Title

Diet of the Tropicalised Herbivore Siganus fuscescens Across a Broad Latitudinal Gradient and Comparisons With Temperate Seagrass-associated Omnivorous Fishes

Date of Award

2018

Degree Type

Thesis

Degree Name

Master of Science (Biological Sciences)

School

School of Science

First Advisor

Professor Glenn Hyndes

Field of Research Code

060205

Abstract

Rising sea temperatures are facilitating range shifts of tropical biota towards higher latitudes. The rabbitfish Siganus fuscescens is a tropical herbivore whose distribution has extended into rocky reefs and seagrass meadows of temperate southwestern Australia. The aim of this study is to determine if the poleward range shift of S. fuscescens could result in competition for food sources with temperate seagrass-associated omnivorous fish and/or increase the grazing rates of seagrasses in temperate seagrass meadows that do not normally experience high levels of grazing. The diet of S. fuscescens was compared across a tropical to temperate gradient, comprising the Kimberley, Shark Bay and Perth. In all regions, red algae were the primary dietary component. Leathery macrophytes were also consumed in the temperate region, while filamentous Sphacelaria was grazed in subtropical Shark Bay and corticated foliose Dictyota in the tropical Kimberley. Seagrass was consumed in all regions, except in the tropics, where meadows are often sparse and ephemeral. The Kimberley was also the only region where fauna were consumed in significant proportions, in the form of epiphytic hydrozoans, most likely due to a lack of diversity of preferred food sources. While S. fuscescens would appear to be primarily algivorous, it will opportunistically consume seagrass and even invertebrates should algae not be available. Large volumes of seagrass consumed by this species in the temperate region indicate that its migration into temperate seagrass meadows will increase consumption rates of seagrass in the region as it establishes its population.

Posidonia-dominated habitat was trawled in two nearshore locations in Perth, WA, to determine whether the diet of S. fuscescens was similar to a range of temperate seagrass-associated omnivorous fish species through gut content and stable isotope analysis. Diets were significantly different between species and locations. While seagrass was consumed by S. fuscescens in large quantities at one location, red (Hypnea and Chondria) and brown (Ecklonia and Sargassum) algae were consumed in greater quantities at the other locations. In comparison, the temperate omnivores Haletta semifasciata, Heteroscarus acroptilus and Scobinichthys granulatus, which were shown to be the most abundant omnivorous fish species in the seagrass meadows, consumed seagrass in moderate quantities, but showed distinct interspecies differences in their diets. These differences were also characterised by different gut length ratios, with S. fuscescens possessing the elongated gastrointestinal tract typical of herbivores. Stable isotope analysis and mixing models showed that S. fuscescens assimilated carbon and nitrogen from red and brown algae, but almost negligibly so from seagrass. While H. acroptilus sourced significant proportions of assimilated nutrients from seagrass, all temperate omnivores primarily utilised animal sources. While there is some dietary overlap between all native species, niche partitioning is likely to reduce competition between the tropical and temperate species. The tropicalisation of S. fuscescens into temperate seagrass meadows is therefore unlikely to impact on native omnivorous species unless the densities of the species become large.

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