Conservation & Science

Keeping up with ocean sunfish

Mola mola are peculiar fish. Shaped like enormous shovels, they can grow to almost 10 feet long. They live throughout the global ocean and sometimes float languidly on their sides at the water’s surface. As charming as they are bizarre, they’re frequent, though temporary, visitors to the living collection at Monterey Bay Aquarium.

Ocean sunfish, Mola mola, on display at the Aquarium.

Because they get so large and grow so fast, molas, also known as ocean sunfish, can’t be kept permanently in the Open Sea exhibit. Senior Aquarist Michael J. Howard and his team collect molas in Monterey Bay, but return them to the wild once they reach about 6.5 feet long.

Monterey Bay Aquarium is, to our knowledge, the only public aquarium in the world that returns exhibit molas to the wild. Until recently, no one knew what became of the individuals after their release.

With the help of colleagues and electronic tracking tags, Michael is starting to get some answers, adding important data to a sparse body of knowledge about the mola’s life history and habits.

Long-term tagging program

Michael has been tagging wild molas in Monterey Bay since 2008. The next year, he started tagging molas captured, and later returned to the wild, by the Aquarium. Now, he’s comparing how released molas behave as compared with never-captured molas.

“Since we have a growing data set on behaviors for wild fish, let’s make sure when we release our exhibit animals, they’re returning to natural behaviors,” he says.

Staff transport an ocean sunfish (Mola mola) into the Open Sea exhibit.

The preliminary results: So far, so good. A review of the data from the molas he’s released from the Open Sea exhibit suggests they’re behaving similarly to their never-captured counterparts.  They tend to chase warmer waters and frequently head south after their release.

The molas carry tags that are programmed to record light levels, water temperature and depth, giving Michael data on where they travel, the temperatures they encounter and their diving behavior. But neither he nor other researchers who study molas extensively in the wild know when or where molas spawn, or even how long they live. To help fill one knowledge gap, the Aquarium team is developing a data-based approach to estimating mola ages.

Many, many baby molas

When molas reproduce, they probably produce a lot of offspring, says Tierney Thys, a marine biologist and ocean sunfish expert who frequently works with the Aquarium.

“They’re the heaviest bony fish, the most fecund, the vertebrate growth champions of the world,” she says. “From hatching size to full adult weight, they put on 60 million times their starting weight, which is monumental!”

Husbandry staff work behind the scenes to tag and release a large ocean sunfish.

Molas have other peculiarities. They’re known for laying on their sides close to the surface of the water—sunning themselves, as their moniker would suggest—primarily to regulate their body temperature or to solicit parasite pickers such as gulls.

“For years and years and years, scientists and fisherfolk thought they were lazy animals that didn’t do much,” Michael says. “But tagging studies like ours and others demonstrate that these fish actually are prolific divers, spending a lot of time searching for prey. They’re diving to deep depths where temperatures can be very cold. They’re actually pretty amazing animals.”

Recouping lost energy

Those cold-water dives cost the cold-blooded fish body heat and energy. This may help explain their sunning behavior at the surface, Michael says.

“Instead of swimming around, they rest at the surface to minimize energy expenditure, and utilize the warmth of the sun’s heat to help them recover their core body temperature,” he says.

Their dietary preferences also fascinate Michael.

Molas eat rich foods like squid when they’re young, then switch to lean jellies as adults.

When they’re young, molas presumably consume rich, energy-dense foods like squid. As they get larger, they shift to food that’s relatively low in calories: gelatinous zooplankton like jellies and siphonophores.

“When they’re young, they are small and vulnerable. Eating calorie-rich prey helps them grow quickly,” Michael says. “Once they mature, their body form—like whales and whale sharks, for example—takes less energy to move through the water. So they can rely on generally underutilized, nutrient-poor prey items like jellies and their kin.”

This poses a challenge for the Aquarium’s husbandry staff, who aim to feed molas enough to stay healthy on exhibit—but not grow too rapidly or become overweight.

“The greatest challenge is to provide enough food each day, about 1-3 percent of their body weight, while keeping the calorie count low,” Michael explains.

Recipe for success

The secret recipe for success: a homemade gelatin. It’s about 90 percent water, to simulate the composition of most gelatinous zooplankton, and enriched with vitamins and minerals.

No new molas are being released from the exhibit right now, so Michael and his team are busy analyzing the data coming in from several active satellite tags. Based on past results, they’ve learned that some wild molas travel from the Monterey Bay to Baja California in about 180 days. Michael says molas released from the Aquarium sometimes do the same.

“Folks all over the world have done tagging programs with molas, but nowhere else has anyone tagged molas that have been held in aquariums,” he says. “I’m looking forward to digging into the data and teasing out the story.”

— Cynthia McKelvey

Learn more about ocean sunfish at the Monterey Bay Aquarium.

2 thoughts on “Keeping up with ocean sunfish”

  1. as a Surf Fisherman I’ve seen them about 18 inches long or so washed up dead on the beaches ocassionally. I actually had one take a chunk out of the back of my leg above the ankle in shallow surf water (about 12 inches deep) when I was fishing last year. Stung like hell and he took a good bite out of me. Not large but right on the Achilles tendon, LOL> He did get away.

    Liked by 1 person

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