Southern opah

Where they come from

Southern opah cruise the open Southern Hemisphere, from cool-temperate to subantarctic waters. Think big, blue, and empty: offshore, far from reefs, often 50-400 m down. They are built for long, steady swimming in chilly water, picking off squid and small fishes in the dim midwater.

Setting up their tank

I helped care for a rescued opah in a public aquarium pelagic exhibit. I would not try this at home. If you are not working with a professional team and a million-liter system, look at the alternatives at the end.

• Space and shape: Round or oval tank, no corners, continuous smooth wall. Think 15-20 m across and 5-8 m deep, bare interior. Any hard edge becomes a nose-scraper.
• Temperature: Chilled 8-12 C for Southern opah. Big chillers and redundancy. Opah handle cool water but hate swings.
• Flow: Strong, mostly laminar current so the fish can cruise without fighting turbulence. Multiple variable-speed pumps to keep the gyre smooth.
• Oxygen: Near saturation at all times. High gas exchange, degassing towers, and backup O2 on a controller.
• Lighting: Dim, blue spectrum. Overhead light spill and reflections trigger panic runs, so baffles and dark lids help.
• Life support: Industrial-scale mechanical + bio + foam fractionation + ozone/UV. Heavy feeding means heavy export.
• Interior finish: Polished, uninterrupted acrylic. No protrusions, intakes guarded, and a soft vinyl lip at the waterline.
• Access: Wide, low-stress transfer doors. Moving an opah is a team job with custom slings and a plan.

What to feed them

Wild opah eat squid, small fishes, and crustaceans. In human care, getting them to accept prepared foods is half the battle. Start with foods that move, then shift to clean, enriched seafood once they are eating confidently.

• Starter options: Live or freshly killed baitfish presented in the current so they glide naturally. Small squid pieces work well.
• Staples once trained: Squid strips, sand lance, capelin, herring, mackerel chunks cut to thumb-size or smaller.
• Variety: Rotate cephalopod and fish to avoid fatty liver and boredom. Avoid only oily fish day after day.
• Supplements: Soak in high-quality marine vitamins with HUFA and extra thiamine (B1), since many frozen baitfish are thiaminase-heavy.
• Feeding style: Multiple small feeds. Use the gyre to carry food past the fish at mouth height. Tongs or broadcast works once they are keyed in.
• Hygiene: Thaw in the fridge, rinse gently, and discard mushy pieces. Keep the skimmer happy.

How they behave and who they get along with

Opah are steady cruisers. They are curious but spook fast if something flashes or booms. They are not bullies, but they will eat bite-sized fish that drift in front of them. Crowd them and they pinball.

• Best kept: As a single showcase animal in a large, low-clutter pelagic tank.
• Possible tankmates (institutional setups only): Other large, fast, non-nippy pelagics that like cool water and will not see the opah as food or a target. Stock lightly to reduce traffic.
• Avoid: Sharks that investigate with their mouth, ramming tunas/bonitos, and any small schooling fish you cannot afford to lose.
• Feeding competition: Give the opah its own lane and timing so quicker fish do not strip every pass.

Breeding tips

There are no confirmed captive breedings of Southern opah. They are broadcast spawners in the open ocean with microscopic eggs and long, drifting larval phases. This is not a realistic project under aquarium conditions.

Common problems to watch for

• Nose and fin abrasions: Usually from chasing reflections or sudden starts. Darken the backdrop, kill glare, and pad the upper lip of the viewing window.
• Refusal to feed after transfer: Start with movement in the current, keep people out of sightlines, and try squid first.
• Temperature and oxygen dips: These show up as frantic surface laps or listless cruising. Put alarms on chillers and O2.
• Digestive upsets: Big, oily chunks lead to regurgitation. Cut smaller, rinse, and slow the feed rate.
• External parasites from raw seafood: Source clean product and run periodic scrapes if you have the staff and scope. Treat in a hospital system, not display.
• Vitamin deficiencies (especially B1): Use a reliable supplement program. Neurological wobble can creep up on you if you slack here.
• Impact stress: Plan every net or sling move. Clear the path, dim the lights, and keep durations short.