If you’ve ever stood in a seafood market holding a plump, silver moonfish steak—firm, buttery, and with a richness that sets it apart from more common commercial catches—chances are you’ve encountered a fish that’s far more complex than its retail presentation suggests. For the past seven years, I’ve run a small moonfish supply business, working directly with longline fishermen in the central Pacific who target these deep-ocean marvels. What I’ve learned isn’t just how to harvest them sustainably, but how they interact with other species in the mesopelagic zone—interactions that shape not just their own lives, but the delicate balance of the ocean they call home. Moonfish
Most people know moonfish (genus Lampris, though the term often refers specifically to the opah, Lampris guttatus) as a solitary, deep-dwelling species. But over hundreds of sea trips, I’ve watched fishermen bring in catches that reveal far more: moonfish are regular, though often unassuming, participants in a network of relationships that stretch from the sunlit surface to 1,000 meters below. One of the most striking interactions I’ve observed is their association with large, pelagic fish like yellowfin tuna and swordfish. Fishermen report that moonfish will frequently swim alongside these larger predators, a behavior scientists call “facultative commensalism” (one species benefits, the other is unharmed and unhelped).
I’ve asked the captains I work with why they think moonfish stick to tuna and swordfish. Theories range from the simple: larger predators stir up small prey as they move, making it easier for moonfish to snap up scraps. There’s also evidence that the deep ocean is a cold place, and unlike most fish, moonfish are regionally endothermic—they can warm their brains, eyes, and muscles to stay active even in 4°C water. Swimming near larger, warmer-bodied fish might help them conserve energy, though that’s still unconfirmed. What I know for sure is that this association creates a side benefit for our supply chain: when we target tuna, we often catch moonfish as bycatch, and we’ve worked with fishermen to refine our methods to minimize accidental harm, turning a byproduct into a reliable, sustainable product.
Another key interaction is moonfish’s relationship with smaller forage species. As mesopelagic fish, they feed on krill, squid, and small fish like lanternfish, but they also become prey themselves. For larger predators like great white sharks, marlin, and even some seabirds, moonfish are a valuable food source. I once saw a 12-foot swordfish brought to deck with a large, intact moonfish in its stomach—proof that moonfish occupy a middle rung in the open-ocean food web. What’s interesting is that moonfish aren’t passive prey. Their round, laterally compressed body and bright, silvery scales act as camouflage in the deep, where light is faint but penetrates from above. Scientists call this “countershading,” and it lets them blend into the dim water when viewed from above, while their pale underbelly matches the faint light from the surface, hiding them from predators below. I’ve noticed that moonfish will also occasionally school with small fish when they’re young—juvenile opah are much smaller and more vulnerable, so grouping together reduces the chance of being picked off. Once they reach about a meter in length, they become more solitary, a shift I’ve tracked by sorting catch sizes over the years.
One interaction that surprised me early on is moonfish’s occasional association with seabirds and marine mammals. Fishermen in the Hawaiian Islands, where we source a portion of our moonfish, have told me that albatross will sometimes dive near moonfish when they surface at night. At first, I thought the birds were targeting the same prey as the fish, but a marine biologist I work with explained that albatross might also pick at parasites on the moonfish’s skin. That’s mutualism—both species benefit: the bird gets a meal, and the moonfish gets rid of unwanted pests. I’ve seen this myself on a few night trips: when we bring up a moonfish, there are often small, winged seabirds circling the boat, some landing on the water near the fish as it’s unhooked, clearly interested in the small crustaceans attached to its sides.
Of course, not all moonfish interactions are peaceful. There’s limited evidence of intraspecific competition—moonfish fighting with other moonfish over food, though it’s rare. Most fishermen report catching moonfish alone, not in groups larger than two or three, so encounters between individuals are infrequent. What’s more common is competition with other large pelagic fish, like bluefin tuna, for the same squid and krill prey. This is why we carefully monitor fishing seasons: during peak squid migrations in late winter, we adjust our longline settings to avoid areas where tuna and moonfish overlap, reducing competition for prey and ensuring that our harvest doesn’t disrupt the natural balance too much.
This understanding of moonfish interactions hasn’t just been interesting from a marine biology perspective—it’s shaped how we run our business. Years ago, we used to harvest moonfish with methods that caught a lot of non-target species, including sea turtles and juvenile sharks. But learning about their associations with larger predators led us to modify our gear: we now use circle hooks instead of J-hooks, which reduces bycatch of both turtles and the larger fish moonfish swim alongside. We also partner with a local marine conservation group to track moonfish migration patterns, so we only fish in areas where their presence is high and other species are scarce. That’s not just good for the ocean—it’s good for our customers, who increasingly care about where their seafood comes from and how it’s caught.
I’ve spent enough time around moonfish to know that their reputation as solitary deep-sea drifters is outdated. They’re dynamic, social creatures that navigate a complex web of relationships, from scavenging scraps off tuna to evading sharks and partnering with seabirds. For anyone who buys our moonfish, that means more than just a high-quality, sustainable product—it means supporting a business that respects the ocean and the life within it. Every moonfish we supply is harvested with an eye toward the species it interacts with, ensuring that we don’t take more than the ocean can give.
If you’re a restaurant buyer, seafood distributor, or retailer looking to source moonfish responsibly, I’d be happy to share more about our practices, our catch locations, and the science behind how we harvest this incredible fish. We work closely with small-scale fishermen who’ve been part of the central Pacific ecosystem for generations, and we’re committed to transparency in every step of the supply chain.
Pacific Mackerel References:
- Cartamil, D. B., et al. (2018). Movements, habitat use, and feeding ecology of opah (Lampris guttatus) in the North Pacific. Marine Ecology Progress Series.
- Kitamura, M., et al. (2015). Regional endothermy in the opah (Lampris guttatus), a large pelagic fish. Journal of Experimental Biology.
- Lowe, C. G., et al. (2020). Bycatch reduction in pelagic longline fisheries: The role of gear modification and species behavior. Fisheries Research.
- Polovina, J. J., et al. (2019). Mesopelagic prey resources and the trophic role of opah in the central North Pacific. Deep-Sea Research Part I: Oceanographic Research Papers.
Changshion Foods Co., Ltd.
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