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Basking Sharks

Basking Sharks

We are passionate about basking sharks, especially as we have the world's biggest hotspot in our backyard! We are lucky to be able to study them and experience stunning encounters with them every summer. They are quite elusive and have a large migration pattern, inhabiting offshore and temperate waters. As such, they have not been studied as extensively as other sharks. We have have prepared some information on the sharks from scientific knowledge and our own learnings to date.

Biology & Morphology

The basking shark (Cetorhinus maximus) is the biggest fish in Scotland and the UK, and one of only three plankton eating (planktivore) sharks worldwide. They are the second biggest shark in the entire ocean- only the tropical whale shark is bigger. These gentle giants grow over 10m (33ft) long and weigh up to several tonnes.

Mouth

Their mouth can be up to 1 metre wide and is lined with a special organ called gill rakers. They are arranged along the gill slits and are specially adapted bones which strain out plankton from the water, whilst allowing the water to pass through. This is similar to the baleen in a filter feeding whale.

Dorsal Fin

Their dorsal fin is one of their most impressive features and on large sharks can be over 1m tall. In some cases the fin flops over at the surface due to its sheer weight and height. During the peak of the shark finning trade one of the largest fins was said to have fetched over $50,000. Thankfully our sharks are now protected species.

Gender

Gender can be identified by the males having two claspers under the anal fins, which vary in size and are not consistent with body size. The maturity of such claspers varies and doesn't seem to have a direction relating to length of shark. Lampreys are a type of parasite which attach themselves to the same area and can sometimes be mistaken for claspers when we are tying to identify the sex. How and where these lampreys attach to the sharks is unknown.

Teeth

Another fact which is relatively unknown about basking sharks is that they do have teeth. They are formed on two cusps on the upper and lower jaw with numerous rows and hundreds of individual teeth. They are only 5-6mm long and hooked backwards. Although they are not used in feeding, they may be used for mating and we have seen scarring on the fins that may indicate this. There is also a suggestion that they are used in oophagy (intra-uterine cannibalism).

Eyesight & Brain Function

To quote the famous poem by Norman MacCaig 'That roomsized monster with a matchbox brain'. Basking sharks have a very small brain (~10cm long) which has evolved due the lack of requirement for hunting and limited energy from their diet. Compare this to a great white shark who has to actively hunt and engage strategy balanced with a high energy (seal) diet. In basking sharks, the nerval mass for smell is actually much larger than the brain, indicating that this has far greater importance and is likely to be used in hunting plankton. The eyes are large and black and eyesight is thought to be fairly poor. It is unlikely that the sharks use eyesight to find plankton in the water but probably use this sense for object avoidance and light level detection. 

Liver

The liver is the biggest organ in their body and can form between a quarter and third of their body weight. It contains an oil called squalene and is used as an energy source (possibly for use during their long migration pattern or times of low plankton abundance) and also for buoyancy in the water as sharks do not have a swim bladder like bony fish. Unfortunately this large organ was one of the reasons why they were extensively hunted as the oil was used in many different industries.

Skin

Like other sharks their skin is formed from placoid scales called dermal denticles. They are similar to shark teeth, covered with a hard enamel and are interlaced like tiles on a roof. These scales are designed for maximum hydrodynamic function so they can move through the water with the most efficiency, along with providing some protection for the shark. If touched they feel very rough in one direction and smooth in the other. The colour of the sharks can vary but they are usually a dark grey with different patterns and markings along their flank.

Classification

Basking sharks have an unusual classification and are part of the order Lamniformes. This order is more commonly known as the mackerel sharks and ranges from great whites to makos, porbeagles and the megamouth. It seems strange to think that the famous seal hunting great white is related to the plankton eating basking shark, however they share a similar fin and body type. When swimming with basking sharks, you can sometimes observe them either swimming with their mouth closed or closing their mouth to swallow food. At this time, with the mouth closed, their outline looks very similar to large predatory sharks and can be quite an exhilarating experience. 

Kingdom: Animalia 
Phlyum: Chordata 
Class: Chondrichthyes 
Subclass: Elasmobranchii 
Order: Lamniformes 
Family: Cetorhinidae 
Genus: Cetorhinus  
Species: Cetorhinus maximus

Food Source 

Scotland has some of the richest cold waters in the world and every spring, oceanic and weather cycles create optimal conditions for explosive blooms of plankton. With increased availability of upwelled nutrients, higher sunlight and warmer temperatures, the plant based phytoplankton generates large blooms.The animal based zooplankton feeds on the phytoplankton and times their cycle to take advantage of this food source. Then in turn we have basking sharks migrating to our water to feed on the rich zooplankton.

 

Zooplankton is a generic term for a multitude of species however there is a specific type of copepod, which is a type of crustacean, that the basking sharks prefer. The Calanus family of copepod and in particular Calanus helgolandicus are understood to be the meal of choice. During blooms of this specific species the water is alive with a red coloured bloom and we have abundant sharks who hone in on this food source. During our own studies we have found that whilst copepods can be abundant all over the coastline, the areas where the larger individuals are located have the most sharks. This does make sense as the sharks are getting more bang for their buck, gaining higher energy input for the same amount of energy used feeding.

Migration Pattern
 
The basking sharks (North Atlantic) have a very interesting migration pattern which was relatively unknown until recent years. Satellite tagging programmes have increased our knowledge on site fidelity of the sharks, where it has been demonstrated that they are seasonally resident around the islands. It has also given us an insight into their autumn and spring migration to and from our islands. Although they are seasonal visitors to our waters, there is evidence of some that stay over winter, with fishermen reporting them in mid winter, divers witnessing them in deep water and even some washing up after bad weather or in poor condition. The main migration in spring coincides with the first plankton bloom of the year with the peak of numbers showing in mid summer, then the southerly migration occurs around September and October. On their winter migration journey the tags that have remained attached have show their journey across the Atlantic towards Newfoundland and offshore towards the Bay of Biscay and Azores. The reason for this stage of their migration is unknown and they are very conspicuous at this time and unaccessible due to their offshore location. Do they feed during this stage and is there available plankton on which they can feed on? As described above, they have a huge energy source in their liver which must be for a specific function such as energy stores for their journey. One of the reasons for the allure of basking sharks is that their migration is a mystery and tracking them is difficult, unlike predatory sharks which are attracted to specific prey location. Protected in Scotland by various Scots and UK law to our statutory limits there is a threat to them during their offshore migration which means it is important that the population is protected not in singular countries but as a migratory species.
 
 

Reproduction

Reproduction is poorly understood in this species as this part of their life cycle is hidden from us. Mating has never been scientifically recorded however there is evidence of sharks rolling around at the surface together as seen by fishermen such as Howard McCrindle (a lucky man to have observed this!). Basking sharks do breach clean of the water and are the largest shark in the world to do so. There seems to be a higher incidence of this is the Hebrides than in other areas and at times of large shark aggregations. Although there is a theory that the breaching may be a way of ridding themselves of the parasitic lamprey, the energy required to breach would not seem to be worth the benefit of increased swimming efficiency. Any other display in nature is usually to do with breeding and it is generally accepted that this behaviour is related to the cycle in some way. We suggest either that the males are showing some type of dominance or attracting the female, or perhaps the females are signaling that they are ready for mating. Breaching is also responsible for the only UK fatality recorded as a shark attack and is known as the Carradale incident. Back in Sept 1937 a family were out fishing in a 15ft dinghy when a shark breached and capsized it. During this incident three of the occupants died.

See below for a video of a basking shark breaching beside our boat during the 2016 summer (taken on an iPhone)

In the Hebrides there are very few small sharks seen and it is thought that pupping occurs elsewhere. The sharks are ovoviviparous meaning they first develop within a egg sac but are born live at approximately 1.5-2 metres long. The only record of this being seen was by a Norwegian fishermen who upon catching a large female shark saw her give birth to 5 live and one still born pups. There seems to be more smaller shark sightings further south in the UK and even into the Mediterranean however this still remains a misunderstood part of their life cycle. 

 

Protection Status & Threats
The sharks were historically hunted in Scotland, even as recently as 1994. They were targeted for their large livers which contain a vast amount of oil. Traditionally this oil was used in lamps, however other industrial uses were found for the shark liver oil such as producing a constituent called squalene for cosmetics, perfumes and lubricants. Once synthetic materials were developed this reduced the demand and thankfully these majestic sharks are now protected.

Over the years almost 100,000 basking sharks have been caught and although the sharks are now protected in the UK, their protection cannot be guaranteed elsewhere around the world. The sharks are still targeted in other countries to use in shark fin soup – it has been reported that one exceptionally large dorsal fin even reached $57,000 USD.  The sharks are listed on the IUCN Red List as ‘Vulnerable’ and also as ‘Endangered’ in the north-east Atlantic. The sharks only produce a small amount of young and can produce litters of up to six live pups which have been said to be around 1.5m long. Similar to other sharks reproductive method (k-selected) this means that the population is highly vulnerable to exploitation.

A more recent threat to basking sharks is from marine debris and microplastics. They filter out their microscopic food source from the water and so much plastic has now entered the ocean and been broken down into small particles, the effect on the sharks' gill rakers or even from ingesting these microplastics is unknown. 

A larger scale threat could be related to their food source. Global warming has been noted to affect the range of their prey copepod and has shifted their distribution northward, possibly replacing them with another warmer water species. These large scale climate and oceanographic changes have unseen effects for the general public but manifest as changes in plankton assemblages and their  life cycles. This in turn has repercussions for large mega-fauna such as basking sharks and possibility affects their food source, migration pattern and distribution. Over the last few years there have been very few shark sightings to the south of the UK which may be a result of this shift in plankton distribution. 

One effect we have seen is that a basking shark had been fouled by some marine debris, possibly a packaging strap. This had gone over the shark's nose and started cutting into the flesh behind the eye which looked bad. Unfortunately on the day we saw the shark we weren't able to find it again to possibly help it. The good news is that the shark was seen again the next year with a nicely healed scar in the same place. Great news for both the shark and an interesting piece of data for the same sharks returning year on year. The lesson here is recycle your waste and make sure you cut up any plastic.

Historical Hunting in Scotland

Unfortunately wherever there is an abundant natural resource, humans have taken advantage of it and exploited it. In Scotland, we have always had abundant basking sharks and over the years there has been various fishing operations for them. 

Maxwell, Geddes, Watkins, O'Conner and Manson all had operations in the Clyde and Hebrides in the 1930's, 40's and 50's, along with visiting Norwegian fleets all of whom took a large number of sharks. The fishery was re-started in the early 1980's by a Clyde fisherman named Howard McCrindle who had a conservation ethos by enforcing a self imposed size limit and working through to the mid 1990's when legislation was passed on the basking shark's protection. 

Basking sharks were hunted using harpoon guns similar to what would have been used in whaling then cut up for their meat, their large liver and fins. There are a number of interesting books available from the fishermen listed above about their exploits. 

The effect of the fishing efforts on the Atlantic population is unknown to an extent but numbers seemed to reduce during the lifetime of the fisheries. Following a period of over 20 years with no targeted fishery in our abundant waters it is hoped the numbers are increasing.