Back To The 80s As Basking Shark Sheds Light On Ocean Fronts

March 23, 2009

Release from: Planet Earth News Online

In June 1982, Monty Priede harpooned a basking shark off the Scottish coast. Not an act you'd normally expect from a young marine biologist, but he wasn't trying to injure the huge fish - just to attach a transmitter to allow groundbreaking research.

For the next 17 days Priede and colleagues enjoyed an unprecedented insight into the shark's behaviour as they followed its movements around Scottish coastal waters. It was the first successful attempt to use a satellite tag to track a fish, and one of the first on any kind of animal.

Now a professor and head of Oceanlab at the University of Aberdeen, Priede recently collaborated with Dr Peter Miller of the remote sensing group at Plymouth Marine Laboratory (PML) to revisit the archived data on the shark's activities and mine it for information about ocean fronts - parts of the sea where masses of warm and cold water meet.

These fronts tend to contain large quantities of plankton, as cold water wells up from the seabed and brings nutrients with it. Miller reconstructed sea surface temperatures back in 1982 from satellite images taken at the time, then analysed this information alongside the shark's movements to shed light on the animal's tendency to swim along ocean fronts in search of the plankton that make up its diet.

The findings appear in Fisheries Research, the same journal that published the original results. They increase our knowledge both of how oceanographic conditions in the area have changed over time, and of how these conditions affect the behaviour of marine animals.

Priede was aware of Miller's previous work on the Ecosystems of the Mid-Atlantic Ridge (ECOMAR) project. Here, Miller used algorithms he had developed to detect thermal fronts in the Atlantic; Priede believed these could be used to shed light on his shark's behaviour, and contacted Miller to explore the possibility of collaborating.

'These days on a research cruise we can provide high-quality sea surface temperature data within an hour of the satellite passing over the area,' Miller explains. 'But in the 80s people thought it was pretty high-tech to get an unprocessed thermal infra-red image once a day. We now have the technology to do far more with this data than would ever have been possible at the time.'

The software he has developed lets him reconstruct sea surface temperatures to within a tenth of a degree, by taking an original thermal image and calibrating and correcting it to remove the distorting effects of factors like atmospheric conditions.

'Once we produced an accurate map of sea surface temperatures we could run the ocean front detection algorithm, which showed us a distinct ocean front that seems to be affecting the movements of the shark,' Miller explains.

On one day during the monitoring period, for example, the fish seems to have encountered an ocean front at 6.16AM, turned left and swam parallel to the invisible line in the water for the rest of the day.

The idea of using animals as platforms for oceanographic research has gained popularity in recent years; for example the Sea Mammal Research Unit in St Andrews has done extensive work on using transmitters attached to seals to gather information about ocean conditions without the need for an expensive cruise in a research ship.

The only problem is that measurements are taken wherever the animal goes, which is not necessarily the same as where the researchers want to take measurements.

The 1982 research was the culmination of a three-year project to build a satellite transmitter small and robust enough to be attached to an animal in the wild.

Satellite tracking is now commonplace, and scientists all over the world use it to keep tabs on creatures ranging from lions to barnacle geese. The ARGOS system, used in Priede's research, is still mounted on satellites and is widely exploited to track everything from animals to competitors in yacht races.

But in 1982 miniaturisation had only just made the devices small enough to be attached to an animal - even one the size of a basking shark, which is the second-largest fish in the world and can grow as long as 12 metres.

Scientists had tried to use NIMBUS, the satellite system that came before ARGOS, to track migrating caribou and turtles, but initially the transmitters weren't much smaller than a suitcase and attaching them to the subject was difficult.

Priede joined the field in 1979 and built a NIMBUS transmitter before having to switch to the ARGOS system, which was run by the French space agency. Even the idea of using satellite transmitters to track living creatures was considered strange; the devices were mostly used on static data-gathering buoys.

'They kept sending me questionnaires asking me about what kind of data buoy I planned to attach the transmitter to,' Priede recalls. 'It took a while to get the message across that it was an animal.'

By 1981 the team had finished building the transmitter but, frustratingly, couldn't find any sharks to attach it to. They had to wait another year before finally finding a suitable shark and managing to attach the transmitter to it.

Harpooning the fish wasn't the original plan, but other ideas for attaching the transmitter, such as using a crossbow, proved ineffective.

Even this method wasn't without its drawbacks. Showing scant regard for the honour of contributing to the march of scientific progress, the shark reacted to being harpooned with a violent swipe of its tail, nearly capsizing the researchers' boat. Luckily they were under no threat from the fish, which eats only plankton despite its huge size, and were able to stabilise the boat and return to shore.

Part of the reason the data from this old research is still useful is that the team were so concerned about getting a signal to the satellite in all conditions that they built their tag using very high-quality transmitters.

The transmitter also signalled its position every time the satellite passed overhead - every 90 minutes or so, far more often than lightweight modern equivalents, which typically send data just a few times a day.

But the advent of plentiful computing power means it's now possible to analyse the information gathered in a way that was never possible in the 1980s. Originally it was stored on a mainframe computer at the University of Dundee; to get at it Priede had to drive to the town and request a printout. Communication with the French space agency was done by telex.

'In a way we went into the technology too early,' says Priede. 'We were trying to do some very clever research, but this was at a time when there were no desktop computers and you had to do all your programming in Fortran. We were just on the cusp of the computer revolution.'

Now the data are held in an archive at Plymouth Marine Laboratory, where any scientist can download them for analysis on their desktop computer.

Miller is now looking for other opportunities to apply his technique for detecting ocean fronts to other datasets. They could help shed light on the behaviour of marine organisms ranging from mammals to plankton.

'Ocean fronts are the boundaries between masses of water, and it is here that you get effects like water column stratification and enhanced biological productivity,' says Miller, explaining that ocean fronts are therefore of great ecological interest. 'We can now get a handle on these things using the technique I have developed.'

He is working with the NERC-funded Oceans 2025 project to investigate how ocean fronts in the north-east Atlantic have changed over the last few decades, as well as preparing for the publication of several papers that will provide more information on the capabilities of his techniques.

'We revisited the work we'd done in 1982, using 2008 technology and software,' says Priede. 'It shows that we don't just have the capability to do spatial work any more - we can also go back in time and pull out information about what conditions were like at a specific point in the past. I'd never have imagined we could go back and say something new about that shark - the message here is never throw away your data!'