Southern Flounder Life History Diversity and Contributions to Fisheries from Differing Estuarine Salinity Zones

Abstract

Otolith chemistry is a useful natural tracer for discerning habitat‐use of estuarine fishes. For Southern Flounder Paralichthys lethostigma, recent otolith chemistry studies have revealed a diversity of residency patterns across salinity gradients. However, the contribution of recruits with specific residency patterns to fisheries is poorly understood. The objectives of this study were to (1) use otolith chemistry from fishery‐independent and fishery‐dependent collections in Mobile Bay, Alabama, to classify lifetime residency patterns (i.e., freshwater, transient, estuarine) in Southern Flounder collected across a large estuarine salinity gradient (0–30 psu); (2) to test if Southern Flounder exhibited resident or migratory behavior by determining if lifetime residency patterns in fishery‐independent samples matched expected salinity patterns in the region of collection after accounting for annual variation in river discharge; and (3) to examine which residency patterns contributed to the commercial and recreational Southern Flounder fisheries in nearby coastal waters. Age‐0 residency patterns in fishery‐independent samples were strongly correlated with region of collection and annual river discharge, suggesting that the majority of Southern Flounder had resided in the same region in which they spent their age‐0 growth phase. Southern Flounder with a combination of freshwater and estuarine salinity signals and classified as “transient” did not appear to have conducted large‐scale movements across salinity gradients, but instead resided in regions of the estuary experiencing seasonal fluctuations in salinity. The majority (57%) of commercially and recreationally harvested Southern Flounder were transients, while a minority (39%) were estuarine residents and lifetime freshwater residents (4%) were rarely harvested. Results from this study suggest that Southern Flounder settle in and remain in the certain habitats during the estuarine residency phase. Given the lack of movement across habitats, future efforts to understand how habitat‐specific conditions (e.g., abiotic, biotic, fishing exploitation) affect vital rates seems warranted for a species currently experiencing population declines.

Isotopes and otolith chemistry provide insight into the biogeochemical history of mercury in southern flounder across a salinity gradient

“…Moreover, southern ounder are fundamental to Alabama's commercial and recreational shery with the majority of those harvested having transient salinity exposure classications. 42 Because transient individuals in this study oen had Hg sources similar to estuarine and freshwater residents collected in similar locations, it is likely that harvested sh contain Hg sources from across the estuarine ecosystem. As such, increases in Hg inputs to this aquatic ecosystem are likely to translate into higher concentrations in Mobile Bay foodwebs, predators that consume prey from this ecosystem, and in human consumers of those sh.…”

“…Concentrations of each element measured in water were converted to molar ratios using Ca as a standard to compare with otolith chemistry. 42,56,57 Using these data, southern ounder were classied into one of three lifetime residency patterns (i.e., freshwater, estuarine or transient) based on lifetime salinity exposure (as inferred from Sr to Ca molar ratios: Sr : Ca) across the otolith transect. To accomplish this, nonlinear models were t to the data to quantify relationships between water Sr : Ca versus ambient sample site salinity.…”

“…Predicted values from these nonlinear models were then multiplied by a partition coefficient to develop a link between expected otolith Sr : Ca and ambient salinity. 42 Finally, a regime shi algorithm was used to detect any signicant shis in Sr : Ca and thus, salinity, across the lifetime of the individual. 58,59 Otolith Sr : Ca equivalent to #1 psu salinity was classied as a freshwater resident for a given time step, while otolith Sr : Ca equivalent to >1 psu salinity was classied as an estuarine resident for a given time step.…”

“…Total Hg, sh total length, d 13 C, d 15 N Source , relative TP, d 202 Hg, D 199 Hg, D 200 Hg, and D 201 Hg were compared for significant differences among southern ounder lifetime residency classications to understand how different habitat uses across the estuary may have inuenced Hg sources to sh tissues (Table 1). 42 To better understand sources of Hg among southern ounder collected across the salinity gradient in Mobile Bay, D 199 Hg with D 201 Hg, D 199 Hg with d 202 Hg, and D 199 Hg with d 13 C, were also analyzed with k-means cluster analysis (a = 0.05; k = 3 clusters). Additionally, a multiple linear regression model (N = 23) was used to predict how lifetime salinity exposure as a proxy for lifetime estuarine habitat use, total length, and trophic position impacted the concentration of total Hg in sh tissue (a = 0.05).…”

“…Sites were sampled in three locations: the Delta (freshwater riverine areas of the Mobile-Tensaw River Delta), Week's Bay (small estuary that drains into lower Mobile Bay and experiences freshwater to polyhaline conditions), and Lower Mobile Bay (sites located within the lower Mobile Bay with salinities ranging from mesohaline-polyhaline). Lifetime salinity exposure classifications are indicated by color on the map and were classified based on otolith chemistry and water chemistry analyses 42.…”