The trigger fish that shouldn't be here

Every August the water off this coast reaches its warmest, and every few Augusts something turns up that has no business being here. The grey trigger fish is the best example — a warm-water species that arrives on the current, hangs about while the sea holds its heat, and cannot stay.

What you are looking at

Balistes capriscus, the grey trigger fish. Brownish green to grey, deep-bodied and flattened side to side, nothing like anything else you meet on these reefs. They reach about 40 cm. You will usually see one rather than a shoal, often hanging near structure, and they are unbothered enough by divers that people get very close.

The name comes from the first dorsal spine. It locks upright, and a second smaller spine behind it acts as the trigger that releases it. A trigger fish wedges itself into a crevice, locks the spine, and becomes very difficult to pull out.

How it gets here

Trigger fish are common in the Mediterranean and along the West African coast. They are also poor swimmers. They do not migrate to Ireland in any meaningful sense — they drift, carried on the Gulf Stream, and end up at the northern edge of a range they never chose.

They spawn only in water above 12 degrees, and Irish water is nowhere near that for most of the year. So they arrive, they live out the warm months, and the population does not establish. Every trigger fish you see here is a visitor that got a lift.

Why late summer, and why more of them

Sea temperature off west Clare peaks in late August and early September, not in July as most people assume. Water holds heat far longer than air does, so the warmest sea of the year arrives weeks after the warmest weather. That lag is exactly when trigger fish show up.

Irish catches rose noticeably between 1989 and 1991, matching a run of records along the coast of Brittany in the same period. The literature is direct about what comes next: numbers around Britain and Ireland are expected to increase as the sea warms.

Where we see them

We dive the whole coast out of Kilkee, and trigger fish are not tied to any one spot. They turn up around reef and structure where there is something to shelter against. They are a late-season fish here, so if you want a chance of one, the back end of the season is when to come.

A trigger fish in Irish water is a thermometer with fins. It tells you the sea is as warm as it gets all year.

The honest caveat

Sightings are unpredictable. Nobody can promise you a trigger fish, and any dive centre that does is selling you something. Some seasons produce several, some none at all. What we can tell you is when the odds are best, which is the warmest water of the year.