Sightings summary for 2023

We are increasingly seeing more common dolphins and Risso?s dolphins and less bottlenose dolphins (see the following images on common and Risso?s dolphins for more information). We used to regularly see the south coasts inshore pod of bottlenose dolphins. However this group seem to spend more time ranging along the whole south coast now and visit us less frequently. Typically they seem to be here more consistently in the winter months.
The bottlenose dolphins we record in the summer are the offshore type. They are the same species but behave differently. They are typically in larger pods and feed on smaller shoaling fish similar to how common dolphins feed. The inshore and offshore pods do not appear to mix.
We have always collected photos to help identify and track bottlenose dolphins passing through Mount?s Bay. We keep our own catalogues but also share these more widely. A recent study has been published, which used our data along with photos collected all along the south coast. The three studies we have been involved with are linked in the photo description for those interested:
Using citizen science data to assess the vulnerability of bottlenose dolphins to human impacts along England’s South Coast. Anim. Conserv – https://marinediscovery.co.uk/wp-content/uploads/sites/6533/2024/01/poster1.png”

[caption id="attachment_2155" align="alignnone" width="1000"] Common dolphin sightings have become increasingly consistent over the last 10 years. This does not necessarily mean there are more common dolphins. It is more likely to be due to shift in their distribution due to changes in food availability. It appears there has been an inshore and northwards shift of this species. The distribution change is probably due to the changing climate across Europe and warming seas both of which have an impact on plankton distributions. Plankton is at the base of the food chain and controls the distribution of the fish the common dolphins feed on.

In 2023, Mount?s Bay experienced an excellent year for spotting Risso?s dolphins, with sightings recorded from April to September, notably consistent between June and late July. This period aligned with the peak spawning time of cuttlefish, a preferred delicacy for Risso?s dolphins in the bay. Notably, cuttlefish tend to lay slightly smaller eggs in warmer water, contributing to increased population and food availability. The rising sea temperatures might explain the upsurge in Risso?s dolphin sightings over the past decade.
Throughout the years, we have expanded our photo identification catalogue for Risso?s dolphins, identifying over 160 individuals. Their distinct scars and patterns allow us to make photo matches, connecting sightings between Mount?s Bay and locations such as the Isle of Man and Bardsey Island.

Mount?s Bay is an important location for harbour porpoises. They occur year round and use the Bay for calving. You can see from the graph that there is a seasonality to their presence. Their numbers build through the Summer and Autumn and are generally lower in the spring. This could be because they are feeding in larger groups in the Summer and Autumn and are therefore easier to detect. In the spring they might be foraging and travelling in smaller groups making them harder to detect. It is likely that Mount?s Bay forms the core of their wider range so it could also be that they are spread across a wider range in spring time. 2023 was typically a year of higher wind speeds and choppier seas. These conditions make porpoises trickier to detect so it is impressive that we still saw them in such high numbers. Duncan completed his thesis investigating harbour porpoise distribution and how it changes across the tide cycle this year. It has taught is about how porpoises use specific oceanographic features to forage at specific time in the Bay. It is likely that the oceanographic processes[ focus plankton (fish food) and that this focuses fish and increases foraging success for porpoises.
Here is a summary of his thesis – https://marinediscovery.co.uk/wp-content/uploads/sites/6533/2024/01/poster1.png”

[caption id="attachment_2158" align="alignnone" width="1000"] Whales are never sighted as consistently as dolphins and porpoises, however they are regular visitors to Mount?s Bay. This year we recorded fin whales and minke whales and as I type this, there are humpback whales off the west Cornish Coast. 2023 was a good year for whales although sightings were slightly fewer than last year. This is probably due to the higher sea states caused by the wind. Minke whales are our most consistently sighted whale and they can be trickier to spot in choppier seas. We had some fantastic fin whale sightings this year.

In the last 8 years bluefin tuna have been returning consistently. We had a brief sighting in April before the quiet period when the tuna are visiting the Mediterranean and Caribbean to spawn. Sightings start to increase in July and peak in the Autumn. The return of the tuna has probably been driven by two things: the recovery of the North East Atlantic population driven by protection and good management along with the same factors driving the common dolphins movement inshore.

Ocean sunfish sightings have decreased in the last few years. We are not entirely sure why, however it could be down to a couple of factors that warrant further investigation. The coastal waters are increasingly warmer during May and June when sunfish sightings typically peaked in Mount?s Bay. Ocean Sunfish need to maintain their body temperature above 15 degrees Celsius. In the past the water in these months would have been slightly colder and the sunfish would need to bask at the surface in order to warm up. Sunfish basking at the surface are easy to detect compared to ones swimming under the surface. Sunfish travel north to exploit foraging opportunities. It is a common misconception that they feed only on jellyfish. However studies have shown that their diet is much more varied particularly for sunfish of the size we see. The most nutritious foraging opportunities might occur at the limit of the sunfish’s thermal range. Warming waters may be pushing this boundary further north in the summer. Both these factors and ones we have not thought of could account for lower sightings of sunfish in the last few years.