Shark research - insights into trophic position of sharks

Scientists at Shanghai Ocean University have used stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) to assess trophic postions of 5 shark species.

Previous work on the trophic levels of sharks has primarily used stomach content analysis to assess where sharks sit in the food web. Assessing trophic position by examining stable isotope ratios is a less invasive method than stomach content analysis.

Sharks are apex predators and play a pivotel role in marine ecosystems. Detailed understand of where apex predators sit in the food web, in relation to each other, gives us a better understanding of the top down effect they may have on an ecosystem.

The five species included in this study were the blue shark (Prionace glauca), bigeye thresher (Alopias superciliosus), silky shark (Carcharhinus falciformis), scalloped hammerhead (Sphyrna lewini) and oceanic whitetip (Carcharhinus longimanus). These five species are all caught by the Chinese longline fleet in the mid-east Pacific.

Stable isotope analysis (SIA) can be used to assess a number of mechanisms in the food chain. By examining δ¹⁵N isotopes in organisms, researchers can make an estimate of the position that organisms inhabits in the food web. δ¹⁵N isotopes move through the food chain as organisms consume each other. Therefore organisms residing in a higher position in the food web will have higher δ¹⁵N values than those further down.

The researchers also examined δ¹³C isotopes; these isotopes originate from the primary producer and remain relatively unchanged as they make their way through the food web; they can be used to give an indication of the source of primary production supporting the entire food web.

From these analyses, the team discovered that A. superciliosus occupies the highest trophic position in the food web; possibly owing to the fact that this species consumes more cephalopods than the others - cephalopods have higher δ¹⁵N and δ¹³C values than fish prey. The team also discovered that A. superciliosus had the largest variation of δ¹⁵N - possibly owing to its highly migratory lifestyle.

C. longimanus occupied the lowest trophic position in the food web.

Li, Yunkai , Gong, Yi , Chen, Xinjun , Dai, Xiaojie and Zhu, Jiangfeng. Trophic ecology of sharks in the mid-east Pacific ocean inferred from stable isotopes. Journal of Ocean University of China 13(2):278-282

Shark research - Sharks get their speed from spring loaded spines

New research has discovered that the vertebral columns of sharks behaves like a spring, providing power when swimming.

Sharks differ from most other fish in that their skeleton is made of cartilage instead of bone. By examining whole body curvature of the spiny dogfish (Squalus acanthias), and strain placed on the vertebrae of sharks during swimming, a team of researchers have discovered that the entire vertebral column bends during locomotion. This indicates that the spine is behaving like a spring and providing power to the sharks when swimming.

The morphology of the spine of cartilaginous fish appears to allow for spring loading. The vertebral column of S. acanthias displayed an open, intracentral canal, covered by a velum layer which effectively strengthens the vertebral column and could facilitate elastic recoil of the entire spine. This recoil in the cartilaginous vertebral column of sharks assists undulatory swimming by providing springback power to the motion of the tail. A shark requires 6% of its metabolic power to flex the tail and load the spring, but this investment is a relatively inexpensive cost for the recoil power generated.  

The results indicate that the bodies of sharks have been positively selected for speed, which is in keeping with the lifestyle of many large, metabolic sharks.

This method of swimming differs from bony fishes, whose spines consist of hardened vertebrae and flexible joints, and may provide a clue for one reason why sharks diverged from the ancestral state of having a bony skeleton.

M.E. Porter, Candido Diaz Jr., Joshua J. Sturm, Sindre Grotmol, A.P. Summers, John H. Long Jr., Built for speed: strain in the cartilaginous vertebral columns of sharks, Zoology, Volume 117, Issue 1, February 2014, Pages 19-27, ISSN 0944-2006, http://dx.doi.org/10.1016/j.zool.2013.10.007

Shark research - Sharks vs magnets 2: visually deprived lemon sharks show heightened awareness of electric stimuli

Following on from their previous research (see The Shark Post 13th Jan), researchers at University of Massachusetts have investigated the reactions of visually impaired lemon sharks to magnets.

The team had already ascertained that C8 barium-ferrite (BaFe12O19) permanent magnets successfully repelled bull sharks, but were unsure as to the success magnets would have on repelling sharks which are more reliant on their visual system. The aims of this study were to ascertain whether magnets would successfully repel visually impaired juvenile lemon sharks (Negaprion brevirostris).

The researchers hypothesised that restricting the vision of lemon sharks would lead to a greater reliance on short range senses such as electro reception - an effect known as context dependent switching. In the wild the visual field of a marine animal could be restricted by bad visibility caused by environmental conditions such as turbidity. In this instance,

The sharks were visually impaired using a short term, simple surgery technique to suture closed the nictitating membrane of the eye. Once the experimental phase was completed, the sutures were removed, restoring the vision of the sharks. All sharks showed successful recovery post-experimentation.

The experimental area was separated into different compartments, both with and without magnets present. The sharks were divided into groups with differing levels of visual impairment. By assessing the behaviour of the different shark types to the magnets, the researchers were able to assess what impact visual impairment had on context dependent switching of senses.

Visually impaired sharks did indeed enter areas with magnets less frequently than sharks with visual ability. These results suggest that magnetic repellents would be more successful when sharks are visually impaired, and this could translate to the visual impairment experienced in turbid waters. Magnets could be an appropriate method of deterring all species of sharks from entering turbid regions, and not just those which are less reliant on their visual system to begin with.  

Connell, C., Guttridge, T., Gruber, S., Brooks, J., Finger, J. and He, P. (2014) Behavioral modification of visually deprived lemon sharks (Negaprion brevirostris) towards magnetic fields. Journal of Experimental Marine Biology and Ecology 453:131-137

Shark Research - new low cost survey techniques could aid shark conservation

Recent research conducted by researchers at l'Acquario e Civica Stazione Idrobiologica di Milano in Italy has led to the development of a low cost method to assess shark assemblages in remote areas.

The research was conducted at 2 reef locations off the coast of Sudan. Shark populations are of concern in this region as there have been reports of illegal fishing of sharks around the Sudanese coast. The authors also comment on a lack of control over Sudanese shark fisheries.

The researchers used stationary underwater cameras to video shark activity during the day, and laser pointers to infer the total length of the sharks filmed. By gathering data in this manner, the team were able to compile information regarding species, total length, sex, ecology and behaviour of pelagic and reef associated sharks in this region. In total 6 species were recorded from the reefs: common thresher shark (Alopias vulpinus), scalloped hammerhead (Sphyrna lewini), whitetip reef shark (Triaenodon obesus), grey reef shark (Carcharhinus amblyrhynchos) and tiger shark (Galeocerdo cuvier).  

The equipment and methods used in this study are relatively inexpensive and non invasive to the marine life. The survey techniques were also suitable for the adverse conditions of the study sites - both sites exhibited strong currents and relatively deep thermoclines which some of the shark species remained beneath. The relative ease with which these methods can be employed could facilitate the monitoring of shark assemblages in remote areas and contribute to data gathering of shark assemblages which presently, may be quite data deficient. This, in turn, could lead to a greater enforcement of shark regulation in these areas.

Danilo Rezzolla, Ginevra Boldrocchi and Tiziano Storai. Evaluation of a low-cost, non-invasive survey technique to assess the relative abundance, diversity and behaviour of sharks on Sudanese reefs (Southern Red Sea) . Journal of the Marine Biological Association of the United Kingdom, available on CJO2014. doi:10.1017/S0025315413001781.    

Shark research - Global warming could threaten whale shark populations

A research article published in Global Change Biology coupled existing whale shark distribution to global climate forecasts to predict how climate change may affect whale shark (Rhincodon typus) populations.

The research, led by Ana Sequeira at The University of Adelaide, found that rising sea temperatures may reduce the available habitat of whale sharks, and are likely to shift current whale shark populations towards the poles.

The researchers used sightings of whale sharks, recorded by tuna purse-seine fisheries, to assume the current global distribution. The data had been collected over different seasons and in some cases recorded whale shark sightings from over 30 years ago.

Currently, the range of the whale shark is commonly between 30-35°N. By combining this data with climate forecasts derived from global circulation models - which predict that existing sea surface temperatures will increase by at least 2°C on average by 2070 - the researchers discovered that the suitable range of the whale shark will be pushed towards the poles by 2070.  

Perhaps more worryingly, the team predicted that the available suitable whale shark habitat will contract with rising sea temperatures. Ectothermic organisms, such as whale sharks, are sensitive to temperature change and rises in sea temperature lead to an increase in metabolic activity which raises food intake requirements. Other threats to whale sharks include illegal fishing, shipping activity and habitat disturbance.

The whale shark is listed as Vulnerable in the IUCN Red List (http://www.iucnredlist.org/details/19488/0) and the results of this research suggest that current management policies of whale sharks should be revised to attempt to guarantee the global survival of whale sharks.

Sequeira, A. M. M., Mellin, C., Fordham, D. A., Meekan, M. G. and Bradshaw, C. J. A. (2014), Predicting current and future global distributions of whale sharks. Global Change Biology, 20: 778–789. doi: 10.1111/gcb.12343

Shark research - Post release mortality rate of shark bycatch higher than first thought

A study conducted primarily by the University of Miami has examined post release mortality in 5 shark species.

Bycatch - where a non-target species is caught by non-discriminatory fishing methods - is a common problem of the fishing industry. Owing to the fact that many species of shark are endangered and for the sake of upkeeping good fishing practice, bycatch is often required to be released alive. Whilst there have been studies on the successful handling and release of shark bycatch, there have been few studies on the post release mortality of sharks.

In their research the team, led by A. J. Gallagher, looked at whole blood lactate, pH values, pCO2 and shark size to assess the physiological disturbance that hooking has on a shark and to ascertain if any of these disturbances linked to post-release mortality. Smart Position or Temperature Transmitting (SPOT) tags were deployed on 3 of the species (bull, tiger and great hammerhead sharks) to assess potential post-release survival.

The researchers included 5 shark species in the study: blacktip shark Carcharhinus limbatus, bull shark Carcharhinus leucas, great hammerhead shark Sphyrna mokarran, lemon shark Negaprion brevirostris and tiger shark Galeocerdo cuvier. The species were caught on baited drumlines using circle hooks - which have been shown to lower mortality rates in sharks. The hooks afforded the sharks a 23m radius to swim around once caught. The hooks were checked periodically and caught sharks were brought alongside the boat in order to take samples and to measure the total length.

The results showed that of the 5 species, tiger sharks were the most likely to survive post release capture with 96.4% of the individuals caught transmitting tag data 4 weeks after the study. Blacktip sharks appeared to be the most vulnerable to hooking, with their physiological disturbances ranking the highest of the 5 species studied. The team also found that of all the measurements, blood lactate levels seemed to correlate closest with post release survival. Species which may need to use short bursts of energy to evade predation from larger sharks - such as the blacktips - displayed high levels of lactate in their blood. Lactate is generated when periods of sustained exercise cause animals to undergo a shift from aerobic to anaerobic respiration. Tiger sharks - which most likely rely on stamina and sustained swimming as oppose to bursts of energy - exhibited very little change in their lactate levels.

These stress responses - such as changes in lactate and pH - can affect the short term and long-term survival of an animal. This study found that some sharks - such as tiger and lemon sharks - are able to recover very quickly from the stress of being hooked. However, other species - such as blacktips and great hammerhead sharks - suffer greatly from post capture stresses as displayed by large changes in their blood lactate levels. This is translated to higher supposed mortality rates post capture - only 53.6% of great hammerhead sharks transmitted tag data after 4 weeks compared to 96.4% of tiger sharks.

The finding here are important as the fishing industry may currently be assuming that if a caught shark swims away from the boat that it will survive. However, the results here show that there are huge variations in the ability of different shark species to recover from the stress of being hooked and the post release survival rate is actually quite low in some species. Owing to the fact that sharks are threatened globally and live release of bycatch is currently an accepted strategy for dealing with unintentionally caught sharks, more data on actual post release survival levels will help determine whether or not hooking certain species of sharks will impact upon the long term survival of the species.

Gallagher, A.J., Serafy, J.E., Cooke, S.J. and Hammerschlag, N. (2014) Physiological stress response, reflex impairment, and survival of five sympatric shark species following experimental capture and release. Marine ecology progress series. 496: 207-218