{"id":714,"date":"2019-01-17T00:18:48","date_gmt":"2019-01-16T15:18:48","guid":{"rendered":"http:\/\/www.oceanecol.com\/?p=714"},"modified":"2019-06-12T19:44:47","modified_gmt":"2019-06-12T10:44:47","slug":"chemically-unravelling-mysteries-surrounding-the-worlds-largest-fish","status":"publish","type":"post","link":"https:\/\/www.oceanecol.com\/?p=714","title":{"rendered":"Chemically unravelling mysteries surrounding the world\u2019s largest fish"},"content":{"rendered":"

Foraging, starvation and herbivory in the globally threatened whale shark<\/h2>\n

Whale sharks, filter feeding sharks that travel tropical oceans in search of their microscopic prey, are globally threatened. Despite being the world\u2019s largest fish, reaching over 12 m in length and 21 tonnes, many facets of the whale shark\u2019s life in the open ocean remain unknown. New research based on a collaboration led by researchers at The University of Tokyo\u2019s Atmosphere and Ocean Research Institute (AORI) with participants from Okinawa Churaumi Aquarium and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) published in the journal Ecological Monographs<\/em><\/a> offers new insights into diets, movement and behaviour of whale sharks. The enhanced chemical approach developed suggests that individual specialisation, starvation, and even herbivory may be prevalent in whale sharks. The research is expected to facilitate better understanding and management of this enigmatic species through more detailed assessments of global changes in their feeding and health.<\/p>\n

\u275dMost significantly, we demonstrated how tissue isotopes are strongly connected to rates of growth and feeding…This suggests that it is impossible to accurately interpret chemical signals in the wild without some idea of growth and feeding rates prior to sampling\u275e<\/p><\/blockquote>\n

The study used long-term observations of captive whale shark feeding to test chemical approaches for measuring whale shark foraging and health. By combining measurements of different forms (isotopes) of carbon and nitrogen in various tissues, the research revealed a number of important caveats for successfully interpreting these chemical markers in the wild. \u201cMost significantly, we demonstrated how tissue isotopes are strongly connected to rates of growth and feeding\u201d said lead author Dr Alex Wyatt. \u201cThis suggests that it is impossible to accurately interpret chemical signals in the wild without some idea of growth and feeding rates prior to sampling\u201d. By correlating growth and feeding with a \u201chealth check-up\u201d for sharks based on a blood test, the researchers propose a framework for enhanced chemical investigation of wild foraging. \u201cSimilar to blood tests performed when you visit the doctor, we are able to assess the health of these sharks based on a large variety of parameters in their blood and then interpret tissue isotopes in the context of this health check-up\u201d.<\/p>\n

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A whale shark being fed in Okinawa Churaumi Aquarium<\/p><\/div>\n

The enhanced approach was applied to wild whale sharks encountered off the coast of Okinawa, Japan, with tissue sampling and a health check-up performed on a number of sharks while freeing them from accidental entanglement in fishing nets. This revealed two distinct foraging strategies in the sharks encountered, identifying individuals that depended or either coastal or open ocean foraging. \u201cBy measuring isotopes in a number of tissues with different rates of chemical change, we could demonstrate that these two foraging strategies were consistent across time for the individuals involved, perhaps reflecting individual differences in geographical origins or foraging specialisation over months to years\u201d Dr Wyatt observed. The study made use of a number of advanced tracers, including tissue radiocarbon which showed the sharks fed over a narrow latitudinal range. Dr Wyatt added that \u201cseveral of the individuals what we encountered appeared to be starving, with blood tests suggesting they may not have eaten for weeks or months prior to sampling\u201d. This could reflect a lack of local prey sources or non-feeding during ocean-basin scale migrations, with the research stressing the need to consider such starvation effects when attempting to chemically identify foraging strategies in this and other poorly understood species.<\/p>\n

\u275dThis is a somewhat surprising and controversial finding, since whale sharks are generally assumed to feed strictly on higher trophic levels (zooplankton up to small fish)\u275e<\/p><\/blockquote>\n

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A whale shark meal composed of two species of krill and some minor amounts of dices fish and fish larvae<\/p><\/div>\n

Exactly what whale sharks feed on during their time at sea remains somewhat of a mystery. \u201cDespite their enormous size, sustained observations of whale shark feeding remain difficult and rare\u201d says Dr Wyatt. Although open ocean feeding observations are largely impossible, observations at coastal aggregation sites have demonstrated that whale sharks can feed on a range of prey from tiny krill and fish eggs, up to small fish and squid. A new finding in this study suggests that whale sharks may, in fact, be more herbivorous than previously assumed, with tissue amino acid isotopes indicating incorporation of primary producers (plants and algae) into all of the sharks tested. \u201cThis is a somewhat surprising and controversial finding, since whale sharks are generally assumed to feed strictly on higher trophic levels (zooplankton up to small fish), although they have been found with seaweed in their stomachs\u201d said Dr Wyatt. In support, recent work published in another study showed that the bonnethead shark, which, like most sharks, is assumed to be strictly carnivorous, can digest seagrass in captivity. \u201cOur chemical estimates of the trophic position of Okinawan whale sharks were consistently low, suggesting that herbivory by whale sharks may be prevalent \u2013 this might make sense if feeding opportunities can become as limited as our blood tests suggest\u201d observed Dr Wyatt. Whether herbivory is a consistent strategy, or a response to prey shortages, will be an important aspect of ongoing global work on this mysterious species.<\/p>\n

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Cartoon Summary. Licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. To view a copy of this license, visit http:\/\/creativecommons.org\/licenses\/by-nc-nd\/4.0\/.<\/p><\/div>\n

The paper, \u2018Enhancing insights into foraging specialization in the world\u2019s largest fish using a multi-tissue, multi-isotope approach\u2019 can be found in the journal\u00a0Ecological Monographs <\/em>(DOI: https:\/\/doi.org\/10.1002\/ecm.1339<\/a>)<\/p>\n","protected":false},"excerpt":{"rendered":"

Foraging, starvation and herbivory in the globally threatened whale shark Whale sharks, filter feeding sharks that travel tropical oceans in search of their microscopic prey, are globally threatened. Despite being the world\u2019s largest fish, reaching over 12 m in length and 21 tonnes, many facets | Click for More \u2192<\/a><\/p>\n","protected":false},"author":1,"featured_media":761,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[15,5],"tags":[59,126,118,117,31,112,114,45,113,30],"class_list":["post-714","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-news","category-research","tag-fish","tag-foraging","tag-health-test","tag-herbivory","tag-isotope","tag-nutritional-history","tag-rhincodon-typus","tag-stable-isotope-analyses","tag-starvation","tag-whale-shark"],"_links":{"self":[{"href":"https:\/\/www.oceanecol.com\/index.php?rest_route=\/wp\/v2\/posts\/714","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.oceanecol.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.oceanecol.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.oceanecol.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.oceanecol.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=714"}],"version-history":[{"count":30,"href":"https:\/\/www.oceanecol.com\/index.php?rest_route=\/wp\/v2\/posts\/714\/revisions"}],"predecessor-version":[{"id":880,"href":"https:\/\/www.oceanecol.com\/index.php?rest_route=\/wp\/v2\/posts\/714\/revisions\/880"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.oceanecol.com\/index.php?rest_route=\/wp\/v2\/media\/761"}],"wp:attachment":[{"href":"https:\/\/www.oceanecol.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=714"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.oceanecol.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=714"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.oceanecol.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=714"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}