Tag Archives: Sharks

Reducing bycatch of skates and rays – stop tickling them!

Bottom-trawl fisheries may supply us with much of the tasty fish we like to enjoy, but it does come with its problems.  Also known as ‘dragging’, bottom trawling essentially involves dragging a large net, held open either with a beam (beam trawling) or large metal/wooden ‘doors’ (otter trawling) along the sea bed, or just above it.  It is used to catch a range of commercial species like cod, shrimp, flounder, and halibut.  One of the problems of trawling is that it is not a very selective form of fishing.  Other species are caught in the process, and this bycatch can include at risk species such as skates, rays and sharks.  As well as ecological implications, bycatch can be bad for fishers, who often end up throwing away bycatch either because it isn’t worth anything, or because they are not allowed to land it.  Bycatch reduction is a win-win for fishers and for the marine life caught.

Reducing bycatch of sharks, rays, and skates (collectively known as elasmobranchs) in bottom trawls is one of the many fishery-related issues on the mind of scientists at Marine Scotland Science.  As this piece of research from the Marine Scotland Science team shows, one possible solution (though not perfect) may not be all that tricky to implement. Continue reading Reducing bycatch of skates and rays – stop tickling them!

The travelling life of the tiger shark

At 9 foot long, not including the tail, tiger shark (Galeocerdo cuvier) Harry Lindo is not exactly on the small side.  It’s not Harry’s size that is exciting scientists and shark enthusiasts, nor a photograph taken in 2009 by Ian Card showing a shark – suspected to be Harry, trying to eat a 150 lb juvenile tiger shark off the coast of Bermuda.  Between 2009 and 2012 researchers tagged 24 tiger sharks with satellite transmitters in the Challenger Bank, which lies just off Bermuda in the Atlantic Ocean.  In study lead by James Lea (The Guy Harvey Research Institute, Nova Southeastern University Oceanographic Center) and team of international collaborators, those shark movements have been compiled and analysed.  Harry, it turns out, is one heck of an ocean wanderer.  In just over 3 years Harry swam over 44,000 kilometres – that’s more than the circumference of the Earth (just over 40,000 kilometres).  Harry’s track is the longest recorded for a tiger shark, and probably the longest ever published for any shark species.

Continue reading The travelling life of the tiger shark

Whose eating all the Menhaden?  Probably not spiny dogfish…

Life’s tough if you’re a menhaden (Brevoortia tyrannus) .  As a forage fish, you are one tasty morsel for somebody, be it a striped bass, tuna, gannet, shark, or human (who as well as eat you and use you for other purposes).  But here’s the thing – menhaden has declined, increasing the competition among its predators.

We don’t like dwindling fish resources.  We also don’t like competition for those resources.  It didn’t take too long before some fingers of blame were pointing towards non-humans, particularly dogfish – an order of shark that consists of some 126 known species.

In this recent piece of research, Charles Bangley and Roger Rulifson from East Carolina University, took at look at the feeding habits of spiny dogfish (Squalus acanthias) – a species that overwinters in North Carolina – through a series of controlled trials.  Their main conclusion on the impact of these critters on the menhaden….

“Spiny Dogfish potentially consumed an equivalent of 1.55–3.33% of the Atlantic Menhaden stock while overwintering in North Carolina waters”

As Charles summaries, “it may be time to find a new scapegoat for issues rebuilding fish stocks”.

Their paper, which was published in the  _North American Journal of Fisheries Management_ is sitting behind a paywall.  But fear not! Charles has written a fantastic summary of the paper over on his blog.  I highly recommend taking a peak.

Image:  A spiny dogfish  photographed in 2011 using NOAA’s ‘Arc’ remotely operated vehicle in waters off the Olympic Peninsula in Washington State  Credit: NOAA’s National Ocean Services Image Gallery (CC BY 2.0)

Citizen Science shows promise for shark monitoring

Earlier this year the IUCN (International Union for Conservation of Nature) Shark Specialist Group took a close look at the status of chondrichthyes -that’s sharks, rays, skates, and chimaeras.  Their findings did not make for happy reading.  Just 23% of the species that make up this group were classified as ‘Least Concern’.  But that doesn’t mean the remaining 77% of sharks are threatened, because the research also revealed that around 46% of the species that make up the chondrichthyes are classed as ‘data deficient’ – meaning we really don’t have enough information to figure out what their population status is.  Given that many chondrichthyes are in bad shape, it is important that we rectify this data deficiency so we can better direct conservation management.  The trouble is, data doesn’t come cheap – especially when it involves obtaining data on marine species.

One of the methods used for studying shark populations is to use acoustic telemetry.  The exact details vary, but the general process is to 1) catch a shark 2) attach an acoustic tag to it 3) deploy acoustic receivers 4) collect ‘ping’ data (when a tagged shark comes in range of a receiver) from the acoustic receiver 5) analyse data to figure out which sharks have been where and when.  Of course this method only tells us when a shark has come across a receiving station, so if a shark happens to spend a lot of time out of the range of a station, we wouldn’t get a ‘ping’ to analyse.  There are other methods of monitoring shark movements that get around this problem but they are costly – more costly than acoustic monitoring which doesn’t come cheap.  This isn’t to say that acoustic monitoring isn’t useful.  If you have an area that is known to have resident sharks, or frequented by visiting sharks, deploying acoustic monitoring in those areas can act as a population monitoring point.  With limited funds for research and conservation, there is always a balance between collecting the best data technology can provide, with collecting the best data that we can afford.  Perhaps monitoring doesn’t always have to involve high-tech gadgets at all.  After all, there are many parts of the world where scuba diving and snorkelling occurs regularly.  Can these human eyes act as reliable data collectors?  Continue reading Citizen Science shows promise for shark monitoring

Shark fishing, bleaching, & cyclones….

Over the years shark fishing – and particularly the practice of finning* – has garnered increasing attention from the public.  It seems that these predatory fish that were once feared and even hated are slowly being seen in a better light, the impetus for their conservation beginning to make ground.  Many shark populations have – and still continue to – experience high levels of overfishing.  The problem with overfishing top predators is that they tend to be slow-growing, long-lived, and don’t produce lots of offspring to allow populations to rebuild quickly.  Their removal can also cause what is known as a tropic cascade – when the removal of predators results in their prey becoming more abundant, which means that the critters they eat become less abundant….and so forth.  However, removal of top predators isn’t the only possible cause for changes in marine ecosystems.  There are a whole host of human stressors that can impact on the marine environment, and there are also natural stressors.  Take coral reefs for example we may add too many nutrients to the water, causing an increase in algae which can dominate corals, we may overfish a whole host of other coral reef species to name a few.  But these reefs may also be impacted by storm damage, flooding, and even ‘natural bleaching’, causing alterations in the benthic habitat (the reef itself).  If we want to understand how overfishing of sharks on reefs has altered the coral ecosystem, we need to do our best to take these other stressors into account.

About 300 km off the northwest coast of Australia lay two groups of uninhabited atoll-like coral reefs – the Rowley Shoals and Scott Reefs.  Scott reef has been, and continues to be, a top location for Indonesian fishers for centuries – including reef sharks.  Rowley Shoals on the other hand lies in what is now a marine protected area, and compliance is thought to be pretty good there, giving us a now unfished reef system.  Using long-term datasets,   Jonathan Ruppert from  the University of Toronto in Canada and fellow researchers took the opportunity to understand the effects of shark fishing, bleaching, and cyclones on changes in fish and habitat on these two reef systems.  As with many field-based studies, they could not pin down any definitive ‘cause and effect’, but they did uncover a number of interesting correlations.

The abundance of corallivores (coral eating) and planktivores (plankton eating) fish species appeared to decline in line with loss of live coral (this is called negatively correlated).  For these critters, their abundance didn’t significantly differ between the fished and unfished reefs.  It seemed that they are more affected by changes in habitat as a result of the natural disturbances then the abundance of sharks.  This makes a sense – they have just lost their food source and shelter.  Herbivorous fish – like parrotfish – on the other hand seemed to fair much better after a these natural disturbance.  When a cyclone or bleaching destroys coral, algae can (but not always) increase in its place.  This is because algae grow much quicker than the corals, resulting in an increase in food for the herbivores.  This is likely the reason why herbivores showed a positive correlation (an increasing trend) with increasing algal cover.  But there were also less herbivores on fished reefs compared to unfished reefs.  The loss of these guys is a particular problem because herbivores are vital for keeping algae in check to stop it completely dominating a coral reef.  If you lose herbivores, you can lose much if not all of a coral reef.  Given that bleaching and cyclones are thought to increase in frequency and intensity as our climate is changes, the role of herbivores may become even more important to reef survival.

As we move up the food chain, we reach the mesopredators – the mid-level predators, which on coral reefs means families like snapper and emperors.  The abundance of these critters didn’t show any significant relationship with benthic habitat, but were much more abundant on fished reefs than unfished reefs.  In a nutshell, the less sharks the more mesopredators, and the less herbivores.  Sounds like a trophic cascade?  Perhaps, but the authors are also quick to note that these differences on fished and unfished reefs could be down to other reasons – remember, their study found relationships (correlations) – not cause and effect.  For example, the authors found some differences in the habitat between the Rowley Shoals and Scott Reefs that they could cause the differences in the fish communities between the two systems.  They also note that variation in larval supply may be a significant factor in the differences seen.

Despite these issues, and the lack of definitive ‘the loss of sharks has directly caused x’, the authors suggest we don’t sit back on our laurels.  We may not be able to make any appreciable difference to the frequency and intensity of bleaching or cyclone events in the short-term, we can reduce the pressure on sharks.  For a marine ecosystem that is widely regarded as at risk on a global scale, it may be that every little bit helps.

The paper is published in the open access journal  PLoS ONE – you can have a read of it here dx.doi.org/10.1371/journal.pone.0074648

Image:  Black tip reef shark (Carcharhinus melanopterus).  Credit Jim H./Flickr (CC BY-SA 2.0)

To find out more about the Black tip reef shark, head over here.
*Shark finning is the practice of removing a sharks fin whilst on board a fishing boat, and then throwing the remainder of the shark back into the sea.  See this excellent blog entry by David Shiffman  on what shark finning actually is.