Trigger happy

THESE ARE EXCITING TIMES for UK divers as "in your face" encounters with Balistes capriscus have become virtually guaranteed. It's not unusual to be surrounded by scores of triggerfish the size and shape of dinner plates.
The inquisitive triggers have no fear, and will crowd in unnervingly close. This amazing underwater spectacle proves to me that even British waters can produce a world-class wow factor.
Grey triggerfish, more commonly found in warmer waters, probably come up from the South Atlantic during the summer months, drifting on the Gulf Stream. Sightings are concentrated along England's south-west coast, but individuals have been found as far away as west Wales and east Sussex.
Some are caught in fishing nets and lobster pots, while others are washed up on the beach.
At a few locations, they seem to congregate en masse. The Royal Adelaide wreck off Chesil Beach in Dorset is one such hotspot.
Numbers vary from year to year, but it's not unusual to see 50 to 100 triggers shoaling around the old iron sailing ship.
I'm not certain why they should be attracted to the Royal Adelaide, but most of the surrounding seabed is low-lying, with no distinctive features, so the wreck may well be the only substantial inshore "reef" system for miles around to offer some kind of protection.
This is one of the most underrated shore-diving sites on the South Coast. The remains of the 70m-long shipwreck lie only 150m off the beach, but are rarely buoyed and can be quite difficult for divers to find.

THE MOST PROMINENT FEATURES are the foredeck winch, a huge anchor and chain, and a section of the starboard bow, which stands about 4m proud of the seabed.
Marine life explodes here in summer. Shoals of pout, pollack and sea bass frequent the wreck. Bottom-dwellers including tompot blennies, lobsters, conger eels, gurnards, flatfish and crabs are seen under the huge steel plates, and even John Dory, cuttlefish and anglerfish guest from time to time.
The seabed consists of a series of descending ledges, basically a continuation of the above-water topography. At around 12m it begins to shelve off gradually.
Maximum depth for the wreck, even on spring tides, rarely exceeds 16m.
The pebble bottom keeps visibility reasonably clear all year, except on plankton blooms, and during calm spells 10m vis is possible.
This site is suitable for most experience levels, though negotiating surf entry/exits can be quite tricky, depending on wind direction. This year I struck lucky, and chose the best three days in September. Calm seas, sunshine and good underwater visibility proved perfect for photography.
I even persuaded my regular dive buddies Brian and David to join me for a dip (my thanks to Brian for the loan of his gear when my inflator-hose exploded!).
The Adelaide wreckage covers a wide debris field, but the most prominent part is confined to an area about 30m square, so it's best to visit in small groups. Too many divers kicking around just spoils the visibility, and can affect marine-life interaction.
In strong currents and low visibility it's possible to miss the wreck altogether, and spend a fruitless hour scouring the seabed. But divers approaching perpendicularly from the shoreline will normally spot the huge anchor, followed by the starboard bow looming behind.
Triggerfish aren't the best of swimmers. They rarely use their tail-fin, preferring to waggle the second dorsal and anal fin from side to side for forward propulsion. This unusual swimming action takes its name from the triggerfish and is known as "balistiform locomotion".
Circumnavigating the little wreck, we found at least 50 triggers sheltering from the current. They weren't at all distracted by our exhaled bubbles, and I'm not sure who was watching whom! It's a spooky feeling being stared at by so many fish. Triggers also have excellent peripheral vision, and can rotate their eyes independently, like chameleons.
Some of the bigger specimens were raising their "trigger" as I moved in for a closer look. When threatened, the fish will normally dart into a crack or crevice, raise its dorsal spine and anchor it into the surrounding reef. A second, smaller spine then locks behind to hold it in position. This effective defence mechanism makes it very difficult for a predator to pull the fish out of a hole.
When danger has passed, the smaller spine is lowered, which in turn acts as a "trigger" release for the larger spine.
Triggers have a pointed snout that ends with a smallish mouth. Inside are eight very sharp incisors, capable of breaking open extremely tough shells such as those of mussels, barnacles and even velvet swimming crabs.
During the spawning season, they double up as formidable weapons.

A CLOSE RELATIVE, the titan triggerfish, has a fearsome reputation, and divers who have unwittingly ventured into their territory bear the scars. The grey triggers that visit our shores have no eggs to protect, however, and are extremely passive.
They were often less than an arm's length away, and I was constantly hassling my photo models, David and Brian, to get even closer.
They were gobsmacked by the experience. Brian even took the next day off work to return.
Trigger season is a popular time for anglers, which can make entries and exits even more fun. We were constantly dodging fishing lines, and the seabed was littered with spiky fishing weights.
The feisty triggerfish supposedly puts up a good fight when hooked. I am also told that it is very nice to eat. The firm, sweet-tasting white flesh has become popular in local restaurants.
I often see triggerfish trailing fishing lines with ungainly hooks dangling from their mouths, but at least this proves that some of the hooked fish have escaped alive.
Why do grey triggerfish visit our shores? It's still a big mystery. Global warming may hold the key. Triggers can't survive our cold winters, so by late October they have disappeared.
Part of me wanted to share this remarkable shore dive with everybody, but I'm worried that this exposure will end up with the site being trashed.
I have clocked up more than 200 dives on the Adelaide over the past 10 years, with rarely another diver in sight.
Either way, I'll be back again this summer, same time same place, retracing my cardiac-inducing trudge up and over Mount Chesil. Perhaps I'll see you there?