Up close with Mexico’s fish-eating bats: Interview with researcher José Juan Flores Martínez

The fish-eating bat (Myotis vivesi) catches fish and crustaceans thanks to its long legs, hook-shaped claws and waterproof fur.
The species is found only on islands in Mexico’s Gulf of California; it’s considered endangered under Mexican law.
Invasive species such as cats and rats threaten the bats.
Researcher José Juan Flores Martínez has been studying fish-eating bats for more than 25 years, and discusses his fascination with the species and the threats it faces.

In the early 2000s, José Juan Flores Martínez was studying for a bachelor’s degree in biology and working as a volunteer in a program designed to control invasive rodents on islands in the Gulf of California in northwestern Mexico, which are home to several seabird colonies. On one of those expeditions, his group traveled to Isla Partida Norte near the city of La Paz, where something surprised them: they heard bats. Accustomed to seeing them in caves, Flores Martínez was intrigued when he saw them coming out from between the rocks and making clicking sounds under his feet.

“It surprised me to find out that they were on an island, in the middle of the desert, under extreme conditions: They can resist sub[-freezing] temperatures and heat above 50 degrees [Celsius, or about 122° Fahrenheit],” said Flores Martínez, now an academic technician at the National Autonomous University of Mexico’s (UNAM) Institute of Biology. “But it was much more surprising to me that, when I began to ask questions, I was told that they feed on fish.”

They were fish-eating bats (Myotis vivesi), and they fascinated Flores Martínez. This fascination led him, along with researcher Gerardo Herrera Montalvo, to embark on a scientific journey that has lasted a quarter of a century and counting. The fish-eating bat is the largest bat in its genus, reaching up to 16 centimeters (about 6 inches) in length. Its long, shiny fur is waterproof, which comes in handy as it maneuvers above the surface of the sea.

To date, this mammal — whose long legs, with flat, hook-shaped claws, allow them to catch fish and crustaceans — has been recorded on more than 45 islands in the Gulf of California. Isla Partida Norte is home to the largest known colony, with tens of thousands of individuals, and is the species’ most important island for breeding.

The fish-eating bat’s long, shiny fur is light brown on its back and whitish on its underside. Its face is triangular, with a wide forehead and narrow snout. Image courtesy of Vladimir Rojas.

Mongabay Latam spoke with Flores Martínez about this species, which is endemic to the Gulf of California. Fish-eating bats are categorized as vulnerable on the IUCN Red List and are considered endangered under Mexican law due to their restricted range and the impacts caused by the introduction of nonnative species into their habitat.

Biologist José Juan Flores Martínez has dedicated more than 25 years to studying the fish-eating bat on islands in the Gulf of California. Image courtesy of Maximiliano López Romero.

Mongabay: What biological — or behavioral — characteristics make Myotis vivesi so special among bats?

José Juan Flores Martínez: I believe that there are many species that are unique, each one in its own region and with its own characteristics, both physical and behavioral or ecological, but Myotis vivesi is an endemic species and is endangered. It is a relatively large bat, with some enormous legs and claws. It is a species that, if it drops into the sea, it can go back to flying a moment later. We did some experiments in that respect with another researcher: It is due to its fur, which is not very silky, but is waterproof.

Its wings are very large, and so is its uropatagium [the membrane between its legs]. The latter acts as a pouch to store the fish or crustaceans that it catches.

Also, it lives underneath the rocks. The area’s climate is complex, and [the rocks] function thermally for them. It’s so interesting. Another thing is that they practically do not drink freshwater. In other words, their kidneys are unique and they are adapted because, when they catch their food, they also ingest water from the sea. With respect to the exchange of nutrients on the islands, we can see that these bats function almost like marine birds: all of their guano is part of the formation of rocks and mounds.

Lastly, these bats are indicators of conserved areas. If we find Myotis vivesi, it is because the area is well-conserved. There are islands in the Gulf of California where there are cats and rodents — invasive exotic species — and there are no bats there, because they would prey on them. They are not the same as a natural predator, such as seagulls or owls. Because of all this, the bat is unique to these islands, due to all these taxonomical, physiological and conservation-related characteristics that we see in them there.

The fish-eating bat is piscivorous. It feeds on small fish and some crustaceans it catches directly from the sea using its long claws and uropatagium, which acts as a pouch to carry the bat’s food back to its island. Image from “The Illustrated Guide to the Bats of the Islands of the Gulf of California” (2021) courtesy of José Juan Flores Martínez.

Mongabay: When did you officially begin to work with this species?

José Juan Flores Martínez: After seeing them for the first time, I was hooked. When we returned to the Institute of Biology, I asked Dr. Bernardo Villa about the bat and he told me that nobody was studying it. He told me that there was a thesis, from approximately 1960, which studied the bat’s biology and a little bit of its ecology. That was what made me interested in Myotis vivesi, because there were no studies in Mexico; [there were] only some by U.S. researchers.

I was finishing my bachelor’s degree with another bat in Yucatán: Artibeus jamaicensis [the Jamaican fruit bat]. I started to look for publications and information about Myotis vivesi, planning to do a master’s program. Around that time, I began looking for a graduate adviser, and I came across Dr. Gerardo Herrera Montalvo, who was arriving at the institute as a researcher, and he did not know about this bat, either.

We contacted Dr. Rodrigo Medellín, from the Institute of Ecology at UNAM, and he put us in contact with Bat Conservation International. They were interested in the topic, and they gave me a scholarship to begin the work on the islands. That was where the adventure began with Gerardo Herrera Montalvo, [and together we have spent] almost 26 years working with the species.

Next, we started to look for resources at the Mexican National Council for Science and Technology [CONACYT]. At the time, there was an agreement with the University of California, and they gave us very generous support, which we used to search for the bat at geographic points based on historical data.

The fish-eating bat has long legs with flat, hook-shaped claws, which allow it to catch fish and crustaceans. Image courtesy of José Juan Flores Martínez.

We confirmed the bat’s presence on some islands and found it on other islands where it had not been reported. But our “natural laboratory” has always been Isla Partida, because the females and their young are there, while the males spread out to other islands. We did a study [focused on] counting the bats using quadrants and estimated a population of 80,000 bats, give or take, on Isla Partida alone.

Obviously, it’s not just the two of us. Other students have joined us to study its biology, reproduction, ecology, physiology, genetics and geography, in collaboration with people from all different parts of the world.

Very interesting projects came about. For example, [one project used] basic telemetry with antennas, during which we saw that bats fly much farther than we thought. Then came another collaboration with another student from the United States, with a grant from National Geographic to study bats’ flight [habits]. They used GPS to see how far they actually fly, and we learned that they fly more than 30 to 40 kilometers [about 19 to 25 miles] per night to feed.

Mongabay: What is the conservation status of Isla Partida? What problems are caused by the presence of invasive exotic animals on this and other islands in the Gulf of California?

José Juan Flores Martínez: We went to Isla Partida last year, and fortunately, there were no cats or rodents. The characteristics of [the island’s] slopes, which are rocks or slabs that are stacked, allow these bats to have their hideouts.

The bats chose Isla Partida because they can have their young there. Specifically, the island gives them shelter and, because of the surrounding marine currents, they always have food. I’m not saying that the other islands do not have these characteristics, because we did see young [bats] on some [other islands], but the number of bats is much lower than on Isla Partida.

Isla Partida Norte is home to the fish-eating bat’s largest maternity colony. Image courtesy of José Juan Flores Martínez.

On other islands, cats and rodents have been reported. On some small islands, [the bats] have been eliminated or are no longer there, but invasive exotic animals are not only a problem for these bats, but for the islands’ wildlife in general.

The National Commission of Protected Natural Areas [CONANP] has programs to mitigate and control the island’s exotic species that affect birds, reptiles and other species. We do not know how much harm they have caused for bats in particular, but there is damage to wildlife as a whole.

The way I see it, the species from these islands are sensitive and fragile. Anything — from climate change to anthropogenic factors to the presence of invasive exotic animals — can cause a lot of harm. And this has already happened: There have already been extinctions due to cats on islands in the gulf.

Biologist José Juan Flores Martínez on Isla Partida Norte in the Gulf of California. Image courtesy of Maximiliano López Romero.

Mongabay: In terms of the conservation of this species, what role do the islands in the Gulf of California play?

José Juan Flores Martínez: The islands are vital for bats. I was trying to find colonies on the coast of Sonora and on the coast of the peninsula, and I didn’t find any.

I believe that the bat has adapted to be in these conditions, and the islands provide them these shelters underneath the rocks. The islands are a vital refuge for this species, and they are similar to how tropical rainforests are for fruit-eating bats: they give them [the proper] humidity, temperature and food.

Myotis vivesi is endemic to the Gulf of California region. Image courtesy of José Juan Flores Martínez.

Mongabay: What can be done to reduce the risks caused by anthropogenic factors?

José Juan Flores Martínez: In published articles, and in the meetings we have had with CONANP in Sonora and in Bahía de los Ángeles, we’ve said that it is important to try to monitor the distribution of this bat on those islands, in terms of anthropogenic factors.

When boaters arrive at Isla Partida, we explain to them how sensitive [the island] is. If boaters do not have adequate information, it isn’t their fault; they go there to work and they need places to stay.

When we used to tell them, “Look, there’s a bat!” They would hear it, they’d be astonished, and they would later say “We have to take care of it. Tell us where we can camp and how far we can go.” I think that is very important.

Mongabay: Are researchers conducting any new studies of the species, or planning to in the future?

José Juan Flores Martínez: Yes. Although several projects have been done with the species, you always find something new. Ever since the COVID-19 pandemic began, and [interest increased in] the transmission of diseases between wildlife and humans, the idea is to assess the conservation status of Myotis vivesi. [The idea] is to see whether they can have an illness that we could contract, and vice versa: whether we could transmit diseases to them. The topic of diseases originating in bats is a current project that we are working on now.

The tail of Myotis vivesi is completely covered by the uropatagium and is longer than its legs. Image courtesy of Vladimir Rojas.

Mongabay: Finally, what do you anticipate for the fish-eating bat in the coming years? What actions or policies do you think are most urgent to ensure its survival?

José Juan Flores Martínez: I think we can expect positive things for the fish-eating bat. First of all, [we will] continue to study it and keep encouraging the conservation of the islands. If we promote the conservation of the islands, not only will we conserve Myotis vivesi, but the entire ecosystem, too.

It is important to maintain communication with CONANP; they are the ones who protect the islands in the Gulf of California. [It is also important to] continue to encourage conservation [and] scientific outreach in the region and, in some way, to reach fishers to tell them that the islands are extremely important in terms of flora and fauna, and not only for this bat, but also for birds, rodents, amphibians and reptiles.

The fish-eating bat is so charismatic and important in this sense that there are always new ideas or studies surrounding it. I believe [the bat] is like a “hook” for many academics and for science.

Myotis vivesi shelters in crevices and holes along the rocky cliffs, often cohabiting with petrels. Image courtesy of José Juan Flores Martínez.

Fortunately, for us Mexicans — and I’m saying this with great pride — this species is symbolic in the Gulf of California, which is known as the “Aquarium of the World.” Myotis vivesi makes up part of that aquarium because it feeds on marine resources, although it lives on land on the islands.

The idea is to continue with a lot of collaboration, because the two of us, Gerardo and I, honestly could not do it ourselves. I say this in plural, because there are students and collaborators [from Mexico and from other countries] who are helping in the study of Myotis vivesi.

I’ve been talking about my experience, but in reality, it is a wide-reaching national and international collaboration, and that is important to highlight, because everyone has — or has had — a very important role.

This story was first published here in Spanish on Nov. 26, 2025.

Banner image: The fish-eating bat has been recorded on more than 45 islands in the Gulf of California. Isla Partida Norte is home to the largest known colony of the species. Image courtesy of José Juan Flores Martínez.

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