Monday, 8 February 2016

What’s happening to the oceans’ phytoplankton?

Research suggests that the oceans’ phytoplankton are declining in abundance in many places.
Global Biosphere September 1997 - August 1998 composite image showing the magnitude and distribution of global oceanic chlorophyll a and terrestrial primary production.

But first of all, what are phytoplankton, and secondly, why should we be interested in what’s happening to them?

Phytoplankton are microalgae that float, drifting in the seas’ sunlit surface. Although these microalgae are too small individually to be seen by your naked eye, collectively they are so numerous that they are the ocean’s main primary producers. Just like plants on land, the phytoplankton uses the energy in sunlight to combine carbon dioxide and water to produce sugar and oxygen in the process we call photosynthesis. To give you an idea of the significance of phytoplankton, it is interesting to compare them to the macroalgae – the seaweeds, with which most people will be more familiar. Collectively, the macroalgae, although much bigger account for less than 5% of the total primary production in the sea each year. As primary producers the phytoplankton underpin the marine food chain, and determines the abundance of other marine life, from the amount of fish in the sea to the number of polar bears on the Arctic ice (see: The importance of plankton).

The phytoplankton do more than underpin the marine food chain however, as they also play a central role in the global carbon cycle by influencing the atmospheric composition of the greenhouse gas carbon dioxide. Over hundreds of millions of years the burial of some of the organic carbon fixed by phytoplankton photosynthesis sequestered carbon in the sediments; some of this became the Earth’s oil and gas reserves. Likewise, over similar time-scales, the burial of inorganic carbon in the calcium carbonate remains of phytoplankton coccolithophores sequestered carbon in deposits we now know as chalk. It is changes in the rates of phytoplankton growth and carbon fixation that are thought to have played an important role as feedback mechanisms driving climate change during glacial / interglacial periods. For example, as global temperatures cooled as the Earth entered a glacial period, it is suggested that steeper temperature gradients would have developed between the poles and the equator strengthening winds, which are thought to have then blown nutrients from the land to the sea. This increase in nutrients could have acted as a fertiliser promoting phytoplankton growth leading to a greater drawdown of carbon dioxide and a further cooling of the atmosphere.

So, as you can see, the phytoplankton play an important role on Earth despite their diminutive size, which is why a paper published by 3 Canadian Scientists in Nature in 2010 (http://bit.ly/1OwkMze and http://bit.ly/1Q4HseL) caused quite a stir. The authors of the Nature paper led by Daniel Boyce used a 100-year data set to see if phytoplankton had changed in their abundance in the sea. Their results suggested that the phytoplankton had reduced in abundance globally by 40% since 1950, or a decline of about 1% per year. They suggested that warming seas due to climate change might have led to increased stratification (layering) reducing the supply of nutrients to the surface from deeper waters; in essence, the supply of fertiliser to the surface promoting phytoplankton growth had reduced. The Canadian scientists study was controversial however. Other scientists thought they saw different results and the study was also criticised on the fact that it combined data on phytoplankton collected in very different ways. The earlier part of the data set used Secchi Depths as a measure of phytoplankton (more about Secchi Depths in a later blog) and the later part used measurements of chlorophyll abundance.

Fast forward to the last three months of 2015 and three new studies on phytoplankton abundance were published in quick succession. The first of these by Cecile Rousseau and Watson Gregg looked at satellite ocean colour and environmental data, and reported that diatoms had declined by more than 1 percent per year from 1998 to 2012 globally (the 15-year period the scientists studied) leading to changes in both phytoplankton abundance and community composition. Losses of diatoms were most significant in the North Pacific, the North Indian, and the Equatorial Indian oceans (http://bit.ly/1Rhd5kg and http://bit.ly/1W5oJOv). The scientists’ study suggested that a likely cause was a shallowing of the mixed-layer by 1.8 meters (5.9 feet). The mixed-layer is the uppermost layer of ocean water and a shallowing would reduce the nutrients available for phytoplankton growth. Why the mixed layer shallowed is still uncertain. One possibility the scientists suggested is a change in wind.
A collection of diatoms ©Richard Kirby. About 50% of the primary production on Earth takes place in the oceans and diatoms are the most important photosynthetic eukaryotes accounting for about 40% of total marine primary production.

However, the second study, also published in October 2015, and led by Michael Behrenfeld (http://bit.ly/1Pgwj74) suggested that there is an inherent error in the algorithm we use to convert satellite measures of ocean colour (an estimation of chlorophyll abundance) into phytoplankton biomass. Phytoplankton can alter the amount of chlorophyll in their cells depending upon light intensity and nutrients. As a result, the scientists suggested that contemporary relationships between chlorophyll changes derived from satellite measures of ocean colour, are not indicative of proportional changes in productivity; light-driven decreases in chlorophyll can be associated with constant or even increased photosynthesis. In other words, failing to take account of this feature introduces a source of error causing temporal anomalies in surface chlorophyll to over-represent associated changes in mixed-layer productivity.

The third study published in November 2015 by lead author Sara Rivero-Calle (http://bit.ly/1PJEzOs and http://bit.ly/1LFI9T5) looked at the abundance of coccolithophores in the North Atlantic over the last 45 years. Coccolithophores are the phytoplankton that surround their cells with plates of calcium carbonate. These authors found that the relative abundance of coccolithophores had increased 10 times, or by an order of magnitude, during this 45-year period. During the same period the authors found that the relative abundance of other species such as diatoms, had declined in some places. To explain the increase in coccolithophores the authors suggested that they may be taking advantage of the extra carbon from carbon dioxide dissolved in seawater as a result of rising levels of atmospheric carbon dioxide. Interestingly, in the geological record, coccolithophores have been typically more abundant during Earth’s warm interglacial and high CO2 periods.

Now, at the beginning of 2016, a new paper titled “A reduction in marine primary productivity driven by rapid warming over the tropical Indian Ocean” has just been published in the Journal Geophysical Research Letters - http://bit.ly/1PvYhcQ. In the paper lead by author Matthew Koll Roxy from the Centre for Climate Change Research at the Indian Institute of Tropical Meteorology, data is presented demonstrating a decline in phytoplankton in the western Indian Ocean by up to 20% over the last 60 years. The western Indian Ocean shows the largest warming trend among the tropical oceans and the study’s authors found the downward trend in phytoplankton could be explained by a reduction in nutrients reaching the surface form deeper waters due to increased stratification of the water column.

So what’s happening to the ocean’s phytoplankton at a global scale? I’d say there is both mounting evidence that phytoplankton populations are changing globally and that we need to understand how we study them, and as a result we urgently need much more research. As I explained above, these tiny organisms are central to the marine food chain and the global carbon cycle and consequently, I’d argue, it is imperative to understand what is happening to help us appreciate the ramifications of climate change, not just in the oceans but also for our planet.

Dr Richard Kirby is a British plankton expert, scientist, author and speaker. Follow Richard @planktonpundit on Twitter. Richard’s book “Ocean Drifters, a secret world beneath the waves” is available on Amazon and as an iBook www.planktonpundit.org.

Sunday, 13 December 2015

Mangroves, an invaluable ally against climate change

Mangroves are the rainforests by the sea, found at the boundary where land meets ocean. They serve a wide range of ecological functions, providing economically valuable products and services. Mangroves, once estimated to cover an area of over 36 million hectares, dominated large stretches of tropical coastline. However, due to ongoing development pressures, mangroves are degraded and their area substantially diminished relative to their historic range, less than 15 million hectares remain.

Mangrove forests are vital for healthy coastal ecosystems. The shallow inter-tidal reaches that characterize mangrove wetlands offer refuge and nursery grounds for juvenile fish, crabs, shrimps, and molluscs, and are prime nesting and migratory sites for hundreds of bird species. Additionally, manatees, crab-eating monkeys, monitor lizards, Bengal tigers, sea turtles and mudskipper fish utilize the mangrove wetlands.

Mangroves play a vital role in protecting sea grasses and coral reefs from sediments and pollution, filtering out heavy metals and halting shoreline erosion. Mangroves buffer against hurricane winds, storm surges and tsunamis, saving thousands of lives, while protecting infrastructure. Mangroves are also invaluable in combating climate change!

Mangroves, tidal marshes and seagrass beds remove massive amounts of carbon from the atmosphere and fix it in mangrove soils, where it can remain for millennia. Unlike terrestrial forests, marine wetlands are constantly building carbon pools, storing large amounts of so-called "blue carbon" in highly organic sediments, storing up to 5-times more carbon per unit area than tropical rainforests. Their carbon sequestration potential is significant in helping to reduce atmospheric carbon dioxide. Including the carbon stored in soils, mangrove forests store the most carbon per hectare of any other forest type.

Deforestation and land-use change currently account for 8-20% of global anthropogenic carbon dioxide (CO2) emissions, second only to fossil fuel combustion. Destruction of mangroves accounts for around 10% of emissions from deforestation globally, despite accounting for just 0.7% of tropical forest area. Moreover, if left undisturbed, the carbon storage by mangroves currently continues to expand through biological sequestration of CO2 and carbon burial. If current trends in conversion continue, however, much of the carbon stored in mangroves along with its future accumulation could be lost.

Mangroves are among the most threatened and rapidly disappearing natural environments worldwide, with a much higher rate of loss than other tropical rainforests. One of the greatest threats to mangroves today is the rapacious shrimp aquaculture industry, which has caused massive mangrove losses in Asia and Latin America. With the current 0.7% rate of loss, most of the world’s mangroves may disappear by the end of this century. Conversion for agriculture or aquaculture, results in massive emissions of greenhouse gases into the atmosphere, as mangroves change from a sink for carbon to a massive source. This greatly exacerbates the problems of global warming.

Restoring mangrove forests would deliver significant benefits in reducing net greenhouse gas emissions, improving food security and livelihoods of coastal communities, increasing resilience in the face of sea level rise and extreme weather events, and improving habitat for many vulnerable species along extremely biodiverse tropical coastlines.

Alfredo Quarto is the Executive Director and co-founder of the Mangrove Action Project. You can follow Alfredo on Twitter @mangroveap. For more information visit MAP’s website www.mangroveactionproject.org.

Saturday, 5 December 2015

Fishing for deadly ghost gear

Ghost Fishing’ is what fishing gear does when it has been lost, dumped or abandoned. Imagine a fishing net that gets snagged on a reef or a wreck and gets detached from the fishing vessel. Nets, longlines, fish traps or any man-made contraptions designed to catch fish or marine organisms are considered capable of ghost fishing when unattended, and without anyone profiting from the catches, they are affecting already depleted commercial fish stocks. Caught fish die and in turn attract scavengers which will get caught in that same net, thus creating a vicious circle.
Ghost fishing longline survey Croatia

















Lost fishing gear, or so-called ‘ghost gear’ is one of the greatest killers in the oceans. Literally hundreds of kilometres of nets and lines get lost every year and due to the nature of the materials used to produce these types of gear, they can and will keep fishing for multiple decades, possibly even centuries.
Ghost fishing net survey Croatia

















Divers are all too familiar with this phenomenon, especially in well-fished areas. As founders of the Ghost Fishing Foundation we were confronted with ghost gear while diving the many wrecks in the Dutch North Sea. In 2009 we were part of a local team of divers who started to clean those wrecks. After some years of local efforts it was time to broaden the horizon and get in touch with like-minded groups all over the world. And so the Ghost Fishing Foundation was born.
Ghost fishing net survey Croatia

















The Ghost Fishing Foundation has been collaborating worldwide with various local groups of divers and salvage companies to remove lost fishing gear. With projects in Netherlands, Belgium, Germany, Croatia, United Kingdom and the United States we work on existing projects, set up new ones and document these through visual media, informing a wide audience and raising social awareness. We exchange solutions and best practices by maintaining a steady stream of information through social media, and a website that offers extensive information and possibilities for interaction.

Ghost fishing net recovery Croatia
The Ghost Fishing Foundation recently launched unique collaborations with several well-known organisations like Healthy Seas Initiative, World Animal Protection and Greenpeace and they are part of the Global Ghost Gear Initiative (GGGI). The GGGI aims to improve the health of marine ecosystems, protect marine animals, and safeguard human health and livelihoods. GGGI was launched in September 2015 and is the first initiative dedicated to tackling the problem of ghost gear on a global scale. The GGGI’s strength lies in the diversity of its participants including the fishing industry, the private sector, academia, governments, intergovernmental and non-governmental organisations.

If you would like to know more about ghost fishing gear visit www.ghostfishing.org or their social media channels on Facebook, Instagram and Twitter.

Pascal van Erp is the founder and chairman of the Ghost Fishing Foundation. You can follow Pascal at @suberp on Twitter.

Saturday, 28 November 2015

Bluefin tuna: a new perspective in the NE Atlantic

 In 2013, Angus Campbell caught a 515lb Atlantic bluefin tuna with a rod and reel off the Isle of Harris, the Outer Hebrides. Although this wasn’t out of the realms of possibility, this had never been done before in Scotland. In 2014, Dr. Francis Neat (Marine Scotland Science) initiated a scientific program on bluefin tuna in Scotland with the aim of finding out three things: 1) how long bluefin resided in Scottish waters, 2) where they went, when they left, and 3) what stock the fish belonged to. That year we successfully tagged three bluefin with miniPAT tags from Wildlife Computers. Although the project was a success, the results from our work were far from conclusive. We’re now in the process of starting a collaborative study with Stanford University, to contribute knowledge on this enigmatic species to the bigger picture of bluefin in the northeast Atlantic.

Atlantic bluefin tuna post-release off the Isle of Harris, Scotland exhibiting miniPAT















Bluefin tuna are commercially important, highly migratory apex predators, split into three geographically distinct species: Atlantic, Southern and Pacific bluefin tuna. Demand for these fish has skyrocketed over the last few decades in line with the rise of the Japanese sushi-sashimi market, in which bluefin is the most highly prized delicacy. The majority of bluefin caught is flash frozen and shipped to Japan for auction, with single fish fetching exorbitant amounts of money: in 2013 a fish weighting 489lbs sold for $1.76m. Although this figure was especially high, fish regularly sell for tens of thousands of dollars.

Graph showing price of inaugural bluefin tuna sold at the Tsuiji fish market, Japan












Atlantic bluefin tuna are comprised of at least two genetically distinct stocks, designated by their spawning region: the eastern stock in the Mediterranean and the western stock in the Gulf of Mexico. Fish from both stocks make seasonal migrations from warm low-latitude waters to highly productive foraging grounds at higher latitudes [1]; although not all fish do this, and whether bluefin migrate or not is related to maturity and body size, with larger fish ranging further. Despite being genetically distinct, migratory fish mix extensively outside of their spawning areas and fish from the western stock can be found in the eastern Atlantic and vice versa [2]. As a result of prolonged overfishing on both sides of the Atlantic, the western and eastern stocks were reduced to 17% and 33% respectively, of 1950’s spawning stock biomass by 2008 [3]. This caused the bluefin regulatory body, ICCAT (the International Commission for the Conservation of Atlantic Tunas) to introduce stock rebuilding programs, ultimately resulting in slashed catch quotas.

Atlantic bluefin tuna, south Donegal, Ireland 2015
The extent of bluefin distribution is limited by temperature, despite their advanced thermoregulatory capacity. Recent reports of bluefin in the Greenland strait (2010), the establishment of small-scale fisheries off Iceland and Norway (2014), increased sightings off Ireland and Scotland (2012-13-14), fish caught off Wales (2015) and even sightings off Cornwall, England (2015) suggests bluefin have repatriated highly productive northern latitudes in significant numbers in recent years. This simple fact would lead us to believe that something has changed; what that ‘something’ is, is cryptic.

Map showing UK and Ireland Atlantic bluefin tuna sightings 2013-15




























We are exploring three possible causes for these recent changes; 1) a warming ocean climate, allowing tuna to exploit areas previously too cold, 2) the forage prey are now ranging further north and in greater abundance than previously believed, e.g. mackerel, or 3) a recovery of the eastern bluefin stock, as has been heralded by ICCAT; this would result in a more significant cohort of larger, more migratory fish. These hypotheses are not mutually exclusive and may all be acting in concert. Consequently, this is just the beginning of tuna research in the UK and Ireland.

Atlantic bluefin tuna feeding on sprat off Donegal, Ireland 2015














Our work would certainly not be possible without the efforts of a number of recreational fishermen, acting responsibly on a catch-and-release basis. This form of fishing represents a hugely sustainable way of gaining revenue from bluefin tuna. A report looking into further developing the existing bluefin recreational fishery in Canada’s Atlantic provinces, estimated that 1 tonne of bluefin quota allocated to a live-release fishery could yield up to $100,000; 6 times that of a capture fishery, whilst having minimal effect on the stock [4]. If the eastern Atlantic bluefin stock has bounced back, such fisheries may have a place in the UK and Ireland, and one of the key project aims of our work in Scotland is to advise as to whether or not this would be a possibility. Well managed catch-and-release fisheries represent a way of providing much needed revenue to often remote coastal communities, whilst also supporting vital scientific research on apex predators and maintaining good fish stocks.

1: Walli, A. et al. PLOS One, 4(7): e6151. (2009) doi: 10.1371/journal.pone.0006151
2: Block, B. et al. Nature, 434: 1121-1127. (2005) doi: 10.1038/nature03463
3: Taylor, N. et al. PLOS One, 6(12): e27693. (2011) doi: 10.1371/journal.pone.0027693

Tom Horton is a marine biologist, wildlife guide and photographer specialising in the spatial ecology of marine megavertebrates around the UK & Ireland. His current work involves basking sharks, ocean sunfish and Atlantic bluefin tuna. You can follow Tom and check out his work and pictures on Twitter @profhorts.

Monday, 9 November 2015

Seagrass meadows – carbon sinks and fishery powerhouses

The most underappreciated of marine ecosystems, the humble seagrass meadow.













I might be biased, but seagrass meadows are just a little bit fantastic. Seagrasses are admirable in that they do a lot just by being themselves; they are the strong silent type, happy for their flashy coastal colleagues the coral reefs to get all the adoration, whilst they quietly continue in the background, churning out the next generation of baby fish, and sinking lethal dissolved carbon into the seabed. 

To borrow a rugby analogy, they are the second rows of the coastal seascape, working hard in the ‘engine room’ whilst it’s the ‘pretty’, ‘flashy’ backs that grab the headlines with their razzle-dazzle. Seagrass meadows are barely recognised on the world stage, and hardly ever make the front page news, but they are essential to our metaphorical team’s success, week in, week out.

Seagrass meadows provide carbon capture storage at rates up to 100 times greater than rainforests. One hectare (10,000 m2) of seagrass can support up to 80,000 fish, and produce up to 100,000 litres of oxygen per day. Put another way, in a recent Plymouth University study, seagrass meadows in the Mediterranean Sea were valued as contributing approximately €190 million per year to local fisheries.














The drawback for seagrass meadows is that they just aren't seen to be as sexy as coral reefs. However, with the passionate work of a handful of individuals we are looking to change that perspective. For example, here in the UK there are a couple of recently formed organisations championing the cause for seagrass meadows.

In the southwest of England there is a fantastic new venture called the ‘Community Seagrass Initiative’ being run out of the National Aquarium (http://www.csi-seagrass.co.uk) in Plymouth. Their CSI project covers the 191 mile stretch of coastline from Looe in Cornwall, to Weymouth in Dorset and is seeking to engage coastal communities with their local seagrass meadows, raising awareness and promoting conservation.

In Cardiff, Wales, one of the UK’s newest marine charities has also recently been born – Project Seagrass (http://www.projectseagrass.org), of which I am a proud founding member.























Project Seagrass is an environmental charity devoted to the conservation of seagrass ecosystems through education, influence, research and action. We’re here to communicate to you that seagrasses both locally and globally are under threat, and as such their capacity to act as both carbon sinks and fisheries powerhouses is being jeopardized by our actions.

So what are these threats? Anchoring and inappropriate moorings scar the seabed and uproot the seagrass; less seagrass equals fewer fish. Furthermore, coastal development, litter, pollution and waste can smother seagrasses reducing their access to the vital sunlight they need for growth; less seagrass growth equals less CO2 absorption.

Protecting seagrass helps to ensure food security and fights climate change. Some of our most iconic sea creatures live in seagrass; seahorses, sea turtles and sea cows all need seagrass meadows. Can you imagine a world without them?

Richard ‘RJ’ Lilley is a British seagrass scientist and science communicator. Follow Richard @rjlilley on Twitter. You can see more images of seagrass and learn more about his work by following @projectseagrass on Facebook, Twitter and Instagram. 

Thursday, 5 November 2015

Understanding climate, plankton and commercial fishing

If you were to ask people what influences the abundance of fish in the sea, most would probably answer ‘commercial fishing’. While it is true that commercial fishing can deplete fish stocks, another important factor, the ‘environment’, or more accurately, environmental variability, is often overlooked as a determinant of fish abundance.

Atlantic cod  (Gadus morhua)

Certainly, when the environment is benign, commercial fishing, referred to as a ‘top-down’ control of fish abundance (think of fishing as a form of predation) can be the most important influence. However, when the environment changes and becomes unfavourable, the environment will become a key driver of fish abundance, and this is referred to as ‘bottom-up’ control. Understanding the interplay of bottom-up and top-down controls is vital for sustainable fisheries, and especially at a time of climate change and warming seas.

It is the phytoplankton and the plankton food web that determines the abundance of fish and all other creatures in the sea (http://blueplanetsociety.blogspot.co.uk/2015/03/the-importance-of-plankton.html).

Stylised diagram of the plankton food web with respect to the herring, a fish that feeds upon plankton throughout its life. The arrows show the interconnections in the food web, ‘who eats who’, revealing the complexity and how it might easily uncouple.

The plankton live at the sea surface and their habitat is likely to warm due to anthropogenic climate change. More and more studies are now providing evidence that the plankton’s distribution, abundance and seasonality is altering as their habitat warms, uncoupling the marine food chain. In the North Atlantic ocean many of these studies have focused upon understanding the population dynamics of the cod Gadus morhua due to its commercial importance and history of population declines.

In the Northeast Atlantic, warm-temperate, pseudo-oceanic species of copepod have moved northwards by about 10° of latitudes over 48 years between 1958 and 2005 (52–62°N;10°W) as the sea surface has warmed, which is a poleward movement of 23.16 km per year (http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2486.2009.01848.x/full). Cold water species of copepod have retracted towards the poles and warm-water species have moved northwards. This movement of copepods has resulted in a 60% reduction in the preferred food of larval cod in the North Sea, the cold water copepod Calanus finmarchicus, affecting cod recruitment (the number of juvenile cod that survive to become adults).

Cod is also a cold water species and the North Sea lies at the southern edge of this fish’s distribution (http://www.int-res.com/articles/cr2010/41/c041p015.pdf).

The thermal niche of the Atlantic cod (blue area) based on mean annual sea surface temperature (SST) during the period 1960 to 2005 and the probability of cod occurrence. Both observed (1960 to 2005, shaded bars, white text and arrows) and projected (based upon climate change scenario A2, 1990 to 2100, black text and arrows) ranges in SST are shown for Iceland (solid vertical lines) and the North Sea (dashed vertical lines), indicating that under climate change scenario A2, the North Sea becomes too warm for high numbers of cod such that it may be an unviable fishery.
So, not only are cod in the North Sea experiencing fishing pressure, but the warming environment is now exerting bottom-up control too, both through the food web and upon cod directly (http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2486.2009.02063.x/full). You might well argue that cod will just move northwards and so ‘all will be OK’. Unfortunately, it is not so simple as both the habitat (bathymetry) and temperature must be suitable. The North Sea is a shallow, nutrient rich sea and so supports a productive plankton food web. (Shallow seas, like nutrient rich upwelling regions, support the world’s most productive fisheries.) Northwards of the North Sea the ocean deepens and is less favourable for plankton and cod. Here, the shallow seas are restricted to continental shelves along the coast of Norway and surrounding Iceland. The next, large, favourable habitat for cod is the Barents Sea.

Cod feed upon crabs, lobsters and shrimps and in regions where cod have declined through overfishing, such as in the Northwest Atlantic, there has been a large increase in these decapods, which may be due to decreased predation pressure upon them (relaxation of a top-down control). In the North Sea, where cod have declined due to the combined effects of fishing and environmental change, decapods have also increased in abundance. Currently, the abundance of decapods in the North Sea is also influenced positively by warming; they produce more offspring when the sea is warmer and warm-water species have also invaded. And so, in the North Sea, the decline of cod and the warming environment may both be favouring decapods. In turn, this has ramifications for other species and the ecology of the North Sea; there are ‘winners and losers’ in this ecosystem (http://blueplanetsociety.blogspot.co.uk/2015/11/plankton-as-indicators-of-ecosystem.html).

Two recent studies of cod have again shed light upon how important the environment is as a driver of an animal’s abundance. These two studies are focused at the northern and southern limits of the distribution of cod in the Northwest Atlantic. Here, at the species’ southern limit in the Gulf of Maine, warming seas are reducing the abundance of cod (http://www.sciencemag.org/content/early/2015/10/28/science.aac9819.full). In contrast, at the northern, cold boundary of the species’ distribution in Newfoundland, warming seas are having a positive impact upon their numbers through the food web (http://www.nrcresearchpress.com/doi/10.1139/cjfas-2015-0346#.VjjRWqS3KPQ). Interestingly, these two studies also reveal that environment is a more important determinant of abundance than controls upon overfishing.

If the global climate and the sea surface continues to warm it does not mean that we will not experience some seasons and years that are colder than others. (Of course, due to the warmer baseline temperature, these colder years will not be as cold as they might have been in the past.) Again, using cod as an example, in these cooler years we may see an increase in cod abundance among populations that reside at the warmer edge of the species’ niche due to more favourable conditions (such as cooler conditions would create in the Gulf of Maine or the North Sea). In these circumstances, if we only consider commercial fishing activity to influence abundance, we may be lulled into a false belief that a fish stock is recovering due to effective fishery management strategies, only to find that we were wrong when the sea temperature increases again.

Ecosystems by their nature are complex with many linkages among the species they contain. Understanding the interactions among species, and how and why they change, which must include an understanding of the environment, is key to their sustainable exploitation. Consideration of environmental changes is absolutely necessary with regard to anthropogenic climate change. While there is no guarantee that setting quotas will enable a stock to resist adverse climatic conditions, an absence of regulation might well precipitate a stock’s collapse, or might cancel any short-term benefit of improved environmental conditions.

Dr Richard Kirby and Dr Grégory Beaugrand are plankton scientists interested in marine ecosystem dynamics and fisheries.