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Science & NatureGreen TalkWeb of Life: Marine Fish

Web of Life: Marine Fish

by Theresa Peluso

Genesis 1:20-21 – Then God said, Let the waters teem with swarms of living creatures, and let birds fly above the earth in the open expanse of the heavens.

As most of us know by now, the waters are no longer teeming with swarms of living creatures, including fish.  Why is that the case?  And does it really matter?

Because there are significant differences between fish in the ocean and those in our rivers and lakes, it seems best to deal with each group separately.  The topic of freshwater fish will be addressed in my next Green Talk column.

As of 2009 it was estimated that there are 10,000 species of marine fish identified with a total global biomass of between 800,000,000 and 2,000,000,000 tonnes ( (ecology)).  By comparison, the human population has a worldwide estimated total biomass of 350,000,000 tonnes.  Estimating just how many fish there are in the ocean is extremely difficult. when you consider that the total area of all the oceans is 362 million square kilometres; the estimated total volume is 1.332 billion cubic km; and the average ocean depth is 3.7 km. Depending on what depth fish live at within the ocean, affects the amount of pressure as well, making it even more difficult to assess the status of marine fish.

Nevertheless, for management purposes, it is essential to assess fish numbers (i.e., fish stocks) – especially for commercially important species. Dividing the oceans into marine fish zones based on physical attributes (depth, currents, etc.) is the common method used to tackle this question.  For instance, one of the “divisions” applied near land is to discriminate between coastal pelagic (“pelagic” means “open seas”) and oceanic pelagic zones. Examples of coastal pelagic fish are anchovies, sardines, and Atlantic herring, which are generally small fish that travel in large schools.  Larger oceanic fish that prey on them can also be found in this zone.

Within the oceanic pelagic zone, the uppermost zone is called the epipelagic zone, and it extends down to 200 metres. This is where you would find sharks, mackerel, flying fish, pilot fish and tuna.  Below that is the mesopelagic zone, (200-1000 m) where fish such as swordfish, cuttlefish, sabretooth fish, and dragon fish live.  Further below that is the bathypelagic zone, which extends from 1,000 metres below the ocean surface to just above the bottom-most zone. The bathypelagic zone is where you’ll find anglerfish, viperfish, and daggertooth fish.  The bottom-most zone is called the benthic zone, and this is where fish such as the orange roughy and the Patagonian toothfish live.   (

Now that we’ve had this quick introduction to marine fish zones, we can better understand the importance of several studies to assess the status of these fish.

One study, carried out during the 2010 Malaspina Expedition, has found that the abundance of mesopelagic fish could be at least 10 times higher than the original estimate of 908,000,000 tonnes. (Source:  Compared with the heavily and often overfished species in shallower zones, what these fish have as an advantage – for now – is that they live at depths that are difficult to access, and are extremely fatty, for the most part, and therefore unpalatable to humans.  But that could very well change, as the more edible and accessible fish disappear and technologies to capture and process these species continue to improve.

Most studies of marine fish have discouraging results to report.  Another report published in February, 2017 by IRIN (United Nations Integrated Regional Information Network), has shown that the world’s ocean fish stocks are “on the verge of collapse”.  (

Many factors are affecting marine fish.  Since the 1970s, the oceans have absorbed more than 93 percent of the heat generated by human activity, primarily through their large surface area, volume, and low albedo (“albedo” means “the proportion of the incident light or radiation that is reflected by a surface”).  The resulting increase in ocean temperatures has given rise to new toxic organisms and algal blooms that poison fish and can even make humans sick.

And we’ve heard about coral bleaching.  When the water is too warm, coral will expel the algae living in their tissues.  Since these algae provide the coral with up to 90 percent of its energy (and colour), expelling the algae results in the coral starving. This also causes the coral structure to turn completely white.  All the fish and other organisms that depend on these coral reefs, for either food or protection, are then left with greatly reduced sources of food and shelter.

Not only the increasing global temperatures, but also an increase in carbon dioxide in the atmosphere (from fossil-fuel burning), have resulted in the oceans absorbing one-quarter of the carbon dioxide produced. This has resulted in ocean acidification, which, in turn, reduces the pH levels in the ocean. The outcome is that marine organisms like sea urchins, mussels, and microscopic plankton, which rely on available calcium to build their shells and other structures, are put at much higher risk.

Industrial, long-distance fishing fleets are largely to blame for damaging the marine food chain. China has been the largest fishery (i.e., largest remover of fish) since the 1990s, and often engages in IUU fishing (Illegal, unreported, unregulated) in other countries’ exclusive economic zones.  Furthermore, super-trawling factory ships, often originating from Europe, and capable of accessing most fish species from all depths, including the ocean floor, have become widespread across the world. These ships can catch up to 360 tonnes of fish every time they lower their nets.  Not all of that catch is used – two-fifths of it is thrown back over the side of the boat, dead, as bycatch.  In 2017 the total amount of fish caught by humans from all sources (not including bycatch and farmed fish) came to approximately 174 million tonnes.

The IRIN report states that thousands of coastal communities in poor countries, dependent on fish for food, are resorting to ever more destructive methods, such as blast fishing (using explosives), as the fish become increasingly scarce, all because of overfishing and climate change.

In fact, according to the IRIN report: “An estimated 70 percent of fish populations are fully used, overused, or in crisis as a result of overfishing and warmer waters.  If the world continues at its current rate of fishing, there will be no fish left by 2050.”

The Pew Environment Group report “Protecting Life in the Sea” (…/protectinglifeintheseapdf.pdf ), published in 2010, explains that nearly one-third of the world supply of commercially caught fish has already collapsed.  Meanwhile, the overfishing issue persists, and within the next 25 years, one-quarter of the Earth’s biodiversity is in danger of extinction solely because of commercial fishing.  As larger fish become extinct, smaller fish are being targeted, damaging biodiversity and the ocean’s ecology. As for trawling, Dr Maria Salta, a biological oceanographer at the University of Southampton, states:  “It’s the equivalent of forest clear-cutting, but in the ocean, because when they trawl the entire bottom, whatever is there, is removed from the environment and changes the entire ecosystem.  Biomass of the deep sea is in sharp decline because of trawling.”

Other threats to the health and survival of marine fish include oil spills from tankers and from aircraft flying over the ocean; and garbage and sewage (often containing persistent toxic chemicals) dumped by ships deliberately into the ocean, as well as by coastal development. The fastest growing issue affecting ocean health is that of plastic in all its forms. It has been found to be potentially lethal in all stages of decomposition – from when larger pieces choke and disable marine life to when it disintegrates into toxic microscopic pieces. At that stage microplastics work destructively, causing a range of conditions (e.g., skin, respiratory, digestive, developmental, reproductive, and visual damage) at all levels of the food chain.  These pollutants, in addition to poisoning marine life, also interfere with the photosynthetic process, by which green plants and some other organisms use sunlight to synthesize nutrients from carbon dioxide and water, because the pollutants prevent sunlight from reaching the surface of the sea.

The pillaging of our oceans is especially horrible when we realize just how abundant ocean fish were in the centuries before the invention of the combustion engine.  Numerous accounts of this bounty exist, such as Captain Cook’s crew trying their hand at fishing, back in 1770 in Botany Bay, on the eastern Australian coast, and catching about 140 kg of fish in three or four hauls with their nets.  Other accounts relate how, in the early 1800s, the waters off Cornwall, England, were dark with large pods of whales, orcas, blue sharks and thresher sharks. The first European explorers to visit the east coast of Canada in the 1500s marvelled that the waters off the shores of Newfoundland were teeming with fish, especially cod – so very, very different from today.  Not only were fish more plentiful; they reached much bigger sizes.  As recently as 1967, the trophy fish caught by sport fishers in Key West, Florida, averaged 20 kg, compared with less than 3 kg in 2007.

We are all well aware of the nutritional value of fish, but are they important in other ways?

Well, fish also recycle nitrogen and phosphorus, crucial nutrients for most organisms at the base of the marine food web.  This they do by storing these nutrients in their tissues, at times transporting them great distances, and excreting them in a form that is readily available to plants and microorganisms in the oceans.  In fact, fish contribute more nutrients to their local ecosystems than any other source. Not only that, but the mesopelagic fish remove CO2 from the air by ingesting organic carbon, such as plants and algae, in the upper layers of the ocean, and then excreting it in the form of feces when they return to their habitual ocean layer, more than 200 metres below the surface.

Reduced fish biomass as a result of overfishing, pollution and habitat destruction reduces this benefit, and further degrades the environment for other marine life, resulting in a trophic (i.e., relating to feeding and nutrition) cascade.   As fish become scarcer, the larger marine predators, such as turtles, seals, seabirds, whales and dolphins will starve.  Also, removing the top predator fish such as tuna, swordfish, marlin, halibut and seabass, will lead to a trophic shift in the marine ecosystem, so that the oceans will fill up with lower food-chain species, such as jellyfish.  Pollution, ocean acidification, global warming and seafloor trawling, in addition to contributing to the decline of fish, will also degrade the quality of the water and seafloor for those species that don’t rely directly on fish for food.  And this doesn’t include the impact on humans, who according to 2016 data, consumed an average of 20 kg of fish (marine and freshwater) per person per year.

Mississippi Mills is hundreds of kilometres away from the nearest seacoast, but it’s important for us to do what we can to halt the depletion of such a critical component of our planet.  It’s important to buy only fish that have been caught sustainably – check for the MSC or ASC symbols on the package before putting it in your shopping cart.  You can also check the eco-friendly status of the fish you’re buying using an app provided by Monterey Bay Aquarium Seafood Watch (

Support environmental organizations such as World Wildlife Fund, the Sustainable Fisheries Foundation, and others, in advocating for stricter fishing regulations and more extensive monitoring and policing of the high seas.  Support political parties with a strong environmental platform.  Don’t take your holidays on cruise ships, which are notorious for discharging high volumes of waste into the ocean.  Also, in connection with holidays, don’t harass fish by signing up for shark-cage diving or swimming or snorkelling near coral reefs. If you’re planning a holiday on the seacoast, make sure your hotel follows environmentally sustainable practices, and try to make your activities as non-motorized as possible.  Participate in local initiatives, and change your lifestyle to reduce carbon emissions, pollution and loss of woodlands, wetlands and natural shorelines.  Each individual impact is small, but if we all take this approach, we can help to bring about a healthier ocean.  At the very least, we can have a clear conscience that we are not contributing to the egregious problem of marine fish decline.





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