Reconstructing the Past Salinities Experienced by a Freshwater and Marine Piscivore in the Mobile-tensaw River Delta Using Otolith Microchemistry

Abstract

In this study, we used changes in otolith microchemistry, with particular emphasis on otolith Sr:Ca, along the otolith growth axis to examine the past salinities, and thus the past environments, experienced by age-0 largemouth bass Micropterus salmoides and age-0 southern flounder Paralichthys lethostigma collected along an upstream to downstream gradient in the Mobile-Tensaw Delta (MTD). Results from a laboratory experiment indicated that there is a significant time lag (~ 21 days) between initial changes in salinity and maximum saturation levels in the otoliths of age-0 largemouth bass. For age-0 largemouth bass collected from the MTD, spatial and temporal variation in salinity, and also Sr:Cawater, resuted in otoliths from spring collected fish having markedly different Sr:Caotolith profilesthan those of fall-collected fish. Spring-collected fish had relatively stable profiles below 1500 µmol mol-1, which is indicative of a freshwater environment. Strontium:Caotolith profiles for fall-collected fish, particularly from sites that experienced increased salinity from late summer until their collection, were stable and below 1500 µmol mol-1 for the first 70% of the profile (i.e., while ambient water was fresh), but showed an abrupt increase for the remaining 30% of the otolith. Further, our ability to classify age-0 largemouth bass to their hatch and collection sites was also driven by spatial and temporal variation in salinity, and, as a result, water chemistry. In the early spring, when largemouth bass hatch, water chemistry in the MTD was relatively homogenous due to freshwater, and thus elementally homogenous, environments that existed from upstream to downstream. Thus, our ability to correctly classify spring-collected fish to their collection sites was poor. However, increased salinity during the fall at downstream sites resulted in a water chemistry that differed from upstream to downstream. As a result, our ability to correctly classify fall-collected fish to their collection sites improved. Based on otolith microchemistry, 68% of age-0 southern flounder appeared to hatch in higher salinity waters before moving into the MTD. For these fish, Sr:Caotolith was high in the otolith core and declined rapidly to = 1500 µmol mol-1 for the remainder of the otolith, indicating a prolonged period of freshwater residency after moving there from the marine environment. Surprisingly, not all southern flounder exhibited this pattern; 32% of fish from the MTD had Sr:Caotolith concentrations that remained = 1500 µmol mol-1 throughout the entire otolith; suggesting that these fish hatched in a reshwater or low salinity environment and remained there for their entire first year of life.