Sunday, February 15, 2015

Over the Moon...fish

If you're at all curious about the ocean, and let's face it you're reading this blog so you probably are, you're most like likely aware of the mola mola. Molas are also called sunfish thanks to their penchant for lounging at the surface like beach goers looking for a tan. 

That's fair, this is about what my thighs look like the first 
time I go out in shorts every year.
Courtesy: Sandip Bhattacharya via Flickr

So if there are sunfish out there, are there moonfish? That may sound like the kind of lame joke your uncle might make at a family dinner, but it's actually a valid question. And yes there are a couple of species commonly called moonfish. My favorite of these, and an animal I only discovered recently, is the opah (Lampris spp.). Opah look a little like mola, but they're completely unrelated.

Oooo, shouldn't have gone with the spray-on tan.

This fish is incredible. Believe it or not they're related to oarfish. If you've never seen them before, oarfish are anything but round and squat. In fact the giant oarfish (Regalecus glesne) is the longest of any bony fish, usually growing to 8 meters. It's believed that giant oarfish originated the legends of giant sea serpents. While they're generally pretty different; if you look at opah and the oarfish (dibs on the band name) side by side you can see a bit of a resemblance.

From Field Book of Giant Fishes 1949 courtesy: Biodiversity Heritage Library 

Both fish have those large dorsal (on their back) fins with a large crest in the front. They've also got long pelvic fins extending down from their bodies. It's going to sound incredibly unscientific to describe it this way but they're also both very shiny. The scales of these fishes are so reflective that their group name (Lampriformes) means the shapes of light. That reflectivity is really useful in both fishes' habitat, the open ocean.

Okay, so why are we not talking the sea serpent one? The opah just looks like someone took a belt sander to the edges of a tuna. Its tail is so small compared to the rest of its body it doesn't even look like it would help the opah swim. Ah, you're right imaginary snarky reader! Opah don't use their tail fin to propel themselves through the water. Instead they use their pectoral fins, flapping them up and down like a penguin! They're also massive, the biggest opah ever caught weighed over 300lbs. Yet despite their size and stubby looking fins opah have been recorded speeding away from predators at four meters per second. That's about twice as fast as most people jog.

In order to move all that mass around opah have powerful muscles behind their pectoral fins. The muscles are so burly that they look, and apparently taste, more like beef than fish; even though the rest of the opah's muscles are similar to those of tuna.

"Moo?"
Courtesy: Io [Public domain], via Wikimedia Commons

It's kind of incredible that we have a lot of information about what Opah taste like, and you can find plenty of recipes online, because we hardly know anything about their biology and life history. Opah live in deep water, rarely coming to anything shallower than 50 meters and they don't live in schools. Both those things make them especially hard to study. It doesn't help that during the day when we humans like to be awake, opah spend time at the deepest parts of their range foraging mostly for squid.

Most encounters with opah come from fisher people long-lining for tuna. Long-lining is a fishing technique where several-hundred meter lengths of line are dropped down covered in hundreds of baited hooks. Inevitably something other than the target species gets caught; thinking they're snagging a free meal. This is referred to as bycatch, and many bycatch species aren't commercially valuable, so they're thrown back dead. Fortunately for fishers opahs' tasty meat makes them very valuable and they're able to keep and sell them. You can watch some guys in Hawaii filleting an opah in the video below. Skip ahead to the 2:50 mark to get past the local TV shenanigans, or enjoy the dorky jokes and watch the whole thing.



The thing is we don't know how this catch affects the opah population. Some people think they're safe because there isn't a targeted fishery, and others think we don't know enough to say whether they're being seriously harmed or not. The Monterey Bay Aquarium's seafood watch program, which is well researched, recommends avoiding eating opah from international sources, and only occasionally enjoying those caught in US waters. This is because the US has some of the best fisheries management in the world, and lots is being done to cut down on bycatch.

References:

Lee, Jane, "Rarely Seen Moonfish, Size of Manhole Cover, Caught on Camera." National Geographic: Weird and Wild, February 5th, 2015

McClain et al., "Sizing Ocean Giants: Patterns of Intraspecific Size Variation in Marine Megafauna", PeerJ, 2015, DOI: 10.7717/peerj.715, Accessed via: https://peerj.com/articles/715/

Polovina, Hawn, & Abecassis, "Vertical Movement and Habitat of Opah (Lampris Guttatus) in the Central North Pacific Recorded with Pop-up Archival Tags.", Marine Biology, 2007 DOI: 10.1007/s00227-007-0801-2

"Order Summary for Lampriformes", Fishbase


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