Marine life in the Sound of Mull

Basking shark graphic
Eight basking sharks are currently being tracked in the Western Isles (July 2012) – visit: basking shark location maps for more details.

The coastal waters of the United Kingdom are amongst the most biologically productive in the world, with seasonality contributing to constantly changing underwater vistas. This productivity is powered by winter storms that recycle the nutrients from the shallow sea bottom. This feeds phytoplankton that form in vast blooms in the early spring, as the water warms and the days lengthen. Unfortunately, this productivity has an adverse effect on water clarity, as the density of planktonic life forms makes the water cloudy.

Above right: Filter feeders, such as the deadmens fingers and tunicate rely on tidal currents bringing food particles to them. Tunicates filter vast volumes of seawater every day for food.

Influenced by the proximity of the Atlantic Ocean, the water clarity on the West Coast of Scotland is generally much clearer than elsewhere in the UK. With this, however, comes the problem of exposure, which makes diving conditions weather dependent. In many ways the Sound of Mull offers the best of both worlds, with clear water that is sheltered from the worst of the Atlantic weather. This provides the Sound of Mull with abundant and diverse marine wildlife, which is easily accessible to divers, snorkelers and beachcombers all year round.

At the start of the year, as the days begin to lengthen in spring, the returning sun warms the water, and enables phytoplankton (minute algae) to reproduce in vast numbers, resulting in a bloom. This serves as a food source for many of the other organisms in the Sound.

Underwater life changes with the seasons

Marine-based animals, such as crabs and urchins, are released as minute larvae to coincide with the phytoplankton bloom, either to feed directly on the algae, or on other animals that are grazing on it. The vast majority of these tiny larvae are eaten, but a small number metamorphose into their adult forms, and colonise new habitats – either swimming, bottom dwelling, digging into the sand, or searching out cracks or crevices for protection. The Sound offers a full range of habitats, including underwater cliffs, sandy beaches and flat muddy sediment traps. Hard substrates such as rocks and cliffs have many things to offer the marine wildlife hunter, as the wildlife cannot burrow out of sight into soft mud, and the rocks provide shelter from bottom trawling, which has destroyed most of the established ecosystems on the soft muddy bottoms around the British Isles.

Above left: Colonies of Tubularia, a hydroid related to the anemonies, are an important component of the animal turf on the wall at Lochaline. They support a wide range of other animals – in this instance it is being grazed by the seaslug Coryphella.

Many organisms, such as sea squirts, fan worms and anemones live on a diet of microscopic plants and animals all the way into the adult stage of their life cycle. These animals remain fixed to the place they settled after changing from their larvae stage. As adults they rely on tidal currents bringing them food particles suspended in the water.

Above right: Sponges can be found both on the wall, and amidst kelp hold-fasts in the rocky zone of the hotel beach.

Stimulated by sunlight during the long days of summer, seaweed grows very quickly. Some species complete their life cycle in a single year, however most will survive longer if not grazed on by mollusks or urchins. The encrusted red algae that grow at greater depths may survive centuries, growing slowly in the diffuse light of the deeps (the deepest points in the Sound of Mull exceed 120m).

Left: Sea pens, such as these Virgularia, are common on sandy or muddy bottoms below 20m depth. These were photographed on the deepest part of the beach below the hotel at Lochaline, but are more common off Fuinary and other coastal dive sites.

While many marine snails are herbivorous, the vast majority of sea slugs or nudibranchs are carnivorous. They can be found in large numbers grazing on sea firs and dead men’s fingers (both are relatives of anemones), but are often difficult to spot, requiring a little patience and a keen eye. Commonly they are easiest to spot in early spring, close to their coiled-egg masses.

Summer is a time of plenty in the Sound, dolphins and porpoises are common sightings, and shoals of saithe and other fish are to be found alongside the perennial wrasse, sheltering in the many wreck-sites in the sound.

The dissolved nutrients in the water are used up by the last phytoplankton blooms of autumn, and plant growth slows as the days shorten into winter. The phytoplankton effectively disappears from the water column, and most of the zooplankton along with it. Given a quiet period in winter, the water in the Sound can approach the gin-like clarity of the tropics.

Growth also slows in the seaweed forests, and the balance between plants and grazers shifts. By early spring the magnificent kelp forests have often been reduced to rags and stiff upstanding stalks. Paradoxically, this rather denuded ecosystem offers a lot of opportunities for the diver. The grazed back kelp is a lot easier to penetrate, whether your interest is in the shallower wreck dives, or in the abundant life that the thick fronds usually conceal.

Above Right: Photograph of kelp taken in early spring at Fuinary. The fronds have been very much reduced by grazing, but snails can still be seen attached to the kelp stipes.

The Sound of Mull offers the perfect location and environment for a diverse range of marine wildlife at all times of the year. If you’re interested in getting up close and exploring the marine environment, check out Lochaline Dive Centre’s scuba diving and snorkelling packages and courses.

Photographs were all taken on location in March 2008. Plans and an extended species list for the Lochaline Hotel beach dive can be found on the MCS Lancashire website:

MCS Lancashire survey