Some sharks use mental maps to navigate; others do not

I know where I'm going

From BBCNews

Some shark species make "mental maps" of their home ranges, allowing them to pin-point destinations up to 50km (30 miles) away, research suggests.

US-based scientists analyzed data from tiger sharks tagged with acoustic transmitters, and found that they took directed paths from place to place.

Other species such as blacktip reef sharks did not show this behaviour.

Writing in the Journal of Animal Ecology, researchers suggest this shows a capacity to store maps of key sites.
In addition, it is further evidence that the great fish can navigate, possibly using the Earth's magnetic field.

Earlier research in Hawaii had shown tiger sharks swimming across deep channels and finding shallow banks rich in food 50km away.
In this project, researchers used statistical techniques to show the journeys were not made by accident; the sharks were following some kind of path.

Blacktips, however, did not.
A third species, thresher sharks, also showed "directed walking" like the tigers, but on much smaller scales.
"Our research shows that, at times, tiger sharks and thresher sharks don't swim randomly but swim to specific locations," said research leader Yannis Papastamatiou from the Florida Museum of Natural History in Gainesville.
"Simply put, they know where they are going."

Maps and magnets

A key question is how they know where they are going.
Sharks are among the wide array of animals that can sense magnetic fields.

But whereas others, such as yellowfin tuna, apparently do this using small amounts of the mineral magnetite in their heads, sharks do not appear to maintain deposits of this magnetic sensor.

Alternative possibilities are that they use signals from ocean currents, water temperature or smell.
"They have to have a pretty good navigation system because the distances are great," Dr Papastamatiou told BBC News.
"Which one it is is open to debate, but the fact that many of these journeys took place at night - you and I would think there's nothing to orientate to, so orientating to magnetic fields is one possibility."

Among thresher sharks, adults made much longer directed journeys than juveniles.
The researchers say this suggests the fish build up mental maps as they mature.

Blacktip reef sharks hunting (Alecconnah/Solentnews/SIPA)

The differences between species are probably explained by the varying ways in which they live.
Blacktip reef sharks (Carcharhinus melanopterus), although widespread around the Pacific, appear to have small ranges within their home reef system.
On the other hand, tiger sharks (Galeocerdo cuvier) can cover huge distances.
Tags have been recovered from individuals more than 3,000km away from where they were attached.

Links :

• ScientificAmerican : Lost? Evidence that sense of direction is innate

Venetian navigators : the voyages of the Zen brothers to the Far North

In his new book, Venetian Navigators, Andrea di Robilant sets out to investigate how much, if any, truth there is to the story that Antonio and Nicolò Zen really discovered the New World before Columbus.

From FT

A curious book was published in Venice in 1558.
Its author was one Nicolò Zen, a well-known official of the Venetian republic.
In it, he made the extraordinary claim that his great-great-great grandfather Antonio and his great-great-great granduncle Nicolò had travelled around the north Atlantic as far as the coast of modern Newfoundland in the late 14th century – a whole century before Christopher Columbus’s American landfall.

A facsimile of the Zeno map of the purported voyage of Nicolo and Antonio Zeno across the North Atlantic to North America in 1380.
The map shows Norvegia (Norway), Denmark, Iceland, Greenland, and the fictitious Frisland.

The book included a map depicting the places allegedly visited by the brothers.
Though clearly inaccurate, this navigation chart, if genuine, is significant for offering the earliest known cartographical representations of the Faroe Islands, Greenland, Iceland and the eastern coast of Canada.

Contemporaries had few doubts about the authenticity of Zen’s account of his ancestors’ travels.
The cartographer Gerard Mercator incorporated some of the Zen map’s features into his own famous world map of 1569.
John Dee, Queen Elizabeth’s favourite astrologer, seized on the Zen narrative to convince the monarch to support voyages of exploration.

As late as 1701, charts still included references to the Zen “discoveries”.
But as the golden age of exploration waned, and as mapmakers depicted the new world with increasing precision, the Zen narrative and its unusual map became footnotes in the history of geographical discovery.

They would have languished in antiquarian obscurity were it not for a Danish geographer, Captain Christian Zahrtmann, who in the mid-19th century set out to prove that the tale was little more than “a tissue of fiction”.

“Carta de Navegar” of Nicolo and Antonio Zeno – (the fictitious ‘Zeno map’). Published in Zeno’s ‘Commentaries’, 1588"

In a report to the Royal Geographical Society, he declared: “It is not from the south that we can expect elucidations on the older north.”
Was the book a factual (if wildly erroneous) account of geographical observations predating Columbus or was it, as another critic claimed, “one of the most successful and obnoxious [literary frauds] on record”?
This is the question that historian Andrea di Robilant sets out to answer in Venetian Navigators.

Di Robilant is, from the outset, prepared to give Nicolò the younger the benefit of the doubt.
He has few compelling reasons to do so, except that it is such a great yarn – one that, if true, would up-end much of what we know about the age of discovery.
While he comes to no definitive conclusions, by the end of his research he has fallen even more firmly into the camp that claims the Zen voyages might have actually happened.

Pursuing clarity about the voyages, di Robilant visits libraries and archives in Venice, Tórshavn (Faroes), Kirkwall (Orkney), Lerwick (Shetland), Iceland’s Reykjavik, and Nuuk in Greenland.
Most of the scholars he meets believe that the Zen story is “pure rubbish”, or “the handiwork of a two-bit Venetian swindler”.

But di Robilant’s own journeys give him – and us – a good feel for the barren landscapes that the Zens might have encountered.
We are introduced to extraordinary characters such as Villi, a blind Icelandic farmer who spends an afternoon discussing Icelandic poetry with di Robilant, and Silverio Scivoli, an Italian who arrived in Greenland 30 years earlier as a touring pianola player and moved in with a local woman.

“I am inclined to believe that Nicolò the younger was a first-class muddler, not a fablemonger,” di Robilant writes, “and that the story he tells of his forefathers offers fascinating glimpses into the past.”
Di Robilant’s book is a bit muddled too – unsure, whether to lean towards scholarly antiquarianism or towards the first-person travelogue.

The author is right, however, in suggesting that the Zen narrative and the debates it generated open a window on to the ways in which knowledge (geographical, in this case) is generated and appropriated.
In setting out to elucidate a cartographical mystery, di Robilant has also shed light on the 16th century mind, caught somewhere between the middle ages and the Renaissance.

Links :

• TheIndependant : Venetian Navigators
• Faber&Faber : Andrea di Robilant on 'Venetian Navigators'
• Cambridge : Voyages of the Venetian Brothers, Nicolò & Antonio Zeno, to the Northern Seas, in the XIVth Century
• Irresistible North, from Venice to Greenland on the trail of the Zen brother
• Earl Henry Sinclair's fictitious trip to America (Wikipedia)
  

Salty dogs

From morning till night some weathered salty sea men fish to classic rock.

World's wind and waves have been rising for decades



Challenges in the use of Satellite Wind and Wave Products in Marine Forecasting (Stanley Wilson) from IODE OceanTeacher

From NewScientist

Wind speeds and wave heights over the world's oceans have been rising for the past quarter-century.
It's unclear if this is a short-term trend, or a symptom of longer-term climatic change.
Either way, more frequent hurricanes and cyclones could be on the horizon.

Ian Young at the Australian National University in Canberra and colleagues analysed satellite data from 1985 to 2008 to calculate wave heights and wind speeds over the world's oceans.
They found that winds had strengthened – speeding up over most of the world's oceans by 0.25 to 0.5 per cent, on average, each year.
Overall, wind speeds were 5 to 10 per cent faster than they had been 20 years earlier.

The trend was most pronounced for the strongest winds.
For instance, the very fastest 1 per cent of winds were getting stronger by 0.75 per cent per year, says Young.

Average wave height was also on the rise, but less so; and the highest waves showed the strongest trend.

The results were compared against conventional measurements taken from deep-water buoys and numerical modelling.
"There is variability, but the same general features are observed," Young says.

From space to sea

Previous attempts to investigate these phenomena used observations from ships and buoys, but these could generally provide only a regional picture.
Using altimeter data from satellites allowed the team to detect decadal trends on a global scale for the first time.

Satellite altimeters use radar to measure the height of points on the Earth's surface, and can measure wave height very precisely.
Measuring the amount of backscattering from the radar signals, meanwhile, can help calculate wind speed.

The global view afforded by the satellites reveals stronger trends in some areas than in others.
For example, both wave height and wind speed have been increasing more rapidly in the oceans of the southern hemisphere than in the north.

Wave driver

Young can only speculate on what is causing the increases.
"If we have oceans that are warming, that energy could feed storms, which increase wind speeds and wave heights," he says.
But with a data series that covers just two decades, it's too early to tell whether there's a long-term trend at work. "We don't know the driving force."

Considering there are so many regional forces influencing waves and wind, "it's surprising that there is such a uniform trend", says Mark Hemer, a wave researcher at the Centre for Australian Weather and Climate Research in Hobart, Tasmania.
Variability in winds and waves associated with weather systems such as El Niño and La Niña, the North Atlantic Oscillation and the Southern Annular Mode could all help to explain it, he says.

In either case, if winds continue to strengthen and waves to rise – even if only for a few years – it suggests more intense storms, hurricanes and cyclones are on the horizon, says Young.

However, Tom Baldock, a coastal engineer at the University of Queensland in St Lucia, Australia, says that although there is no reason to doubt the analysis, it doesn't mean more coastal natural disasters will necessarily ensue.
"Tornados, hurricanes and cyclones occur through complicated regional weather conditions, and are not just related to wind speed and wave height," he says.
For example, there are higher wind speeds at high latitudes, but most cyclones hit around the equator.

The new study may be more relevant to the burgeoning offshore gas, wind, wave and tidal power industries, Baldock thinks.
"Larger waves are a hazard for any offshore construction."

Links :

• ScienceMag : Global trends in wind speed and wave height
• ScienceNews : Global gale warning
  

Big spring tide extremes in time-lapse

Grande Marée (coefficient 118) from Benoit Stichelbaut
Coincidence of perigean and spring tidal conditions

resulting in the highest high and the lowest low tides

 Anse du Minaouët (Concarneau) with the Marine GeoGarage

-> localization in the Marine GeoGarage

Links :

• Marine GeoGarage blog : Super full moon
  
• Wikipedia