The Florida Everglades: Lair of the Bull Shark

It’s always fascinating to watch different species of fish arrive seasonally at the spots you frequent. Last weekend, while tagging sharks for research in the Everglades, it became clear to me that this very phenomenon was occurring, with blacktip sharks beginning to trickle back into the habitats, being followed around by even bigger local predators.

The Everglades is a fascinating spot to work because there are lots of predators – and those predators vary in size, number, and relative power over each other. At the apex – literally the top of the food chain – are the bull sharks. The Everglades is their lair. Blacktip and lemon sharks are also predatory sharks, but the blacktips are potential prey for the bulls, making them the proverbial “middle man on the totem pole.” The blacktips are usually the largest in number, and are often smaller than both the lemons and the bulls. Indeed, parts of the Everglades and Florida Bay give sub-adult blacktip sharks refuge from the risky, open water of the Gulf where they risk being consumed by larger predators. However, in nature, there is no free lunch, so by trading off open ocean habitats for the Everglades ecosystem, blacktip sharks gain some refuge and feeding opportunities, but subject themselves to a smaller number of large predators – the bulls – which are cruising the river mouths, coastlines, and estuarine areas for small prey. In fact, a recent study published in PLoS ONE by R.J. Dunlap Director Dr. Hammerschlag and colleauges found that bull sharks in the Everglades cause other prey species (such as tarpon) to alter their behavior when swimming through areas of high bull shark abundance. These “risk effects” are especially difficult to detect with large predatory fishes, and this study is the one of the first of its kind to detect these often overlooked measures of predator-prey relationships in marine settings. Another reason for this change in the shark presences is due to the prevailing water conditions– strong oxygen content, slightly cooler temperatures that we see from July – September, and stable salinity.

Most visits to Everglades National Park usually produce a large bull shark for our research. It is critical to establish estimates of how many bull sharks in the area, what they are eating, and if/how they can tolerate human-induced changes in the ecosystem. I encourage all of the readers to visit the Everglades and explore – the ecosystem is starting to ignite, and it is awesome to watch the predator-prey interactions between different coastal shark species. If you plan on fishing, I urge you to practice catch and release, as the seasonal blacktip aggregation already gets enough pressure from the bull sharks in the area, and the bull sharks themselves are in smaller numbers because they are the top predator. Release is a great option for these species, because both bulls and blacktips do relatively well with responsible catch and release.

-Austin Gallagher
PhD Student, Research Assistant
RJ Dunlap Marine Conservation Program
Abess Center for Ecosystem Science and Policy
www.austingallagher.com

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