An analysis of shark bite scars on the Sarasota Bay resident bottlenose dolphin community and implications for habitat use

Feb 24, 2013 No comments By

 

Predator-prey relationships have long been an interest of ecologists.  Such relationships are dynamic.  One false move may result in an individual being taken out of a community, or a predator may go hungry if unsuccessful.

Predation itself is difficult to study due to predation events rarely being observed.  We are left to rely on evidence of predation attempts through wounds and scars on the surviving prey, and on wounds on carcasses.

The frequency of scars and wounds has been used to measure predation risk, with an obvious disadvantage in that we only have evidence of failed predation attempts.  Thus rate of predation will be greater than that measured by wound and scar frequencies. Regardless of this disadvantage, bite frequency is still a useful measure to determine relative risk and proves useful in comparisons among populations and allows us to further our understanding of these complex species interactions.

calf 1331 with shark bite wound

Young calf 1331 with shark bite wound in 2012

Predator-prey interactions are evident among sharks and cetaceans, and frequency of shark-inflicted wounds and scars has been reported from many locations, such as South Africa and Australia.  In Sarasota Bay, the bull shark (Carcharhinus leucas), is thought to be the most frequent predator to Sarasota Bay’s bottlenose dolphin residents.  A 1998 analysis by Kim Urian and colleagues of shark-inflicted injuries on resident Sarasota Bay dolphins found that of 151 dolphins sampled during 1975-1997, 31% had shark bite scars, and a male sex-bias in scaring frequency was observed.

One objective of my Master’s thesis is to update these analyses with more recent data from health assessments through 2012.   Furthermore, I propose to answer the following questions, 1) what relative threat are sharks to the Sarasota Bay resident bottlenose dolphins and 2) is there a differential occurrence of shark bites for animals with home ranges in different habitat types?

Scar and wound data gathered over 30 years of dolphin health assessments conducted in Sarasota Bay, along with more than 25 years of stranding data collected by Mote Marine Laboratory’s Stranding Investigations Program will be examined.  I will build on previous analyses to determine shark-inflicted bite mark frequency.  In addition, I will quantify the number of bites per body area to clarify if the distribution of bites on the body is equal.  I will also determine if differential bite frequencies exist for different sex-and age-classes, determine a rate of accumulation of scars for animals with multiple bites, and investigate if frequency of shark-inflicted injuries has changed through time.  These data will assist me in assessing the risk sharks pose to dolphins in the Sarasota Bay area.  I will then investigate if individual dolphin home ranges in differing habitat types are correlated to presence or absence of shark bite scars, to see if some areas are less desirable ranges from a predation risk perspective.

Asking questions in regards to predator-prey interactions have several important ecological and behavioral implications.  Predator-prey dynamics are often taken into consideration while looking at population dynamics, growth rates, evolutionary functional morphology and reproductive success within an ecosystem.  Predation risk has also been suggested by as an important element shaping dolphin group size, habitat use, and distribution for Sarasota Bay animals.  Ultimately these ecological and behavioral characteristics of complex ecosystems will help us understand and predict how animals will respond to the inevitable environmental and anthropogenic changes in their environments.

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This article was published on page 21 in the January 2013 Nicks n Notches.

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