Environmental impacts of trawling and longline fishing

This is the second article in a series that I’m writing for a Chinese magazine targeting wildlife conservation. As you may

Photo courtesy of the NOAA photo library (www.photolib.noaa.gov) Photographer: Robert K. Brigham

Photo courtesy of the NOAA photo library (www.photolib.noaa.gov)
Photographer: Robert K. Brigham

guess, they started with Panda conservation, so the magazine is called Giant Panda, but they are running a series on exploitation of natural resources. In the first I covered overfishing. This current article delves a bit deeper into some of the bigger impacts (certainly not all of them!) of trawling and longline fishing. This is an abridged version.

In the previous article I discussed why overfishing is such a harmful and global issue and how it is leading to negative changes in marine ecosystems. It is not only this overall effect that we should be concerned about, however, because some fishing practices can have large negative impacts on other species, such as sea birds and turtles, or the environment, even if they are not overfished.

What many people don’t realise is that many fishing techniques have some level of unintended negative impacts. The two biggest impacts are 1) the destruction of benthic habitats and 2) bycatch. Benthic habitats are the habitats on the sea floor such as kelp forests or coral reefs which support many other species and are essential to functioning of the ecosystem. Bycatch is the unintentional catch of species that are not of commercial value, not of interest to the fishermen, cannot be sold under their fishing licence or because it is a protected species. It is estimated that approximately 40% of the global fisheries catch is bycatch and discarded back into the ocean. The major issues with bycatch are that it is discarded back into the water, usually dead, contributing to the decline of the ecosystem. Below, I discuss two globally common fishing techniques and some of their impacts.

Longline fishing

Longline fishing is a technique where long fishing lines, up to 10 km long, and containing thousands of baited fishing hooks, are floated along the surface of the ocean to catch pelagic fish species such as tuna or marlin. These lines are set in place for many hours to days and left to drift on the ocean to catch their prey. Over this time, however, many other species are both exposed to and attracted to the baited hooks, meaning that longline fisheries are responsible for a large amount of unwanted bycatch. Unlike trawl fisheries which catch big and small species (explained below), longline fisheries have the dubious record of killing larger animals such as seabirds, turtles, sharks and whales. For example, it has been estimated that global longline fisheries kill somewhere between 160,000 – 320,000 seabirds annually. Unfortunately, many of these species are protected or endangered and such bycatch only serves to increase the pace at which their populations decline.

Another impact that some fisheries, including longline fisheries, have on marine ecosystems is a phenomenon known as ghost fishing. This is when the fishing gear is lost and not collected by the fishermen, allowing it to float around the world’s oceans indiscriminately catching and killing marine life. For longlines it is easy to see how this can occur, as the lines are kilometres long and can be lost when other ships run over the lines, cutting them and separating them from the marker buoys so the fishermen cannot find them again. Unlike bycatch which is pulled from the ocean within a matter of hours, however, such “ghost gear” will continue to kill animals until it degrades and breaks up, usually several years after it is lost.

Trawl fishing

Much of the world’s seafood is caught by trawling, where fishing vessels (trawlers) drag large nets through the water to catch the target species. There are broadly two main types of trawling, pelagic, where the net is dragged through the water column, and benthic, where the net is dragged along the bottom. While a common practice and quite cost-effective for the fishing industry, trawling has two large negatives, 1) a very large bycatch and, 2) for benthic trawls, damage to the seabed.

While effective in catching the target species, the nets used for trawling are not selective and catch many of the animals which are in their path. Imagine a net that can be as much as 100 m wide at the mouth, travelling faster than most fish can swim and it is easy to see that most organisms cannot escape the net, becoming bycatch. The extent of this bycatch can be astonishing. In some regions of the world up to 64 % of the catch is discarded back into the ocean, dead. In the trawl fisheries of the Gulf of Mexico alone, for example, bycatch is estimated to be the equivalent of 1 billion meals a year3! In addition to this wastage, large species such as dolphins, sharks, turtles and seals are often caught in the nets and drown, severely impacting their populations and causing them great suffering. For example, a single prawn fishery that does not employ Turtle Exclusion Devices (TEDs) can catch more than 50,000 turtles per year. Not only are some of these species in decline and protected by law but, as discussed in my previous article, they perform important roles in regulating the function of marine ecosystems on which we depend.

The other major negative impact of trawling is damage to the seabed. When nets are dragged along the seabed they not only catch the species that is being targeted, but they also rip up the seabed itself. Indeed, many fisheries, such as prawn or shrimp trawl fisheries, use chains on the bottom of the net to drag along the seabed and scare the prawns up into the net to be caught. Unfortunately, this type of trawling is now very common and in some regions the seabed is highly impacted. An example is the North Sea, much of which is turned over every year, some areas up to three times per year. Such intense disturbance corresponds with a decline in faunal abundance and species diversity, meaning that over 100 years of intense trawl activity in the North Sea has led to marked declines in species diversity.

This type of physical disturbance of the seabed also, leads to dramatic changes in benthic habitats – larger structures are gradually removed or broken leading to homogenous habitats which are less suitable for most species. Biological habitats such as seagrasses or deep sea sponge beds are destroyed. Unfortunately, many of these types of biological habitats are extremely slow-growing and can take 100’s – 1000’s of years to regrow, assuming that they are not disturbed again in that time.

Are there solutions?

Bycatch is a problem of massive scale which requires a global effort to improve. Thankfully, there are emerging fishing techniques, practices and gear which will start the process of limiting some bycatch. One of the best examples for trawl nets is the inclusion of a device known as a Turtle Exclusion Device, or TED, which also work for excluding other large animals like dolphins and sharks. These devices are large metal bars which cross the opening of the net allowing small species, like prawns or shrimp, into the net so they are caught but larger animals, like turtles, are allowed to escape and are free to swim away. In fisheries where TEDs are now compulsory the bycatch of turtles has deceased by up to 100 %. But this is only one species being excluded.

There are also changes to longline fisheries that can be implemented to reduce bycatch, in particular of seabirds. For instance, setting the baited hooks deeper in the water rather than on the surface and only setting lines at night when the birds aren’t feeding have shown to be effective to some degree.

Limiting the damage of benthic habitats by trawling is more difficult. Changing some practices, such as not using chain on the bottom of the net, can reduce the impact a little, but it is unlikely that these techniques will be broadly implemented as they also reduce the catch. As such, one way to limit damage to benthic habitats is to reduce the frequency with which an area is trawled to allow habitats to recover, but even this will not be effective when geological features or long-lived biological habitats are destroyed. This means that the only truly effective way to ensure the protection of these habitats is by implementing systems of Marine Protected Areas in international waters, currently a very difficult task.

As with all fishing activities, however, regulating and policing these techniques is extremely difficult, especially in international waters. In addition, some fishermen, especially in developing nations, incorrectly think that using these techniques (such as TEDs) will reduce their catch. These people are often already poor and desperate to feed their families and therefore afraid to make any changes that could further harm their families’ health. Until we move towards complete implementation of these techniques, or even improvements on them, the impact of fishing will go beyond what we see on the target fish stock and continue to degrade marine ecosystems. One effective way that you can help towards implementing these changes is by only eating or purchasing seafood from restaurants and shops that support sustainably managed fisheries. Making this choice is becoming more and more viable (see and example guidebook here) and by doing so this will only continue to improve, meaning that you too can help improve the health of our oceans.

When global warming and shifting-baselines syndrome collide

We are having a strange summer in South Australia. First it was mild, then it was late, now it’s hot. So, the weather is a hot topic (pardon the pun) in every conversation. Invariably, conversation then leads onto climate and global warming. And that’s where things get interesting because, as I’ve discussed before, humans and all other organisms experience weather, not climate. In one such conversation with a friend I brought up an excellent article published recently in The Conversation. The article outlines a scary truth; by the end of February 2015 the global temperature has been above the long-term average for 30 years (see the second figure from NOAA, below). My friend said to me, in a very tongue-in-cheek way, “well, I’m 30-ish, which means that they ARE average temperatures to me!”

And that is part of the problem with climate change. It is now easy to demonstrate that temperatures are warming. In Australia, we’re starting to get used to hot summers and bush fires. Even amongst normal inter-annual variation, it’s certainly not difficult to see where the temperature trend is going from the temperature records:

This pattern is repeated globally:

But why can’t we seem to accept the data to all agree that the earth is warming and that we’re the cause?

The problem is three-fold. First, there is the shifting-baselines syndrome. Basically, the idea behind this syndrome is that what you experience in your lifetime is “normal” to you. As with my friend, if you’re only 30 years old (or younger!) then these current temperatures are “normal”. But that doesn’t mean that they ARE normal; the data clearly show that we’re warming outside pre-industrial climatic patterns.

Second, and related to the first, is that we only experience weather. If it rains, we get wet. If it’s winter, we put on a jacket. If it’s summer, we go swimming. We don’t experience “averages.” Some colleagues and I recently published a paper explaining the different effects of climate and weather, noting that without understanding these differences we will not be able to predict what will happen to our marine ecosystems. Yet, policy-makers generally conflate climate with weather, and so we continue to hold to bad policy.

The third, and possibly worst reason, is that in an attempt to “sell the story” the global media still pretends to provide a balanced report. What this means for them is that one person who speaks out against the science underpinning our understanding of climate change gets equal voice to the thousands of scientists who recognise the rigor of this science. That is not only unbalanced, but simply confuses the public into thinking that there is some debate. There is not. To paraphrase the start of the Conversation article, let’s call it, the climate has changed and we’re the cause.

Write English well? Help get published someone who doesn’t

The following post is from Prof. Corey Bradshaw, who writes for his blog, Conservation Bytes. Corey’s experience with international collaborators, in particular in China, so mirrored mine that I thought I should share. Over the last year, I have discovered the joy of working with some of the excellent scientists in China; they truly are brilliant at what they do and I would encourage anyone to work with them. Spending time in China and writing papers with Prof. Yunwei Dong, Prof. Kunshan Gao, and their research groups has (and continues to be) an amazing experience.


imagesI’ve written before about how sometimes I can feel a little exasperated by what seems to be a constant barrage of bad English from some of my co-authors. No, I’m not focussing solely on students, or even native English speakers for that matter. In fact, one of the best (English) science writers with whom I’ve had the pleasure of working is a Spaniard (he also happens to write particularly well in Castellano). He was also fairly high up on the command-of-English ladder when he started out as my PhD student. So. There.

In other words, just because you grew up speaking the Queen’s doesn’t automatically guarantee that you’ll bust a phrase as easily as Shakespeare, Tolkien, Gould or Flannery; in fact, it might put you at a decided disadvantage compared to your English-as-a-second- (-third-, -fourth-, -fifth- …) language peers because they avoided learning all those terrible habits you picked…

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Can nature compensate for human impacts?

Algal turfs dominating under acidified conditions at cold-water (temperate) CO2 seeps, which we use at "natural experiments". You can just see the fronds of a solitary kelp plant in the right of the photo, otherwise they are rare at the site (when they should be 8 - 10 plants per metre!).

Algal turfs dominating under acidified conditions at cold-water (temperate) CO2 vents, which we use at “natural experiments” to try and understand the effects of carbon emissions on our oceans. You can just see the fronds of a solitary kelp plant in the right of the photo, otherwise they are rare at the site (when they should be 8 – 10 plants per metre!). This is a system that has been pushed past its ability to resist or compensate for human activities.

One thing that humans are really good at is having an impact on the environment through their activities. The problem is that we generally don’t realise that we’re having an impact until something changes in a drastic way. We talk about things called phase-shifts, where the environment changes from one “phase” to another. Good (and unfortunately common) examples are the loss of kelp forests for bare reef, seagrass meadows for bare sand, or coral reefs for algal habitats. In all of these cases, the environment has been degraded to the point where it no longer functions as it should, meaning that biodiversity and productivity are massively reduced.

There are two questions to ask here, (1) why don’t we see these phase-shifts coming, and (2) does nature have any resistance to them? A new paper by one of my PhD students, Giulia Ghedini, shows that nature may actually try to resist human-caused stressors (such as increased nutrient pollution, ocean acidification, warming) by increasing the strength of compensation. In this case, Giulia found that the compounding effects of multiple disturbances increasingly promoted the expansion of weedy algal turfs (which replace kelp forests), but that this response was countered by a proportional increase in grazing of those same turfs by gastropods. This is a natural compensatory mechanism, but it has limits.

What does this mean for our understanding of phase-shifts? First, it means that nature is stronger at resisting than we realised. BUT, because it is extremely difficult to either see or quantify this resistance we generally don’t realise it is happening…. until it stops. Then, once we push the systems past their ability to compensate for the increased pressure we place on them we see a sudden shift. It’s like watching a duck on a river – it may look extremely calm on the surface, seemingly stationary, but underneath it is paddling extremely hard. At some point the current strengthens too much and it can’t paddle harder and so, seemingly suddenly, the duck begins to float down the river.

Unfortunately, when put together, this means that more systems may be more stressed than we realise, and the only way to stop detrimental phase-shifts is to take the conservative approach and start to reduce our impacts on these systems. For example, we know that nutrient pollution, carbon emissions, overfishing and many other activities have damaged marine ecosystems, why not begin to reduce our impacts before we add more systems to the list of those we didn’t realise were at breaking point?

Overfishing: a problem for everyone

I am currently writing a series of articles for a Chinese produced magazine which targets wildlife conservation. As you may guess, they started with Panda conservation, so the magazine is called Giant Panda, but they are running a series on over exploitation of natural resources. Which is where I come in, contributing a series on fisheries. Over the next few months I will post abridged versions of these articles here. The first, as the title suggests, is about overfishing.


It shouldn’t be a surprise to most people that many of the world’s fisheries are overexploited. Most of the world’s population eats seafood. In fact, the amount of seafood that each person eats, on average, has risen to 19.2 kg per person per year, with over 1 billion people relying on seafood for their primary source of protein. This means that seafood is an extremely important part of our lives. The problem is that over 90% of the world’s fisheries are either fully exploited or overfished (FAO 2014 report on the global fisheries), meaning that if we take any more from those fish stocks they will collapse, perhaps forever.


Over fishing and fisheries collapse

Overfishing has a long history. One of the best documented cases of a fish stock collapse is that of the Atlantic Cod. When the fishery was discovered in the late 1400’s the cod were so plentiful that it was assumed that the stock was unending. There are stories of people dipping a basket into the ocean and pulling it out full of cod! Catches of cod steadily increased from the early 1500’s, supplying a major proportion of the world’s protein, but were relatively small until industrialisation meant that catches increased dramatically. In the late 1960’s the annual catch peaked at over 1.5 million tonnes, an unsustainable catch. Years of overfishing caused the stock to collapse, and despite ever-improving fishing technology and manpower, the catches continued to decline until the fishery was forced closed in 1992. By that time, the total biomass of cod remaining in the Atlantic was estimated to be less than 1% of the original stock, and still has not and may never recover.  (for a great read on this topic pick up the book “Cod: A biography of the fish that changed the world” by Mark Kurlansky).

The most important lesson to learn from the Atlantic Cod fishery is that any fishery which is overfished can and will collapse. In the last decade alone, many important fisheries have been listed as overfished, including the Largehead hairtail (Trichiurus lepturus), of which over 1 million tons is caught in Asian waters annually, the Mediterranean hake (Merluccius merluccius) and red mullet (Mullus barbatus), Cunene Horse Mackerel (Trachurus trecae), White Grouper (Epinephelus aenus), a number of shrimp species, the list is extensive, and most countries in the world feature at least one fishery. As mentioned above, over 90% of the world’s fisheries are already heading in the direction of being overfished and without good management they too will collapse. Unfortunately, the true frequency with which fisheries collapse can be masked by catch statistics. Global annual fisheries production has been relatively stable since the 1990’s. On the surface, it would appear that fisheries are well-managed and sustainable. What happens in reality, however, is that as we overfish one stock and it becomes unviable, either economically or biologically, so it is replaced by another, new fishery. So, the overall global catch stays the same but we have simply shifted the damage. Usually this means that we are doing something known as “fishing down food webs”, whereby we overfish one stock and then move on to fish a different species further down the food web, often the food of the species that is now over fished! This leads to a situation where the productivity of the oceans as a whole has reduced because the catch is now coming from a previously unfished source. Over time, this continual overfishing causes not only a decline in fish abundance but also massive damage to the ocean ecosystems (which will be topics of future articles in this series).

Fishing down foodwebs

Ecosystem Impacts

Overfishing doesn’t only impact the particular fish species that is over exploited; it is not simply a matter of thinking “it is only one fish species, we will do better next time”. Removing a species from an ecosystem is like removing one cog from a finely tuned machine – it stops working properly. This is especially the case because many of the species that we prize play critical roles in regulating the function of ecosystems. When these species are removed from the ecosystem it begins to become unwell, not providing all of the ecosystem services that we take for granted. Then, as we fish down the food web and remove more species, the ecosystem degrades further.  A very good example of this is shark fishing. Sharks are usually the top predators in ecosystems and control how it functions. To be healthy and function properly, marine ecosystems need these top predators. However, nearly all shark fisheries in the world are over fished, with some species of shark becoming extremely rare. As most species of shark are long-lived they tend to be particularly susceptible to over fishing, and the only way that their populations will recover is by not fishing them.

Indeed, some of the most dramatic changes we see in ecosystems are because of over fishing. A good example from colder oceans would be the overfishing of large predatory fish such as snapper, which are prized by humans to eat, allowing species like sea urchins to become overly abundant because normally the predatory fish would keep their numbers in balance. While sea urchins are a natural part of the ecosystem, in large numbers they completely consume kelp forests, which are the base of the food chain and removing them causes the loss of hundreds of species. Unfortunately, these are not isolated examples, and every country in the world has examples of ecosystems which are degraded by overfishing.


Why aren’t fisheries sustainable?

The answer to this question is that they actually are sustainable, as long as we do not take too much. In fact, the goal of fisheries managers is to maintain catches at the Maximum Sustainable Yield (MSY), or the catch that you can take from a particular fishery forever. In its simplest form, the MSY is an easy concept – you just need to harvest slightly less than the total number of fish which recruit to the fishery each year. It is, however, exceptionally hard to calculate the MSY for a fishery for a number of reasons, in particular that (1) we cannot know how many fish there actually are because we cannot actually count them all, (2) the number of fish which recruit into a fishery, the number we need to know so that we can set catch limits, is dependent not only on how many fish are in the stock, but also a myriad of environmental factors, and (3) we don’t really know how many fish are being taken from a stock because of unmonitored recreational and illegal fishing. This third pressure can be very problematic as people often take fish that are too small, and taking fish before they are able to reproduce (that is, they are immature) means that they cannot contribute young to the next generation before they are caught. In addition to these factors, governments, businesses and the public in many countries often place immense pressure on fisheries managers and fishermen to take more fish to keep supply high. Ultimately, this proves to be counterproductive as when a fishery becomes fully exploited, catches begin to decline and prices rise. Increasing fishing effort at this point leads to overfishing and extremely high prices, making that particular species unavailable to everyone, from the consumer who can’t afford to buy it to the fisherman who can no longer make a living and also the forgotten victim – the ecosystem itself.


What’s the solution?

Contrary to what we used to believe, the oceans are not an endless supply of resources; the ocean has a limited productivity budget. But, this doesn’t mean that we cannot sustainably harvest seafood from the oceans, we just need to ensure we don’t take too much.

What does this mean for the future? At a time when the consumption of seafood is increasing, 90% of the world’s fisheries cannot produce any more, meaning that we need to look to other ways to produce our seafood and reduce consumption. The logical way to do this is through environmentally sustainable aquaculture, or farming of seafood. Aquaculture is already common around the world, making up over 40% of total seafood production, but there is still a lot of room for sustainable expansion.

How can you help? The best way to help is to be a discerning consumer. Rather than not eating seafood, ask where it comes from. Is it from a wild fishery? If so, is it sustainably managed? Is the fish you’re eating grown in aquaculture in a sustainable manner? While it may be hard to get the answers to these questions, if you ask at restaurants or where you buy your seafood you will then force the suppliers to ask the same questions. This will then force industries to become more responsible and manage fisheries in a sustainable manner. In some countries, this public pressure has shown to be an effective way to change fishing practices.


Next time

In the next article I will discuss two different types of fishing, trawling and long-line fishing, and the damage that they cause to marine ecosystems.

What’s in a little noise?

Image source: http://www.marineinsight.com/marine/environment/effects-of-noise-pollution-from-ships-on-marine-life/

Different sources of noise in the marine environment. Image source: http://www.marineinsight.com/marine/environment/effects-of-noise-pollution-from-ships-on-marine-life/

Everyone has seen some sort of human impact in the ocean, from plastic washed up on the beach, to a plankton bloom driven by nutrient pollution, possibly even something as confronting as a fish kill (or even dolphins!). But what about the things you can’t see, say some noise?

Marine noise pollution has again become topical in South Australia, with the announcement that seismic surveys in the waters south and west of Kangaroo Island will begin in 2015. But this raises the question, what do we know about the effects of seismic surveys? The answer is…. not much. There is obviously immense community concern, and I was lucky enough to talk about it on ABC radio today.

For those of you who don’t know, the most common method of seismic surveys in marine waters is to use an array of air guns that are towed below the surface (at say, 8 m depth) behind a ship, firing in a sequence at intervals from seconds to minutes. The sound that is reflected back is then analysed to tell you what is on and under the sea floor, important information if you’re looking to extract resources. These surveys can span hundreds of square kilometres and run for months.

There is some literature on the effect of these surveys, but woefully little, and none in this region. The little information that we do have suggests that the effects will be variable, depending on taxa. Whales and dolphins seem to alter the way they communicate and potentially migration routes or residency patterns, at least in the short term, which is concerning because of the seasonal Blue Whale and Southern Right Whale populations in this region. Fish may become stressed and migrate away from the testing area, which includes important fisheries for species such as the Southern Bluefin Tuna. In contrast, it seems that at least some invertebrates may not be affected. I would reiterate, however, that the evidence in either direction is extremely sparse, which concerns me because this region (South Australia) is a global hotspot for species diversity and endemism.

This is where the discussion collides with another topical issue in Australia – how much information do we need to properly assess applications to develop marine resources, and which activities should we allow in our marine (and terrestrial) environments in the name of “progress”? Although some development and an increase in productivity is good, there is more and more support from the scientific community to make sure we don’t damage our environment beyond repair. I won’t go into detail on this, however, as others have written about this topic in much more depth. But, I note that other countries are taking the issue of marine noise seriously, and discussing it, so why aren’t we?

Declining productivity

We’ve all heard about productivity, but I suspect that the only context most people have heard the term used in is about the productivity of the workplace, or perhaps the economy.

Phytoplankton may be tiny but they are the base for much of what we see and use in the ocean!

Phytoplankton may be tiny but they are the base for much of what we see and use in the ocean!

Economists and governments are certainly concerned with productivity. But, we should all be concerned with productivity – of the oceans.
As we burn more fossil fuels and pump carbon dioxide into the atmosphere we are making astonishing changes to the global climate systems. Not the least of these is the addition of billions of tons of CO2 to the surface waters of the ocean. What does this mean for productivity of the oceans? A cursory analysis would lead you to believe that because many photosynthetic plants and algae can use in photosynthesis that productivity would increase. As the oceans produce about 50% of the oxygen we breathe and provide us with a substantial amount of food and other resources you may think that this would be a good thing. Unfortunately, the evidence is stacking up that productivity won’t increase, and in fact it is likely to decrease.
I have previously posted on work by my research group where we experimentally project that ecosystem productivity in temperate waters is likely to decrease because of an indirect effect whereby highly productive kelp forests will be replaced by lower productivity systems dominated by algal mats. Of potentially greater concern, however, is the emerging data from open-ocean pelagic systems. Recent work by Professor Kunshan Gao from the State Key Laboratory of Marine Environmental Science, Xiamen University, has demonstrated that the projected concentrations of CO2 in our oceans by 2050 (assuming we don’t suddenly decide to stop burning carbon!) will actually cause a decrease in the productivity of phytoplankton. And, the situation was even worse when the phytoplankton were exposed to increased light intensity, which will happen as the upper ocean that they live in shoals towards the surface. This result was initially surprising given that both light and CO2 are required for photosynthesis. In combination and high enough concentrations, however, they inhibit photosynthesis, leading to a decline in productivity.

What does all this mean? The changes that are happening in the ocean because of changes to our climatic systems, including (but not limited to) increased availability of CO2, ocean acidification and warming are going to be with for a very long time. The resources that we currently expect from the oceans will change, many declining. How do we stop this? By being a little smart – let’s stop burning carbon for fuel!