Basking Shark science

We have been studying the impact of microplastics around tidal fronts where basking sharks are feeding. We have comparative studies of plastic abundance between the Clyde and the Hebrides, which have shown increased plastic pollution in areas of higher population density.
Basking sharks are at risk of ingesting microplastics that are concentrated in the same part of the water column as their food. We have been trying to quantify the levels of plastics in these areas and therefore the assumed risk of ingestion by basking sharks. See here for more information on our results so far.

During our 2016-2018 research voyages, we conducted a specific zooplankton study looking at both the abundance and size of basking shark food. Basking sharks feed on tiny Calanoid copepods, usually Calanus helgolandicus, but also Calanus finmarchicus. We collected samples of zooplankton at shark feeding sites, and compared them to samples from control sites, one nautical mile away. We quantified the abundance and the average size of the copepods within these samples. We found that the sharks were feeding in an area where there was both a higher abundance of copepods and larger individual copepod size.

From the surface we can collect data on; shark number, size, time of sighting, tides, sea state and weather conditions, along with any notable behaviour. During our in-water encounters we can collect additional data, including the shark’s sex, markings, behaviour and presence of parasites.
This information, in addition to that provided by the public, is building up a sizeable data set of basking shark distribution and abundance. This has been used in a variety of ways, such as providing evidence for the basking shark Marine Protected Area (MPA) and studies on site fidelity around the Inner Hebrides.
Sightings data contributes greatly to scientific research. One particular success was sighting a tagged basking shark and confirming the tag details. This shark was tagged by The Irish Basking Shark Group earlier that year and was the first movement between Ireland & Scotland, confirmed by physical sighting. Read more about the story here.

Summer basking shark aggregations show a number of behaviours that could be interpreted as courtship. This is difficult to monitor and definitively describe. Whilst early surface-following behaviour sightings were deemed likely for feeding purposes, there have been other behaviours thought to be related to courtship. Observations across Scotland & Ireland have revealed multiple sharks swimming in a wheel-like circle, now named ‘torus’. This is now thought to be group courtship behaviour which may lead to mating. Our observations were included in a paper published by Dr David Sims.
Basking sharks are the biggest breaching shark in the world and we see this behaviour often around the Hebrides. It usually occurs when there are large aggregations of sharks in the area and is thought to be a visual or acoustic communication. We log all sightings of breaching, and see this occur at a range of depths and conditions. We have discussed breaching in more detail on our blog and podcast.

We were the first in the world to film basking sharks with a drone, and have been doing so since 2014. We have developed a survey protocol for assessing their behaviour, which has contributed to both research and novel findings. Having a bird’s eye view gives us a unique perspective and is a highly valuable tool that produces dramatic and exciting results. The footage was used in our first collaborative published paper with Prof Mauvis Gore & Prof Rupert Ormond.

In collaboration with Prof Les Noble et al, we are contributing to a basking shark genetics programme. The aim is to try and sample as many basking sharks as possible to determine how genetically closely related they are. Our methodology involves creating a basking shark passport from obtained sightings data. In the water, we can collect samples of shark slime, the mucus layer of the skin, and surrounding water to extract basking shark DNA. This is a passive and painless process. These samples are fixed in preservative and then stored for analysis at the lab. Preliminary findings have shown new genetic links between groups of sharks, and site fidelity, we are excited to see the next development in this ongoing research.
Listen to our interview with Professor Les Noble here.

Basking sharks are affected by large eel-like parasites called sea lampreys. They attach using a fearsome array of teeth and feed from the shark. How they affect the sharks, and if indeed they affect them at all is unknown. We endeavour to monitor their presence, how many are attached to individual sharks and location on the shark, comparing this to the rest of the distribution information we gather above. This data collection contributes to a collaborative study with University College Dublin and we are looking forward to the published results.

The dorsal fin of a basking shark is unique to each individual, like a human fingerprint. They can have notches, cuts, blotches, striations and scars from parasites. These features can all be entered into one large database for year-on-year comparisons for returning individuals. We have a large database from the Hebrides, along with a specific catalogue from our Clyde surveys.

Thankfully this is a rare sighting, we observed a shark in 2013 that had a plastic packing strap caught around its nose, digging into the flesh. Unfortunately, we did not see ‘Sore Nose’ again that season, however the images and video were utilised as an educational tool for marine debris. In 2014 we re-sighted ‘Sore Nose’ within 10 days and 5 miles from our last data entry. Luckily his nose had healed, but scarring and comparison to previous photos confirmed this was the same individual. This was both great news for the shark’s welfare but also interesting early evidence of site fidelity in basking sharks.

For over 10 years we have been observing a variety of fish species piloting alongside basking sharks. This is more commonly seen in tropical waters where pilot fish follow other shark species such as oceanic white tips. We have observed rare sightings of temperate and sub-tropical fish exhibiting the same following behaviour with basking sharks, such as horse mackerel, barrelfish and bluefin tuna.