Time to Move? Sharks and Environmental Drivers

Category: General Article

Back to Tracking Sharks on the Great Barrier Reef

Date: Thursday, August 31, 2017

Submitted by: Dr. Michelle Heupel, Senior Research Scientist and Team Leader for Ecological and Biological Monitoring, Australian Institute of Marine Science.

How animals respond to their environment is a key question, but one that can be difficult to answer.

To try to understand this, we must record the conditions where the animal is living as well as what it is doing. These studies aim to understand whether animals change their behavior when conditions change. There is currently a lot of evidence of animals changing their behavior based on the environment, such as seasonal migrations where animals move from colder to warmer waters. Like whales, sharks show these kinds of migrations. For example, blacktip sharks on the east coast of the US are known to move south toward the Florida Keys during winter months. This includes individuals less than one year old. Acoustic tracking of young blacktip sharks in a nursery area on the Gulf of Mexico coast of Florida revealed that young sharks left their nursery as winter cold fronts approached and were not present when water temperatures were below 70 degrees. The disappearance of sharks from the area has been known for many years and assumed to be related to water temperature, but tracking data revealed the direct link between shark presence and water temperature. 

While seasonal changes in behavior and movement are reasonably easy to identify and predict, sudden changes based on storms or other events are much more difficult to study. Coastal areas are most affected by storm events and after years of tracking sharks in Florida, we have gained an understanding of how some sharks respond to sudden changes in their environment. The first example is a storm event causing high rainfall and flooding. High rainfall can change the salinity, or saltiness, of the water. Most sharks cannot tolerate freshwater or river conditions, so if salinity declines too low the sharks cannot survive. So what do they do? In the case of bonnethead sharks (a small hammerhead), they move away from freshwater to stay in saltier conditions. While tracking bonnethead sharks in Florida, we saw exactly this response with sharks moving away from a river mouth to avoid freshwater flooding conditions. Interestingly, sharks did not stay in their new location. When the flood finished and the water returned to marine conditions, the bonnethead sharks moved back to where they were before the flood. This result not only tells us how these sharks responded to this sudden change in their environment, it also tells us they know where their home area is and will return to it when conditions normalize.

A more extreme example of response to change was recorded in blacktip sharks in Florida. In this case, tracking data revealed that young sharks sensed the approach of a tropical storm and moved out their home area to avoid it. Tracking data showed all of the monitored sharks left their nursery area at least seven hours before Tropical Storm Gabrielle made landfall approximately 30 miles to the south. Analysis of conditions in the study site indicate that sharks were responding to changes in air pressure (barometric pressure) as the storm approached. This suggests sharks are highly in tune with their environment and will move to avoid potentially dangerous conditions such as strong storm waves, increased freshwater input, etc. Like the bonnetheads, all tracked blacktip sharks returned to the nursery after the storm passed and conditions returned to normal. Because these animals were all newborns at the time of the storm, we can conclude that this move was instinct rather than a learned behavior. 

These examples, and my studies on the Great Barrier Reef, show that sharks can have specific conditions that they prefer and will move if local conditions change. The application of acoustic tracking has given us an opportunity to better understand how well sharks understand their environment and how they will respond to changes. However, in most cases sharks returned to their home site after the disturbance and once conditions return to normal which suggests we need to make sure these habitats are maintained and protected. 



A Cooperative Program

This virtual exploration is made possible through the kind cooperation of the Australian Institute of Marine Science. To learn more about the institute's work, visit them online at www.aims.gov.au.

A Note About Dates and Times

Due to time zone differences between the United States and Australia, please be aware that responses to any online questions to Dr. Heupel may be delayed by several hours or more. Thank you for your patience and understanding.