Fish diurnal

Pacific Gregory

Plectroglyphidodon fasciolatus

Meet the feisty gardener of the reef. The Pacific Gregory is a small damselfish with a massive personality, known for cultivating its own algae gardens and defending them against all comers.

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Quick Identification

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Size

Typically 10-15 cm (4-6 inches), reaching a maximum length of 17 cm (6.7 inches).

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Colors

Dusky brownish-gray to near-black overall; body often displays 5-7 faint, dark vertical bars; juveniles may show blue specks.

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Key Features

Deep, oval-shaped body profile
Faint dark vertical bands on the flanks
Small, terminal mouth for grazing
Extremely aggressive territorial behavior

When You’ll See Them

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Activity pattern diurnal

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Peak hours 8 AM - 5 PM

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Season Year-round

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Diet Primarily herbivorous, grazing on filamentous algae that they cultivate and protect within their established territories.

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Habitat Shallow tropical reefs, rocky shorelines, and surge zones with plenty of cracks and crevices for cover.

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Behavior

The Pacific Gregory is a remarkably feisty member of the damselfish family, famous for its 'farming' behavior. Unlike many fish that wander the reef in search of food, the Gregory claims a specific patch of rock or coral as its own. It actively 'weeds' its territory by removing unpalatable algae and debris, encouraging the growth of high-quality filamentous algae which serves as its primary food source.

This species is intensely territorial and will defend its garden against fish many times its size. They are known to charge at parrotfish, surgeonfish, and even human divers who venture too close. While they live in high densities on many Indo-Pacific reefs, they are solitary in their management, with each individual maintaining a strict 'no-trespassing' zone around its specific crevice and grazing patch.

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Camera Tips

To capture the Pacific Gregory on a backyard coastal or reef camera, you first need to identify their 'farm.' Look for a patch of reef that appears slightly greener or more lush than the surrounding area, usually guarded by a dark, oval-shaped fish darting in and out of a nearby hole. Place your underwater housing or AI-powered marine camera on a weighted tripod approximately 2 to 3 feet away from this territory. Avoid placing the camera directly on the algae patch, as this will trigger a constant defensive response that might obscure the lens.

Lighting is the biggest challenge when filming the Gregory, as their dark coloration can easily turn into a featureless silhouette against the bright reef background. Aim to film during the middle of the day when the sun is directly overhead to get maximum light penetration. If you are using an AI-camera, ensure it is set to a high contrast mode. Using a red filter can help restore natural colors if your camera is deeper than 10 feet, though these fish are most commonly found in very shallow water where natural light is abundant.

Because Pacific Gregorys are naturally curious and highly defensive, you don't actually need bait to attract them. The presence of the camera itself is often seen as a challenge or an intruder, and the fish will frequently swim directly up to the lens to inspect it or display its fins. For the best AI detection results, try to position the camera at a side-on angle to the fish’s favorite grazing spot to capture its full profile and those subtle vertical bands. Ensure your camera is securely anchored; the surge zones these fish prefer can easily tip over lightweight equipment.

Similar Species

Species that look similar or are commonly confused with Pacific Gregory.

Dusky Damselfish

The Dusky Damselfish often has a more rounded tail and lacks the subtle vertical banding seen on the Pacific Gregory.

Jewel Damselfish

Juvenile Jewel Damselfish have bright blue spots, whereas Pacific Gregory juveniles are more uniformly dark with fewer markings.

Frequently Asked Questions

Pacific Gregory are strictly diurnal, meaning they are active during daylight hours. They spend most of their day patrolling their territory and grazing on algae, usually retreating into reef crevices once the sun sets.

The best way to 'attract' them is to place your camera near their established algae patch. They are highly territorial, so they will naturally come to investigate the camera as a potential intruder in their garden.

They primarily eat filamentous algae. Interestingly, they 'farm' this algae, keeping their territory clean of debris and other organisms to ensure their preferred food source grows well.

They are common in coastal reef areas across the tropical Indo-Pacific. If your 'backyard' includes a coral reef or a rocky tropical shoreline, you are very likely to see them in the shallow water near the tide line.

Look for their uniform dusky brown or charcoal color and the faint vertical bars on their sides. Unlike many colorful reef fish, the Pacific Gregory is quite plain, but its aggressive 'charging' behavior is a dead giveaway.

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