Dolphinfish

They don’t look alike and aren’t in the same taxonomic group, yet they share a name and cause plenty of confusion. The dolphinfish is a brightly colored teleost (ray-finned fish) – much smaller than the typical dolphin mammal – and sports a blunt, flat forehead in contrast to the dolphin’s beaked face. Dolphins are protected from capture under the Marine Mammal Protection Act, but dolphinfish are heavily harvested and marketed. While restaurants may have formerly referred to them as “dolphin” or “dolphinfish” on menus, many have switched to mahi-mahi or dorado to avoid confusion and outrage among guests. Mahi-Mahi comes from the Hawaiian language and literally means strong-strong, but is normally interpreted as “very strong.” Dorado is Spanish for golden, referring to the fish’s characteristic color. Few answers exist about how and why the term dolphin became attached to this fish. In fact, “dolphin” has roots in the Greek delphis (related to delphys) meaning "womb," possibly since they give birth to live young. But dolphinfish don’t have wombs. They do, however, make similar high-pitched noises to communicate, so the dolphinfish may be named for this dolphin-like trait.

There are only two species of dolphinfish: the common dolphinfish, Coryphaena hippurus, and the pompano dolphin, Coryphaena equiselis. Pompano dolphins do not grow to the sizes that common dolphinfish do, and they are often mistaken for juvenile or female common dolphinfish. Besides size, they can be differentiated by the shape of a tooth patch on their tongue (the pompano has a square patch, and the common has a round patch) and the shape of the anal fin (the pompano has a convex, rounded anterior end, and the common’s is concave).

Common dolphinfish can reach seven feet in length and weigh about ninety pounds, though they more commonly reach just over three feet. The all-tackle game fish record is 87 pounds, caught in Papagayo Gulf, Costa Rica, in 1976. Individuals can live up to four years, but generally live less than two. Their coloration is striking with golden hues on the sides, irregular blotches of metallic blue and greens on the back and sides, and white and yellow on the underside. Small individuals and juveniles have well-defined alternating light and dark vertical bars on their sides. These brilliant colors fade to silvery grey with black spots and dark fins soon after death. Mature males (6+ months old) have a prominent bony crest at the front of the head and are physically larger and heavier than females of the same age.

Both species are widespread in tropical and temperate waters and occur in the Atlantic, Indian, and Pacific Oceans, and the Mediterranean, most commonly in waters between 70 and 86°F. They are abundant in the Gulf of Mexico, the Florida Current, and throughout the Caribbean. Though occasionally found near the coast or caught from an ocean pier, dolphinfish are basically a pelagic species, inhabiting the surface of the open ocean down to about 280 feet. They have a particular affinity for swimming beneath floating objects: buoys, seaweed, logs, ships, etc. Due to their oxygen needs, dolphinfish must swim continuously to ventilate their gills, and thus have gills with a larger surface area than most other teleosts, which also contributes to a higher energy expenditure.

Dolphinfish are generalists, feeding on all forms of fish, zooplankton, crustaceans, and squid. With their speed, agility, and a large mouth equipped with many bands of fine teeth, they are able to overcome most prey items. They are a top predator in the Atlantic Sargassum, where they prey mostly upon smaller fishes and invertebrates. Though they are not selective in their feeding habits, their diets do change with growth. Larvae and juveniles feed primarily on crustaceans, especially copepods. Adults feed mostly on bony fishes, with a special taste for flying fish. Sargassum is frequently found in the stomachs, but this is probably just a side effect of foraging in the sargassum communities. Dolphinfish usually hunt in pairs or small packs during the day, as they rely upon vision (as well as their lateral line system) to detect prey. (The lateral line is a system of tactile sense organs that detect movements and pressure changes in the surrounding water.) Males are apparently more active feeders than females, evidenced by the larger amounts of food found in their stomachs. Predators of the dolphinfish include swordfish, sailfish, sharks, porpoises, albacore tuna, yellowfin tuna, blue marlin, and even seabirds when they’re young. They are sometimes observed with holes pierced through them by marlin bills.

Spawning is probably year-round at water temperatures greater than 70°F and occurs in the open water when temperatures rise. Dolphinfish spawn in pairs, rather than communally, and will spawn two or three times during each spawning period (those that live where spawning isn’t year-round). Eggs and larvae are pelagic. Larvae hatch at approximately 4 mm total length, and reach a length of 5.7 mm within four days. At fifteen days, the larvae are approximately 15 mm long. Vague adult characteristics are visible at this size, and the eyes and mouth are fully developed. In the Gulf of Mexico, sexual maturity is reached at three to four months, between sixteen and nineteen inches. These fish live life in the fast lane, in more ways than one.

They are extremely fast swimmers, reaching speeds up to 50 mph in short bursts. Small dolphinfish often travel in schools from just a few fish to over fifty individuals. Larger adults can be seen traveling alone or in pairs. Dolphinfish are considered a highly migratory species, and as such are accorded special protection under the 1982 United Nations Convention on the Law of the Sea (UNCLOS). Also called the Law of the Sea Convention or the Law of the Sea Treaty, UNCLOS is “an international agreement that sets forth a comprehensive legal framework for the use and protection of the sea, the seabed and subsoil, and the marine environment, including both natural and cultural resources. Through a wide range of provisions, UNCLOS establishes clear guidelines with respect to states' navigational rights, maritime zones and boundaries, and economic jurisdiction, while also providing member states a mechanism for international cooperation and dispute resolution.” (NOAA) The convention replaces four 1958 treaties and has been ratified by over 150 countries. Though the U.S. complies with the Convention’s provisions and even invokes UNCLOS to assert freedom of navigation and challenge excessive maritime claims, the treaty has yet to gather the Congressional support necessary for U.S. accession.

Dolphinfish are harvested throughout their circumglobal range. There are some localized declines that may be related to overfishing. However, there is no indication that this species is undergoing significant population declines. Because they can be locally abundant and are fast-growing, early-maturing, and short-lived, they are listed as Least Concern by the IUCN Red List. A popular fishing method is to troll surface baits. If the first dolphin caught is kept in the water, it will sometimes hold the school, and often others will come near enough to be caught by casting. Researchers at the Harte Research Institute for Gulf of Mexico Studies are waiting on returns from tagged dolphinfish, so if you catch one, you can report the tag at sportfishcenter.org. Tagging studies have already provided information on feeding ecology, as well as age and growth rates, but there’s still a lot to learn about this wide-ranging species. Participation of recreational anglers in fisheries science is a significant advantage in the management of gamefish populations.

In addition to being a highly rated game fish, they are also a delicious food fish, though there are reports of ciguatera poisoning from consumption. Ciguatera poisoning is caused by the bioaccumulation of ciguatoxins in the flesh of tropical marine fishes. Ciguatoxins are produced by marine dinoflagellates that grow attached to marine algae. Apex reef predators concentrate the ciguatoxin by feeding on other members of the reef community who fed on the contaminated algae. Unfortunately, there’s no sure-fire method to avoid ciguatera poisoning. It’s not related to inadequate food handling, storage, preparation, or procurement methods for contaminated fish – it’s heat-stable, so cooking, boiling, freezing, baking, and/or frying does not eliminate or destroy the toxin. Contaminated fish might taste peppery, sharp, metallic, or bitter, but could just as easily look and taste normal. Really the only way to avoid it is to avoid eating large pelagic and tropical reef fish, though it’s also been suggested that you can limit the risk by eating only small portions of any one fish, and avoiding the same fish over successive days. The toxin is also more concentrated in the liver, roe, head, and viscera, so those should also be avoided in high-risk fish. Luckily, dolphinfish aren’t usually on the high-risk lists.

When you see a fish, you don't think of its scales, do you? You think of its speed, its floating, flashing body seen through the water...the flash of its spirit.
~ Constantin Brancusi, Romanian sculptor (1876-1957)

Where I learned about dolphinfish, and you can too!

World Register of Marine Species
www.marinespecies.org/aphia.php?p=taxdetails&id=126846

FishBase
www.fishbase.de/summary/coryphaena-hippurus.html

Texas Marine Species
txmarspecies.tamug.edu/fishdetails.cfm?scinameID=Coryphaena%20hippurus

TPWD
tpwd.texas.gov/fishing/sea-center-texas/flora-fauna-guide/gulf-waters/animals-of-the-gulf-waters/dolphin-fish

Center for Sportfish Science & Conservation
www.sportfishcenter.org/sites/default/files/2018-07/young_thesis.pdf

Florida Fish & Wildlife Conseravation Commission
myfwc.com/fishing/saltwater/recreational/dolphin/

University of Florida Museum
www.floridamuseum.ufl.edu/fish/discover/species-profiles/coryphaena-hippurus/

Encyclopedia of Life
eol.org/pages/356300/overview

Animal Diversity Web
animaldiversity.org/accounts/Coryphaena_hippurus/

IGFA World Records Database
wrec.igfa.org/WRecordsList.aspx?lc=AllTackle&cn=Dolphinfish

Sciencing
sciencing.com/difference-dolphin-fish-dolphin-mammal-8393806.html
sciencing.com/mahi-mahi-called-dolphin-12036934.html

Online Etymology Dictionary
www.etymonline.com/word/dolphin

US National Library of Medicine
www.ncbi.nlm.nih.gov/pmc/articles/PMC2579736/

United Nations
www.un.org/Depts/los/convention_agreements/convention_overview_convention.htm

NOAA
www.gc.noaa.gov/gcil_los.html

The Diplomat
thediplomat.com/2017/05/u-s-ratification-of-the-law-of-the-sea-convention/