Question: how would scentist be able to find out how some animals navigate and migrate huge distances

Edward Codling answered on 12 Jun 2011:

A great question to ask – although I’ll have to give a longish answer I’m afraid!

To study this problem there are various approaches we can take. Many scientists do what are known as ‘observational studies’ – i.e. they simply observe the animals they are interested in in their natural habitats as they move around. One of the great recent new developments is the use of satellite tags – these are like tiny GPS systems that we can use to track how animals move. Using the data that the tag records we can then work out where the animals have moved. This has given us some great new insights into how far certain animals move (much further than we once thought) – e.g. http://www.bbc.co.uk/radio4/worldonthemove/reports/oceanic-odyssey-of-the-whale-shark/

Of course, just tracking where the animals have moved doesn’t really tell us that much about HOW they might do this. Animals move for various reasons – to find food, mates or to avoid predators. One of the key natural events that I am interested in is large scale migrations – animals can do this for various reasons. I’ll discuss a few examples that I’m interested in:

1. How does Nemo get home?
If you’ve watched Finding Nemo (go and watch it if you haven’t!) then you recognise the stripy orange clownfish: http://en.wikipedia.org/wiki/Orange_clownfish

Clownfish (and other similar coral reef fish – like Dory the Blue Tang) have what is known as a ‘bi-partite’ (2-stage) life history. The eggs are laid on the coral reef but when they hatch the tiny baby clownfish (known as ‘larval’ fish) leave the reef and head out to the open ocean (the ‘pelagic’ environment). The reason they do this is because if they stay on the reef they’ll very quickly get eaten by bigger fish (which can include their own parents if they are unlucky!) The baby fish then live for a few weeks out in the open ocean eating plankton (tiny bugs and organisms that live in the water). Once they reach a certain size they then need to get back to a coral reef to ‘settle’. This settement involves a process known as ‘metamorphosis’ where the fish actually physically change (e.g. they go from see-through to having colour) – this is a bit like when tadpoles change into adult frogs. Unfortunately for the baby fish there is a very short time limit for them to do this – if they don’t find the reef in time then they’ll die!

The key problem we are interested in is how do the baby larval fish find a coral reef to get back to when they need to settle? Clearly they are only very small and people used to think it was just random whether they would return to a reef (e.g. if they were lucky enough to find a favourable current in the sea). However, recent work by my colleague Steve Simpson (and a few other people) have shown that the larval fish actually seem to respond to the sound of the coral reef (which is actually very noisy – fish ‘talk’ to each other all the time using croaks and ‘burps’!).

We have done some theoretical simulation work showing how important it is that they respond to sound and navigate towards it – this can greatly increase their chances of surviving. One of the things we are now interested in is whether the baby larval fish navigate to the coral reef as a group or as individuals – if we can determine this then it could have big implications about how we should manage and maintain coral reef ecosystems. We have a PhD student studying this problem at the moment – in fact he is about to go off to Indonesia for 2 months to study fish in their natural environment (while I have to stay at home unfortunately!).

2. Eels – the biggest mystery in nature?
One of the biggest mysteries in nature is how eels navigate and find their way to their spawning ground in the Sargasso Sea (near central America). Eels are a very important fish with really interesting biology and behaviour. They breed in the Sargasso Sea and the baby eels drift back to Europe on the ‘Gulf Stream’ ocean current. The baby eels then develop and move inland to freshwater (this is very unusual – most fish can only live in salt or freshwater, not both). The eels then live in lakes and rivers for a long time (as many as 30-40 years) before they finally decide to migrate to breed (why they decide to go is a mystery – some go when fairly young, some are much older). Once the eel starts to migrate, its body changes so that it is ready to spawn (breed), and part of this means it loses its stomach so it can’t eat. Hence, the breeding migration is a one-way trip – once the eel spawns it dies so it only has one chance to get it right!

However, given what we do know about eels and how important they are, the incredible thing is that no adult eel has EVER been found in the open ocean! 😮
We know roughly where they go but nobody has found the exact location of their breeding grounds, while nobody has ever seen how the adult eels get there – it is a huge mystery. I always find this incredible given the technology we now have and the amount of research done in this area!

One of the most recent findings is by my colleague Dave Righton at CEFAS (the English fisheries lab in Lowestoft in East Anglia). He tagged adult eels and managed to track them part way into their migration journey. Unfortunately he only managed to follow them as far as the edge of Europe – hence the full journey is STILL a mystery!
See:
http://news.bbc.co.uk/earth/hi/earth_news/newsid_8273000/8273877.stm
http://www.bbc.co.uk/radio4/worldonthemove/species/european-eel/

I am working with Dave Righton and a few others at the moment to try and understand more about this amazing migration journey. One of the things we are trying to do is to set up a computer simulation to try out various possible ways that the eel might migrate. Using a simulation means we are able to explore various possible strategies that the eel might use and see which ones might work. Of course, we won’t know for definite what really happens just by using a simulation, but we may get some extra insights that help us when we do further tracking studies.

It is really important that we find out about eels as soon as possible as they have massively declined recently (by 98% in 5 years!) and we don’t know why. One of our most common (and interesting) fishes could very soon become extinct so it is quite urgent that we find out what is going on. It would be awful if they died out completely before we could understand their really interesting behaviour.
See http://news.bbc.co.uk/1/hi/england/london/8473965.stm

Let me know if this answers your question? Hopefully it has given you a few ideas to think about!