The West Coast supports two distinct communities of Bottlenose dolphin, one localised group of about 12 individuals residing in and around the isles of Barra and South Uist in the Outer Hebrides and the “Inner Hebrides pod “which ranges from Arran to North Skye. It is this Inner Hebrides pod, consisting of 30-40 individuals which this year are frequently seen in the Sound.
Weighing between 400-500kg, they are as heavy as a horse, and are 2.5-3.8 m in length.
Lifespan 45-50 years.
Generally slow swimmers but can reach speeds of up to 30km/hr, and dive down to a depth of 250m. When fishing, they can be seen leaping out of the water and tail slapping to spook and stun their prey.
The Bottlenose dolphins have no distinct markings, being a uniform dark grey on their upper body fading to a lighter grey flank and paler lower body.
In comparison, the harbour porpoise which is seen more frequently in the Sound, has a smaller more rounded dorsal fin and is 1½-2m in length, weighing 60-70kg.
Common dolphins traditionally are summer visitors to the West Coast.
They are smaller than Bottlenose dolphins, up 1.7- 2.4 m in size and weighing around 150kg.
These also have the characteristic creamy yellow hourglass pattern on their flanks.
Inner Hebrides Bottlenose dolphin behaviour
Individuals in a group of dolphins will constantly change throughout the day (referred to as a fission-fusion society) although are most usually seen travelling in groups of 3-10 individuals.
Mother-calf bonds are the most stable with calves staying close to their mothers for up to 5 years learning hunting techniques, navigational and communication skills. Grandmothers, mothers and daughters form close long-term bonds to protect calves and assist with rearing.
Sexual maturity is reached between 8-13 years of age with gestation lasting 12 months with calves normally being born between May and September. Due to the maternal effort in rearing and a 20month suckling period they will produce a calf once every 3-6 years and have an 80% survival rate in their first year (although first born calf mortality is much higher).
Newborn calves are usually about 1m in length, weighing 15-30kg and are born tail first, a process which takes between 30 and 60 minutes.
It takes 3 weeks to perfect the smooth gliding breath characterised by adults. Initially calves will breathe by taking a relatively clumsy belly flop out of the water.
In the photograph the calf’s mother is close at its side which allows it to slipstream in her wake and conserve energy.
Males of similar ages form a “bromance”, forming a lifelong intensely loyal pair or trio, cooperating closely whilst fishing, defending territory and corralling fertile females.
They are highly territorial towards other males and frequently show white scars on their dorsal fins from these aggressive encounters.
The relatively cold waters off the West Coast can be demanding in terms of energy requirements and our dolphins are known to eat between 15kg and 25kg of fish per day with nursing mothers needing twice this amount.
Staple species include whiting, cod and pollack whilst herring and mackerel are consumed in the summer. Squid, octopus and cuttlefish are also taken.
Identification of individuals
To assist our knowledge of this population, The Hebridean Whale and Dolphin Trust are compiling an individual ID catalogue using individual dorsal fin shape and characteristics, to identify and study the population, furthering our understanding of behaviour and enhancing our ability to monitor the population.
Threats and conservation
The West Coast Bottlenose dolphins are a relatively small population which inevitably makes them vulnerable.
Chemical pollution is an area of concern in this population. Chemicals such as the now banned polychlorinated biphenyl (PCB,s) being fat soluble accumulate through the food chain and ultimately in marine mammal blubber. The higher up the food chain the more of this toxin accumulates affecting fertility. PFAS (forever chemicals / polyfluroalkyl substances from many industrial processes are an emerging threat having wide ranging effects on fertility, immunity and the nervous system.
Mortality of firstborn calves is around 50% (vs 20% mortality for subsequent calves) and one reason for this is that the mother’s first milk has a particularly high concentration of fat soluble toxins which poisons the calf.
Underwater noise from powered vessels, construction, seismic surveys and fish farm acoustic deterrent devices (now banned) will all impact the ability to hunt and communicate and may cause abandonment of prime habitat.
Single boats quietly observing dolphins are of little consequence compared to chasing or other forms of harassment. Entanglement with ropes and nets is thankfully a very rare occurrence.
The future of this iconic species in our waters is dependent on us keeping the marine habitat viable, reducing pollution and disturbance, and protecting their prey. Monitoring the population, such as reporting sightings and providing photographs to the Hebridean Whale and Dolphin Trust is a way in which we can all contribute.