You are browsing the archive for 2009 October.

The Great White Shark

October 13, 2009 in Articles 2009-10

The great white shark is one of the most fearsome predators in our seas.  Averaging about 12 to 16 feet in length as adults1 they are the third largest carnivorous animal in the ocean only being out done by orca’s and the sperm whale. They inhabit waters all over the world, from New Zealand to the Mediterranean and prey mostly on seals and sea lions that inhabit these temperate waters. To catch such elusive and agile animals such as sea lions, great white sharks have evolved several physiological features and behavioural adaptations.

known distribution of great white sharks

The most recently noted behavioural adaptations of great white sharks is called anchoring2 and has been noted by Neal Hammerschlag of the University of Miami along with partners from the University of British Columbia and and texas state after their experiments in False Bay in South Africa3. Anchoring is the use of a specific position on the sea floor that a great white can stalk unnoticed by its prey before turning upwards and striking it from below in the hope of injuring or killing its victim. The researchers found after monitoring 3404 attacks by great whites that the sharks use certain anchor points under water to position themselves to catch their prey. The underwater anchor points were discovered to be specific to a certain shark, and that great whites do not just attack at random5. Neal Hammerschlag found that an anchor point that an individual great white uses is related to several factors, for example dominance. They found that the most successful anchor points were used by the biggest and most experienced sharks. These dominant sharks found the best anchor points in relation to competition for prey, environmental factors and the distribution of Cape fur seals6. However it was found that the position of the anchor points did not relate to the distribution of the seal population. The older sharks therefore seem to be the better hunters, either because after more experience they learned where the best anchor points were to catch prey, or these larger sharks were trying to exclude the smaller, younger sharks from the best hunting areas, so that they have more food for themselves7.

There are several physiological features that great whites have evolved similarly to other sharks that allow them to obtain food. One such feature is the Ampullae of Lorenzini which is a form of electro receptor that detects electrical currents created by other organisms. This helps to detect any potential prey. This sensory feature is extremely sensitive as a great white can detect minute currents.

The great white’s colouration acts as camouflage from both looking down on to the shark and looking up. The dorsal side of the sharks is a very dark blue which when looking down upon simulates the darkness of the sea water. And from below looking up on the ventral side the colouration is white to replicate the sun’s rays down through the water.

great white shark colouration
known distribution of great white sharks

The great white shark is also extremely streamlined, and has tremendous power due to its high metabolic rate. When attacking prey from below their power through the water often causes the sharks to completely breach. In cases like this, the shark will cover its eyes with protective slits just before it strikes its prey to prevent any damage. To generate such thrust through the water, the great white shark has two main muscles that run the length of its body8. This muscle is divided into two types, red and white. The white muscle is what helps in the short sharp bursts that the great white uses to strike its prey. Momentum is created as the length of muscle on one side contracts whilst the other relaxes, then the other side contracts whilst the other relaxes. This action causes the tail to move from side to side. The head stays straight to reduce the effects of drag as the tail moves.red/white muscle These are simply a few features that aid the great white shark in finding and catching prey to make it one of the most successful predators in the our oceans.

References

1)      http://www.geocities.com/RainForest/Andes/2771/info.html

2)      http://www.newscientist.com/article/dn17345-great-white-sharks-are-serial-killers-of-the-sea.html

3)      http://www.sciencedaily.com/releases/2009/06/090621195618.htm

4)      http://www.geocities.com/RainForest/Andes/2771/info.html

5)      http://www.newscientist.com/article/dn17345-great-white-sharks-are-serial-killers-of-the-sea.html

6)      http://www.sciencedaily.com/releases/2009/06/090621195618.htm

7)      Journal of Zoology, DOI: 10.1111/j.1469-7998.2009.00586.x

8)      http://www.diveplanet.co.nz/dplfoxgwresearch/anatomy/muscle.asp

Photo References

1)      http://scienceray.com/biology/marine-biology/the-truth-about-the-great-white-shark

2)    http://www.networkroundtable.org/wp-content/uploads/2009/01/great-white-shark-002.jpg

3)     http://www.diveplanet.co.nz/dplfoxgwresearch/anatomy/muscle.asp

The future of cod in the North Sea

October 13, 2009 in Uncategorized

North Sea cod. Photograph: Corbis

North Sea cod. Photograph: Corbis

North sea cod. Photograph: Simon Price/AP

North sea cod. Photograph: Simon Price/AP

The future of cod in the North Sea


For years, there has been worries about the future for cod in the North Sea. Until recently it was thought that this was down to overfishing; but some scientists, including Dr Richard Kirby and Dr Grégory Beaugrand believe that that even if cod fishing was outlawed – that cod could disappear completely from the North Sea. This is down to rising temperatures in the North Sea due to global warming.

In the past 40 years, a 1oc average temperature increase has been recorded in the North Sea, leading to the current average temperature of 6oc in winter and 17oc in summer increase [1]. This increase is double the increase in the Earth’s surface temperature of approximately 0.5oc in less than half the time it took [2].

The change in temperature is dramatically affecting the numbers of cod who live in the North Sea and especially the survival of cod through the first year of their live. There are a number of reasons why they are affected like this, including:

1)      Young cod feed mainly on copepods, along with many other species of small crustaceans. The copepods that these cod eat prefer colder waters, and have been moving north as the sea warms by 1200km, with their numbers declining by 60% in the past four decades. With the main source of food for young cod becoming much rarer in the sea, it is only natural that cod are less likely to survive to the stage where they are large enough to eat what adult cod eat [3].

2) Adult cod will eat crabs and shrimp on the seabed. Both cod and shrimp eat plankton, competing with cod larvae for food. With less cod surviving to reach adulthood, there has been an increase in the numbers of crab and shrimp, therefore increasing the competition that cod larvae has to face in order to feed. As a bonus, crabs and shrimp also spawn more larvae in warmer waters. So with the number of crustaceans blooming in the North Sea, there is a lot more competition for food with young cod.

In addition to these two reasons: cod, like many other fish, have their favoured temperature range for spawning and may not spawn at all if the temperature isn’t right; cod eggs and young cod are very susceptible to changes in water temperature, which can even kill them [4].

norths4

Work undertaken by scientists at the Cefas Lowestoft Laboratory have shown a relationship between the average sea-surface temperature of the North Sea during spring, and the numbers of cod surviving their first year [5]

As these two graphs show, the overall pattern is that the higher the temperature of the water, the less cod survive to be 1-year old.

So if cod will disappear from the North Sea, where will they go? The most popular opinion is that the Barents Sea, north of Russia and Norway, will be the most likely choice.

Reference list:

1) http://www.mumm.ac.be/EN/NorthSea/facts.php, undated.

2) http://earthobservatory.nasa.gov/Features/OceanClimate/, undated.

3) http://www.telegraph.co.uk/earth/environment/globalwarming/6162167/North-Sea-cod-could-disappear-even-if-fishing-outlawed.html, 09/09/09

4) http://www.stemnet.nf.ca/cod/history4.htm, undated

5) http://www.cefas.co.uk/publications/miscellaneous-publications/so-what%27s-happening-to-north-sea-cod.aspx, 09/06/08