THE OCEAN DEEP

Mobula is a genus of ray in the family Myliobatidae (eagle rays). Their appearance is similar to that of Manta rays, which are in the same family. The Devil fish can attain a disc width of up to 5.2 meters (17 feet) and probably can weigh over a ton, second only to the Manta in size. Despite their size, this genus is quite little-known. Mobula rays in the Gulf of California (Sea of Cortez) have been reported to breach as high as 2 metres above the sea,. Although manta rays (Family Myliobatidae, Subfamily Mobulinae) are fascinating and beautiful fishes, almost nothing is known about basic aspects of their ecology, population biology, movement patterns, and migrations. Available information is typically anecdotal or based on dead specimens providing little insight into the biology of living manta rays. This lack of information on natural history and behavior is critical in light of their very low fecundity (~1 pup per year) and resulting vulnerability to over-exploitation.

Mantas are the subject of an intensive artesanal fishery in the Gulf of California and they are a common component of bycatch in the tuna purse seine fishery and high–seas gill net fishery. Anecdotal reports suggest that populations in the Gulf of California have declined dramatically over the last 20 years. Unfortunately it is only in the last three years that efforts have been made to collect data on catch statistics. As a result, it is almost impossible to track long-term changes in the population. In addition, lack of understanding of the basic ecology precludes the development of a realistic management program. The goal of our project is to initiate a collaborative research program focussing on the reproduction, ecology, population structure, migration patterns, and fishing mortality of mantas in the Gulf of California. These data will provide essential informationfor the development of management and conservation plans. Unfortunately,fisheries managers are generally ignorantof (or choose to ignore) the vulnerable nature of species with low fecundity and high adult survival such as mantas.
Five of the ten species of manta ray occur in the Gulf of California (Mobula japanica, M. munkiana, M. thurstoni, M. tarapacana and Manta birostris). Few data exist for these or any other manta species. For this study our efforts will focus on M. munkiana and M. japonica. Both species appear in the Gulf of California seasonally and their occurrence seems to coincide with the large influx of warm water apparent each spring. M. japanica attains a larger maximum size (3.1 m maximum wing span), may swim alone or in small groups and is often seen and caught in surface waters. M. munkiana forms schools and is generally caught near the bottom. This is the smallest species in the Gulf of California with a maximum wingspan of 1.1m. In one of the few studies conducted on Mobula spp.in the Gulf of California, Notarbartolo di Sciara examined 262 Mobula spp. between 1981 and 1984 at various fish camps. His surveys indicated that M. japanica feed primarily on one species of krill, Nyctiphanes simplex. M. munkiana, in contrast, feeds primarily on mysid shrimp, suggesting that they prefer a neritic habitat. While this study yields a coarse picture of the biology of these rays in the Gulf, details of movements, behaviors, and habitat use cannot be resolved. In addition, Nortarbartolo di Sciara’s data contrast markedly with those we collected at a fish camp in June 2000. In one day we examined over 130 mobuline rays of which over 75% were M. munkiana (compared to 9% reported by Notarbartolo-di-Sciara, 1988). A cursory examination of stomach contents indicated that M. munkiana was feeding primarily on N. simplex. More information is required to understand the ecological niches and interrelationships of these rays.
With funding from the National Geographic Society and UC Mexus, we have developed a novel approach to tag and track mantas, and examine their foraging behavior and movement patterns within an ecological context. Working closely with local fishermen we are now able to reliably capture manta rays, deploy instruments, and sample environmental variables including primary production, water temperature structure, and prey distribution. Croll has used a similar approach to understand the foraging ecology of baleen whales.
The studies to date in the Gulf of California have determined that seasonal pulses in krill abundance occur in the spring. These pulses are an important resource for a number of large planktivores, including blue and fin whales. Dense krill swarms are generally found down current of upwelling sites in areas with steep bathymetric features. Whales feed intensively on these swarms aggregated between 150 and 250 m during the day. At night, krill disperse near the surface to graze, and whales appear to quit foraging. Our deployment of tags on foraging M. japanica indicated that, in contrast to the whales, mantas forage on krill while they are at the surface at night (Figure 1A). Interestingly, tagged mantas did not always remain in areas where krill density was high and traveled up to 30 miles in 24 hours (Figure 1B). While these studies have laid the foundation for understanding the relationship between foraging mantas and their prey, more detailed information is needed. Determination of critical locations and densities of krill where mantas feed provides a powerful tool for predicting their distribution and abundance. Such information is essential for effective fishery management.
Foraging ecology and habitat use is just one part of the information needed for effective management. To reliably ascertain the impacts of fisheries on Mobula populations in the Gulf of California, information on population structure, reproductive biology, survival, and fishing mortality is needed. Information on movements outside the Gulf must be also determined. We have received funding from the UC-MEXUS program for a collaborative study with Dr. Felipe Galvan (Centro Interdisciplinario de Ciencias Marinas, La Paz, Baja California Sur, Mexico) and Dr. Oscar Sosa (Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California, México) to survey fish camps that target mantas to acquire this information. Together, this information will lay the foundation for the development of a fishery management plan for Mobula mantas.