Ecological Niche of an Abundant Teleost Pelates Octolineatus in a Subtropical Seagrass Ecosystem
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Vol. 541: 195–204, 2015 MARINE ECOLOGY PROGRESS SERIES Published December 15 doi: 10.3354/meps11542 Mar Ecol Prog Ser Ecological niche of an abundant teleost Pelates octolineatus in a subtropical seagrass ecosystem C. Bessey1,2,3,*, M. R. Heithaus1 1Department of Biological Sciences, Marine Science Program, Florida International University, 3000 NE 151 Street, North Miami, FL 33181, USA 2School of Plant Biology, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia 3Present address: Coastal Ecosystems and Modelling, Oceans and Atmosphere, Commonwealth Scientific and Industrial Research Organisation, 147 Underwood Avenue, Floreat, WA 6014, Australia ABSTRACT: This study used primary producer abundance surveys, stomach content analysis, stable isotope analysis, and mixing models to investigate the ecological niche of western striped trumpeters Pelates octolineatus in the subtropical seagrass ecosystem of Shark Bay, Australia. The percent cover of seagrass and macroalgae in the study area was 35.9 ± 4.3 and 28.59 ± 3.8% (mean ± SE), respectively. Of the 2 dominant seagrass species, Amphibolis antarctica was present at a higher number of stations than Posidonia australia, and although fast-growing, smaller-bodied seagrass species occurred infrequently, only 3 stations surveyed did not contain seagrass. Algae also occurred at the majority of stations (47 of 49 stations). P. octolineatus stomach contents con- sisted mainly of primary producers (87.7%). The mean relative amount of seagrass and algae in P. octolineatus stomachs was similar (43.7 ± 44.1 and 44.0 ± 43.8%, respectively), although there was considerable variation among individuals. There was a positive relationship between fish length and percentage of seagrass consumed. Stable isotope values suggested that algae contributed a larger portion of assimilated food rather than seagrass, but predictions from mixing models more closely coincided with stomach content analysis when the standard deviation of potential food sources was increased. P. octolineatus exhibited a similar, but smaller, isotopic niche space to sym- patric, largely herbivorous, green turtles Chelonia mydas (27.3 and 62.4 units, respectively). These data suggest that P. octolineatus function as herbivores in this subtropical seagrass system, thereby having the potential to structure primary producer communities and facilitate the transfer of primary production to higher trophic levels. KEY WORDS: Stomach content analysis · Stable isotope analysis · Mixing models · Primary producer · Herbivory Resale or republication not permitted without written consent of the publisher INTRODUCTION Keys (Kirsch et al. 200) and 73% off the northeast coast of Spain (Tomas et al. 2005), and may modify Herbivores play a critical role in food web dyna - plant traits that indirectly affect other species (Pages mics by transferring primary production to higher et al. 2012). Understanding the role of teleost con- trophic levels, yet the importance of teleost her- sumption in seagrass ecosystems is important be - bivory in seagrass ecosystems not associated with cause seagrasses (1) are the foundation of highly reefs has generally been considered low (Thayer et productive ecosystems that provide critical habitat al. 1984, White et al. 2011, Poore et al. 2012). Some in the form of shelter and foraging sites (Connolly studies, however, have challenged this view (Heck 1994, Heck et al. 2003), (2) are an important carbon & Valentine 2006, Valentine & Duffy 2006). Indeed, store (Fourqurean et al. 2012), and (3) are now herbivorous fish can consume substantial propor- among the most threatened ecosystems on earth tions of seagrass production, e.g. 80% in the Florida (Waycott et al. 2009). *Corresponding author: [email protected] © Inter-Research 2015 · www.int-res.com 196 Mar Ecol Prog Ser 541: 195–204, 2015 A critical first step in understanding the effects of June to August (cold season) (Heithaus et al. 2012). teleost consumption on primary producers in sea- The study site consists of shallow seagrass banks grass communities is to identify herbivorous species (1.5− 4.5 m depth) with the occasional sand patch and quantify their diets. While stomach content (Burkholder et al. 2013a). The community is domi- analysis is the most common method for elucidating nated by 2 slow-growing and large-bodied seagrass teleost diets and can provide detailed information on species (Amphibolis antarctica and Posidonia aus- taxa that are consumed, it provides only a snapshot of tralis), with fast-growing, small-bodied seagrass spe- an individual’s diet (Hyslop 1980). Complementary cies (Cymodocea angustata, Cymodocea serrulata, insights into trophic interactions of species can be Halodule uninervis, Halophila ovalis, Halophila spin- obtained using stable isotope analysis (SIA) (Peter- ulosa, and Syringodium isoetifolium) (Walker et al. son & Fry 1987, Layman et al. 2012). SIA provides 1988, Burkholder et al. 2013a) occurring less fre- insights into relative trophic level using δ15N values quently. Benthic communities in Shark Bay also host and the sources of carbon supporting diets using δ13C ca. 160 taxa of macroalgae (Kendrick et al. 1990). values (Peterson & Fry 1987, Layman et al. 2012). Epi phytic red algae (Rhodophyta) are most speciose Shark Bay, Western Australia, offers a model sys- but green algae (Chlorophyta) are the most conspic- tem for investigating teleost consumption of primary uous (Kendrick et al. 1990). The brown alga Dictyota producers in a seagrass system; it is one of the largest furcellata (Heterokontophyta) is also common. seagrass ecosystems in the world and features large Fish communities inside the study area are domi- populations of teleosts, as well as large-bodied herbi- nated by the western striped trumpeter Pelates octo- vores, piscivores, and top predators (Heithaus et al. lineatus, previously referred to as P. sexlineatus (Heit - 2012). The western striped trumpeter Pelates octolin- haus 2004). This demersal species is commonly found eatus (Jenyns, 1840) of the Family Terapontidae is a in estuaries, lagoons, and seagrass/algae beds around medium-sized teleost (max. length = 28 cm) that has the southwestern coast of Australia, from Broome to been observed consuming seagrasses (Burkholder et eastern South Australia (Gomon et al. 1994). They al. 2012). It is the most abundant medium-sized tele - reach a maximum length of approximately 28.0 cm ost in the study site by an order of magnitude (Heit - (Gomon et al. 1994) and are reportedly omnivores haus 2004) and, therefore, could substantially im pact (Paxton et al. 1989). Beyond observations of P. octolin- seagrass and algal communities. In addition, Shark eatus consuming uprooted primary producers (Burk- Bay has been the site of multiple studies investigat- holder et al. 2012) however, the importance of primary ing the trophic interactions of a variety of species, producers in their diets is unexplored. including megagrazers such as green turtles Chelo- nia mydas (Burkholder et al. 2011) and du gongs Dugong dugon (Wirsing et al. 2007), facilitating a Primary producer surveys community-level understanding of trophic inter - actions. This study used a combination of primary To estimate the relative abundance of seagrass and producer surveys, stomach content, and SIA to inves- macroalgae within the study location, 49 stations tigate the ecological niche of P. octolineatus and to were surveyed across 3 offshore banks between 24 compare their isotopic niche with another sympatric March and 30 July 2012. The survey focused on the herbivore, the green turtle C. mydas, in the Shark most commonly occurring species: 5 seagrass species Bay system. (Magnoliophyta: Amphibolis antarctica, Posidonia australis, Cymodocea angustata, Halodule uninervis, Halophila ovalis), 2 red algae species (Rhodophyta: MATERIALS AND METHODS Laurencia sp., Haliptilon roseum), and 2 brown algae species (Heterokontophyta: Dictyota furcellata, Sar- Study site gassum sp.). Specific stations were determined by stopping the vessel every 400 m along pre-estab- Shark Bay (25° 45’ S, 113° 44’ E) is a ca. 13 000 km2 lished transects (Heithaus & Dill 2006) of each bank subtropical embayment featuring ~4000 km2 of sea- and surveyed using snorkel or hookah diving. Per- grass beds, located along the central coast of Western cent cover was estimated by an observer using a 60 × Australia. The study site was in the Eastern Gulf, off- 60 cm quadrat dropped haphazardly 3 times at the shore of Monkey Mia, where water temperatures are station. The average percent cover was calculated for generally high (>20°C) during September to May seagrass and algae from the 3 quadrat drops at each (warm season) and drop to as low as 12°C during station. It is important to note that primary producer Bessey & Heithaus: Teleost herbivory in a seagrass ecosystem 197 surveys were conducted after a ‘marine heat wave’, egory by the total stomach weight. We then used in which unprecedented water temperatures ex - non-parametric sign-ranked tests to determine if the ceeded 3°C above the long-term average over an category proportion from our visual estimates were extended area of Shark Bay during February and statistically different than our category proportions March 2011 (Pearce et al. 2011) impacting seagrass, obtained by weight. There was no significant differ- as well as potentially impacting macroalgal commu- ence between our visual estimation and weight nities (Thomson et al. 2015). methods for proportion