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 ( 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 (, 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 (

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 ( 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 (

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 ( 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 (

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 ( 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 ( 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.

Monday, 2 November 2015

Plankton as indicators of ecosystem change

From the crabs on the seabed to the seagulls in the sky.

Angular crab larvae

I explained in my previous article why there wouldn't be any polar bears on the ice without any plankton at the sea surface. In this blog I want to begin to tell you how studying the plankton has helped to understand climate-induced changes in the North Sea, a productive marine ecosystem that once provided 5% of the total global seafood harvest. Here, a recent warming of this shallow sea has altered the ecosystem from the animals that live on the seabed to the the seagulls in the sky above, and these changes were first noticed in the plankton.

Analysis of long-term plankton samples collected in the North Sea by the Continuous Plankton Recorder Survey ( revealed that the number of decapod larvae, and particularly the larvae of swimming crabs, had increased in abundance since the mid 1980s. This increase appears to be related to temperature. Following warm winters there are more decapod larvae in the plankton than following cold winters, and as the North Sea has warmed since the mid 1980s the number of decapod larvae has increased. (As you can see from the correlation between sea temperature and decapod larvae in the graph below).

Analysis of crab larvae confirmed that not only had swimming crab larvae increased in abundance, but the North Sea had also been colonised by two warm-water species, the swimming crab Polybius henslowii and the angular Goneplax rhomboides ( Swimming crabs are a good food source for lesser black-backed gulls (especially, during the breeding season), and their numbers have increased following the increase in swimming crabs (

Swimming crab (Polybius henslowii)

A simple bioenergetic model produced by German scientists showed that the 22,000 individual lesser black-backed gulls in the most important breeding colony in the south-eastern North Sea consumed approximately 35 million swimming crabs annually (i.e. 1590 swimming crabs per individual gull) during the breeding period (

While the increase in swimming crabs may be good news for lesser black-backed gulls, the general increase in decapods in the North Sea suggested from plankton analysis may not be such good news for their prey species, such as bivalves and flatfish, which have both shown a decline in abundance as the number of decapod larve in the plankton have increased. The decapod megalopa larva is a voracious predator in the plankton and the newly settled decapods prey upon young bivalves and flatfish recruits on the seabed.

So, I hope that this small story shows how by studying the plankton, in this case the abundance of decapod larvae, we can not only detect early signs of change, but perhaps also understand how changes in the marine ecosystem may extend from the seabed to the skies above.

Dr Richard Kirby is a British plankton expert, scientist, author and speaker. Follow Richard @planktonpundit on Twitter. You can see more images of plankton and learn more about them in Dr Richard Kirby’s book “Ocean Drifters, a secret world beneath the waves” available on Amazon and as an iBook.