A Seasonally Dynamic Estuarine Ecosystem Provides a Diverse Prey Base for Elasmobranchs

Niche partitioning between river shark species is driven by seasonal fluctuations in environmental salinity

“…As we did not attempt to remove urea or TMAO prior to SIA, it cannot be discounted that potential interspecific differences in circulating TMAO levels or TMAO production capacity and variable urea may contribute to observed differences in δ 15 N levels (Carlisle et al., 2017; Shipley et al., 2017). However, our findings are in line with a study conducted on the South Alligator River (Northern Territory, Australia), which used δ 15 N and δ 13 C values contained in washed muscle tissue (in conjunction with fatty acid analysis) to show that C. leucas were utilizing food webs across the marine to freshwater spectrum, whereas G. glyphis were likely targeting lower order benthic prey from estuarine/coastal areas (Every et al., 2017, 2019). However, species in our study system exhibited greater overlap in the δ 15 N and δ 13 C values contained in fin, RBC and plasma tissues, suggesting a greater degree of interspecific competition for a shared dietary resource.…”

“…For example, Tag code 1550 moved between salinities of 0.1 and 15.9 on 19 August 2017, and Tag code 12715 moved between salinities of 7.4 and 22.7 on 19 October 2017. In addition, tissues sampled from juvenile C. leucas captured in low salinity environments in Florida, USA (Matich & Heithaus, 2014), the Northern Territory, Australia (Every et al., 2019) and in the present study were observed having δ 13 C signatures that were indicative of sharks foraging in marine food webs. While this discrepancy in δ 13 C signatures versus capture location could also be influenced by high levels of prey movement from the lower estuary (Every et al., 2019), our movement and blood chemistry data support the theory that juvenile C. leucas resident in freshwater will make occasional higher risk trips into estuarine waters; possibly to access food resources (Matich et al., 2010) or to reduce physiological stress associated with living in a low‐salinity environment (Gleiss et al., 2015; Pillans & Franklin, 2004; Simpfendorfer et al., 2005).…”

“…Stable isotope values of samples were calculated as parts per thousand (‰) relative to international standards (Pee Dee Belemnite carbonate and atmospheric nitrogen). Because previous studies have found elasmobranch blood and fin tissue to have a low‐lipid content (Every et al., 2019; Logan & Lutcavage, 2010; Matich & Heithaus, 2014; Shipley et al., 2017), we did not remove lipids from tissue samples prior to SIA or mathematically correct δ 13 C values in these tissues for the effects of lipids. We did not attempt to remove excess urea from our samples prior to SIA.…”

“…Reductions in salinity can also reduce the efficiency of the ampullae of Lorenzini, a sensory organ found in elasmobranchs that detect bioelectric currents given off by prey (Rigg, Peverell, Hearndon, & Seymour, 2009). Morphological and/or behavioural differences between species could permit sympatric elasmobranchs to specialize on different prey items (Kinney, Hussey, Fisk, Tobin, & Simpfendorfer, 2011; Yick, Tracey, & White, 2011), while moving in response to environmental change and/or separating along environmental gradients may allow sympatric species to partition resources on both temporal and spatial scales thereby negating competitive interactions associated with resource use overlap (Every, Fulton, Pethybridge, Kyne, & Crook, 2019; Matich & Heithaus, 2014; Ross, 1986). By linking physiological approaches with observations of how animal movements and local environmental parameters change in space and time, it may be possible to reveal mechanistic insights into how environmental variation drives patterns of habitat use, river connectivity and community structure in elasmobranch nursery habitats.…”

“…Gleiss et al., 2015; Heupel & Simpfendorfer, 2011; Matich & Heithaus, 2014; Pillans & Franklin, 2004; Reilly et al., 2011); however, most aspects of the biology of sharks in the genus Glyphis are virtually unknown (Grant et al., 2019). While several authors have suggested that both juvenile C. leucas and G. glyphis are euryhaline piscivores (Martin, 2005; Snelson, Mulligan, & Williams, 1984; Thorburn & Morgan, 2004), comparative studies utilizing fatty acids and stable isotopes have revealed low dietary overlap between juveniles of these species (Every et al., 2019; Every, Pethybridge, Fulton, Kyne, & Crook, 2017). Furthermore, studies conducted in Queensland (Lyon et al., 2017) and the Northern Territory (Field et al., 2013) observed that while the two species occurred in the same river systems, G. glyphis were rarely captured where C. leucas were captured.…”