Osmoregulation and Salinity Preference in Juvenile Sandbar Sharks (Carcharhinus plumbeus) in Winyah Bay, SC, USA

Juvenile sandbar sharks (Carcharhinus plumbeus) have been caught in salinities ranging from 7 – 40. In Winyah Bay, a partially mixed estuary in Northeast SC, juvenile sandbar sharks tidally alternate between higher tides in middle bay and lower tides in lower bay. To assess salinity preference and duration in eight acoustically-tagged juvenile sandbar sharks in different salinities, acoustic receivers with salinity loggers were placed throughout Winyah Bay. Juvenile sandbar sharks were caught in salinities from 17.2 to 36.1 and acoustic detections were recorded from 11.5 to 24.7 by salinity loggers in middle bay. Smaller juvenile sandbar sharks used lower salinities, presumably to decrease osmoregulatory costs and predation, and used tidal currents to move throughout the bay, which also decreased energy expenditure. Acoustically tagged sharks spent most of their time in middle Winyah Bay at high tide or tidal phases immediately before or after high tide, whereas when these sharks were present at the mouth of the bay, they spent more time at tides related to low tide. To test whether duration spent in lower salinities was sufficient to change plasma osmolality and osmolyte concentrations, we measured sodium, chloride, urea, TMAO, and potassium concentrations and total osmolality in plasma of juvenile sandbar sharks caught on longlines set at either flood or ebb tide from May-August, 2018. All variables differed significantly (p \u3c 0.05; ANOVA) between salinity groups (17 – 21.9; n = 14, 22 – 26.9; n = 9, 27 – 31.9; n = 9, and \u3e 32; n = 11). Sodium and chloride concentrations in the lowest salinity group (LSG) were 243.15 ± 2.82 and 241.91 ± 2.86 mM, respectively, and increased to 279.84 ± 2.06 and 280.73 ± 1.72 mM, in the highest salinity group (HSG). Between the LSG and the HSG, urea increased from 269.34 ± 5.97 mM to 352.25 ± 5.95 mM, and TMAO increased from 49.69 ± 2.59 mM to 81.15 ± 3.92 mM. Potassium increased from 4.35 ± 0.21 mM (LSG) to 5.09 ± 0.10 mM (HSG). Total osmolality in the HSG was 998.73 ± 12.61 mOsm/kg and 822.24 ± 11.62 mOsm/kg in the LSG. Post-hoc Tukey tests of all variables revealed that the HSG was significantly different than the other three salinity groupings in all osmotic components except potassium, whose contribution to osmoregulation is minimal. This study further supports that juvenile sandbar sharks seek out brackish salinities and that salinity becomes a smaller factor in movement as they grow. It is the first to suggest that juvenile sandbar sharks can partially osmo- and ionoconform in a similar manner to juvenile bull sharks