Habitat-associations of turban snails on intertidal and subtidal rocky reefs.

Patchiness of habitat has important influences on distributions and abundances of organisms. Given the increasing threat of loss and alteration of habitats due to pressures associated with humans, there is a need for ecologists to understand species' requirements for habitat and to predict changes to taxa under various future environmental conditions. This study tested hypotheses about the generality of patterns described for one species of marine intertidal turban snail for a different, yet closely-related species in subtidal habitats along the coast of New South Wales, Australia. These two closely-related species live in similar habitats, yet under quite different conditions, which provided an opportunity to investigate how similar types of habitats influence patterns of distribution, abundance and size-structure in intertidal versus subtidal environments. For each species, there were similar associations between biogenically structured habitat and densities. The intertidal species, Turbo undulates, were more abundant, with greater proportions of small individuals in habitats formed by the canopy-forming alga, Hormosira banksii, the solitary ascidian, Pyura stolonifera or the turfing red alga, Corallina officinalis compared to simple habitat (bare rock). Similarly, more Turbo torquatus were found in biogenically structured subtidal habitat, i.e. canopy-forming algae, Ecklonia radiata, mixed algal communities ('fringe'), or turfing red algae (Corallina officinalis and Amphiroa aniceps) than where habitat is simple (barrens). Small T. torquatus were more abundant in areas of turf and 'fringe', while large snails were more abundant in areas of kelp and barrens. These patterns were found at each location sampled (i.e. eight intertidal and two subtidal rocky reefs) and at all times of sampling, across each environment. This study highlighted the consistent influence of biogenically structured habitats on the distribution, abundance and size-structure of intertidal and subtidal turban snails and forms a basis for increasing the understanding of the potential underlying processes causing such patterns

Number of quadrats and site(s) sampled in the Sydney Region, NSW, Australia on each intertidal rocky-shore from North to South.⁺

+<p>Mona Vale (33°40′33.46″S, 151°18′23.51″E), North Narrabeen (33°42′23.44″S, 151°17′18.1″E), Tamarama (33°53′52.8″S, 151°16′4.8″E), Cronulla – north and south (34°3′26.78″S, 151°9′7.88″E), Era (151°04′E, 34°09′S), Coal Cliff (34°14′0″S, 150°58′0″E) and Bulli Point (34°19′59.23″S, 150°55′7.14″E; see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061257#pone-0061257-g001" target="_blank">Figure 1</a>). All locations are on the open-coast with medium to heavy exposure to waves.</p>✓<p>indicates the habitat sampled.</p

Analyses of the proportion of small (and thus, large) <i>T. undulatus</i> in intertidal, structured habitats at each location and time of sampling.⁺

+<p>Due to the limited number of snails in unstructured habitat, they were not analysed.</p>**<p><i>P</i><0.01,</p>***<p><i>P</i><0.001 and – denotes no data available.</p

Mean density of <i>Turbo undulatus</i> on intertidal rocky reefs.

<p>Mean density (+ S.E.; <i>n</i> = 8) of <i>T. undulatus</i> in areas of (a) <i>Hormosira</i> (black bars; quadrat 0.25 m²) and non-<i>Hormosira</i> (white bars), (b) <i>Pyura</i> (dark grey bars; quadrat 0.04 m²) and non-<i>Pyura</i> (white bars) and (c) <i>Corallina</i> (<i>n</i> = 6; quadrat 0.25 m², grey bars) and non-<i>Corallina</i> (white bars), at each location and at each time of sampling. In areas of unstructured habitat (e.g. non-<i>Hormosira</i>), mean density of individuals are presented above the columns, where needed, due to the small number of individuals.</p

Mean density of <i>Turbo torquatus</i> on subtidal rocky reefs.

<p>Mean density (+ S.E.; <i>n</i> = 7) of <i>T. torquatus</i> in 5×1 m transects in each of three representative sites of each habitat at Cape Banks at (a) time 1 and (b) time 2.</p

Analyses of densities of <i>T. torquatus</i> among subtidal habitats at (a)⁺Cape Banks during each time of sampling and (b)⁺⁺Cape Banks and Bare Island.

+<p>Habitat, fixed 4 levels, Site, random, nested in Habitat, 3 levels, <i>n</i> = 7 and (b).</p>++<p>Cape Banks and Bare Island; Location, random 2 levels, Habitat, fixed, orthogonal, 3 levels, Site, random, nested in (Location x Habitat), <i>n</i> = 7. One site of each type of habitat was removed randomly from Cape Banks, for each time of sampling to be comparable with Bare Island.</p>a<p>Variances were heterogeneous and were stabilised, where possible, using a forth root transformation (X^0.25). Significant differences in means were compared using Student-Newman-Keuls (SNK) tests. NS denotes not significant,</p>*<p><i>P</i><0.05,</p>**<p><i>P</i><0.01,</p>***<p><i>P</i><0.001. For SNK comparisons: B, barrens; K, kelp; F, fringe; T, turf.</p>X<p>Denotes <i>post-hoc</i> pooling, <i>P</i>>0.25. New <i>F</i>-values are given for those tested against the pooled term.</p>X<p>Tested against Si(Lo x Ha), Lo x Ha and Residual.</p

Analyses of the proportion of small (and thus, large) <i>T. torquatus</i> in structured and unstructured habitats on subtidal rocky reefs at each location and time of sampling.⁺

+<p>** <i>P</i><0.01 and *** <i>P</i><0.001.</p

Map of locations studied.

<p>Map of locations studied.</p

Patterns of Occurrence of Sharks in Sydney Harbour, a Large Urbanised Estuary.

Information about spatial and temporal variability in the distribution and abundance of shark-populations are required for their conservation, management and to update measures designed to mitigate human-shark interactions. However, because some species of sharks are mobile, migratory and occur in relatively small numbers, estimating their patterns of distribution and abundance can be very difficult. In this study, we used a hierarchical sampling design to examine differences in the composition of species, size- and sex-structures of sharks sampled with bottom-set longlines in three different areas with increasing distance from the entrance of Sydney Harbour, a large urbanised estuary. During two years of sampling, we obtained data for four species of sharks (Port Jackson, Heterodontus portusjacksoni; wobbegong, Orectolobus maculatus; dusky whaler, Carcharhinus obscurus and bull shark, Carcharhinus leucas). Only a few O. maculatus and C. obscurus were caught, all in the area closest to the entrance of the Harbour. O. maculatus were caught in all seasons, except summer, while C. obscurus was only caught in summer. Heterodontus portusjacksoni were the most abundant species, caught in the entrance location mostly between July to November, when water temperature was below 21.5°C. This pattern was consistent across both years. C. leucas, the second most abundant species, were captured in all areas of Sydney Harbour but only in summer and autumn when water temperatures were above 23°C. This study quantified, for this first time, how different species utilise different areas of Sydney Harbour, at different times of the year. This information has implications for the management of human-shark interactions, by enabling creation of education programs to modify human behaviour in times of increased risk of potentially dangerous sharks