A Tale of Two Timescales: Using Otolith Microchemistry to Improve Our Understanding of Alligator Gar Movement in the Lower Trinity River, Texas

Telemetry-based study of alligator gar (Atractosteus spatula) movement in the lower Trinity River, Texas, indicated that fish primarily re- mained within discrete home ranges less than 60 river kilometers (rkm), supporting the potential for local-scale management. However, the temporal scale of inference was limited (22 months), which may inadequately represent fish movements and home range size at the lifetime (i.e., ≥50 years) scale. Therefore, we used otolith microchemistry to examine the long-term movements of alligator gar (n = 59; total length range 1152 to 2420 mm, age range 4 to 60 years) between the lower Trinity River and Trinity-Galveston Bay system. Strontium:calcium (Sr:Ca) concentrations were measured along laser-ablated transects from the otolith core (i.e., time at hatch) to the edge (i.e., time at capture) for fish collected throughout the system, document- ing movements between the river (freshwater) and bay (saltwater). We identified two residence contingents among fish in the lower Trinity River that differed in prevalence across the system. Multiple logistic regression indicated that river residence, in which fish remained in the river over their entire lifetimes, was most common at the upstream end of the study reach (63% of fish). In contrast, transience, in which fish moved between the river and bay, was prevalent nearest the river mouth (82% of fish). Although our inferences from the otolith data suggest a somewhat greater degree of homogeni- zation across the system than was captured via telemetry, our results generally suggest localized management of alligator gar in the lower Trinity River could be appropriate.

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