Shark Research - Mercury levels in shark fin soup

Researchers in the United States have examined bowls of shark fin soup from 50 US restaurants to assess the levels of monomethylmercury (MMHg) in each one.

MMHg is produced in aquatic food webs; owing to the fact that MMHg is not easily eliminated, it accumulates in organisms and species higher up the food chain will tend to exhibit higher levels of MMHg than those at the bottom. MMHg poses health risks to adults such as an increased chance of heart attack; it it also of particular concern in pregnant women as it can also transverse the placenta and has been linked to neurological development issues in the foetus.

Most fish and shark meat is known to contain high levels of MMHg. The US Environment Protection Agency recommends a daily consumption of no more than 7.4μg a day for a 74kg female. To give you some perspective, a 200g blue shark steak can contain between 36-390μg!

The researchers discovered that whilst average levels of MMHg found in shark fin soup were not incredibly high (they measured an average of 1.1μg MMHg in a 240ml bowl of soup), there was a large degree of fluctuation in the levels of MMHg, depending on which species the fin came from. Large, pelagic sharks which occupy higher trophic levels in the food chain have a greater dose of MMHg in their fins, whilst those lower down the food chain had less MMHg. The highest value recorded in a bowl of shark fin soup in this study was 8.2μg which exceeds the safe, recommended daily dose of MMHg.

This study highlights the fact that the consumer does not always know if the bowl of shark fin soup they are going to consume will pose a risk to their health, or will be within safe limits. If a consumer were to eat a bowl of shark fin soup as an appetiser and then follow it with a main course of fish it is highly likely that they would be ingesting a dose of MMHg far above the recommended daily level. Given the huge ecological impact shark fin soup has, this new data should be further reason to avoid this product!

The full article can be found here.

Shark Research - Insights into mortality of whale sharks in Atlantic and Indian Ocean tuna fisheries

A recent research article in Biological Conservation has examined mortality rates of whale sharks (Rhincodon typus) as a result of the European tuna purse seine fisheries in the Atlantic Ocean and Indian Ocean.

Tuna often associate with large marine species such as dolphins and whale sharks; tuna fishermen sometimes rely on sightings of these species to indicate the presence of large schools of tuna. Once tuna have been sighted, a net is set to encircle the shoal and the bottom of the net is drawn together, like a purse, to trap the fish. Owing to the fact that there are often large marine fauna close by, these animals can also become trapped along with the tuna.

This study used log book data and observer data to assess the number of whale sharks sighted, caught and killed in purse seine nets in the Atlantic Ocean and the Indian Ocean. The biggest incidence of whale shark mortality occurred in certain hotspots where whale sharks overlapped with the purse seine fishers. Overall the incidence of whale shark mortality in the Atlantic and Indian Oceans was relatively low - from 1995 to 2011, 2 whale sharks were reported to have died out of 186 encircled by the nets. In recent times, the purse seine fisheries have implemented new strategies for ensuring the survival of whale sharks caught in nets, such as the cutting of the net to allow the shark to escape.

This data is in stark contrast to whale shark mortality in the Pacific Ocean provided by the Western and Central Pacific Fisheries Commission (WCPFC). Between 2007-2009, 56 whale sharks died in purse seine nets - the most common reason cited was that the fishermen were not aware of the presence of the shark in the net before mortality occurred. As a result of this, the WCPFC has banned the setting of nets around whale sharks since the beginning of 2014.

Additionally, both the Indian Ocean Tuna Commission (IOTC) and the WCPFC have admitted that accurate data on the number of whale sharks encircled and killed by purse seine fisheries is difficult to obtain. The data for this study relied on the accurate reporting of whale shark mortality by purse seine skippers and observers - which only cover 10% of the European fleet.

More analysis of the tuna purse seine fishery and better observer coverage is needed in order to obtain a better picture of the mortality of large pelagics in the tuna purse seine industry.

Shark Research - Lateral line scales can make sharks fly

New research published in the Journal of Fish Biology has examined the scale pattern around the lateral line pores of the spiny dogfish shark (Squalus acanthias) and the shortfin mako shark (Isistius oxyrinchus).

The lateral line system is an important sensory organ in fish and is used to detect vibrations and movement in the surrounding water; this sense can be utilised in predator avoidance, schooling and prey detection. The lateral line runs down the side of a fish and comprises a series of sensory neuromasts, connected by tubules. Whilst some neuromasts remain below the dermis, at sections of the lateral line superficial neuromasts will be exposed to the outside environment via pores in the lateral line system.

Surrounding the pores of the lateral line system are the sharks' scales; unlike the scales of teleost fish sharks possess rough, placoid scales which (as well as being biologically similar to our teeth!) contribute to reducing drag whilst swimming and give sharks a rough, sandpaper like texture. The channelling of water over the neuromasts by the surrounding placoid scales will, of course, have an affect on the sensitivity of these sensory organs.

The fast swimming mako shark has a very different lifestyle to the spiny dogfish, which inhabits the ocean floor and has a much more sedentary lifestyle. The study found that the patterning of placoid scales around the neuromast pores in the mako shark may reduce turbulence in the water immediately surrounding the lateral line; this will affect lateral line sensitivity.

The arrangement of placoid scales around the neuromast pores in the spiny dogfish was entirely different, with S. acanthias exhibiting an entirely different divergent pattern to I. oxyrinchus. The placoid scales of I. oxyrinchus were orientated dorsally and ventrally from the pore, whilst those of S. acanthias were orientated exclusively ventrally away from the pore.

Whilst further research is required to fully analyse these differences across shark species, the results here suggest that the different lifestyles of these 2 shark species may be contributing to different sensory requirements in the lateral line and therefore different arrangement of the scales around the sensory organs.

Mckenzie, R. W., Motta, P. J. and Rohr, J. R. (2014), Comparative squamation of the lateral line canal pores in sharks. Journal of Fish Biology, 84: 1300–1311. doi: 10.1111/jfb.12353

Shark news - Megamouth shark caught off the coast of Japan

Following on from Tuesdays exciting news about the sighting of a goblin shark, a rare megamouth shark (Megachasma pelagios) has been caught off the coast of Japan. The specimen - which is only the 58th to have ever been found - was caught off the waters of Shizuoka last month and is currently on display at Marine Science Museum in Shizuoka.

Very little is known about these sharks but they are believed to be a filter feeding species which feeds primarily on krill. A single specimen was tagged and followed for 2 days and exhibited patterns of diel vertical migration (spending the daylight hours at depth and returning to shallower waters at night).

The megamouth shark is currently classified alongside white sharks (Carcharodon carcharias) and mako sharks (Isurus oxyrinchus) in the order Lamniformes (or mackerel sharks).

News source with pictures: http://www.wptv.com/news/local-news/water-cooler/rare-megamouth-shark-caught-in-waters-off-japan

Shark News - Drum lines removed from WA waters

At 2pm yesterday, the Western Australian Government removed the drumlines which had been set to catch target sharks off the coast of WA, Australia. Whilst this news will be warmly welcomed by the majority of the public - who overwhelmingly oppose the shark cull - there is a chance that the drumlines could be back in the water by the end of the year.

The initial policy to place drumlines in the water around Perth had always been a short term policy set to run for 13 and a half weeks the intended deadline for the removal of the drumlines set as 30th April. By negotiating the placement of the drumlines on a short term, "trial" basis, Colin Barnett was able to circumnavigate some of the laws which would otherwise prevent such a policy going ahead - such as the fact that the great white shark (Carcharodon carcharias) is a protect species in Australian waters.

Currently the Environmental Protection Agency (EPA) is assessing an application by the Barnett Government to extend the drumline policy for a further 3 years. The new submission to the EPA requests permission to put drumlines in the water for 4 and a half months of each year for 3 years and is requested to begin on November 15th 2014. Since the new proposal is for a much longer time period than the initial setting of the drum lines, the EPA has decreed that the policy must be subjected to the highest level of assessment.

Given a very strong public and scientific objection to the drumline policy, the hope is that the EPA will not allow the Barnett Government to continue with this policy long term.

You can read more about the assessment here