I’ve visited Waimea Bay countless times over the years, but it’s always been in winter when the surf, and surfers, are breathtaking to watch. Last week, though, after my friends raved about the great snorkeling there, I visited our famous North Shore bay and this time got in the water.

The minute I put my face down, I spotted several mature parrotfish, a rare sight in Hawaii these days because parrotfish have been overfished in nonprotected zones. In addition to their beauty, algae-eating parrotfish are crucial in helping keep coral reefs from being overgrown with seaweed. As a bonus, the bits of coral rock parrotfish bite off while grazing comes out the anus as a treasured end product of digestion: white sand.

Another joyful sight I found in Waimea was a dark reef about the size of a bus just off the shoreline. The presence of a cluster of hook-and-line fishers, as well as the time of year, gave me a clue as to the identity of this shape-shifting reef. Sure enough, as I swam toward it, I ran into a wall of silvery fish called mackerel scad, known in Hawaii as opelu.

Fishing is prohibited here in the Pupukea Marine Life Conservation District with the exception of Waimea Bay where shoreline hook-and-line fishing is allowed. In August and September fishers can also net opelu there, and in November and December they can net akule (bigeye scad), known as halalu when 6 to 7 inches long.

If the names of these fish are hard to sort out, join the club. Hawaiians gave different names to different stages of growth of the same fish, and changing English and scientific names further muddy the waters.

One easy word to remember, though, is jack, the family name of all of the above fish. Although jacks are variable in size and shape, all are carnivores, and most are silvery with a beautiful iridescent sheen. Of the 140 jack species worldwide, Hawaii hosts 23.

The ones called scads are five of those. The sparkly silver scads form schools from dozens to hundreds to thousands, coming inshore in summer and fall to spawn.

Jacks are a gutsy bunch. Opelu, which grow about 12 inches long, occasionally strike sharks and barracudas, repeatedly hitting, and clearly annoying, the large predators. No one knows whether the little fish are trying to dislodge parasites or scare away a hunter, but in either case it’s a bold move.

Opelu, however, are no match for their big cousins, the greater amberjacks, or kahala, which grow to 6 feet long. While I watched, several amberjacks cruised outside the ball of opelu, biding their time for a sneak attack.

The Pupukea MLCD was created in 1983, but when lenient fishing rules led to widespread cheating, citizen groups and state workers struggled for years to impose stricter fishing laws and expand the protected area. In 2003 it finally happened.

Now, 15 years later, parrotfish are so numerous in Waimea Bay their brightly colored bodies might as well be sandwich boards advertising the success of marine life conservation districts. It’s a strong case for making more.

To reach Susan Scott, go to susanscott.net and click on “Contact” at the top of her home page.