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.

Trawler

Photo courtesy of the NOAA photo library (www.photolib.noaa.gov) 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.

Don’t forget to remember the past

I have recently returned from the 10th International Temperate Reefs Symposium in Perth. It was great to spend a week talking good science

Amblypneustes pallidus in a Posodonia seagrass meadow. Photo: Owen Burnell

Seagrass may increase their productivity in the future as they use CO2 for photosynthesis.
Photo: Owen Burnell

with a vibrant group of great scientists. There was an array of talks from classical marine ecology (which is great to see!) to novel modelling approaches and plenty of discussion of human impacts in marine systems. In the rare moments of quiet since my return I’ve been thinking about the main message that I took away from the meeting, and it’s this: anthropogenic climate change may be new to the planet, but we were studying the effects of human activities on ecosystems for several decades before we even realised that climate change was happening. So why is it that we seem to have abandoned ecology in our race to understand climate change?

While I was writing my talk for the conference I realised that, in general, research into the effects of climate change in marine ecosystems has been hampered by not looking at the literature on other human impacts. For example, there is a rich and abundant literature on how excess nutrient loads degrade ecosystems and change their structure and function. Yet, it is only recently that we have realised that CO2 is a “nutrient” or resource in marine systems. This seems logical; after all, plants use inorganic carbon for photosynthesis.  However, the story isn’t that simple, with different algae and seagrasses using different forms of carbon for photosynthesis. Even more confusing is that it looks like the “weedy” species will benefit by switching to the most abundant source of carbon and start to dominate ecosystems (see some of my papers and Harley et al. for the ecosystem effects and Raven & Hurd for the physiological aspects)! But I digress….

The point is that for some reason we don’t seem to draw on this older literature for the general principles of what we may expect to see as CO2 concentrations increase in the oceans. We’re starting to catch up, but the lost time is frustrating – let’s not make the mistakes of past generations but rather learn by them.

Digital library links for: Connell & Russell 2010

Mediation of global change by local biotic and abiotic interactions

Dr Laura FalkenbergThis post is basically a short synopsis of the work done by one of my (now ex-) Ph.D. students, Dr Laura Falkenberg. Laura’s work has turned much of what we thought we knew about the effect of increased CO2 and nutrients on its head; we found synergies where we didn’t expect them (reviewed in a book chapter) and system resilience and resistance to change beyond what we hoped (via strong competitive interaction and trophic links; published in Oecologia, PLoS One and Marine Ecology Progress Series). Laura has certainly helped us look at things in new ways and given us hope that in marine systems where synergies between stressors exist that management of local conditions could potentially buy us some time in mitigating climate change (e.g. reducing nutrient flows into the marine environment, in Journal of Applied Ecology).

Ph.D. thesis: Mediation of global change by local biotic and abiotic interactions
by Dr Laura Falkenberg.

Throughout my Ph.D., I assessed the conceptual model that while cross-scale abiotic stressors can combine to synergistically favour shifts in marine habitats from kelp forests to mats of turfing algae, management of local conditions can counter this change. My experimental manipulations found broad support for the hypotheses that; 1) cross-scale factors (i.e. local and global) can have interactive effects which increase the probability of expansion of turfs but not kelp and, 2) management of local conditions (e.g. maintaining intact forests, limiting nutrient enrichment) can dampen the effects of global change (e.g. forecasted carbon dioxide). I published the results from my thesis in four papers. In the first, I showed that experimental enrichment of CO2 and nutrients influence the biomass accumulation of turf and kelp differently, with turf responding positively to enrichment of both resources while kelp responded to enrichment of nutrients but not CO2. Given that such direct responses could be mediated by interactions with other taxa, in the second paper I considered a key competitive interaction and revealed that the presence of kelp can inhibit the synergistic positive effect of resource enrichment (i.e. CO2 and nutrients) on their turf competitors. Similarly, in the third paper I highlighted the importance of herbivory by showing that under enriched CO2 conditions rates of this process were increased to counter the expansion of turfs. Finally, in the fourth paper, I considered a scenario in which these biotic controls were absent and identified that where multiple resources had been enriched and prompted a synergistic response (i.e. the expansion of turf where CO2 and nutrients are modified), subsequent reduction of the locally-determined factor alone (i.e. nutrients) substantially slowed further expansion of turf algae, but that the legacy of nutrient enrichment was not entirely eradicated. Together, these results represent progress in ecological tests of hypotheses regarding global climate change as they incorporate comprehensive sets of abiotic and biotic community drivers.

You can access all of Laura’s publications from the University of Adelaide’s digital library, or email her for a copy.

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.

Disrupting synergies – making things not so bad.

Healthy forest of the kelp Ecklonia radiataI’ve posted on synergies between environmental stressors (what most people think of as pollution) before. Basically, a synergy is when the impact of the two stressors, say increased CO2 nd nutrients, is greater than the sum of their individual impacts. Once you recognise that synergies can occur, and are often much worse than we predict, the next question is can we do anything to stop them? The short answer is in most cases yes.

The way that synergies work means that, theoretically, if you remove one of the stressors then the “extra” impact should also be removed. In essence, if a synergy is 1 + 1 = 5, then removing 1 means that 1 + 0 = 1. When you’re talking about impacts to ecosystems that are essential to our GDP and way of life (not to mention that they have intrinsic value anyway) that is a really big consideration.

One of my Ph.D. students has just published a rather elegant study demonstrating it is possible to disrupt a synergy between CO2 and nutrients that has the potential to cause the loss of our kelp forest ecosystems. Basically, where CO2 and nutrients cause the synergistic growth of “weedy” species of algae you can remove the nutrients and remove the synergy. There is, however, a caveat. If you wait to remove the nutrients from the system then a large part of the impact will remain – things won’t go back to normal.

The thing that I like most about this outcome is that it provides useful information to the people who manage our coastal waters. If you are concerned that increasing concentrations of CO2 will have a negative impact in areas around major population centres then recycling and redirecting treated waste water away from the ocean, such as into industry or agriculture, can increase the resilience of marine systems. But, timing matters. Sooner is better.

Common synergies

Image

Algal turfs (brown fuzzy stuff) are overgrowing the hard corals at high CO2 concentrations near volcanic vents – a good “natural” experiment. Note also that the seagrass (green, long leaves) are also doing well – a subject for another post.

We frequently hear about “climate change” in the media these days. How could you avoid it? If you do a search of the scientific literature there are thousands of publications a year on the topic. When thinking about the worlds oceans, the most common things that we hear about are ocean warming or ocean acidification (aka. the “evil twin” of warming). Do you notice somthing about this statement? We hear about warming OR acidification. But is this a realistic scenario?

We as scientists commonly break things down into their components and try to understand them one at a time. This is understandable, because the best way to comprehend the functioning of amazingly complex systems is to break them down their component parts and then put them back together again. In this case, however, we are just starting to understand that by breaking things down to individual conditions, either temperature or acidification, we may be missing the most important part of the study. My research group started to realise this in 2009 when we discovered that, when increased in combination, carbon dioxide and nutrients had a massive effect on the growth of “weedy” species of algae which can help to maintain the loss of kelp forests (download the paper here). In hindsight, this result should not be so surprising – both carbon and nitrogen are resources which the algae use to grow. Isn’t hindsight a wonderful thing?

What was surprising is that when carbon dioxide was elevated in the absence of nutrients these algae didn’t respond by grow faster. In fact, they didn’t respond at all to the increased availability of carbon. This means that CO2 and nutrients cause a synergistic response in these algae – where the response to the combined conditions is greater than the sum of responses to the individual conditions (see here for a good review on the topic).

What now worries us is that increasing availability of carbon in the oceans will happen with ocean warming – these are not either/or conditions. Indeed, the first warning shots were fired when we discovered that these same “weedy” turf algae showed the same synergistic growth in response to combined CO2 and warming (see our results published here). We can do something about nutrient pollution (something I will post on in the near future), but CO2 and warming are inherently linked. I think it is time to not talk about warming or acidification but rather to discuss them in tandem.