Breaching Basking Sharks Have A Good Side

By Caroline Lynam, Marine Biology Postgraduate, Exeter University Penryn Campus, Cornwall, England & Shark Angel Intern.
Scientific study: High resolution biologging of breaching by the world’s second largest shark species By Jessica Rudd, Marine Biologist University of Exeter, England.

Background

Basking sharks are the second largest fish in the world. Listed as globally endangered on the IUCN Red List after two centuries of exploitation for their oily livers.  Basking sharks can grow up to 12 meters/ 39ft 4” in length and 8818 .5 pounds in weight.  These friendly giants are filter feeders and can be seen feeding on zooplankton at the waters surface. In the past ten years scientists have been researching the movements of these sharks. Basking Sharks occur seasonally throughout the Atlantic where large numbers of these sharks can be found feeding off the west coast of Scotland in the North Eastern Atlantic Ocean during summer. However, there is still a lot we don’t know about these sharks such as their mating behaviours.

Studying These Animals

Advances in Biologging, a technique which uses miniaturized tags attached to the study animal to record data about movements, behaviour, physiology and environment, has led to a better insight into the lives of basking sharks. Many species of sharks, whales and fish have all been recorded demonstrating breaching behaviours. For example, white sharks breach when they are pursuing prey. Breaching is a highly energetic activity as it consists of the animals propelling themselves out of the water. There have been many hypothesized reasons for breaching behaviours such as epiparasite removal, hunting and courtship behaviours. Although, there has been accounts of basking sharks breaching before, this study is the first to record and describe the breaching basking sharks.

Methods

This study followed three Basking sharks for 41 days. These sharks were externally tagged with Daily Diary tags which recorded temperature, time and accelerometery. These tags are essentially Fitbits for sharks. Accelerometery is the study of the change in velocity of an object (or the speed and direction an object is moving in). The sharks were also tagged with wet-dry sensors to help the researchers identify when the sharks had left the water. Once the tags were retrieved the researchers analyzed the data to look for evidence of breaching events. They analyzed depth, tail beat amplitude, speed and ascent angle to describe each breaching event.

Findings & Results

This study recorded a total of 67 breaches made by three sharks over the 41 day period. Half of the breaches were single breaches, 13 breaches were double breaches, 3 were triple breaches and one shark was recorded doing four breaches in a 47 second period. These basking sharks were recorded breaching during both day and night, with 4am being recorded as the peak time for breaching. Compared to other breaching animals, Basking sharks started their breaching from a much shallower depth of approximately 20 meters/ 65ft 7” (compared to White sharks who start from much deeper depths of >100 meters/328ft 1”). Basking sharks also showed evidence of breaching at a greater speed than White sharks, this is incredible as the basking shark is a much larger shark than White sharks.

The researchers also found that Basking sharks may have a preferred side. For 67% of the breaches the basking sharks rolled to the right side. The researchers have hypothesized this could be evidence for lateralisation, which is the preference for breaching on one side consistently across events.

One question still remains, why do Basking sharks breach? The answer is still unknown. Breaching is a very energetically costly activity; Basking sharks must be doing it for a specific reason. The scientists have hypothesized it may be due to acoustic communication associated with mating events. There is little known about the mating behaviours of Basking Sharks. Previous research has shown this usually solitary species occur annually in waters off the west coast of Scotland, for feeding events. Behaviours associated with mating such as close following and breaching have been witnessed in Scottish waters which gives scientists reason to believe that mating events could be occurring.

The breaching basking sharks could be signaling they are ready to mate or could be communicating with other distant basking sharks. There is still a lot more work to be done to uncover the hidden world of basking sharks. However, this study has shown that not only are basking sharks breaching but can breach multiple times and breach at night. This study hypothesized that these incredible breaching events could be associated with mating behaviors.