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An unexpected journey (of eels)

29 05 2023

The way that eels migrate along rivers and seas is mesmerising. There has been scientific agreement since the turn of the 20^th Century that the Sargasso Sea is the breeding home to the sole European species. But it has taken more than two centuries since Carl Linnaeus gave this snake-shaped fish its scientific name before an adult was discovered in the area where they mate and spawn.

Even among nomadic people, the average human walks no more than a few dozen kilometres in a single trip. In comparison, the animal kingdom is rife with migratory species that traverse continents, oceans, and even the entire planet (1).

The European eel (Anguilla anguilla) is an outstanding example. Adults migrate up to 5000 km from the rivers and coastal wetlands of Europe and northern Africa to reproduce, lay their eggs, and die in the Sargasso Sea — an algae-covered sea delimited by oceanic currents in the North Atlantic.

_{The European eel (Anguilla Anguilla) is an omnivorous fish that migrates from European and North African rivers to the Sargasso Sea to mate and die (18). Each individual experiences 4 distinct developmental phases, which look so different that they have been described as three distinct species (19): A planktonic, leaf-like larva (i lecocephalus phase) emerges from each egg and takes up to 3 years to cross the Atlantic. Off the Afro-European coasts, the larva transforms into a semi-transparent tiny eel (ii glass phase) that enters wetlands and estuaries, and travels up the rivers as it gains weight and pigment (iii yellow phase). They remain there for up to 20 years, rarely growing larger than 1 m in length and 4 kg in weight (females are larger than males) — see underwater footage here and here. Sexual maturity ultimately begins to adjust to the migration to the sea: a darker, saltier, and deeper environment than the river. Their back and belly turn bronze and silver (iv silver phase), respectively, the eyes increase in size and the number of photoreceptors multiplies (function = submarine vision), the stomach shrinks and loses its digestive function, the walls of the swim bladder thicken (function = floating in the water column), and the fat content of tissues increases by up to 30% of body weight (function = fuel for transoceanic travelling). And finally, the reproductive system will gradually develop while eels navigate to the Sargasso Sea — a trip during which they fast. Photos courtesy of Sune Riis Sørensen (2-day embryo raised at www.eel-hatch.dk and leptocephalus from the Sargasso Sea) and Lluís Zamora (Ter River, Girona, Spain: glass eels in Torroella de Montgrí, 70 cm yellow female in Bonmatí, and 40 cm silver male showing eye enlargement in Bescanó). Eggs and sperm are only known from in vitro fertilisation in laboratories and fish farms (20).}

As larvae emerge, they drift with the prevailing marine currents over the Atlantic to the European and African coasts (2). The location of the breeding area was unveiled in the early 20^th Century as a result of the observation that the size of the larvae caught in research surveys gradually decreased from Afro-European land towards the Sargasso Sea (3, 4). Adult eels had been tracked by telemetry in their migration route converging on the Azores Archipelago (5), but none had been recorded beyond until recently.

Crossing the Atlantic

To complete this piece of the puzzle, Rosalind Wright and collaborators placed transmitters in 21 silver females and released them in the Azores (6). These individuals travelled between 300 and 2300 km, averaging 7 km each day. Five arrived in the Sargasso Sea, and one of them, after a swim of 243 days (from November 2019 to July 2020), reached what for many years had been the hypothetical core of the breeding area (3, 4). It is the first direct record of a European eel ending its reproductive journey.

Eels use the magnetic fields in their way back to the Sargasso Sea and rely on an internal compass that records the route they made as larvae (7). The speed of navigation recorded by Wright is slower than in many long-distance migratory vertebrates like birds, yet it is consistent across the 16 known eel species (8).

_{Telemetry (6) and fisheries (14) of European eel (Anguilla anguilla). Eel silhouettes indicate the release point of 21 silver females in Azores in 2018 (orange) and 2019 (yellow), the circles show the position where their transmitters stopped sending signals, and the grey background darkens with water depth. The diagrams display the distance travelled and the speed per eel, where the circle with bold border represents the female that reached the centre of the hypothetical spawning area in the Sargasso Sea (dashed lines in the map) (3). Blue, green and pink symbols indicate the final location of eels equipped with teletransmitters in previous studies, finding no individual giving location signals beyond the Azores Archipelago (6). The barplot shows commercial catches (1978-2021) of yellow+silver eels in those European countries with historical landings exceeding 30,000 t (no data available for France prior to 1986), plus Spain (6120 t from 1951) — excluding recreational fishery and farming which, in 2020, totalled 300 and 4600 t, respectively (14). Red circles represent glass-eel catches added up for France (> 90% of all-country landings), Great Britain, Portugal, and Spain. Catches have kept declining since the 1980s. One kg of glass eels contains some 3000 individuals, so the glass-eel fishery has a far greater impact on stocks than the adult fishery.}

Wright claimed that, instead of swiftly migrating for early spawning, eels engage in a protracted migration at depth. This behaviour serves to conserve their energy and minimises the risk of dying (6). The delay also allows them to reach full reproductive potential since, during migration, eels stop eating and mobilise all their resources to swim and reproduce (9).

Other studies have revealed that adults move in deep waters in daylight but in shallow waters at night, and that some individuals are faster than others (3 to 47 km per day) (5). Considering that (i) this fish departs Europe and Africa between August and December and (ii) spawning occurs in the Sargasso Sea from December to May, it is unknown whether different individuals might breed 1 or 2 years after they begin their oceanic migration.

Management as complex as life itself

The European eel started showing the first signs of decline at the end of the 19^th Century (10, 11). In 2008, the species was listed as Critically Endangered by the IUCN, and its conservation status has since remained in that category — worse than that of the giant panda (Ailuropoda melanoleuca) or the Iberian lynx (Lynx pardinus).

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Tags: Anguilla, CITES, European Union, fish migration, hydroelectric dams, ICES, migration, Overfishing, Sargasso Sea
Categories : aquaculture, connectivity, conservation, decline, ecology, environmental policy, Europe, exploitation, fish, fisheries, food, habitat loss, harvest, illegal fishing, impact, IUU fishing, management, marine, metapopulation, recovery, threatened species

Australia: the world’s unsustainable ‘mine’

16 09 2020

The COVID-19 pandemic has finally woken a few people up in this country. The closure of our automotive industry, the volatility of the mining sector, the deteriorating relations with our largest trading partner (China) — all these have seem to have acted like smelling salts for our semi-conscious leaders.

Australia has an abysmal manufacturing capacity, and I know that trying to fix this is very much on the table now at the highest levels. Australia is for the most part a 7.7 million km² ‘mine’ to the world — we of course dig up our minerals and ship them overseas, and we export shit-tonnes of coal.

But much of our agricultural produce goes overseas too, including the very poorly valued live-export industry that takes the little water and minerals already in Australian soils and turns them inefficiently into livestock that is then sold overseas whole and living. Even putting aside the woeful animal-welfare issues this entails, it’s not much of a value-add and really a poor business model.

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Tags: deforestation, environmental sustainability, export, manufacturing, mining, soil degradation
Categories : agriculture, aquaculture, Australia, biodiversity, deforestation, food

Influential conservation ecology papers of 2018

17 12 2018

For the last five years I’ve published a retrospective list of the ‘top’ 20 influential papers of the year as assessed by experts in F1000 Prime — so, I’m doing so again for 2018 (interesting side note: six of the twenty papers highlighted here for 2018 appear in Science magazine). See previous years’ posts here: 2017, 2016, 2015, 2014, and 2013.

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Tags: biodiversity hotspots, biodiversity loss, climate breakdown, climate change, cognition, coral reefs, disease, fieldwork, Fish, fisheries, fishing impacts, Great Barrier Reef, killer whales, mammals, nuisance species, plastics, predators, productivity, richness-productivity relationship, sharks, Species richness, wild areas
Categories : anthropocene, aquaculture, Australia, biodiversity, biowealth, carbon, climate change, climate shift, community ecology, connectivity, conservation, conservation biology, conservation ecology, coral reefs, decline, deforestation, demography, ecology, economics, ecosystem, ecosystem function, ecosystem services, environmental policy, environmental science, exploitation, extinction, extinction debt, fish, fisheries, food, fragmentation, function, habitat loss, harvest, human overpopulation, invasive species, management, marine, marine protected area, modelling, pollination, pollution

Offshore Energy & Marine Spatial Planning

22 02 2018

I have the pleasure (and relief) of announcing a new book that’s nearly ready to buy, and I think many readers of CB.com might be interested in what it describes. I know it might be a bit premature to announce it, but given that we’ve just finished the last few details (e.g., and index) and the book is ready to pre-order online, I don’t think it’s too precocious to advertise now.

A little history is in order. The brilliant and hard-working Katherine Yates (now at the University of Salford in Manchester, UK) approached me back in 2014 to assist her with co-editing the volume that she wanted to propose for the Routledge Earthscan Ocean series. I admit that I reluctantly agreed at the time, knowing full well what was in store (anyone who has already edited a book will know what I mean). Being an active researcher in energy and biodiversity (perhaps not so much on the ‘planning’ side per se) certainly helped in my decision.

And yes, there were ups and downs, and sometimes it was a helluva lot of work, but Katherine certainly made my life easier, and she has finally driven the whole thing to completion. She deserves most of the credit.

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Tags: acoustic, aquaculture, benthic communities, benthos, biodiversity, birds, buffer zones, cables, climate change, co-location, collision, conflict, earthquake, ecosystem services, energy, energy extraction, energy security, entanglement, epifauna, eutrophication, exclusion, exclusive economic zone, Fish, fisheries, fishing, food security, gas leak, impact, infauna, intertidal, invasive species, Law of the Sea, local knowledge, marine, marine mammals, marine spatial planning, mooring lines, noise, objective function, oil and gas, oil platforms, oil spill, OSPAR, partnerships, pile-driving, pipelines, planning, platforms, pollution, portfolio theory, production, productivity, Renewable energy, renewables, resilience, resuspension, sediments, social licence, spatial planning, stakeholder engagement, stakeholders, tidal energy, trade-offs, values, wave energy, wind farms
Categories : acidification, alien species, aquaculture, biodiversity, biowealth, birds, book, carbon, climate change, climate shift, community ecology, connectivity, conservation, development, economics, ecosystem function, ecosystem services, environmental economics, environmental policy, eutrophication, exploitation, fish, fisheries, food, habitat loss, harvest, invasive species, investment, management, marine protected area, modelling, monitoring, MPA, ocean, planning, pollution, prioritisation, research, seagrass, sustainability, toxicology, toxification

Scoping the future threats and solutions to biodiversity conservation

4 12 2009

Way back in 1989, Jared Diamond defined the ‘evil quartet’ of habitat destruction, over-exploitation, introduced species and extinction cascades as the principal drivers of modern extinctions. I think we could easily update this to the ‘evil quintet’ that includes climate change, and I would even go so far as to add extinction synergies as a the sixth member of the ‘evil sextet’.

But the future could hold quite a few more latent threats to biodiversity, and a corresponding number of potential solutions to its degradation. That’s why Bill Sutherland of Cambridge University recently got together with some other well-known scientists and technology leaders to do a ‘horizon scanning’ exercise to define what these threats and solutions might be in the immediate future. It’s an interesting, eclectic and somewhat enigmatic list, so I thought I’d summarise it here. The paper is entitled A horizon scan of global conservation issues for 2010 and was recently published online in Trends in Ecology and Evolution.

In no particular order or relative rank, Sutherland and colleagues list the following 15 ‘issues’ that I’ve broadly divided into ‘Emerging Threats’ and ‘Potential Solutions’:

Emerging Threats

Microplastic pollution – The massive increase in plastics found in the world’s waterways and oceans really doesn’t have much focus right now in conservation research, but it should. We really don’t know how much we’re potentially threatening species with this source of pollution.
Nanosilver in wastewater – The ubiquity of antimicrobial silver oxide or ions in products these days needs careful consideration for what the waste might be doing to our microbial communities that keep ecosystems alive and functioning.
Stratospheric aerosols – A simultaneous solution and threat. Creating what would in effect be an artificial global cooling by injecting particles like sulphate aerosols into the stratosphere might work to cool the planet down somewhat. However, it would not reduce carbon dioxide, ocean acidification or other greenhouse gas-related changes. This strikes me as a potential for serious mucking up of the global climate and only a band-aid solution to the real problem.
Deoxygenation of the oceans – Very scary. Ironically today I was listening to a talk by Martin Kennedy on the deep-time past of ocean hypoxia and he suggests we’re well on our way to a situation where our shelf waters could essentially become too anoxic for marine life to persist. It’s happened before, and rapid climate change makes the prospect plausible within less than a century. And you thought acidification was scary.
Changes in denitrifying bacteria – Just like we’re changing the carbon cycle, we’re buggering up the nitrogen cycle as well. Changing our water bodies to nitrogen sources rather than sinks could fundamentally change marine ecosystems for the worse.
High-latitude volcanism – One of these horrible positive feedback ideas. Reducing high-latitude ice cover exposes all these slumbering volcanoes that once ‘released’, start increasing atmospheric gas concentrations and contributing to faster ice melt and sea level rise.
Trans-Arctic dispersal and colonisation – Warming polar seas and less ice mean fewer barriers to species movements. Expect Arctic ecosystems to be a hotbed of invasion, regime shifts and community reshuffling as a result.
Invasive Indo-Pacific lionfish – Not one I would have focussed on, but interesting. These spiny, venomous fish like to eat a lot of other species, and so represent a potentially important invasive species in the marine realm.
REDD and non-forested ecosystems – Heralded as a great potential coup for forest preservation and climate change mitigation, focussing on maintaining forests for their carbon sequestration value might divert pressure toward non-forested habitats and ironically, threaten a whole new sphere of species.
International land acquisition – Global financial crises and dwindling food supplies mean that governments are acquiring more and more huge tracts of land for agricultural development. While this might solve some immediate issues, it could potentially threaten a lot more undeveloped land in the long run, putting even more pressure on habitats.

Potential Solutions

Synthetic meat – Ever thought about eating a sausage grown in a vat rather than cut from a dead pig? It could become the norm and a way of reducing the huge pressure on terrestrial and aquatic systems for the production of livestock and fish for human protein provision.
Artificial life – Both a risk and a potential solution. While I’ve commented before on the pointlessness of cloning technology for conservation, the ability to create genomes and reinvigorate species on the brink is an exciting prospect. It’s also frightening as hell because we don’t know how all these custom-made genomes might react and transform naturally evolved ones.
Biochar – Burn organic material (e.g., plant matter) in the absence of oxygen, you get biochar. This essentially sequesters a lot of carbon that can then be put underground. The upshot is that agricultural yields can also increase. Would there be a trade-off though between land available for biochar sequestration and natural habitats?
Mobile-sensing technology – Not so much a solution per se, but the rapid acceleration of remote technology will make our ability to measure and predict the subtleties of ecosystem and climate change much more precise. A lot more work and application required here.
Assisted colonisation – I’ve blogged about this before. With such rapid shifts in climate, we might be obliged to move species around so that they can keep up with rapidly changing conditions. Many pros and cons here, not least of which is exacerbating the invasive species problems around the globe.

Certainly some interesting ideas here and worth a thought or two. I wonder if the discipline of ‘conservation biology’ might even exist in 50-100 years – we might all end up being climate or agricultural engineers with a focus on biodiversity-friendly technology. Who knows?

CJA Bradshaw

Sutherland, W., Clout, M., Côté, I., Daszak, P., Depledge, M., Fellman, L., Fleishman, E., Garthwaite, R., Gibbons, D., & De Lurio, J. (2009). A horizon scan of global conservation issues for 2010 Trends in Ecology & Evolution DOI: 10.1016/j.tree.2009.10.003

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Tags: biodiversity, conservation, horizon scanning, science
Categories : acidification, agriculture, alien species, aquaculture, Arctic, biocarbon, biodiversity, biosequestration, carbon, carbon trading, climate change, cloning, conservation, deforestation, environmental engineering, fish, fisheries, habitat loss, invasive species, marine, pollution, research, threatened species, trophic cascades

Sleuthing the Chinese green slime monster

21 10 2009

greenslimemonster I just returned from a week-long scientific mission in China sponsored by the Australian Academy of Science, the Australian Academy of Technological Sciences and Engineering and the Chinese Academy of Sciences. I was invited to attend a special symposium on Marine and Deltaic Systems where research synergies between Australian and Chinese scientists were to be explored. The respective academies really rolled out the red carpet for the 30 or so Australian scientists on board, so I feel very honoured to have been invited.

During our marine workshop, one of my Chinese counterparts, Dongyan Liu from the Yantai Institute for Coastal Zone Research, presented a brilliant piece of ecological sleuthing that I must share with readers of ConservationBytes.com.

The first time you go to China the thing that strikes you is that everything is big – big population, big cities, big buildings, big projects, big budgets and big, big, big environmental problems. After many years of overt environmental destruction in the name of development, the Chinese government (aided by some very capable scientists) is now starting to address the sins of the past.

Liu and colleagues published their work earlier this year in Marine Pollution Bulletin in a paper entitled World’s largest macroalgal bloom caused by expansion of seaweed aquaculture in China, which describes a bloody massive outbreak of a particularly nasty ‘green tide’.

What’s a ‘green tide’? In late June 2008 in the coastal city of Qingdao not far from Beijing (and just before the 2008 Olympics), a whopping 1 million tonnes of green muck washed up along approximately 400 km² of coastline. It took 10,000 volunteers 2 weeks to clean up the mess. At the time, many blamed the rising eutrophication of coastal China as the root cause, and a lot of people got their arse kicked over it. However, the reality was that it wasn’t so simple.

The Yellow Sea abutting this part of the Chinese coast is so named because of its relatively high productivity. Warm waters combined with good mixing mean that there are plenty of essential nutrients for green things to grow. So, adding thousands of tonnes of fertilisers from Chinese agricultural run-off seems like a logical explanation for the bloom.

However, it turns out that the bulk of the green slime was comprised of a species called Enteromorpha prolifera, and it just so happens that this particularly unsavoury seaweed loves to grow on the infrastructure used for the aquaculture of nori (a.k.a. amanori or zicai) seaweed (mainly, Porphyra yezoensis). Problem is, P. yezoensis is grown mainly on the coast hundreds of kilometres to the south.

Liu and colleagues examined both satellite imagery and detailed oceanographic data from the period prior to the green tide and not only spotted green splotches many kilometres long, they also determined that the current flow and wind direction placed the trajectory of any green slime mats straight for Qingdao.

So, how does it happen? Biofouling by E. prolifera on P. yezoensis aquaculture frames is dealt with mainly by manual cleaning and then dumping the unwanted muck on the tidal flats. When the tide comes back in, it washes many thousands of kilos of this stuff back out to sea, which then accumulates in rafts and continues to grow in the warm, rich seas. Subsequent genetic work also confirmed that the muck at sea was the same stock as the stuff growing on the aquaculture frames.

Apart from some lovely sleuthing work, the implications are pretty important from a biodiversity perspective. Massive eutrophication coupled with aquaculture that inadvertently spawns a particularly nasty biofouling species is a good recipe for oxygen depletion in areas where the eventual slime monster starts to decay. This can lead to so-called ‘dead’ zones that can kill off huge numbers of marine species. So, the proper management of aquaculture in the hungry Goliath that is China becomes essential to reduce the incidence of dead zones.

Fortunately, it looks like Liu and colleagues’ work is being taken seriously by the Chinese government who is now contemplating financial support for aquaculturists to clean their infrastructure properly without dumping the sludge to sea. A simple policy shift could save a lot of species, a lot of money, and a lot of embarrassment (not to mention prevent a lot of bad smells).

CJA Bradshaw

Liu, D., Keesing, J., Xing, Q., & Shi, P. (2009). World’s largest macroalgal bloom caused by expansion of seaweed aquaculture in China Marine Pollution Bulletin, 58 (6), 888-895 DOI: 10.1016/j.marpolbul.2009.01.013