Global analysis reveals how sharks travel the oceans to find food

A major international collaboration, including AORI researcher Dr. Alex Wyatt and authors from 20 other countries, could help global efforts to overturn recent declines in the world’s shark population by providing greater insight into the feeding habits of the world’s most misunderstood fish.

Led by Dr. Christopher Bird from the University of Southampton, the study published in Nature Ecology & Evolution, used chemical markers in the form of carbon isotopes found in sharks to investigate where in the world they have been feeding – an unresolved question for many shark species. Knowing which parts of the global ocean are important shark feeding areas may help to design more effective conservation measures to protect declining shark populations.

Distribution of compiled shark data and plankton isotopes from Bird et al. (2018) Nature Ecology & Evolution.

All life depends on carbon at the bottom of the food chain. Carbon comes in three forms or isotopes, and the proportions of two of the most common isotopes vary across the world’s ocean. In the study, 73 scientists from 21 countries compared the carbon isotopes from more than 5000 sharks from 114 species across the globe with those from phytoplankton at the bottom of the food web.

“If an animal feeds in the same place where it was caught, the carbon isotope signals in the shark and phytoplankton will match.,” says Dr Bird whose PhD research was focused on deep-sea sharks. “However, if the shark has moved between feeding and where it was caught, then the signals will be different.

“You’ve heard of ‘you are what you eat’ – well this is more ‘you are where you ate’”

“You’ve heard of ‘you are what you eat’ – well this is more ‘you are where you ate’”, Bird continued. “We were able to show that sharks living close to land and those that live in the open ocean have very different ways of feeding.  The global study used muscle tissues to show that sharks living near to the coast feed locally across a range of different food webs –this is like people living in a city with access to lots of different restaurants in the neighbourhood and no need to travel far to find the food they want. On the other hand, oceanic sharks that are found throughout the world’s oceans, appear to get most of their food from specific areas of cooler water in the northern and southern hemispheres. This is more like travelling long distances from rural areas to spend lots of time eating in a few restaurants in a distant city.

“Our results suggest that for whale sharks, we can isotopically capture feeding that occurred in the months to years prior to sampling by sampling a range of slow and fast tissues, like blood plasma and fin cartilage, respectively”. Dr Wyatt added that future work looking at multiple tissues is expected to add a temporal dimension, “in this case, not only are ‘you what and where you ate’ but also ‘when you ate’”.

Carbon isotopes of different types of sharks from Bird et al. (2018) Nature Ecology & Evolution.

These findings are very important for Dr Wyatt, who is using isotopes to better understand the open ocean movements and foraging strategies of planktivorous sharks and rays like the whale shark.  “Our results from captive whale sharks, in collaboration with Okinawa Churaumi Aquarium, have demonstrated that we can use carbon isotopes measured in different tissues of one shark to show changes in feeding activity of that individual over time” says Dr Wyatt.

This is because different tissues have different rates of metabolic turnover, and can thus reflect feeding over different time scales.  “Our results suggest that for whale sharks, we can isotopically capture feeding that occurred in the months to years prior to sampling by sampling a range of slow and fast tissues, like blood plasma and fin cartilage, respectively”. Dr Wyatt added that future work looking at multiple tissues is expected to add a temporal dimension, “in this case, not only are ‘you what and where you ate’ but also ‘when you ate’”.

Such increased understanding of global shark feeding is considered vital for their conservation. “With over 500 known species around the world, sharks are certainly amongst our most diverse and misunderstood group of fish but we still have limited knowledge of their habits and behaviours, particularly relating to feeding and movement|” said Dr. Christopher Bird. “Over the last 50 years, the pressures of fishing and habitat degradation have resulted in declines amongst some of the world’s shark populations, the effects of which are also not fully understood.”

Cartoon summarising the research (credit: Clive Trueman)

Senior author Dr Clive Trueman, Associate Professor in Marine Ecology from the University of Southampton added, “The results have important implications for conservation. Globally, sharks are not doing well. Many shark populations have declined in the last few decades, particularly in the wide-ranging oceanic sharks that are targeted by fishing boats and caught accidentally in tuna fisheries as “by-catch”. Governments are now creating large marine protected areas around the globe, which help to reduce fishing, but most of these protected areas are in tropical waters, and may not provide effective protection for oceanic sharks.”

“Sharks urgently need our help, but to help them we also need to understand them. Our study has helped by identifying important shark feeding grounds. New technologies like satellite and isotope tracking are giving us the information we need to turn the tide on these beautiful and fascinating animals.”

The paper ‘A global perspective on the trophic geography of sharks’ is published in the February issue of Nature Ecology & Evolution (doi 10.1038.s41559-017-0432-z).


View the Nature blog post here:

https://goo.gl/BF73Uk

View the article here:

https://www.nature.com/articles/s41559-017-0432-z

View the cartoon here:

https://goo.gl/iS8voV

More information on the Chondrichthyan Stable Isotope Data Project (CSIDP):

https://goo.gl/Wmmzk3


 

New paper on the trophic geography of sharks (Bird et al., Nature Ecology & Evolution)

A global perspective on the trophic geography of sharks

Christopher S. Bird 1,71*, Ana Veríssimo2,3, Sarah Magozzi1, Kátya G. Abrantes4, Alex Aguilar5, Hassan Al-Reasi6, Adam Barnett4, Dana M. Bethea7,72, Gérard Biais8, Asuncion Borrell 5, Marc Bouchoucha9, Mariah Boyle10, Edward J. Brooks11, Juerg Brunnschweiler12, Paco Bustamante 13, Aaron Carlisle14, Diana Catarino 15, Stéphane Caut16, Yves Cherel17, Tiphaine Chouvelon18, Diana Churchill19, Javier Ciancio20, Julien Claes21, Ana Colaço15, Dean L. Courtney 22,73, Pierre Cresson23, Ryan Daly24,25, Leigh de Necker26, Tetsuya Endo27, Ivone Figueiredo28, Ashley J. Frisch29, Joan Holst Hansen30, Michael Heithaus31, Nigel E. Hussey32, Johannes Iitembu33, Francis Juanes34, Michael J. Kinney 35, Jeremy J. Kiszka 36, Sebastian A. Klarian37, Dorothée Kopp38, Robert Leaf39, Yunkai Li40, Anne Lorrain41, Daniel J. Madigan42, Aleksandra Maljković43, Luis Malpica-Cruz44, Philip Matich45,46, Mark G. Meekan47, Frédéric Ménard48, Gui M. Menezes15, Samantha E. M. Munroe49, Michael C. Newman50, Yannis P. Papastamatiou51,52, Heidi Pethybridge53, Jeffrey D. Plumlee54,55, Carlos Polo-Silva56, Katie Quaeck-Davies1, Vincent Raoult 57, Jonathan Reum58, Yassir Eden Torres-Rojas59, David S. Shiffman60, Oliver N. Shipley61, Conrad W. Speed47, Michelle D. Staudinger62,63, Amy K. Teffer64, Alexander Tilley 65, Maria Valls66, Jeremy J. Vaudo67, Tak-Cheung Wai68, R. J. David Wells54,55, Alex S. J. Wyatt 69, Andrew Yool70 and Clive N. Trueman1*

[Author affiliations]

Sharks are a diverse group of mobile predators that forage across varied spatial scales and have the potential to influence food web dynamics. The ecological consequences of recent declines in shark biomass may extend across broader geographic ranges if shark taxa display common behavioural traits. By tracking the original site of photosynthetic fixation of carbon atoms that were ultimately assimilated into muscle tissues of 5,394 sharks from 114 species, we identify globally consistent biogeographic traits in trophic interactions between sharks found in different habitats. We show that populations of shelf-dwelling sharks derive a substantial proportion of their carbon from regional pelagic sources, but contain individuals that forage within additional isotopically diverse local food webs, such as those supported by terrestrial plant sources, benthic production and macrophytes. In contrast, oceanic sharks seem to use carbon derived from between 30° and 50° of latitude. Global-scale compilations of stable isotope data combined with biogeochemical modelling generate hypotheses regarding animal behaviours that can be tested with other methodological approaches.


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Impacts of internal waves on the twilight zone @ ICRS 2016, Hawaii (Dr Wyatt)

Ecological and biogeochemical impacts of internal waves on mesophotic coral ecosystems: testing eddy correlation and isotope approaches, Iriomote, Japan

Alex S.J. Wyatt1*, Toshihiro Miyajima1, James J. Leichter2, Tohru Naruse3, Tomohiro Kuwae4, Shoji Yamamoto5, Naomi Satoh1, Toshi Nagata1

1Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, JAPAN
2Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California, USA
3Tropical Biosphere Research Center, University of the Ryukyus, Taketomi, Japan
4Coastal and Estuarine Environment Research Group, Port and Airport Research Institute (PARI), Nagase, Yokosuka, JAPAN
5Department of Earth and Planetary Science, The University of Tokyo, Bunkyo-ku, Tokyo, Japan

While mesophotic coral ecosystems (MCE) may be protected or damped from disturbances impacting shallower reefs insufficient information is available on the environmental conditions supporting these ‘deep water refugia’. Nutrient inputs and recycling have rarely been quantified over MCE but may differ fundamentally to that of shallow counterparts due to the reduction in light and increasing reliance on oceanic nutrients, leading to increased heterotrophy over autotrophy at species and ecosystem levels and stronger links to oceanic processes. For instance, due to the depth of MCE relative to typical water column density stratification, internal waves may be a highly significant process depending on community aspect and exposure. Preliminary observations of MCE along a continuum of oceanic exposure in Funauki Bay, Iriomote, Japan indicate that ocean-exposed MCE are subject to semi-diurnal temperature oscillations of up to 4 C during summer (range 23 – 29 deg C), while inner MCE occur shallower in more turbid but stable environments. Oceanic exposure along the bay may determine both the distribution and function of spatially extensive, but relatively homogenous, communities dominated by Leptoseris sp. or Acropora ?horrida. Combining bulk and compound-specific stable isotope analyses, depth-specific radioisotope markers such as radiocarbon, and eddy correlation experiments in these habitat promises a useful approach for elucidating the functional importance of internal waves in the development and persistence of MCE at local to regional scales.

Formation and maintenance of high-nitrate, low pH layers (Waite et al., Biogeosciences)

Formation and maintenance of high-nitrate, low pH layers in the eastern Indian Ocean and the role of nitrogen fixation

A. M. Waite, V. Rossi, M. Roughan, B. Tilbrook, P. A. Thompson, M. Feng, A. S. J. Wyatt, and E. J. Raes

We investigated the biogeochemistry of low dissolved oxygen high-nitrate (LDOHN) layers forming against the backdrop of several interleaving regional water masses in the eastern Indian Ocean, off northwest Australia adjacent to Ningaloo Reef. These water masses, including the forming Leeuwin Current, have been shown directly to impact the ecological function of Ningaloo Reef and other iconic coastal habitats downstream. Our results indicate that LDOHN layers are formed from multiple subduction events of the Eastern Gyral Current beneath the Leeuwin Current (LC); the LC originates from both the Indonesian Throughflow and tropical Indian Ocean. Density differences of up to 0.025 kg m−3 between the Eastern Gyral Current and the Leeuwin Current produce sharp gradients that can trap high concentrations of particles (measured as low transmission) along the density interfaces. The oxidation of the trapped particulate matter results in local depletion of dissolved oxygen and regeneration of dissolved nitrate (nitrification). We document an associated increase in total dissolved carbon dioxide, which lowers the seawater pH by 0.04 units. Based on isotopic measurements (δ15N and δ18O) of dissolved nitrate, we determine that ~ 40–100% of the nitrate found in LDOHN layers is likely to originate from nitrogen fixation, and that, regionally, the importance of N-fixation in contributing to LDOHN layers is likely to be highest at the surface and offshore.

Picoplankton and virus uptake by a coral reef (Patten et al., Coral Reefs)

Uptake of picophytoplankton, bacterioplankton and virioplankton by a fringing coral reef community (Ningaloo Reef, Australia)

Nicole L. Patten, Alex S.J. Wyatt, Ryan J. Lowe, Anya M. Waite

We examined the importance of picoplankton and virioplankton to reef trophodynamics at Ningaloo Reef, (north-western Australia), in May and November 2008. Picophytoplankton (Prochlorococcus,Synechococcus and picoeukaryotes), bacterioplankton (inclusive of bacteria and Archaea), virioplankton and chlorophyll a (Chl a) were measured at five stations following the consistent wave-driven unidirectional mean flow path of seawater across the reef and into the lagoon.Prochlorococcus, Synechococcus, picoeukaryotes and bacterioplankton were depleted to similar levels (~40% on average) over the fore reef, reef crest and reef flat (=‘active reef’), with negligible uptake occurring over the sandy bottom lagoon. Depletion of virioplankton also occurred but to more variable levels. Highest uptake rates, m, of picoplankton occurred over the reef crest, while uptake coefficients, S (independent of cell concentration), were similarly scaled over the reef zones, indicating no preferential uptake of any one group. Collectively, picophytoplankton, bacterioplankton and virioplankton accounted for the uptake of 29 mmol C m−2 day−1, with Synechococcuscontributing the highest proportion of the removed C. Picoplankton and virioplankton accounted for 1–5 mmol N m−2 day−1 of the removed N, with bacterioplankton estimated to be a highly rich source of N. Results indicate the importance of ocean–reef interactions and the dependence of certain reef organisms on picoplanktonic supply for reef-level biogeochemistry processes.

Keywords: Coral reef, Picoplankton, Virus, Uptake, Ningaloo Reef, Indian Ocean

Coral reef phytoplankton fluxes (Wyatt et al., MEPS)

Particulate nutrient fluxes over a fringing coral reef: relevant scales of phytoplankton production and mechanisms of supply

Alex S. J. Wyatt, Ryan J. Lowe, Stuart Humphries, Anya M. Waite

Seasonal observations of phytoplankton uptake at Ningaloo Reef, Western Australia, reinforce the importance of particulate organic nitrogen (PON) and carbon (POC) in reef nutrient budgets and identify wave action and the dynamics of regional currents (over a range of temporal and spatial scales) as important factors determining plankton supply to the reef. Phytoplankton uptake rates, calculated from declining chlorophyll a concentrations as water moved over the reef, appeared to be near the physical limits of mass transfer. Phytoplankton-derived PON flux of 2 to 5 mmol N m–2 d–1 was on the order of that typical for dissolved N uptake—confirming that particle feeding may supply the N missing in reef N budgets—while POC flux of 14 to 27 mmol C m–2 d–1 was on the order of net community metabolism. Phytoplankton supply was highly variable at daily-to-seasonal time scales in response to the dynamics of a regional current system dominated by the downwelling-favourable Leeuwin Current (LC). Acceleration of the LC in the austral autumn may supply as much phytoplankton to the reef as sporadic upwelling associated with the Ningaloo Current (NC) in summer. The ocean catchment concept is introduced as a basis for examining the spatial scale of pelagic processes influencing benthic systems: every day, Ningaloo may completely consume the phytoplankton over 87 km2 of LC water, compared to only 20 km2 of NC water. Production within this catchment appears insufficient to maintain offshore phytoplankton concentrations, and advection of remotely sourced production into the catchment is required to balance reef uptake. A functional dependence by reef organisms on externally sourced ocean productivity increases the potential scale at which human- or climatically induced changes may affect reef communities and suggests that processes such as changes in offshore currents and plankton communities require further consideration in reef-level biogeochemistry.

KEY WORDS: Ningaloo Reef · Nutrient budget · Oceanographic forcing · Particulate organic carbon · Particulate organic nitrogen · Leeuwin Current · Ocean catchment · Upwelling