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Trophic Ecology of Large Predatory Reef Fishes: Energy Pathways, Trophic Level, and Implications for Fisheries in a Changing Climate

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Trophic Ecology of Large Predatory Reef Fishes: Energy Pathways, Trophic Level, and Implications for Fisheries in a Changing Climate Trophic ecology of large predatory reef fishes: energy pathways, trophic level, and implications for fisheries in a changing climate Ashley J. Frisch, Matthew Ireland & Ronald Baker Marine Biology International Journal on Life in Oceans and Coastal Waters ISSN 0025-3162 Volume 161 Number 1 Mar Biol (2014) 161:61-73 DOI 10.1007/s00227-013-2315-4 1 23 Your article is protected by copyright and all rights are held exclusively by Springer- Verlag Berlin Heidelberg. This e-offprint is for personal use only and shall not be self- archived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”. 1 23 Author's personal copy Mar Biol (2014) 161:61–73 DOI 10.1007/s00227-013-2315-4 ORIGINAL PAPER Trophic ecology of large predatory reef fishes: energy pathways, trophic level, and implications for fisheries in a changing climate Ashley J. Frisch · Matthew Ireland · Ronald Baker Received: 25 June 2013 / Accepted: 20 August 2013 / Published online: 12 September 2013 © Springer-Verlag Berlin Heidelberg 2013 Abstract Large predatory fishes are disproportionately approach to fisheries management. Unexpectedly, there targeted by reef fisheries, but little is known about their was little isotopic niche overlap between three of four trophic ecology, which inhibits understanding of commu- focal predators, suggesting that inter-specific competition nity dynamics and the potential effects of climate change. for prey may be low or absent. δ15Nitrogen indicated that In this study, stable isotope analyses were used to infer the closely related P. leopardus and P. maculatus are apex trophic ecology of a guild of large predatory fishes that are predators (trophic level > 4), while δ13C indicated that each targeted by fisheries on the Great Barrier Reef, Australia. species has a different diet and degree of trophic speciali- Each of four focal predators (Plectropomus leopardus, sation. In view of these divergent trophic ecologies, each Plectropomus maculatus, Lethrinus miniatus and Lutjanus of the four focal predators (and the associated fisheries) are carponotatus) was found to have a distinct isotopic signa- anticipated to be differentially affected by climate-induced ture in terms of δ13C and δ15N. A two-source mixing model disturbances. Thus, the results presented herein provide (benthic reef-based versus pelagic) indicated that P. leop- a useful starting point for precautionary management of ardus and L. miniatus derive the majority (72 and 62 %, exploited predator populations in a changing climate. respectively) of their production from planktonic sources, while P. maculatus and L. carponotatus derive the major- ity (89 and 74 %, respectively) of their production from Introduction benthic reef-based sources. This indicates that planktonic production is important for sustaining key species in reef Trophic interactions can have profound influences on fisheries and highlights the need for a whole-ecosystem demography, habitat structure and fishery yields (Myers et al. 2007; Estes et al. 2011) and thus are a cornerstone of community ecology and a fundamental consideration for Ashley J. Frisch and Matthew Ireland have contributed equally to ecosystem-based management. Food webs provide a useful the research. framework to understand trophic interactions because they depict important trophic properties such as diet, source of Communicated by K. D. Clements. primary production (e.g. algae, phytoplankton, coral) and A. J. Frisch (*) trophic level (TL), which is a continuous measure of the ARC Centre of Excellence for Coral Reef Studies, James Cook number of energy transfers from producer to consumer University, Townsville, QLD 4811, Australia (herbivore TL 2, predator TL 3–5). Sound knowledge = = e-mail: [email protected] of trophic interactions is imperative for defining species’ M. Ireland · R. Baker roles within ecological communities (Ginzberg and Arditi School of Marine and Tropical Biology, James Cook University, 2012), assessing the implications of harvesting key func- Townsville, QLD 4811, Australia tional groups such as apex predators (Heithaus et al. 2008), and evaluating the sustainability of multispecies fisheries R. Baker Commonwealth Scientific and Industrial Research Organisation (Pauly and Watson 2005). Despite these important applica- (CSIRO) Land and Water, Townsville, QLD 4811, Australia tions, the trophic ecology of many exploited fishes remains 1 3 Author's personal copy 62 Mar Biol (2014) 161:61–73 poorly understood, which inhibits awareness of commu- (3) define the trophic niche of targeted predatory fishes and nity dynamics and limits capacity for ecosystem-based thus evaluate the potential for dietary overlap (Vaudo and management. Heithaus 2011). Integration of this information will pro- Marine apex predators are large fishes and sharks that vide insights into the dependency (or lack thereof) of reef occupy the top level of food webs (TL > 4). They often fisheries on reef-based primary producers (e.g. coral and have disproportionate influences on ecosystem proper- macroalgae), which are forecast to undergo radical demo- ties via ‘top-down’ control of prey abundance and behav- graphic changes in the near future due to global warming iour (Heithaus et al. 2008; McCauley et al. 2010). In and ocean acidification (Hoegh-Guldberg et al. 2007). some cases, altering the density of apex predators invokes Australia’s Great Barrier Reef (GBR) supports an exten- community-wide trophic cascades that involve multiple sive line-fishery involving up to 369 licensed commer- intervening trophic levels. In the north-west Atlantic, for cial motherships and up to 80,000 registered recreational example, over-fishing of large sharks caused the ‘release’ vessels (Lunow and Holmes 2011; Taylor et al. 2012). of cownose ray, which led to increased predation on scal- Although >100 species are harvested, most of the catch is lops and, ultimately, collapse of a scallop fishery (Myers comprised of leopard coral trout, Plectropomus leopardus et al. 2007). Depletion of apex predators can therefore have (~50 %); bar-cheek coral trout, Plectropomus maculatus far-reaching, detrimental consequences that threaten the (~10 %); red-throat emperor, Lethrinus miniatus (~15 %); supply of fisheries resources and the human livelihoods that and stripey snapper, Lutjanus carponotatus (~10 %) (Frisch depend on them. et al. 2008; Welch et al. 2008). These four species are dom- Due to the intensifying worldwide decline of coral reefs inant predators, both at the GBR and at many other reefs in (Hughes 1994; Bruno and Selig 2007; De’Ath et al. 2013), the Indo-Pacific region (Williams and Hatcher1983 ; Blaber there is an urgent need to improve management of local et al. 1990; Newman et al. 1997; Watson et al. 2009). At and regional anthropogenic pressures such as fishing in some locations, coral trouts are the largest conspicuous order to strengthen reef resilience and to offset the effects predators, thereby prompting some authors to assume they of increasing global pressures such as climate change are apex predators (Goeden 1982; Huntsman et al. 1999). (Hughes et al. 2010). The contribution of large predatory Although limited information is available regarding the fishes to maintenance of reef resilience is unclear (Heithaus diets of coral trouts and other targeted species, nothing is et al. 2008; Nystrom et al. 2008), but is potentially very known about the ultimate source of primary production important. Recently, it was demonstrated that protected (reef or pelagic), and it is yet to be demonstrated whether reefs (which have higher density of large predatory fishes coral trouts are indeed apex predators (TL > 4). For these than do fished reefs) suffer fewer outbreaks of coral-eating reasons, P. leopardus, P. maculatus, L. miniatus and L. car- starfish and coral disease (Sweatman 2008; Raymundo ponotatus constitute the foci of the present study (i.e. the et al. 2009). Thus, a better understanding of large predatory focal predators). Due to the broad geographic distributions fishes and their ecological functions is imperative for guid- of these species, our results will serve as a useful model ing management actions that aim to preserve or enhance to explore the trophic ecology of predatory fish communi- ecosystem resilience. ties on coral reefs throughout the Indo-Pacific region and Reef fisheries tend to remove a disproportionate quan- elsewhere. tity of large predators relative to other trophic groups (Dalzell 1996; Bellwood et al. 2004). As a result, fishing has severely reduced the densities of large predators such Materials and methods as groupers (Epinephelidae), emperors (Lethrinidae) and snappers (Lutjanidae) in many parts of the world (Hunts- Method selection man et al. 1999; Sadovy 2005). Little is known about the trophic ecology of these fishes, which makes it difficult to The classical approach to trophic ecology has been stom- understand the dynamics of reef communities and to pre- ach content analysis, but this method provides only a snap- dict the broader
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