By Kayleigh Michaelides
Marine conservation student
The Manta Ray:
The giant manta ray (Manta birostris) is the world’s largest ray species with a disc width of up to 7m from wing tip to wing tip (Graham et al. 2012). Despite their large size, mantas are harmless to humans. Manta rays have a broad distribution throughout the world’s tropical and temperate oceans, and little is known about the size of the global population (Marshall et al. 2011). Manta rays aggregate in large groups in areas off the coasts of Mexico, Mozambique, Hawaii, and the Maldives. It is thought they gather in these areas to breed and visit cleaning stations (Graham et al. 2012). Manta rays are long lived, late to mature, and produce 1 to 2 live pups every few years, making them vulnerable to fishing pressure and slow to recover from population threats or disturbances (Marshall et al. 2012). The rate of population decline in the past 75 years in parts of their range may be as high as 80%. Globally, it is suspected that manta rays are declining at an estimated 30%. This population decline has been attributed mainly to both targeted fishing and incidental (by-catch) mortality (Marshall et al. 2012). Currently, M. birostris is listed as “vulnerable” on the International Union for Conservation of Nature’s Red List.
The leading threat to manta rays is fishing, both targeted and incidental. Manta rays are often caught as by-catch in gillnet, purse seine, longline, and trawl fisheries (Marshall et al. 2012). Aside from incidental catch, mantas are also targeted in many areas of the world for their gill rakers, the stiff projections in their gills that filter food particles. The gill rakers are highly valuable in Asian markets, where they are used in traditional Chinese medicine. The value of the international gill raker trade is estimated to be about $11 million per year (Heinrichs et al. 2011). Along with demand for gill rakers, manta rays are also fished for their skin, which is either consumed or made into leather, and their cartilage, which is used as a filler or a cheap substitute for shark cartilage in shark fin soup (White et al. 2006). Other threats manta rays face include boat strikes, marine debris ingestion, and entanglement in mooring lines, anchor lines, or fishing lines which can result in damage to the cephalic fins or head and can impair the animal’s ability to feed and swim (Deakos 2011).
The study entitled Satellite tracking of manta rays highlights challenges to their conservation by Graham et al. (2012) is the first to satellite tag the giant manta ray in the southern Gulf of Mexico near the Yucatan peninsula. The purpose of the study was to gather insight about manta ray movements, environmental preferences, and spatial ecology through the use of real-time satellite tagging. From the data collected by the satellite tags, the authors were able to examine the distribution of the species and how often they occurred or remained in Marine Protected Areas (MPAs) and Mexico’s Exclusive Economic Zone (EEZ). An MPA is an area where natural/cultural resources are given greater protection than the waters surrounding that area (National Marine Protected Areas Center 2010). EEZs are zones which extend 200 square miles seaward beyond the edge of the constituent state’s territorial zone that nations claim special rights over concerning the use of marine resources and fisheries management (Glossary of Environment Statistics 1997). Manta ray occurrence and the duration of time spent in these areas may play a crucial role in the way MPAs and EEZs consider existing management plans and implement new ones in respect to this species.
Because manta rays are a somewhat cryptic species, only 6 individuals were tagged over the 13-day research cruise in the southern Gulf of Mexico near the Yucatan peninsula. Satellite tags were attached by a swimmer located slightly behind and above the animal. At the time of tag attachment, body size of each individual was estimated and sex was determined. Manta rays are filter feeders. In order to get more information on the diets of manta rays, plankton tows, efficient ways to collect plankton in the water, were performed when manta rays were observed filter feeding in surface waters. The satellite tags were programmed to record position and ambient temperature as they transmitted the information continuously when at the sea surface. From the satellite data, movement patterns, speed, distance traveled, and other environmental parameters were determined for each satellite-transmitted position of tagged individuals. Statistical and geographic analyses were conducted in order to assess the data, and the proportion of times the manta ray tags were located in MPAs and Mexico’s EEZ was calculated.
The tagged individuals consisted of 4 adult females, 1 adult male, and 1 juvenile of undetermined sex. The results indicate that the tagged individuals remained in the frontal zones off the Yucatan peninsula and within Mexico’s EEZ for a majority of the time the tags were in place, which ranged from 2 to 64 days. Manta rays moved up to 116 km away from the sites of tag attachment, and 92% of all of the manta ray locations were more than 20 km offshore. However, only 11.5% of all location transmissions occurred within MPAs. A total of 83% of all locations occurred in waters shallower than 50 m deep, and the majority of manta ray locations occurred in waters warmer than 26.1°C. Manta rays were observed filter feeding in both less productive waters during a seasonal spawning event of a prey item and in highly productive waters during a seasonal upwelling event, in which large concentrations of nutrients and zooplankton are brought to surface waters by a combination of wind and current factors. The locations and movement patterns derived from this study also coincided with some of the region’s busiest and most prominent shipping routes. The impact of this spatial overlap of manta locations and shipping routes in the area off the Yucatan remains unknown, but the overlap could be a factor in future management and conservation efforts.
While the results of this study are not comprehensive, they suggest that manta rays have a somewhat strong site fidelity to foraging areas, meaning that mantas return to the same areas to feed, as well as a strong association with frontal zones. However, mantas forage over large spatial areas that are too extensive and far off shore to be included in the existing MPA systems. This information can provide some critical insight into which locations should be of most concern regarding the establishment of new MPAs or more effective management within existing MPAs and EEZs. The data support other studies conducted in Hawaii, Mozambique, the Maldives, and Indonesia that have also suggested strong site fidelity to foraging areas (Marshall et al. 2012). Because the data were collected over the months of July to October of 2010, it is inconclusive as to whether this site fidelity is simply seasonal or if it is influenced by any other factors. It is still unknown whether manta rays undergo long-distance migrations, and studies that examine longer tracking periods of more tagged individuals could provide more conclusive data to inform manta ray management and conservation. As this study provides a glimpse into the spatial ecology of these animals, much more information is needed in order to better understand what is necessary to protect this species from the current threats they face and those that they may face in the future. This novel study is important as it is the first to satellite tag manta rays and examine manta ray movements in relation to shipping lanes, and it may be used as a model for further satellite tagging research of giant manta rays.
Deakos MH, Baker JD, Bejder L (2011) Characteristics of a manta ray Manta alfredi population off Maui, Hawaii and implications for management. Marine Ecology Progress Series 420:245-260 doi: 10.3354/meps09085
Glossary of Environment Statistics (1997) Studies in Methods, Series F, No. 67. Accessed 26 Sept. http://stats.oecd.org/glossary/detail.asp?ID=884
Graham RT, Witt MJ, Castellanos DW, Remolina F, Maxwell S, et al. (2012) Satellite Tracking of Manta Rays Highlights Challenges to Their Conservation. PLoS ONE 7(5): e36834. doi:10.1371/journal.pone.0036834
Heinrichs S, O’Malley M, Medd H, Hilton P (2011) The global threat to Manta and Mobula rays. Manta ray of Hope. Accessed 25 Sept. http://www.mantarayofhope.com/learn/
Marshall A, Bennett MB, Kodja G, Hinojosa-Alvarez S, Galvan-Magana F, Harding M, Stevens G, Kashiwagi T (2011) Manta birostris. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.1. Accessed 27 Sept. www.iucnredlist.org
National Marine Protected Areas Center (2010) MPA definition. US Department of Commerce and National Oceanic and Atmospheric Administration. Accessed 26 Sept. http://www.mpa.gov/aboutmpas/definition/
Wildlife Conservation Society (2012) First satellite tag study for manta rays reveals habits and hidden journeys of ocean giants. ScienceDaily. Accessed 11 Sept. http://www.sciencedaily.com /releases/2012/05/120511122228.htm
White WT, Giles J, Dharmadi, Potter IC (2006) Data on the bycatch and reproductive biology of mobulid rays (Myliobatiformes) in Indonesia. Fisheries Research 82(1- 3): 65-73. doi:10.1016/j.fishres.2006.08.008