Jump to: Seals; Dolphins; Basking Sharks; Birds.
Here I gather what I have observed in my many trips around the Irish Sea and nearby waters. See my trips. I am not a marine biologist but have a keen interest in what can be seen at sea. Here I focus on life visible above water (so not diving related).
There are two indigenous seal species around Britain: common and grey. The main difference between the two species of seal is that the grey seal has a much longer snout than the common seal, and the nostrils of the grey seal are spaced far apart, while they are close together in the common seal. Seen sideways, the grey seal has its eyes halfway between the end of its nose and the back of its head. The common seal (also called harbour seal, which has its eyes nearer the end of its nose and is smaller) is actually less common except further north.
The atlantic grey seal is common around the coasts of the Irish Sea. They breed on offshore islands and on secluded beaches. There are also non-breeding colonies: the one I am most familiar with is on the Hoyle Bank at the mouth of the Dee Estuary. I have often visited the Hoyle Bank (which only appears below half tide) and photographed the seals. I am always careful to stay far enough away from them so that they are not disturbed. There can be many hundreds of seals here. At high water, from Hilbre Island, some of these seals can often be seen swimming and diving for food. They are also often met out in Liverpool Bay.
Hoyle Bank images here: Seals resting; Seal close up;   Audio file of moaning seals.
Other areas where seals are usually to be seen include the
SE tip of Walney Island: image.
The NE tip of Puffin Island: seal
parked on rock.
Calf Sound (Calf of Man side) where there is a seal nursery that I
have dived: watching the pups playing under the protection of their mothers.
seal underwater.
Another breeding area that I have visited is the shore of Cardigan Bay west of Newquay. There are stony beaches at the foot of the cliffs and these can have seal pups in the autumn. The seal pups are left above the high tide line by their mother, as she goes hunting for food. The little bundles of white fluff look abandoned ( pup on beach) - but they don't need "rescuing". Seal and dolphin watching boats go out from Newquay to this area.
Dundrum Bay (Ballykinler beach): image. ( enlargement). This site has one of the few locations in the Irish Sea with a significant common seal population: they give birth in June/July to pups that can swim almost immediately and are not white.
Porpoises are smaller than dolphins (with a less pronounced dorsal
fin) and tend to be more wary of boats. One mostly sees them some way
away. On one occasion I managed to video a pair fairly close to me:
Porpoises in Menai straits(near Beaumaris).
There were a pair of porpoises off New Brighton for a few years also.
Dolphins are found all round the western shores of Britain and Ireland. These are mostly either bottlenose (with "grin" and nose as described) or common (smaller with creamish patches). To see dolphins, one needs fairly calm weather since they do not stand out sufficiently among waves. Often one just sees some in the distance as they jump up to take a breath. In the Irish sea, most often seen are Bottlenose Dolphins.
One area with a resident (breeding) population is in Cardigan Bay: between Newquay and Cardigan Island. Boat trips to view these dolphins go from Newquay.
Dolphins have good hearing, so can hear a boat engine quite far away. They also can hear frequencies up to 150 kHz so may be able to detect sonar (depth sounder) signals also.
They will come close to boats, even riding the bow wave. Their normal swimming speed is 3-7 knots, but they can swim in short bursts much faster. They seem to like a boat with a steady direction, not pitching, displacement rather than planing, and not too fast (7 knots works well) - so that they get a "free ride" from the water being pushed forward by the bow.
They have been observed to push each other about to get to the best spot for this "free ride". It is thought that they bow ride purely for fun - but no-one really knows.
On two occasions I have encountered groups in Liverpool Bay which
stayed close to the boat:
The first encounter was near the wreck of the Alarm (East of
Douglas Accommodation Rig):
Liverpool Bay 2014
(swimming near boat);
The second was between the Burbo Extension and the Gwynt y Mor windfarms:
Liverpool Bay 2018(riding
bow wave).
I have also had a group of dolphins (common?) riding my bow wave off
SW Ireland:
Crossing Dingle
Bay (SW Ireland)
Basking sharks are large, slow filter feeders. They swim slowly near the surface straining vast amounts of seawater through their huge mouths to extract the plankton. They have a rather "floppy" triangular dorsal fin and their nose and tail often break water as well.
They are seen mostly off the Isle of Man and off the West of
Scotland. I have seen one off Coll many years ago. On my trip to St.
Kilda, I passed close west of the lighthouse at Skerryvore and came
upon a family group:
Near Skerryvore ( More ).
I am not a keen bird-watcher, but I take notice if something unusual
appears. At sea, usual is: gannets, various gulls, kittiwakes, guillemots and razorbills, shags and
cormorants
[and here], terns and occasional
fulmars. Watching the gannets
dive is always a pleasure.
  In spring to early summer, there are breeding colonies of
seabirds on sheer cliffs: around my area, at Great Orme's Head; Puffin Island; South Stack (Anglesey).
Coastal birds: Turnstones at Rhyl Harbour, Oystercatchers on Hoyle Bank, Eider Duck near Walney Island.
Gannetries: Grassholm (W of Pembrokeshire); Ailsa Craig (Clyde Estuary); Little Skellig (SW Ireland); Boreray (near St Kilda).
Puffins: Puffins on Skomer; Puffins in South Haven, Skomer; Puffin on Great Skellig.
Great skua: Boreray
Black guillemot: Giles Quay(Ireland).
I have seen (no photo) in Liverpool Bay, an arctic skua attack a (much larger) gannet and get it to release a fish.
I have also seen a white-tailed sea eagle(no photo) in Loch Dunvegan (Skye) recently.