ITS SUMMER AGAIN. Clearer, calmer waters are back, and so too are the fish.
If you dive in the UK all year round, you are likely to have noticed in winter that there are fewer fish around than in the warmer months. This is the sort of thing that it is easy to accept without question, but the reasons are interesting.
Those reasons are simple enough, and yet, as always appears to be the case in the cool marine environment around our shores, also complicated.
It is a reasonable presumption that fish go away in winter because the water is colder and rougher, so they are seeking somewhere where conditions are better. After all, birds travel immense distances with the seasons, so why not fish
However, while to say that fish migrate would be fairly accurate for some of them, it does not apply to all. True migrations would involve a journey to another location, followed by a journey back.
Many fish undertake seasonal migrations so that they can reproduce, or to ensure a plentiful supply of food for themselves and their offspring. Thisis especially important in our own temperate waters, where planktonic blooms (the plankton which wrecks our underwater visibility is food for the newly hatched fish) are seasonal.
Plaice usually spawn in winter. The adult plaice migrate to distinct spawning areas, where each female can produce up to half a million floating eggs. These areas allow both eggs and young plaice the greatest chance of being carried by currents to suitable nursery areas, such as estuaries and bays, where there is lots of food for them.
This doesnt always work, and if the plankton bloom is late, or the weather alters the current patterns in the sea, this can have catastrophic effects, with many of the young fish starving or not even reaching their nursery area. In fact only around one in more than two million eggs ever survives to become an adult fish.
The nursery areas often cannot provide suitable food for the larger fish, and later in the year, as the plaice grow, they again migrate further offshore into deeper water.
This explains why many of the fish that you may see during your summer dives, such as the shoals of young coalfish, pollack, whiting, bib, poor cod, cod and haddock, disappear as the onset of winter approaches, many never to return to inshore waters.
This is why divers rarely see the larger, adult fish. Each species has different areas in which they spawn and slightly different spawning times too, so shoals of juvenile fish arrive at different times throughout the spring and summer months.
Britains largest fish, and the second largest in the world (only the whale shark is bigger), is another well-known migrant. Basking sharks of 12m (weighing 2-3 tonnes) are not uncommon and there have even been reports of 18m monsters! In British waters, basking sharks of 4-6m are more common during the summer months.
Basking sharks are harmless and gentle creatures which feed on plankton. They are slow swimmers, cruising at around 2 knots with their metre-wide mouths wide open, filtering plankton through their gill-rakers. A basking shark can filter a huge volume of water, equivalent to that contained in a 50m swimming pool, in just one hour.
While basking sharks are migrational summer visitors, very little is known about where they go in winter. Its possible that they lose their gill-rakers in autumn and winter, and then have a resting, non-feeding time in deeper water. New gill-rakers are believed to develop by late February, in time for the spring plankton bloom.
Cuckoo wrasses are well known by divers and can be very common during the summer. The males are also the most inquisitive and persistent of fish, and in certain popular dive sites can be a real nuisance. They will often swim up to divers and gaze into their masks, possibly because the diver is intruding on the fishs territory or, in some places, because divers regularly feed the fish.
Bizarrely, all cuckoo wrasses are born female, but some will eventually change sex and become males. They spawn in pairs after an elaborate courtship during which the male will excavate a depression in some gravel and aggressively chase off any other males that invade his territory. The female lays her eggs in the depression in early summer, and they are fertilised and protected by the male. All hatch into females, which are dispersed in the plankton by currents.
Most females can change sex from about the age of six onwards. If a dominant male dies, the most senior female in his territory will change sex and take over. Somehow the male must be able to stop females from changing sex, possibly by releasing biological chemicals (pheromones) in his territory.
Cuckoo wrasses migrate into deeper water during winter, probably to depths of up to 200m, usually well beyond the reach of most of us. In winter, shallow water is affected by constant gales and storms, and the coldest water is also found in the top few metres. In contrast, the deeper water is hardly affected by the weather and is generally warmer too. The deeper water becomes a sanctuary where fish can lie low, using little energy, and wait for spring to head back to the action-packed shallows.
The ballan wrasse has a similar life history, and spawns in early summer. This can result in huge numbers of young fish in early autumn.
Sudden periods of cold, before the youngsters have migrated into deeper water, have been known to kill large numbers of these wrasses.
Less is known about the smaller goldsinny, rock cook and corkwing wrasses. These are not thought to change sex as the ballan and cuckoo wrasses do.
In the summer months they are often very common on boulder slopes and in seagrass beds. During winter it is assumed that they move offshore to avoid extremes of temperature.
However, goldsinny wrasse can be found hiding under small stones in water less than 10m deep during the middle of winter, so perhaps not all migrate.
Some fish dont return from their journeys, and these are not true migrations but are known as dispersals. Conger eels can be found all around Great Britain and Ireland, usually in rocky areas where there are plenty of nooks and crannies in which to hide.
Congers spawn only once in their lifetime - somewhere between Gibraltar and the Azores - and then they die.
They become sexually mature at 5-15 years old and then start their dispersal to their spawning areas. During this time their gonads dramatically increase in size, their gut degenerates, their teeth fall out and their bones lose calcium and become soft and gelatinous.
On arrival, in summer, each female lays several million eggs in midwater, at depths of around 3000m, and then dies. The eggs hatch into small fish that look nothing like their parents. In fact they are so different that they were once thought to be a separate species.
Over the next one to two years the larvae drift back inshore and change into the typical eel-shape of congers with which we are all familiar.
During the summer months it is not uncommon to see exotic fish species such as tunny, swordfish, wreckfish, triggerfish and various sharks. Nearly all these migrants (or dispersants, as they may never return home) come from the south and dont breed in our more northern waters.
Of these, the most unusual has to be the sunfish. This is the largest bony fish in the world, growing as big as a Mini (it also produces the most eggs, at 300 million). It appears to have two large pointed fins at the top and bottom, but no tail. It is occasionally seen swimming rather feebly in the surface waters off the western coasts of Britain and Ireland.
Those fish that you do see (which may appear to be residents) may well migrate, but perhaps not so far. Some may move their location only to another within our diving range, and this is not always very noticeable. In winter, any fish that you do see while diving are likely to be far less active, simply because of the lower water temperatures.
The temperate undersea world around our coasts is a strange place, often poorly understood and appreciated even by those of us who are able to see it firsthand.
Just as above water, there are seasons under the sea, with spring bringing forth luxuriant new growths of plants and animals. In early summer many species of fish are just beginning to reappear to harvest this wealth of food and make our diving even more spectacular.