Movements of bonnetheads, Sphyrna tiburo, as a response to salinity change in a Florida estuary
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The movement of bonnetheads, Sphyrna tiburo, within an estuarine system on the Gulf of Mexico coast of Florida was examined to define response to salinity change. Shark presence and movements were evaluated by acoustic monitoring and gillnet sampling. Acoustic monitoring data were used to investigate active selection of different zones within the estuary based on differences in salinity among zones. Sharks were monitored for 187 days in 2003 and 217 days in 2004 in salinities ranging from 11.0 to 31.0 ppt in 2003 and 15.8 to 34.6 ppt in 2004. Monitoring data supported the hypothesis that salinity played a role in the distribution and movement of S. tiburo. Catch per unit effort (CPUE) data obtained from gillnet sampling from 1995 to 2004 were examined to determine affinity or avoidance of specific salinities within the study site as calculated using an electivity index. Electivity analysis showed almost no affinity or avoidance for specific salinity values. The difference in results between the CPUE and acoustic monitoring in relation to the potential effects of salinity likely relate to the nature of the data, with acoustic monitoring providing continuous data and CPUE providing snapshot location data. The results of this study suggest that although S. tiburo are collected within a wide range of salinity levels, salinity may affect movement and distribution. Salinity effects may be more pronounced during periods of prolonged and/or large changes in salinity as detected by long-term monitoring.
KeywordsAbiotic factors Acoustic telemetry Electivity Shark
Funding for the acoustic monitoring portion of this research was provided by the Mote Scientific Foundation. Relative abundance sampling was supporting by funding to Robert Hueter via National Marine Fisheries Service grants. Targeted fishing surveys were completed under various funding sources. We thank the many MML staff who were involved in collecting these data: M. Amato, A. Collins, M. Friday, J. Gelsleichter, C. Manire, J. Morris, J. Tyminski, T. Wiley and B. Yeiser. We also thank the numerous student interns who participated in this fieldwork. Thanks Dr. R.F. Noss for his helpful review of this manuscript.
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