The blue shadows of the deep
Blue sharks. One of the UK’s most prevalent migratory open-ocean shark species.
Footage of a blue shark off the coast of Penzance, shot by Kelly Greener.
An Introduction
Blue sharks get their name from their beautiful blue colour. They have distinct indigo blue on their backs, vibrant sapphire blue along their sides and white underbellies.
They’re curious, and have an inquisitive nature and need to gently investigate everything and anything that crosses their path, and so are often called the “blue dogs” or “puppies of the sea”.
These worldly circumnavigators live throughout the global ocean, from the tropics to cold temperate waters, , and spend most of their lives far from the coast. The IUCN lists the waters of over 150 countries as blue sharks native habitat but they’re also highly migratory, and have been tracked swimming over 9,000 kilometres during pupping season!
Blue sharks are phenomenally evolved swimmers. Their sleek silhouette makes them incredibly graceful cruisers (perfect for long migrations in the open ocean) and agile hunters. Although blue shark generally swim slowly to conserve energy they are capable of fast bursts of speed when hunting, and have even been documented jumping out of the water for food.
In general, blue sharks are opportunistic feeders but they often feed on mackerel and herring as well as squid in deeper waters and sometimes hunt larger fish such as cod, pollock and coalfish.
Footage of a blue shark off the coast of Penzance, shot by Kelly Greener.
The Reality
First, sharks are top predators. Top predators – those animals at the apex of the food web – can shape how ecosystems look. Not necessarily only by controlling population numbers by eating prey, but also by influencing how other animals in that ecosystem behave.
Specifically looking at the UK and Europe, blue sharks have in history been a target for anglers. They’re big, they’re blue, and they’re feisty, and are often considered a ‘high prize’.
In the Mediterranean, blue shark was placed in the highest risk category of Critically Endangered and the IUCN noted that there are no recovery plans or fishery management schemes in place to allow stocks to recover.
As well as targeted for angling, they also have a target on their back in parts of the world for fins and their meat. Read ‘Predator Turn Prey’ for more information on that. However, despite these pressures one of their biggest killers is from the issue of by-catch: i.e., landing from targeted longline fisheries for tuna and swordfish usually (Figure 2).
Due to similar lifestyle and movement patterns to swordfish and tuna, blue sharks interact with these fisheries and are attracted to the bait set out on the longlines. Sharks can be fatally injured by the hooks, become entangled in the line while trying to free themselves, and attacked by larger predatory sharks because they are vulnerable when they on a hook. They can also get caught up in trawlers nets depending on what they’re eating.
All these pressures contribute to the species becoming vulnerable to overfishing and its decline in population. (Note catching fish is not inherently bad for the ocean, except for when species are caught faster than stocks can replenish, called overfishing). Fishing is also a complex topic and will discussed in a later blog.
“If they’re caught in nets though, then isn’t it more sustainable just to eat them?”
Well sharks are often not considered good fish to eat. It is well documented that sharks are high in mercury, so can poison those who eat them and as mentioned top predators are needed to regulate the food chain. More reasons for why shark is not a fish to eat can be found here.
One of the things it comes down to … fishing methods
Figure 2: Image of the longline fishing method.
The number of overfished stocks globally has tripled in half a century and today one-third of the world's assessed fisheries are currently pushed beyond their biological limits, according to the Food and Agriculture Organisation of the United Nations.
One of the main reasons causing overfishing is by-catch, caused by the types of fishing methods used. This is the main issue for the blue shark. There are other issues that run alongside this, e.g. some of the big fishing companies but that will be covered in the next blog.
Longline fisheries along with trawling have a reputation, for good reason, for being some of the most damaging fishing methods worldwide for by catch. A recent World Wildlife Fund (WWF) Report estimated that bycatch represents 40.4% of global marine catches, a huge percentage.
Global bycatch in commercial marine fisheries has been estimated as anywhere from 37-6.8 million tons per annum (Kelleher, 2005) and these figures differ based on the definition of bycatch only, so the true amount could be much more. There are other kinds of fishing methods that contribute to these by catch statistics include trawlers, drift netting and gillnets. More information on harmful fishing practises can be found here.
However there are fishing methods that can be used that have been found to produce less by-catch. More on this in the next blog post!
So why is this important?
When it comes to fishing, the damage done by overfishing goes beyond the marine environment. Billions of people rely on fish for protein, and fishing is the principal livelihood for millions of people around the world. If there’s no fish, then there’s no fishermen.
Alongside this, just for some perspective blue shark adults become sexually mature when they reach two to three meters long, which typically occurs around four to six years of age. After breeding, the female will go through a gestation period of nine to twelve months. That is nearly up to 7 years until pups can be born. That’s a long time for a shark that is patrolling the deep with sharp hooks waiting around every corner…
So the effects of by-catch and overfishing who doesnt sexually mature for many years is particularly worrisome and can have detrimental impacts on the species, the ecosystem, and wider world. Like all sharks, blues play an important role in ocean ecosystems. The issue around by-catch, especially for a top predator species such as the blue shark highlights the need for more vital conversations to be had around the issue of sustainable fishing practises – in terms of how more sustainable fishing methods can be implemented, and how people can make better, more sustainable choices when deciding on their next seafood meal.
In the UK alone, we are fortunate that that these travellers choose to make our waters their home during the summer months. Therefore we have a duty to protect it.
Despite continuing commercial exploitation and the worldwide reduction of many shark stocks, blue sharks have actually bounced back in the UK due to changes in attitude and education around the species, with a lot of support from the anglers themselves. This just goes to show not all hope is lost, and some change can make a world of difference for species such as the Blue Shark.
This is just one species of shark and one species of fish found in our waters affected by multiple pressure including by-catch, all of which could change with some informed simple solutions such as fishing practises. The species might be in good shape for now, but its future success hinges on us changing our methods, by implementing more sustainable fishing practices, educating ourselves, and cutting down our by-catch rates.
This is just an introduction into some pressures felt by our fish, more details to come in future blogs!
