Losing Nemo

I was resting on the surface between dives when I was startled by a yellow flash in my peripheral vision. It was close to my mask and bright, taxi-cab yellow. I thought it was the sun reflecting off the yellow housing of the strobe attached to my camera and ignored it. I was freediving around one of the large rocks at Los Arcos south of Puerto Vallarta, exploring a series of ledges that stair-stepped down the wall. As I descended on my next dive, a yellow spot appeared in front of my mask. It was a small fish less than 5 cm (2 in) long, and it was leading me to the bottom.

Golden trevally swimming near my face

I recognized the fish instantly, although I had never seen one this small. It was longer than it was tall, thin and bright yellow with black vertical stripes. It was a young golden trevally (jurel dorado in Mexico), less than a year old and a member of the jack family (Carangidae). It had adopted me and was using my body for protection while it hunted small zooplankton. While I was swimming, it hung out around my head, riding my bow wave, saving energy and seeing a lot more ocean than if it had followed a fish on the reef. When I rested on the surface catching my breath between dives, it swam around me, but never far from my body.

Golden trevally swimming around my legs

The little trevally stayed with me for over two hours during which I made upwards of 20 dives and crossed more than 100 m (109 yd) of open water. The whole time, the bright yellow fish zigged and zagged in front of my face, occasionally darting out to attack small plankters invisible to me. At one point, we passed through a school of larval fishes close to the surface. They were less than 10 mm (0.4 in) long and all I could see was their eyes and gut; the rest of their body was transparent. The little trevally dashed into the school, but I didn't see it caught a larva. They were several times larger and much faster than the plankters it had been feeding on, but the instinct was there and it didn’t hesitate to attack when it saw an opportunity. The trevally followed me to the shore as I ended my morning in the water. I left it among the boulders and algae where, hopefully, it could find a real fish to follow.

Golden trevally near my fins

When I returned from diving, I told Rande the story and showed her the pictures.  “It’s Nemo!” she exclaimed laughing. Nemo is an ocellaris clownfish (Pomacentridae; link) and not a close relative of the trevally, but I could see the resemblance.

Juvenile golden trevallies; the largest individuals are twice the size of the smallest

This happened to me twice more: once at Tamarindo near La Manzanilla on the mainland, and again at El Califin, a reef north of La Paz in Baja. At Tamarindo, I was diving out of a kayak among a group offshore rocks and a small trevally followed me on several dives; I lost it when I got back in the kayak. At El Califin, I was diving with a speargun when two small trevallies adopted me while I was hunting. They faithfully followed me on every dive I made that morning and I left them among the boulders and algae at the shoreline.

Juvenile golden trevallies (10-13 cm, 4-5 in) encountering a school of wavyline grunts

Carangids (jacks, pompanos, jack mackerels) are schooling fishes. Some, like the well-known pilot fish, spend their lives accompanying large, pelagic animals, including sharks, bony fishes and turtles, in what is described as a “semi-obligate commensal relationship” (link) – the pilot fish benefits while the fish that is being followed is not affected. The pilot fish is protected from its predators by the fish it is “piloting,” and consumes scraps of food when the larger fish feeds or catches its own as it cruises through the water. The pilot fish is too quick and agile to be consumed by its protector, and may clean its ectoparasites.

Oceanic whitetip shark and pilot fish in the Red Sea, Egypt; photo by Peter Koelbl (link)

Several species of jacks follow other fishes when they are young and school with their own kind as juveniles and young adults. On several dives at sites along mainland Mexico, I encountered juvenile Pacific crevalle jack (Carangidae), a cousin of the golden trevally, following individual fish or small groups of fishes. Sometimes the juvenile crevalle jack was almost as large as the fish it was following and was probably looking for prey flushed by the larger fish.

Pacific crevalle jack following 18-cm (7-in) banded wrasse

Pacific crevalle jack following 15-cm (6-in) Mexican goatfish

I also encountered juvenile crevalle jacks foraging with schools of other species. The jacks receive some protection from being in the school and access to prey targeted by the group. I wrote about my encounters with schools of Pacific crevalle jacks in a post in June 2014 (link).

Pacific crevalle jack (upper center) foraging with 15-18-cm (6-7-in) Mexican goatfish

Golden trevally grow to 120 cm (47 in) and 15 kg (33 lb). Large adults tend to be more silvery with the bars faded or absent; they occur singly and in small groups. The species is widely distributed in coastal waters throughout the Pacific and Indian oceans. In the Eastern Pacific Ocean, golden trevallies are found over rocky reefs and sandy substrates from the Gulf of California south to Columbia. Juveniles occur in schools that may follow (or "pilot") larger fish such as groupers and sharks (link). They are also known to follow divers (link; link).

Young adult golden trevally

The golden trevally’s mouth is a defining feature of the species. Trevallies have protrusible jaws and fleshy lips, and individuals larger than 90 mm (3.5 in) have no teeth in their jaws. Smaller individuals have bands of teeth in both jaws. Golden trevallies feed by extending the jaws rapidly forming a tube and creating a vacuum that can suck prey from sand, rock or algal substrates (link).

Protrusible mouth of golden trevally (60 cm, 24 in); photo by John Snow (link)

When feeding, sediments and debris are filtered out by the gill rakers and prey items are retained and swallowed. Golden trevally eat crustaceans, molluscs and small fishes and, unlike most other members of the jack family, normally do not target individual prey (link). Its Latin binomial, Gnathanodon speciosus, translates as “toothless jaws” (genus) and “beautiful” (species). It is the only species in the genus, which attests to its unique mouth structure and feeding habits (link). Gnathanodon is most closely related to the genus Caranx, which contains 18 species of jacks, all of which have teeth in their jaws (the Pacific crevalle jack is Caranx caninus) (link).

Juvenile golden trevallies feeding in mixed sand-cobble substrate; fish in the center is expelling sediments through its gills

Juvenile golden trevallies feeding in mixed sand-cobble substrate; fish on the right is expelling sediments through its gills

Golden trevallies are targeted by commercial fisheries in the Middle East, Australia and Singapore. They are also popular with hook-and-liners and spearfishermen (link; link). I shot the fish below at El Califin North of La Paz in Baja. It was swimming in a school of a dozen similar-sized individuals along the edge of the rocky reef in 10 m (33 ft) of water. An older man fishing with a rod and reel in a skiff nearby caught one soon after I speared this fish; it was probably from the same school.

Adult golden trevally (male) speared at El Califin in Baja; it was 69 cm (28 in) long and weighed 4.3 kg (9.5 lbs)