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 20th 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 20th 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 19th 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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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

Neo-colonialist attitudes ignoring poachernomics will ensure more extinctions

14 01 2022

No matter most people’s best intentions, poaching of species in Sub-Saharan Africa for horn and ivory continues unabated. Despite decades of policies, restrictions, interventions, protections, and incentives, many species of elephant and rhino are still hurtling toward extinction primarily because of poaching.

Clearly, we’re doing something heinously wrong.

Collectively, we have to take a long, hard look in the conservation mirror and ask ourselves some difficult questions. Why haven’t we been able to put any real dent in the illegal trade of poached elephant ivory and rhino horn? How many millions (billions?) of dollars have we spent seemingly to little avail? Why haven’t trade bans and intensive security measures done the trick?

The reasons are many, but they boil down to two main culprits:

  1. neo-colonialist sentiments driven by the best intentions of mainly overseas NGOs have inadvertently created the ideal conditions for the poaching economy — what we term poachernomics — to thrive by ensuring the continued restriction of legal supply of wildlife products; and
  2. shutting off conservation areas to local people and directing the bulk of ecotourism profits away from source communities have maintained steady poaching incentives in the absence of other non-destructive livelihoods.

In our new paper — Dismantling the poachernomics of the illegal wildlife trade (led by Enrico Di Minin of the Universities of Helsinki and KwaZulu-Natal, and co-authored by Michael ‘t Sas-Rolfes of the University of Oxford, Jeanetta Selier of the South African National Biodiversity Institute, Maxi Louis of the Namibian Association of Community-Based Natural Resources Management Support Organizations, and me) — published quietly in late 2021, we describe how poachernomics works, and why our efforts to incapacitate it have been so ineffectual.

First, what is poachernomics?

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Categories : Africa, Asia, China, conservation, conservation biology, corruption, culture gap, development, economics, environmental economics, environmental policy, exploitation, extinction, extinction vortex, governance, harvest, human-wildlife conflict, mammal, management, poaching, poverty, preservationist, propaganda, protected area, reserve, sustainability, threatened species, tourism

Fancy a pangolin infected with coronavirus? Apparently, many people do

30 12 2021

The logic of money contradicts the logic of species conservation and human health. As illegal trade has driven pangolins to near extinction, their hunting and market value has kept increasing ― even when we have known that they act as coronavirus reservoirs in the middle of the Covid-19 pandemic.

Sunda pangolin (Manis javanica) in a monsoon forest (Sumba Island, Indonesia). With adult weights up to 10 kg and body lengths around half a metre, these animals are mostly solitary and nocturnal, feed on ants and termites, and love tree climbing using bark hollows to shelter and give birth to singletons. The species occurs across mainland and islands of South East Asia, and became ‘Endangered’ in 2008 and ‘Critically Endangered’ in 2014, following a 80% decline in the last 20 years due to hunting and poaching. It has been the most heavily trafficked Asian species, and the IUCN’s assessment states: “… the incentives for harvesting and illegally trading in the species are universally high based on the high financial value of pangolin parts and derivatives”. Captive breeding is unlikely to deter wild collection because (among other reasons) farming costs are high (more so on a large scale) and, even if the species could be traded legally, wild versus farmed pangolin products and individuals are difficult to distinguish (23). Photo courtesy of Michael Pitts

Urbanites are attracted to exotic species, materials, and places. Our purchasing power seems to give us the right to buy any ‘object’ that we can pay for, no matter how exotic the object might be. In such a capitalist rationale, it is no surprise that > 150 thousand illegal cargos with wild animals and plants have been confiscated in 149 countries over the last two decades, moving some 6000 species from one place of the planet to another (1).

Social networks show people interacting with all kinds of fauna, creating the illusion that any animal can become a pet (2). And there’s a multi-$billion market of wildlife for a diverse array of uses including collecting, food, ornamentation, leisure, clothing and medicine (3-5). The paradox is that the rarer a species is, the higher its market value runs and the more lucrative selling it turns out to be, leading to more exploitation and rocketing extinction risk (6).

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Categories : Africa, Asia, biodiversity, China, conservation, conservation biology, corruption, decline, disease, economics, environmental policy, exploitation, extinction, harvest, Malaysia, mammal, poaching, poverty, South East Asia, threatened species, tropical

How much is that iguana in the window?

25 08 2020

In our latest study, we examine the downstream effects of publicising an elevated species description for a reptile that is highly prized in the international commercial wildlife trade.

We describe how iguanas from an insular population of the common green iguana (Iguana iguana) entered commercial trade shortly after an announcement was made indicating that the population would be described as a new species.

The international commercial wildlife trade presents a known risk factor for wild populations of threatened species. One organisation in particular regulates the international trade in species — the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).

Although most people probably know about the illegal practices involving iconic elephants and rhinos, reptiles are also targeted and traded. For example, after its discovery and description in 2016, and even though locality data were safeguarded, China’s endemic Mountain spiny crocodile newt (Echinotriton maxiquadratus) quickly entered the trade. This put conservation pressure on this small-range species (1, 2). Therefore, CITES signatory countries placed this species on its Appendix II in 2019, which lists animals and plants in need of protection.  

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Categories : alien species, captive breeding, conservation, conservation biology, environmental policy, exploitation, tropical

Crocodiles, spiders and leeches

11 04 2011

I just wrote a fun little piece for a new section in the journal Frontiers in Ecology and the Environment that they’re calling Trails and Tribulations. The basic idea is that the author recounts a particularly interesting field-related experience through which an ecological concept is woven.

Editor-in-Chief Sue Silver said that I could reproduce my article here as long as I acknowledged Frontiers and the Ecological Society of America. It was fun to write, and I hope you enjoy it too [the PDF of the article is available free of charge here].

—

“So does each team get a hand gun?”

“No, you get an oar”

“What good is an oar?”

“Listen, mate. When a 3-metre croc jumps out of the swamp at you, there is nothing more natural in the world than to thump him with a big stick. It’s an autonomous response. With a gun, IF you manage to keep it dry, and IF you manage to get it out in time before the croc bites off your head, chances are you’ll just shoot the bloke in front of you anyway. So you get an oar.”

“Fair enough”.

That is an approximate, paraphrased reproduction of the initial conversation I had with renowned Australian crocodile biologist, Grahame Webb, just prior to my first (and as it turns out, only) trip to collect crocodile eggs for his Darwin wildlife park and crocodile farm. I volunteered to take part in the collection because I had recently begun working with Grahame and his team tracking the world’s largest crocodile species – the saltwater or estuarine crocodile Crocodylus porosus – and modelling aspects of its populations (Bradshaw et al. 2006). Having already been out on several occasions to harpoon and satellite-tag animals (some measuring > 4 m) on the Mary River, and cage-trap others in Kakadu National Park, I thought a little egg collection would be a proverbial walk in the park. Little did I know that it would end up being one of my more memorable experiences.

Let me walk you through the process. First, you wait until the height of the wet season and drive out as far as you can toward the breeding swamp of interest (in this case, Melacca Swamp in the Adelaide River flood plain, about one hour’s drive from Darwin). Then you and two other loonies pile into a small helicopter equipped with landing pontoons which ferries you to one of many previously identified crocodile nests. Because there is usually too much vegetation around the nest itself, the helicopter must land about 100-300 m away. Clothed only in long pants, a long-sleeved shirt and cotton gloves to protect your skin from the slicing blade grass, you jump off the helicopter’s pontoons into impenetrably murky, chest-deep water. One of the team drags an esky (chiller box into which eggs will be placed) and another carries an oar. As the noise of the departing helicopter becomes a faint buzz, you suddenly realise via the rapid expansion of your terminal sphincter that you are in the middle of a crocodile-filled swamp – and you are holding an oar. Read the rest of this entry »


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Categories : Australia, celebrity, conservation, conservation biology, environmental policy, exploitation, extinction, harvest, management, predator, research


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