Scientists Encounter The Greatest Gathering Of Whale Sharks Ever Recorded

By McKenna Hardie, B.S. Graduate Student, Florida Institute of Technology & Shark Angels Intern
Scientific study: An Unprecedented Aggregation of Whale Sharks, Rhincodon typus, in Mexican Coastal Waters of the Caribbean Sea by Dr. Dove Alistair, Marine Biologist

Background

The whale shark, Rhincodon typus, is a solitary animal that will occasionally come together in large clusters to feed in coastal waters. The clustering of many individuals is typically referred to as an aggregation. Feeding activity usually takes place on the surface using a variety of filter feeding behaviors such as vertical, active, and passive methods. Whale sharks often correspond their movements with productivity events in order to maximize how many opportunities they get to feed since they are such large animals. Productivity events such as fish spawning give whale sharks the opportunity to feed on tiny organisms like plankton, small fish, and small shrimp. Productivity events provide a big source of food for a wide range of organisms like whale sharks. Aggregations of whale sharks are common in the Cabo Catoche, on the Yucatán Peninsula of Mexico due to the seasonal high productivity of crustacean zooplankton in that area. Fishermen and those involved in the ecotourism industry reported another large aggregation event taking place on the tip of the Yucatán Peninsula even farther offshore than the Cabo Catoche. The new location, known as the “Afuera”, neighbors northeast of Cabo Catoche and is close to Isla Mujeres. As the Afuera attracted more whale sharks, the Cabo Catoche area seemed to be decreasing in the amount of whale sharks present. The Afuera appeared to have much less productivity indicated by the bright blue water in the area. The green water of the Cabo Catoche indicated it was abundant with crustacean zooplankton (high productivity). Researchers were interested in what could have attracted whale sharks to this new aggregation site considering it appeared to have significantly less productivity than the Cabo Catoche.

Methods

The research team used an aircraft to perform aerial surveys to photograph and observe the aggregation of whale sharks in the Cabo Catoche and the Afuera. The aircraft was flown on a zigzag flight pattern using fixed GPS coordinates to make sure the area was completely covered. Occasionally, the aggregation at The Afuera was so large, that the zigzag pattern was altered to a tight circular pattern in order to record all of the individuals. The team on the aircraft consisted of: two observers/spotters, one camera person, and one author. Throughout the study, a total of 61 flights occurred which all took place in the morning before mid-day (when the whale sharks typically stopped feeding). The researchers conducted flights over the Afuera 34 times (averaging 67.5 whale sharks per flight) These flights totaled 105 hours of transect time, with an average of 57 minutes of transect time per flight. A transect is a fixed path that observers follow when observing a species of interest. Along the transect, observers counted the number of observed whale sharks, took aerial photographs, and recorded the latitude and longitude during each flight.

Researchers also collected environmental data, whale shark behavior, whale shark factors data (such as size, sex, and wounds), and conducted plankton tows while on a personal boat. Plankton tows are a great tool to use in order to collect small organisms like plankton and determine their abundance in the area. The Afuera was so dense with fish eggs that the plankton tow time had to be lessened to prevent the net from clogging. By using mitochondrial DNA barcoding, researchers were able to identify which species the fish eggs in The Afuera belonged to. This is a method used in genetics that matches an organism to its species by using unique markers in the organism’s DNA. From the boat, sharks were tagged on their dorsal fin using a pole spear and stainless steel-headed dart tags. The sharks were tagged in order to track their movements during the study.

Findings & Results

A total of 2,295 whale sharks were recorded during the entirety of this study from May of 2005 to September of 2009. In 2006, five aerial surveys were conducted and resulted in a total of 480 sightings. In 2007, there were very few individuals identified which could’ve been the result of the observers missing the aggregation or failure of the aggregation to come about in the Afuera. During 2008, the sightings of whale sharks were more consistent. This included a high peak in the number of whale sharks occurring in August. The largest whale shark aggregation event ever recorded occurred in 2009 with a total of 420 whale sharks observed during one flight. The second highest number of whale sharks recorded on a single flight in 2009 was 389 individuals. In total, 81 sharks were tagged from both the Cabo Catoche and The Afuera. Of the 81 tagged sharks, only one was re-sighted at the Cabo Catoche. Three sharks that were tagged at Cabo Catoche were not sighted at The Afuera. The fourteen sharks tagged at Cabo Catoche in 2009 were re-sighted at The Afuera. Overall, not a lot of movement between the two sites was seen based on the whale sharks that were tagged. Of the tagged whale sharks, 57 were male, 20 were female, and 3 were undetermined. The sex ratio at the Cabo Catoche was extremely similar. A significant number of whale sharks observed in both areas were young (juvenile) males. DNA barcoding was used to identify which species produced the fish eggs in The Afuera. The results were clear that a majority of the fish eggs were produced from little tunny, Euthynnus alletteratus, which are a type of tuna that usually spawn in the Mediterranean between May and September.

At Cabo Catoche and the Afuera, deep nutrient-rich water is being brought up to the surface from a process called upwelling. Upwelling creates an area that is high in productivity which attracts a wide array of marine life such as manta rays, cownose rays, devil rays, sea turtles, and whale sharks. At first, researchers were confused as to why whale sharks were aggregating to the blue water of The Afuera, a seemingly less productive area when compared to the rich, green water of Cabo Catoche. However, the fish eggs that are present in the Afuera are translucent and cannot be seen in the water, leaving the water color blue. Both sites are high in productivity, however, evidence from the study suggests that the Afuera site had a much denser food source available. This evidence includes: the amount of eggs retrieved from plankton tows, the high number of sharks feeding at any one time, and the duration of the aggregation. The largest aggregation of whale sharks (2009) may have been due to little tunny having an exceptional year for spawning. Another possibility is that more whale sharks may have been drawn away from other sites in previous years possibly due to changes in quantity and quality of that prior food source. Little tunny was not previously known to spawn in areas outside of the Yucatán Peninsula. However, data collected on little tunny from this study are similar to data collected in other little tunny spawning grounds, specifically the Mediterranean. This data includes: the time of the year, prevailing conditions, and duration of spawning events collected for. The 2009 whale shark increase at The Afuera coincides with the decrease of whale sharks at Cabo Catoche. This suggests that the whale sharks from Cabo Catoche may have relocated to the Afuera, an area with a denser production of food. However, the sharks that were tagged show very little movement of individuals between the two sites in 2009. New studies suggests that the whale sharks that aggregate to the Afuera likely travel along the Mesoamerican reef from Belize and Honduras, and proceed to follow the Yucatán current into the Gulf of Mexico. Not all whale sharks stick to this path, some whale sharks will travel to the Afuera from the open ocean in the Atlantic. 

Conclusions

After this study was performed, the group of researchers strongly advised that conservation efforts should be enforced upon the Yucatán Peninsula due to the high biodiversity and productivity in this area. There are two major feeding grounds that are within the Yucatán Peninsula that are quite possibly subject to negative consequences from tourism. Cancun among many other tropical tourist destinations are very close to the Yucatán Peninsula. This study found that the Afuera is a consistent spot where little tunny among other fish go to spawn, resulting in the area attracting a wide array of marine organisms looking for food. The Whale Shark Biosphere Reserve (WSBR) was made following this study but did not include the Afuera. Later efforts proved successful; the Afuera is now within the largest Marine Protected Area (MPA) in Mexico, the Mexican Caribbean Biosphere Reserve (MCBR). Efforts from the Georgia Aquarium, Mote Marine Laboratory, and CONANP (Mexico’s Natural Protected Areas Commission) led to the formation of this MPA in 2009 which is currently administered and monitored by CONANP. The Navy is in charge of enforcing protection of this MPA. The Mexican Caribbean Biosphere Reserve no longer supports petroleum exploitation, cargo traffic, cruise ships, and inexperienced divers who could possibly cause damage to the reef in this area. It is with high hopes that this new MPA can promote safety and protection over the whale sharks and many other species that travel long distances in order to mate and feed in this area.