Five-fold increase in the abundance of juvenile bull sharks over the last 20 years in Mobile Bay coincides with increasing sea surface temperatures
A bull shark. Photo: iStock The attack on 34-year-old fisherman Vikas Suresh Powari by a bull shark (Carcharhinus leucas) in the Vaitarna River near Mumbai on Valentine’s Day this year caused a sensation. While many attacks by the species go unreported in the Global South, interactions between it and humans are expected to rise. The reason: Warming sea surface temperatures, according to a new study.
There has been a five-fold increase in the abundance of juvenile bull sharks over the last two decades in Mobile Bay. These two decades have also witnessed increasing sea surface temperatures in the Alabama estuary.
Researcher Lindsay Mullins and her colleagues measured changes in the distribution and abundance of bull sharks in Mobile Bay using data from 440 individuals captured and released during surveys conducted between 2003 and 2020.
They investigated the environmental factors associated with these changes using remote sensing data collected throughout the same period, according to a statement.
The scientists found that the number of individuals captured per hour of surveying increased five-fold between 2003 and 2020.
All bull sharks surveyed during the study period were juveniles. This coincided with an increase in the mean sea surface temperature in Mobile Bay from 22.3 degrees Celsius (ºC) in 2001 to 23ºC in 2020.
The authors then performed computer modeling. It showed that sea surface temperatures above 22.5ºC were associated with an increased likelihood of bull shark presence.
Read Vaitarna river attack: Why the bull shark is among the most dangerous animals in the world
The average probability of capturing a bull shark during surveys increased throughout Mobile Bay between 2003 and 2020. The highest incidence of this was recorded near the city of Daphne and along the western shoreline of the bay.
“…changes in apex predator abundance may have significant ecological and social consequences, altering the structure of the community through predation and risk mediation behavior of prey, while leading to increased human-wildlife interactions,” the scientists noted.
Can this spell trouble for both humans and sharks?
Bull sharks are unique in the shark world in that they can live in three types of water: fresh water of rivers and lakes, the saline water of the oceans and the brackish water of estuaries, where rivers meet the sea.
“Its large size, proclivity for freshwater, abundance and proximity to human populations, particularly in the tropics, makes it more of a potential threat than is either the White shark or the Tiger shark,” according to the Florida Museum of Natural History, University of Florida.
However, the authors noted that the increase in shark numbers had local fishers worried more about their stocks:
Unlike the ‘Jaws Effect’ that has historically plagued shark management initiatives, surveys of recreational fishermen find that the impacts they perceive sharks will have on fishing opportunities are what primarily hinder their willingness to support shark protection and conservation initiatives.
They added that given that bull sharks had been identified as common depredators in the north-central Gulf of Mexico, these changes could hinder stakeholder willingness to support shark conservation and management plans.
It is also unclear how the population may respond to further increases in sea surface temperature.
“Continued monitoring of the local population is imperative for assessing the scope of these potential impacts and when coupled with species distribution modeling, can be used to anticipate changes in distribution due to the impacts of climate change, equipping managers with flexible and proactive means necessary to mediate the impacts of climate change,” they said in the study.
We are a voice to you; you have been a support to us. Together we build journalism that is independent, credible and fearless. You can further help us by making a donation. This will mean a lot for our ability to bring you news, perspectives and analysis from the ground so that we can make change together.
.png)
Comments are moderated and will be published only after the site moderator’s approval. Please use a genuine email ID and provide your name. Selected comments may also be used in the ‘Letters’ section of the Down To Earth print edition.