisiana estuaries showed that riverine and upper-estuarine fish consistently had low d 34 S values (25% to +5%) in contrast to fish from the lower, more marine portion of estuaries that had higher d 34 S values (11-17%). Residents were identified using tests of normality within community-level d 34 S 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 d 34 S 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, O of the estuarine fish fauna was resident and M transient. Transients were a minority in most species but commonly included small as well as large fish. This novel d 34 S approach may be a general technique for evaluating fish residency and movement across salinity zones in estuaries.Station and habitat evaluations play an important role in conservation and restoration decisions. In estuaries with a typical mix of marsh and open water areas spread over a freshwater to marine salinity gradient of 0 to 35, it is often unclear which sites and habitats are most important for fish productivity and biodiversity. Both refuge and feeding opportunities are aspects of estuarine habitats and stations important to fish (Boesch and Turner 1984), and higher fish abundances are often taken as evidence that sites are important or essential habitat (Minello et al. 2003). Further concerns center around how habitats support growth or decrease predation risk, but these aspects are harder to measure and evaluate (Beck et al. 2001). A more recent approach to habitat evaluation is to use chemical markers such as stable isotopes and trace metals to determine which habitats have relatively high importance for fish, especially habitats that have important nursery or productivity functions (Thorrold et al. 2002;Wissel and Fry 2005). Residency is potentially an additional way to assess habitat quality, with low residency and high transience likely indicating suboptimal habitats (Hoffman et al. 2007). The advantages residents may have include better avoiding local predation and better accessing local nutritious foods. The chemical marker approach can be strong for studying residency (Fry and Ewel 2003), and identifying stations where fish isotope chemistry indic...

Tracking variations in wetland use by breeding flamingos using stable isotope signatures of feather and blood

“…Sulphur isotope ratios (d 34 S) provide in formation on dietary protein source and geographical origin (Richards et al, 2003) and have been used to identify patterns relating to the salinity of feeding locations (e.g. Fry and Chumchal, 2011). As dietary nutrients are incorporated at different times depending on growth, metabolism and protein turnover of the tissue considered, different tissues from the same individual may provide information on dietary source at different phases of life history.…”

Preliminary assessment of the trophic structure of demersal fish community in the Sea of Oman

“…The combination of stomach content with δ 13 C and δ 15 N analyses is now recognized as a pertinent methodology in order to quantify fish diet and address its spatial and temporal variability in a given ecosystem (Hamidan et al, 2016;Keller et al, 2016;Malek et al, 2016;Olson et al, 2016;Weidner et al, 2017). In addition to carbon and nitrogen isotopic ratios, sulfur isotopes are also useful to trace the origin of the prey consumed by consumers in aquatic ecosystems (Peterson, 1999;Post, 2002;Fry and Chumchal, 2011). The relative composition of 34 S/ 33 S (δ 34 S) in fish muscles, in particular, provides additional information to carbon and nitrogen isotopic ratios to unravel the identity and origin of basal resources used by mobile consumers, in particular prey origin, i.e., benthic or pelagic, as well as the diversity of potential resources constitutive of a consumer's diet (Peterson, 1999;Connolly et al, 2004;Jardine et al, 2012;Colborne et al, 2016;Morkūnė et al, 2016).…”

Rescaling stable isotope data for standardized evaluations of food webs and species niches

“…Comparisons were also made between the pooled reserve sites and several individual sites. Fish from these estuarine studies were generally 5 to 50 cm in length (Fry & Chumchal 2011, Davis et al 2014.…”

“…Data for 3 food webs from Barataria Bay, Louisiana (USA), were taken from Fry & Chumchal (2011). These food webs were arranged along a salinity gradient, with average salinities at the times of fish collections of 6, 9 and 17 for Fisherman's Point (FP), Manilla Village (MV) and Grande Terre (GT), respectively (Fry 2013b); salinity of full-strength seawater is 36.…”

“…The studies included (1) 4 east-coast African estuary food webs where fish functional groups were sampled before and after flooding (Abrantes et al 2013), (2) 3 estuarine food webs from turbid, lower salinity Louisiana systems (Fry & Chumchal 2011), (3) 6 estuarine food webs from higher-salinity and clearerwater Australian systems (Davis et al 2014), (4) 3 deep-sea food webs from the eastern, central and western Tasman Sea (Flynn & Kloser 2012), (5) seasonal studies of the deep-sea fish food webs in the Mediterranean Sea (Papiol et al 2013), (6) river food webs at dredged versus un dredged sites (Freedman et al 2013) and (7) 2 freshwater pond food webs representing a control food web and a food web after addition of an invasive fish species (Britton et al 2010). These diverse food webs showed a wide range in average S:N ratio estimates, and in each study, we also counted the proportions of observations that had S:N <1 and >1 in food web comparisons made over time or space.…”