Article: McCauley, D., et al. (2014). “Reliance of mobile species on sensitive habitats: a case study of manta rays (Manta alfredi) and lagoons.” Marine Biology: 1-12.
It is typically thought that anthropogenic change (or human induced change) is going to have the biggest impact on sessile, or non-moving, species; after all, they can’t just pick up and leave if their habitat becomes unsuitable. However, researchers are starting to recognize that highly mobile species may be just as susceptible to climate and anthropogenic change. For example, animals traveling between habitats have an increased probability of encountering areas that are affected by change.
Our oceanic ecosystems contain many highly mobile, nomadic species like whales, tuna, sharks, and manta rays. These organisms are constantly patrolling the ocean but often take pit stops at islands and atolls to rest, feed, mate, and escape predation. This behavior is well documented in many marine species, but we have yet to quantify just how important these habitats are.
Manta rays are large elasmobranchs, or cartilaginous fish, like sharks. The reef manta ray, Manta alfredi, can have a wingspan (fin-span) of up to 18 feet (Figs 1 and 2). Despite their large size, they glide gracefully through the water, filter-feeding for small prey like zooplankton and krill. This species of manta ray has been observed in a variety of habitats and is known to travel distances greater than 100 km. Manta alfredi is known to use open ocean habitat as well as atolls and lagoons. As a result, researchers used this species in a case study to determine the reliance of mobile organisms on lagoon habitats.
In the middle of the Pacific Ocean is the Palmyra Atoll (Fig 3 and 4). This atoll’s lagoons and surrounding waters have been protected as a US National Wildlife Refuge since 2001. This site was chosen by researchers to investigate how Manta alfredi utilize these habitats. The lagoons provide refuge, food, and a place for juveniles to grow. In order to assess how important the lagoons are to mobile species, the following questions were proposed and a variety of techniques were used to answer them:
1. What percent of a manta ray’s diet is made up of food from the lagoon?
Stable isotope analysis was used to answer this question. Food from different locations will have varying isotopic ratios (variants of chemical elements). Using muscle tissue from rays, researchers analyzed the isotope ratios and were able to determine where the rays’ food came from.
2. How do manta rays use space within the lagoons?
Manta rays were tagged and tracked with acoustic transmitters. This allowed researchers to determine where in the water column the rays were and provide insight as to how they utilize lagoon space.
3. How do manta rays move between the lagoon and the open ocean?
Using sonar, researchers were able to track the movement of individuals and large populations moving between the two habitat types.
4. What are the population characteristics of manta rays using the lagoons?
A combination of photography and lasers allowed researchers to gather images of individual manta rays, determine the sex, and determine the size. Lasers were used calibrate photographs.
It was found that lagoons provide about 80% of a manta ray’s diet, compared to only 20% from the open ocean (Fig 4). Acoustic tracking of rays indicated that individuals spent most of their time in the upper 10m of the water but would migrate up and down throughout the day. It was found that rays typically utilized a core space (or where they spend the majority of their time) of 0.02 km2 – 0.32km2. Manta rays were found in the main channel between the open ocean and the lagoons regardless of the time of day or the tide, suggesting that there is always movement between the two habitats. Finally, photographic methods were able to detect individuals, their size, and their sex (Fig 5). As a result, researchers were able to tell how often new rays entered the lagoon and how often rays returned. There were a large number of individuals that returned to the same location.
It has been known for some time that mobile species use these habitats for shelter and food, but the actual importance and utilization had yet to be quantified. This study has begun to piece this together. By showing that a huge proportion of manta ray diet comes from this habitat and the documentation of large numbers of returning individuals indicates that these habitats are not just a place to stop; they are vital habitats for mobile species that can act as a home base.
Atolls and islands are highly vulnerable to human impacts. These sites can be heavily fished and are exposed to large amounts of ship traffic. By quantifying the utilization of these habitats by mobile species like manta rays, we now know just how important they are. Studies like this will hopefully lead to increased protection of these habitats and further monitoring of movement behaviors. Understanding where mobile species travel and how they utilize different habitats will allow for a more complete conservation efforts. If these sites become altered by humans or climate change they will likely lose the services they provide mobile species with, and in such vast oceans, the effects of losing these habitats would be catastrophic.