Over the last few weeks, Blue Planet II has been impressing viewers around the country, quickly becoming the most-watched television show of 2017.

Of course, the show wouldn’t exist without fascinating wildlife populating the ocean. Yet, mankind has put the inhabitants of the Big Blue under threat.

During the last episode of the series, David Attenborough issues a warning to viewers, detailing how overfishing, plastic pollution, and climate change are all damaging ocean habitats.

“For years we thought the oceans were so vast and the inhabitants so infinitely numerous that nothing we could do could have an effect upon them. But now we know that was wrong,” Attenborough says.

“It is now clear our actions are having a significant impact on the world’s oceans. [They] are under threat now as never before in human history. Many people believe the oceans have reached a crisis point.

“Surely we have a responsibility to care for our blue planet. The future of humanity, and indeed all life on Earth, now depends on us.”

Blue Planet II – in pictures Show all 10 1 /10 Blue Planet II – in pictures Blue Planet II – in pictures Broadclub cuttlefish "A broadclub cuttlefish (Sepia latimanus) in Indonesia. Its skin contains millions of pigment cells with which it can create ever-changing colours and patterns to apparently mesmerise their prey." BBC Blue Planet II – in pictures Coral spawning on the Great Barrier Reef, Australia "Once a year, responding to cues from the cycle of the moon and the temperature of the water, corals simultaneously release their eggs and sperm into the ocean. Tiny coral larvae drift in the ocean currents, some for days, some for weeks, before sinking back down to the ocean floor and settling to become new corals." BBC Blue Planet II – in pictures An aggregation of marbled grouper (Epinephelus polyphekadion), French Polynesia "Thousands of groupers gather here in one of the few pristine spawning aggregations for this species remaining in the world. By taking up the position closest to a female, the male stands the best chance of fertilising her eggs." BBC Blue Planet II – in pictures Saddleback clownfish (Amphiprion polymnus) "A male saddleback clownfish uses all the strength it can muster to push a coconut shell to its anemone in Borneo. Unlike their reef relatives, saddleback clownfish live around anemones that are found in the sand, far away from the solid structure of the coral reef. Male saddleback clownfish must use their ingenuity to find a hard substrate upon which the females can lay their eggs." BBC Blue Planet II – in pictures Peters' monocle breams (Scolopsis affinis) "A group of Peters' monocle breams blow water-jets onto a hiding bobbit worm (Eunice aphroditois) to expose its ferocious jaws. In so doing, they alert other fish to the worm's location, ruining the predator’s chance of a surprise attack. This new behaviour was published for the first time by Jose Lachat and Daniel Haag-Wackernagel in 2016." BBC Blue Planet II – in pictures Green turtles (Chelonia mydas) Green turtles in Sipadan, Borneo, jostling for their place at a cleaning station. Here, turtles are serviced by blennies and surgeonfish who rid them of algal growth, parasites and dead skin. In return, these fish receive a nutritious meal. BBC Blue Planet II – in pictures Bumphead parrotfish (Bolbometopon muricatum) "Bumphead parrotfish feeding on coral and algae in Borneo. These parrotfish use their large forehead to ram corals, thereby breaking them into pieces that are more easily ingested. Each fish ingests over 5 tons of structural reef carbonates per year and after they digest the edible portions from the rock, they excrete it as sand, helping to create small islands and the sandy beaches." BBC Blue Planet II – in pictures Coral garden "A fish's-eye view of a coral garden on the Great Barrier Reef in Australia. New underwater probe lenses, developed for Blue Planet II, allow the viewer to immerse themselves into the coral reef city like never before." BBC Blue Planet II – in pictures A coral grouper (Plectropomus leopardus) "A coral grouper on the Great Barrier Reef in Northern Australia. Groupers use a gesture dubbed the ‘headstand signal’ to reach across the vertebrate-invertebrate divide and encourage another species to help it hunt. Gestures such as this are thought to only occur in the largest-brained species. The discovery of this behaviour in groupers indicates that some fish are able to think flexibly to achieve their goals." BBC Blue Planet II – in pictures Bigeye trevally (Caranx sexfasciatus) "A school of Bigeye trevally gather at the outer edge of the reef wall in Sipadan, Borneo. These trevally are a schooling species, widespread in the tropics and usually found in large- slow moving schools during the day. They eat a variety of smaller fish, crustaceans and marine invertebrates." BBC

The BBC were reportedly nervous certain viewers would see the series as too politicised by taking a stand, ordering a fact check. “We just couldn’t ignore it – it wouldn’t be a truthful portrayal of the world’s oceans,” producer Mark Brownlow told The Guardian. “We are not out there to campaign. We are just showing it as it is and it is quite shocking.”

Brownlow also revealed that the team saw albatross chicks being killed after eating plastic they mistook for food — they decided the scenes were too upsetting to broadcast.

Blue Planet II: Humphead Parrotfish feed on coral making use of incredibly hard beaks

Blue Planet has issued an article explaining what can be done to help preserve the sea, the first piece of advice — and quickest for those who live by the sea — being to simply pick up litter at the beach. They even call on those who do take part to Instagram and Tweet #2MinuteBeachClean.

Another tip is eating sustainably caught fish — which ensures wildlife and habitats aren’t affected by fishing. The Marine Conservation Society has a handy guide for finding out whether what you’re eating is sustainable.

For those watching Blue Planet II, you’ll be happy to know every piece of plastic the crew came across was removed from the ocean and recycled.

"There would rarely be a dive where I wouldn't find some form of plastic from a thread of plastic fishing line, sweet wrappers or plastic bottles," assistant producer Sarah Conner told BBC Newsbeat.