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.

Recovery of seagrass from overgrazing depends on species morphology.

Above: A meadow of seagrass (Amphibolis antarctica) that has been heavily grazed by sea-urchins to the point where only dead shoots and detritus remain. Below: A moderately dense meadow of Posidonia sp. with aggregations of sea-urchins (Amblypneustes pallidus). This genus of seagrass appears to have a much greater capacity to recover from grazing than Amphibolis antarctica. Photo credits: Andrew Irving (Above), Owen Burnell (Below)

Above: A meadow of seagrass (Amphibolis antarctica) that has been heavily grazed by sea-urchins to the point where only dead shoots and detritus remain. Below: A moderately dense meadow of Posidonia sp. with aggregations of sea-urchins (Amblypneustes pallidus). This genus of seagrass appears to have a much greater capacity to recover from grazing than Amphibolis antarctica. Photo credits: Andrew Irving (Above), Owen Burnell (Below)

Following on from my last post on how sea-urchins alter how much they eat in response to nutrients and CO2, here Owen Burnell describes his latest paper (in as many months!) that shows why Amphibolis antarctica and other morphologically similar species of seagrass may be so susceptible to grazing.

Sea urchins are important marine invertebrates, which in many parts of the world can shape sub-tidal habitats via their grazing. In South Australian seagrass meadows the short-spined sea urchin Amblypneustes pallidus generally occurs in low densities, however, population increases of the species have recently been documented by researchers at The University of Adelaide.

It was observed that the grazing activity of these urchins was impacting seagrass meadows, in particular the species Amphibolis antarctica, when compared with adjacent Posidonia spp.  By manipulating urchin density to measure seagrass loss and then simulating urchin grazing to study seagrass recovery, we found that while urchins grazed equally upon both seagrass species, Posidonia sinuosa recovered much faster from simulated grazing than Amphibolis antarctica. It appears the different morphology of these two seagrass species, in particular the meristem location (or centre of growth) of Amphibolis spp., which is elevated within the canopy and thus exposed to grazers, is likely to be the cause of these asymmetric grazing impacts. In essence, if the urchins eat the meristem that seagrasses grow from they don’t recover as quickly!

While sea-urchins are by no means a rampant force destroying local seagrass meadows, population expansions such as these are important to document, particularly if they have deep seated connections with changing trophic interactions or urchin fecundity. In many marine systems worldwide population expansion of macro-grazers such as urchins can be linked to over-exploitation of their predators (e.g. fish or crustaceans, or before their protection, sea otters!) or changes to temperature that affect their reproduction and metabolism.

For more information, check out the abstract (below), journal website (subscription required), or link to the full manuscript

The persistence of seagrass meadows reflects variation in factors that influence their productivity and consumption. Sea urchins (Amblypneustes pallidus) can over-graze seagrass (Amphibolis antarctica) to create sparse meadows in South Australia, but this effect is not observed in adjacent Posidonia sinuosa meadows despite greater densities of inhabiting urchins. To test the effect of urchin grazing on seagrass biomass, we elevated the density of urchins in meadows of A. antarctica and P. sinuosa and quantified seagrass decline. Urchins removed similar amounts of biomass from both seagrass species, but the loss of leaf meristems was 11-times greater in A. antarctica than P. sinuosa. In a second experiment to assess the recovery of seagrass, we simulated urchin grazing by clipping seagrass to mimic impacts measured in the first experiment, as well as completely removing all above ground biomass in one treatment. Following simulated grazing, P. sinuosa showed a rapid trajectory toward recovery, while A. antarctica meadows continued to decline relative to control treatments. While both A. antarctica and P. sinuosa were susceptible to heavy grazing loss, consumption of the exposed meristems of A. antarctica appears to reduce its capacity to recover, which may increase its vulnerability to long-term habitat phase-shifts and associated cascading ecosystem changes.

Super trawler, Super bad?

Over the past week there has been a lot of media attention thrown at the imminent arrival of a super trawler in Australian waters. It seems that there is strong objection from a lot of the population but objections are far from unanimous. So, what are the issues surrounding super trawlers? Well, it depends on your point of view:
1. Bycatch.

Bycatch, or non-target species caught in the nets, is an issue with trawlers – all trawlers. The size of the nets on super trawlers will mean that there is a large amount of bycatch (e.g. dolphins, turtles, sea birds) regardless of devices designed to limit bycatch being placed in the nets. This is sad and wasteful, but an issue with all trawling activities.

As an aside, I personally don’t believe that the use of the term “gently” with reference to these excluder devices
(“excluder device inside the net guides the animals gently up to an escape hatch on the top panel on the net” – see the article on ABC news for the full quote)

2. Jobs.

If I were the owner of a smaller trawler, I wouldn’t like to be sharing my fishery with this boat. Hundreds of fishermen around the world have been put out of business by big boats and collapsing fisheries……

3. Overfishing.

This is an interesting one. TECHNICALLY, if an effective quota system is in place on this fishery then the addition of this super trawler will not increase the likelihood of over fishing. However, I think this is the main issue in the debate. If you look at global fisheries, the vast majority are either fully or over exploited already. A lot are in an unarrested free-fall. Why? Because we’ve become way too good at catching fish. Our current level of technology means that if we don’t have accurate assessments of fish stocks, and believe me this is a supremely difficult task (so all credit to the people who have to figure it out), then we have the industrial power to over fish a stock before we even know it’s happening. In my mind, this is the real issue with such large boats.

But, this also brings me back to one of the issues I have with the way that society currently looks at problems. If you break something, or there is too little of what you want, don’t change what you’re doing just engineer a better way. With fishing that means bigger, more efficient boats. With climate change, it means geoengineering or carbon mitigation. But, why capture carbon dioxide through a chemical reaction and inject it deep into the ocean rather than switching to non-carbon based sources of fuel? At best you’re buying time to make appropriate changes, at worst you’re creating a false sense of security that will lead to disaster.

Are we risking the same choice with the super trawler?