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<p>The Ten Thousand Islands Estuary of Southwest Florida is the site of ongoing long-term hydrologic restoration. It features gradients of human impacts to freshwater input, from restricted to enhanced flows, including areas with relatively small human effects on water delivery. Thus, it is an excellent location to investigate the biotic and abiotic factors influencing the abundances and size structure of bull shark populations within these estuarine nurseries. We used long-term monitoring data to determine how bull shark abundance varied across three bays differing in human alterations to freshwater flow, which feature spatiotemporal variation in abiotic (salinity, temperature, dissolved oxygen, depth) and biotic (prey abundance and presence of competing species) factors. Boosted Regression Tree analysis revealed that juvenile bull shark abundance was influenced primarily by minimum monthly salinity (relative influence = 42.1). There was also considerable interannual (RI = 29.1%) and monthly (RI = 18%) variation. Minimum temperature (6.6%), average dissolved oxygen (2.1%), and prey abundance (2%) had smaller effects. Spatiotemporal fluctuations in shark abundance relative to abiotic conditions drove seasonally divergent patterns in abundance relative to human water management. While dry season catch rates were similar across all three bays, during the wet season, catches of bull sharks in the bay with the lowest mean salinities due to artificially enhanced freshwater flow were more than 2.5 times that of the other two bays. Overall, our findings suggest that upstream restoration of estuaries will have considerable influence on the suitability of estuarine nurseries for bull sharks, increasing suitability in areas where freshwater flow is enhanced but likely decreasing abundances in areas where inputs are reduced.</p>

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Legacies of Ecosystem Modification: Factors Affecting Long-Term Variation in the Abundance of Juvenile Bull Sharks in a Subtropical Estuary

  • 期刊论文

  • 发表日期：

    2025年3月13日

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  • Kristine Zikmanis,
  • Patrick O’Donnell,
  • Simon Dedman,
  • Michael R. Heithaus

摘要

The Ten Thousand Islands Estuary of Southwest Florida is the site of ongoing long-term hydrologic restoration. It features gradients of human impacts to freshwater input, from restricted to enhanced flows, including areas with relatively small human effects on water delivery. Thus, it is an excellent location to investigate the biotic and abiotic factors influencing the abundances and size structure of bull shark populations within these estuarine nurseries. We used long-term monitoring data to determine how bull shark abundance varied across three bays differing in human alterations to freshwater flow, which feature spatiotemporal variation in abiotic (salinity, temperature, dissolved oxygen, depth) and biotic (prey abundance and presence of competing species) factors. Boosted Regression Tree analysis revealed that juvenile bull shark abundance was influenced primarily by minimum monthly salinity (relative influence = 42.1). There was also considerable interannual (RI = 29.1%) and monthly (RI = 18%) variation. Minimum temperature (6.6%), average dissolved oxygen (2.1%), and prey abundance (2%) had smaller effects. Spatiotemporal fluctuations in shark abundance relative to abiotic conditions drove seasonally divergent patterns in abundance relative to human water management. While dry season catch rates were similar across all three bays, during the wet season, catches of bull sharks in the bay with the lowest mean salinities due to artificially enhanced freshwater flow were more than 2.5 times that of the other two bays. Overall, our findings suggest that upstream restoration of estuaries will have considerable influence on the suitability of estuarine nurseries for bull sharks, increasing suitability in areas where freshwater flow is enhanced but likely decreasing abundances in areas where inputs are reduced.