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


 

Adding to our isotope axes @ IsoEcol 2016 (Dr Wyatt)

Amino acid and radiocarbon insights from captive whale sharks

Alex S.J. WYATT1*, Rui Matsumoto2, Yoshito Chikaraishi3, Yosuke Miyari1, Yusuke Yokoyama1, Keiichi Sato2, Nao Ohkouchi3, Toshi Nagata1

1Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, JAPAN
2Okinawa Churaumi Aquarium, Motobu, Okinawa, JAPAN
3Japan Agency for Marine-Earth Science and Technology, Yokosuka, JAPAN
*Presenting author

Stable isotope analyses (SIA) have the potential to provide novel insights into spatial and temporal patterns in the trophic ecology of poorly understood organisms like whale sharks Rhincodon typus. However, interpreting SIA depends on accurate diet-tissue discrimination factors (DTDF) to quantify diets and trophic positions, with experimental derivations of DTDF rare for such large-bodied organisms. Captive R. typus have provided a unique opportunity to validate a range of SIA, compound-specific isotope analyses (CSIA) and radioisotope approaches in the world’s largest fish and one of three planktivorous sharks. Diet records over the past five years revealed a diet dominated by North Pacific and Antarctic krill, 44% and 49% of weighted diet for Euphausia pacifica and E. superba, respectively. Despite the well-known diet, SIA of fin tissue from three captive R. typus (7.1, 7.2, and 8.4 m in length) proved hard to reconcile, especially for bulk carbon. In contrast, CSIA of amino acid (AA) nitrogen in the sharks’ tissue was relatively stable over time, despite evidence of variation in AA compositions and δ15N-AA of diet components. Tissue radiocarbon further suggested either long turnover in fin tissues (27 months), or the preferential assimilation of the smaller E. pacifica14C of 3 ‰ compared to -112 ‰ for E. superba). Daily-scale analysis of radiocarbon in R. typus faeces may support the preferential assimilation hypothesis, faeces generally being depleted relative to diet. Together, CSIA-AA and radiocarbon analyses add multiple addtional axes to our isotope space and may alleviate some of the complications involved in interpreting bulk SIA in ecological studies.

Isotopic tools for planktivorous megafauna @ ASLO 2015 (Dr Wyatt)

Isotopic Tools for Assessing Oceanic Versus Reef-Scale Drivers of Planktivorous Megauna Aggregations

Alex S.J. WYATT1*, Rui Matsumoto2, Yoshito Chikaraishi3, Keiichi Sato2, Nao Ohkouchi3, Toshi Nagata1

1Marine Biogeochemistry Laboratory, Department of Chemical Oceanography, Atmosphere & Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, JAPAN.
2Okinawa Churaumi Aquarium, Motobu, Okinawa, JAPAN.
3Japan Agency for Marine-Earth Science and Technology, Yokosuka, JAPAN
*Presenting author

Stable isotope analyses (SIA) have the potential to provide novel insights into spatial and temporal patterns in the trophic ecology of poorly understood planktivorous megafauna, especially the regional oceanic versus local reef-scale drivers of whale shark and manta ray aggregations that occur along coral reefs worldwide. However, interpreting SIA depends on accurate diet-tissue discrimination factors (DTDF) to quantify diets and trophic positions, with experimental derivations of DTDF rare for such large-bodied organisms. Captive whale sharks Rhincodon typus have provided a unique opportunity to validate a range of SIA, compound-specific isotope analyses (CSIA) and radioisotope approaches in the world’s largest fish and one of three planktivorous sharks. Combining SIA and CSIA with depth-specific radioisotope markers such as iodine ratios (129I/127I) are expected to offer a promising path towards elucidating the regional to local scale divers of planktivore aggregations Although I will focus on the implications of multi-tissue differences in DTDF and turnover times in three captive whale sharks (7.1, 7.2, and 8.4 m in length) the concepts and techniques are highly applicable to studying a wide range of species in diverse environments. An example will be provided of application to a wild caught (4.4 m) specimen of the smallest planktivorous shark, the rarely encountered megamouth shark Megachasma pelagios.

Isotope discrimination in captive whale sharks @ IsoEcol 2014 (Dr Wyatt)

Isotope Discrimination in Planktivorous Elasmobranchs Focusing on the World’s Largest Fish, Captive Whale Sharks Rhincodon typus

Alex S.J. WYATT1* Rui Matsumoto2 Yoshito Chikaraishi3 Keiichi Sato2 Nao Ohkouchi3 Toshi Nagata1

1Marine Biogeochemistry Laboratory, Department of Chemical Oceanography, Atmosphere & Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, JAPAN.
2Okinawa Churaumi Aquarium, Motobu, Okinawa, JAPAN.
3Japan Agency for Marine-Earth Science and Technology, Yokosuka, JAPAN
*Presenting author

Accurate diet-tissue discrimination factors (DTDF) are essential for quantifying diets and trophic positions (TP) using stable isotope analyses (SIA), with potential variation between diets, tissues, organisms and environments arguing against untested application of meta-analysis averages (e.g. 3.4 ‰ for bulk nitrogen (Δ15Nbulk), ~0.5 ‰ for bulk carbon (Δ13Cbulk), and 7.6 ‰ and 0.4 ‰ for nitrogen of glutamic acid (Δ15Nglu) and phenylalanine (Δ15Nphe), respectively). Experimental derivations of DTDF in elasmobranchs (sharks and rays) are scarce, with large-bodied organisms difficult to maintain in captivity and non-lethal multi-tissue sampling problematic for both captive and wild individuals. SIA of captive whale sharks Rhincodon typus, one male (8.5 m in length) and two females (7.1 and 7.2 m), fed a mixed diet composed mainly (~ 48 % each) of Antarctic krill Euphausia superba15N = 3.45 ‰, δ13C = -26.3 ‰) and North Pacific krill E. pacifica15N = 5.88 ‰, δ13C = -21.6 ‰), provide an opportunity to examine DTDF in the world’s largest fish and one of three planktivorous sharks. DTDFs estimated based on temporally averaged diets for easily sampled but slow turnover fin tissue were close to previous observations, but varied between individuals, perhaps reflecting differing growth rates with size or physiological differences between the sexes: Δ15Nbulk (2.6, 3.3, 3.1 ‰), Δ13Cbulk (3.9, 4.5, 5.9 ‰), Δ15Nglu (7.6, 6.5, n.d. ‰) and Δ15Nphe (0.3, 0.2, n.d. ‰). Short turnover tissues, such as liver or blood, may be difficult or impossible to obtain for these species, requiring non-lethal isotopic proxies to examine diet and TP at higher temporal resolution. For instance, SIA of faecal material was highly variable but reflected day-to-day variation in minor (<3 %) components of the sharks’ diets. DTDF will be discussed in the context of sampling constraints related to multi-tissue SIA and recent radioisotope approaches for understanding feeding and aggregations of planktivorous elasmobranchs, including recent application to a wild caught (4.4 m) specimen of the smallest planktivorous shark, the rare megamouth shark Megachasma pelagios.