Ecosystems Impacts of Overfishing

Project Title: Assessing the cascading ecosystem impacts of marine predator declines as a result of overfishing


Overfishing is among the greatest threats facing marine biodiversity. Many species of top ocean predators, such as billfish, tunas and sharks, are experiencing unprecedented population declines worldwide. Sharks are among the most threatened marine animals worldwide. Recent estimates suggest that populations of many large sharks have declined by 90% or more in areas where they were once abundant. We are conducting a series of integrated field and laboratory studies in the Florida Keys to understand the ecosystem role of marine predators and the potential cascading effects of their declines on fish communities. This work will provide insights for predicting how both predators and prey are likely to respond to overfishing with implications for developing effective conservation and management strategies. Some of the primary questions of this work include:

  1. Can changes in the presence and absence of sharks impact the community structure of coral reef fishes?
  2. Can changes in the abundance or movements of sharks effect the feeding behaviors and diets of coral reef fishes?
  3. Can changes in the abundance or behavior of sharks effect the physiology and/or reproductive behavior of coral reef fishes?

Recent Study Highlights & Selected Examples

Changes in the movement patterns of predators can cause prey to alter their habitat use, which can subsequently trigger trophic cascades or domino effects to other organisms in the community. In a paper published in the journal PLoS ONE titled, “A Comparison of Spatial and Movement Patterns between Sympatric Predators: Bull Sharks (Carcharhinus leucas) and Atlantic Tarpon (Megalops atlanticus),” our team and collaborators conducted a satellite tagging study to investigate the potential predator-prey interactions between bull sharks and tarpon in Southern Florida waters. SPOT satellite tags were deployed on 18 adult bull sharks and 10 adult tarpon to examine their movement patterns. The tracking data showed that even though bull sharks and tarpon share similar diets and generally occupying similar areas, there was little overlap in their habitat use.

Bull_tarpon_figure1 Figure 1 (above) Predator-prey interactions between bull sharks and Atlantic tarpon in the southern Florida ecosystem. Colored contours indicate core areas of shark density (kernel estimate) ranging from high density (red and orange colors) to low density (blue and purple colors). Pink circles represent tarpon locations and size of circles represent rate of tarpon movement (small circles = slow movements, large circless = fast movements). The figure shows that tarpon mainly occupied shallow, coastal areas, where they generally moved slowly. Tarpon avoided open areas, where bull shark density was highest. When moving over areas of high density, tarpon rate of movement was highest. A tarpon released on 23 May, 2011, was likely attacked (red triangle) by a shark on 28 May in the core area of shark use.

Bull sharks were present in southern Florida waters year-round, but the occurrence of the largest shark (>230 cm) peaked when tarpon abundance was highest. The study revealed that tarpon generally avoided areas of high bull shark density, despite high food availability occurring there. In fact, when moving over deep ocean waters, where shark abundance was highest, tarpon swam at high movement rates, in straight lines, until reaching shallower and more structurally-complex areas (where risk from sharks was lowest), at which point tarpon returned to a normal, slower, swimming speed with behaviors indicative of feeding. The tarpon also occasionally swam up rivers, where tracked bull sharks were absent. Based on these results, we propose that tarpon trade-off energetic costs of food (spending more time in less productive feeding areas) and osmoregulation (moving upstream into freshwater rivers requires extra energy to maintain the correct amount of water inside the fish’s body) to reduce their risk of bull shark predation.

Bull_tarpon_figure2 Figure 2 (above) Evidence of a bull shark attacking a tarpon. The figure shows data recorded by a satellite tag deployed on a mature tarpon between May 13th and 29th, 2007, showing: (a) depth; and, (b) light-level recorded every second. The low to absent light-levels shown in panel (a), and the abrupt changes in depth shown in panel (b) indicate the tag (and tarpon) was likely ingested by a shark just after the tarpon was released on May 25th. Examples of recovered tags from tagged tarpon that had likely fallen prey to sharks: (c) PAT tag; and, (d) SPOT tag, both bear the tell-tale teeth marks (based on spacing and serration) of a shark. Although we cannot identify the species of shark by the bite marks on the tag, we speculate a bull shark was responsible.

Hammerschlag N, Meÿer M, Seakamela S M, Kirkman S, Fallows C, Creel S. (2017) Physiological stress responses to natural variation in predation risk: evidence from white sharks and seals. Ecology, 98: 3199–3210.

Hammerschlag N, Broderick AC, Coker JW, Coyne MS, Dodd M, Frick MG, Godfrey MH, Godley BJ, Griffin DB, Hartog K, Murphy SR, Murphy TM, Nelson ER, Williams KL, Witt MJ, Hawkes LA (2015). Evaluating the landscape of fear between apex predatory sharks and mobile sea turtles across a large dynamic seascape. Ecology, 96(8): 2117-2126.

Creel S, Becker M, Christianson D, Dröge E, Hammerschlag N, Haward MW, Karanth U, Loveridge A, Macdonald DW, Wigganson M, M’soka J, Murray D, Rosenblatt E, Schuette P. (2015) Questionable policy for large carnivore hunting. Science, 350(6267): 147-1475

Hammerschlag N, Luo J, Irschick DJ, Ault JS (2012) A Comparison of Spatial and Movement Patterns between Sympatric Predators: Bull Sharks (Carcharhinus leucas) and Atlantic Tarpon (Megalops atlanticus). PLoS ONE 7(9): e45958. doi:10.1371/journal.pone.0045958

Hammerschlag N, Trussell G. 2011. Beyond the Body Count: Behavioral Downgrading of Planet Earth. Science. (E-Letter, 11 November 2011)

Watch a VIDEO about our research on bull shark and tarpon predation and interaction. Now available on our Vimeo Channel.


Listen to a PODCAST about shark stress physiology in the player below. Available for download on SoundCloud.

This project will increase our understanding of how predators can influence coral reef ecosystems. Such an understanding will aid scientists and managers in predicting how ecosystems may change following declines or recoveries of marine predators.