Climate change and the collapse of fisheries

This is the thiFishing down foodwebsrd article in the series that I’m writing for a Chinese magazine targeting 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 exploitation of natural resources. So far I have covered overfishing, trawling and longline fishing. In this current article we discuss the interaction between fishing and climate change. I say we, because this article was led by Charlee Corra, a postgraduate student of mine. Charlee really deserves the credit for this one!

The first article in this series discussed the effects of overfishing and how it causes degradation to the environment. However, changes to the environment also affect fisheries and their sustainability. In any ecosystem, the survival of a species is dependent on its ability to grow to maturity and reproduce, which is in turn dependent on many factors such as environmental conditions that support healthy physiological functioning. Temperature, salinity, and water quality are all examples of integral abiotic factors that can have the power to support life or pose a serious threat. Global climate change is rapidly altering these environmental conditions, and thus altering marine communities in a way that scientists, fishermen, fisheries managers, and policy makers must understand in order to predict future stocks and improve sustainable practices

How the environment affects plants and animals
Physiology:  Marine organisms differ widely in their tolerance of environmental conditions. Some animals can survive better under stress than others. These differences in biological responses determine where an organism can live. For example, in the intertidal zone temperature sets the upper limits of species distributions such that barnacles, mussels or oysters with a greater heat tolerance live higher on the shore than those with a lower tolerance. While almost every organism has the ability to withstand heat stress to varying degrees, most organisms are also adapted to the temperatures in their particular habitat. Thus, many species, and even populations, have different thermal limits beyond which survival is brought into question. As an extreme example, imagine that you grew up in the polar regions and summer for you only gets as hot as, say, 10°C and you were put in the desert in summer – you would be above your thermal tolerance and likely would die.

The water chemistry of our oceans also heavily affects physiological functions. Calcifying organisms, such as corals, oysters, mussels, and some crustaceans, rely on specific levels of CO2 (usually low) and several other chemical compounds (usually high) in order to induce the chemical reaction that allows them to make their skeletons and shells. Changes to the water’s chemistry can compromise the structural integrity of these essential parts. Of particular concern is that constant and increasing CO2 emissions are causing more CO2 to dissolve into the ocean, causing Ocean Acidification (OA). OA is already making it difficult for shelled organisms to make their shells in some parts of the ocean! You can do a small experiment to demonstrate this effect: put a small seashell or piece of egg shell into a glass of an acidic liquid like Cola or vinegar and watch it slowly dissolve (this can take a day or two).

Climate change and long-term climate shifts: Climate fluctuates and changes naturally across many different time scales from seasonal to multi-decadal and millennial. However, in the last two centuries industrial activity has begun to influence these cycles, mostly because of emissions of greenhouse gases such as CO2 into the atmosphere. Of particular concern is that in addition to causing OA this CO2 also causes the earth’s atmosphere to warm, in turn warming the ocean. Unless something is done to change this trajectory, CO2 levels will continue to rise, negative effects on the environment will become stronger, and the impacts on marine habitats and communities will become more visible.

Shifts in distribution of plants and animals: As environmental conditions change, and especially as oceans warm up, many species are predicted to move poleward to higher latitudes to live in more optimal conditions. These range shifts are not always consistent or predictable among organisms or across regions due to complex ecological interactions with other physical and biological factors such as currents and larval dispersal, competitors and predators. Importantly, as the distributions of different species change, the balance of ecosystems is upset and their function is degraded.

Just as with other species, climate change will invariably impact fish populations and dynamics. For example, fish populations may either get smaller where they currently are or move to a new area. Adjusting fishing practices and quotas to these changes is essential for the future of sustainable fisheries.


Photo courtesy of the NOAA photo library ( Photographer: Robert K. Brigham

Effects of climate change on fisheries
Range shifts represent a huge threat to the productivity and success of fisheries, especially when they occur to economically and socially important species. In addition to losing an important species as its range shifts poleward, fisheries may be further affected by the opening of a gap in the ecosystem that can become occupied by a new species. This ultimately changes the structure and function of the ecosystem, potentially reducing the productivity of not only that single fishery but also the ecosystem overall.

In addition to range shifts, decreases in abundance of fish may also occur simultaneously. For example, warming has already caused decreases in populations of Norwegian Cod, leading to a less sustainable fishery. In such cases, the fishermen must either change to another fishery or risk damage to the fishery, degradation of the ecosystem and going out of business.

Together, the combination of range shifts and declining abundance has the potential to be devastating to fisheries if vulnerable fish stocks are fished at the same intensity.  Particularly sensitive fish stocks could easily collapse under these combined pressures. Considering that over 80% of the world’s fisheries are either already fully fished or over-exploited, collapses will become more likely under future conditions. However, armed with more accurate knowledge of how fished populations will be impacted, fishing regulations could be fine-tuned to protect the viability of fished species and avoid such a bleak future.

Predicting future stocks
Knowing that these issues exist, a lot of research is currently being done to predict the trajectory of future fish stocks and assist in managing fisheries in a more sustainable way. Because we are trying to predict what will happen in the future, one of the common techniques is to use computer-generated models which use complex calculations based on as many environmental and biological variables as possible to predict the effects of climate change on fish populations. These models take into account the physiological effects of climate change (mentioned above) on the targeted species to predict parameters such as growth, survival, and reproductive output to determine the future supply of adults. Then, in combination with experiments to test the outputs of these models, managers and policy makers decide how many and what type of fish can be caught annually to avoid depleting populations but also to maximize profits and food security. Importantly, these models can, if used properly, help managers prepare for the future of fisheries and to hopefully avoid more fisheries collapsing. However, it is extremely important to remember that predictions are not certainties and models, while very powerful tools, are far from perfect. There will always be variability across regions and habitats due to the interaction of many different factors and projections might represent some outcomes but not all.

It is important to remember that we can formulate all the regulations that want, but unless we are also simultaneously making an effort to decrease or mitigate the impacts of a changing climate on the ocean and its ecosystems, fisheries will continue to decline. The ocean is an important source of food for humans. In many countries seafood is a way of life. Many smaller communities rely exclusively on fish and other marine organisms for protein.  Therefore, it is important for everyone to understand how climate change will impact on the ability of marine organisms to survive because our fate is inextricably intertwined with that of the marine environment.

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.