by Andrew Marbury
The St. Johns River (SJR) was historically home to the southern-most native population of Atlantic strain Striped Bass. However, natural reproduction of this stock was likely low compared to more northern rivers and was thought to have ceased completely by the 1970’s. Since the FWC and USFWS have stocked both Striped Bass (Morone saxatilis) and Hybrid Striped Bass (M. chrysops x M. saxatilis) into the system as a means of replacing this bygone fishery.
While these species are wide-ranging and can tolerate the mild water temperatures of winter, the Florida summer brings extremes that confine Morones to cool water refuges such as freshwater springs.
Years of directed electrofishing and snorkeling surveys have led FWC biologists to believe that the “chimney” spring boil at Silver Glen Springs (SGS) holds one of the largest summer aggregations of Morones. In the past, researchers gauged the abundance and health of these fish through snorkel surveys at SGS from May to August, while utilization of the spring was at its highest. While not recorded, it was assumed that as temperatures in surrounding waters cooled in the fall, the fish would emigrate from SGS to the productive waters of the SJR.
To test this hypothesis, Morone residence has been recorded year-round at SGS since May 2016. Surveys involved snorkeling with underwater cameras in the summer months and the use of waders and an extendable “camera pole” throughout the winter. Typical summer aggregations (> 1,000) were present through October 2016 with fish health depreciating noticeably over this time, likely due to the lack of forage in the spring boil. By November 2016, numbers fell significantly to around 100 individuals but surprisingly, have remained at this level throughout the entire winter (through March 2017). Interestingly, fish health has improved over this time, suggesting some level of foraging.
We will continue to monitor seasonal fluctuations of Morone abundance to determine when the majority of fish immigrate back to SGS. A telemetry study has been proposed for 2018 to determine both the temporal and spatial movements of Morones throughout the SGS and SJR systems. This should help to answer questions regarding residence time and foraging efforts while in thermal refuges, ultimately better informing managers on the feasibility of Morone stocking programs.