Whitemouth jack

Where they come from

Whitemouth jack (Uraspis uraspis) is a wide-ranging Indo-Pacific pelagic jack. You see them off reef drop-offs and around offshore banks, cruising in the blue water between 10 and a few hundred meters. Think fast, open-water predator that swings in along current edges to pick off small fish and squid.

Setting up their tank

I kept one in an oval tank a bit over 2,000 gallons. Even at that size, it used every inch. These fish are built to swim. Straight runs and rounded corners matter more than fancy rockwork.

• Footprint and shape: go oval or round with no sharp corners. 8-12 ft long is a starting point for a single subadult. Bigger is better.
• Aquascape: open water. If you plan any rock, keep it low and smooth. Bare-bottom or smooth sand. Pad overflows and edges with plastic mesh or foam to prevent face rub.
• Flow and oxygen: high, continuous, and linear if possible. Aim 15-25x turnover with broad, guarded outlets so they do not slam into pumps. Strong skimming and redundant aeration.
• Lid: tight-fitting, heavy-duty net or solid cover. They jump at lights-out or spook.
• Temp and water: 24-26 C (75-79 F), salinity 1.025-1.026, pH 8.1-8.4. Keep nitrate under 20 ppm and avoid ammonia spikes at all costs.
• Filtration: oversized skimmer, big bio capacity, and frequent water changes. They eat a lot and make a mess.
• Lighting: moderate. They do not need reef lighting. Keep a gentle ramp-up/down to reduce startle jumps.
• Quarantine space: a long, open QT with heavy aeration. Standard small hospital tanks are too cramped for a jack this active.

What to feed them

New arrivals are often rattled and may refuse food for a few days. Current helps a ton. Toss food into the flow so it drifts by like fleeing prey.

• Staples: chopped marine fish (mackerel, sardine, mahi offcuts), squid strips, whole lancefish or silversides, raw shrimp (shell off).
• Enrichment: soak foods a few times a week with a HUFA/vitamin supplement; add vitamin C and a thiamine (B1) source to counter thiaminase-heavy items like silversides.
• Pellets: some individuals take large, sinking marine carnivore pellets once settled. Offer after they are eating frozen reliably.
• Frequency: 2-4 small feedings per day for subadults. Adults can go 1-2x daily but still appreciate multiple passes. Watch the girth behind the head; they crash fast if underfed.
• Technique: use a feeding stick for strips to prevent face-banging on the glass. Let food ride the current; no hand-waving in front of them that triggers panic dashes.

How they behave and who they get along with

They are perpetual swimmers and very visual feeders. Mine paced the long axis and surged into the flow during feeds. They spook hard at sudden shadows, then settle fast.

• Temperament: not mean for a jack, but anything bite-sized is food. They will outcompete slow fish.
• Best companions: other large, fast, tough swimmers in a huge tank (large amberjacks/trevallies, large tunas are a no-go in home settings, robust cobia, big snappers). Public aquaria often mix them with sharks in very large tanks.
• Avoid: small fish, ornamental crustaceans, and slow or delicate species (batfish, lionfish, angels, butterflies). They get picked off or starved out.
• Solo vs group: a single is simplest. Groups of 3+ can work only with massive volume and lots of flow; pairs tend to bicker.

Breeding tips

They are pelagic spawners with tiny eggs and a long, delicate larval phase. As far as I know, no hobby-level captive breeding records exist. You would need a huge, round spawning tank and specialized larval systems with live plankton rearing. File this under not realistic for home aquaria.

Common problems to watch for

• Shipping shock and refusal to eat: heavy aeration and low-stress surroundings. Start with squid strips and oily fish in the current.
• External parasites: ich and velvet hit jacks hard. Quarantine 4-6 weeks. Treat with chelated copper or chloroquine in a well-aerated, roomy hospital tank. Monitor levels with a reliable test.
• Mouth and head abrasions: from glass and hardware. Pad overflows and protect pump intakes. Keep water pristine to prevent secondary infections; have a broad-spectrum antibiotic on hand.
• HLLE (head and lateral line erosion): linked to poor diet, stray voltage, and dusty carbon. Use varied foods, vitamins, and rinse carbon thoroughly or switch media.
• Low oxygen stress: fast gilling, surface gulping. Add air stones, increase skimmer air draw, and reduce temperature a notch within safe range.
• Thiaminase issues: long-term diets heavy in silversides or raw shrimp can cause neurological signs and wasting. Rotate foods and use vitamin B1.