Diversity in immature-shark communities along a tropical coastline

Abstract

Effective conservation and management of shark populations is complicated by our limited understanding of their spatial ecology. For example, there are scarce data on diversity in community structure and nursery function across broader geographic scales (e.g. across multiple inshore systems) and the implications of this diversity for shark populations. Accordingly, fishery-independent surveys were undertaken to investigate shark communities along ,400 km of the tropical eastern coast of Australia (18.1-20.68S, 146.0-148.88E). A variety of shark species were encountered, with 19 species of Carcharhiniformes contributing 99.2% of the total shark catch. Of the 1806 sharks captured, 567 were immature, including 336 young-of-the-year individuals. Immature sharks from 18 species were present; however, interspecific variation in the proportions of life-history stages was apparent. Multivariate analyses identified significant spatial heterogeneity in immature-shark communities. Results also highlighted the importance of tropical coastal habitats for numerous shark species, and indicated community-wide spatial structuring of sharks on the basis of body size rather than life-history stage. In addition to building on traditional shark-nursery paradigms, these results demonstrated that data on nursery function from restricted areas may not accurately portray patterns occurring over broader geographic scales, and this diversity may provide population-level benefits for sharks.

Spatio-Temporal Occurrence Patterns of Young Sharks in Tropical Coastal Waters

“…Similarly, variable depth utilisation by immature sandbar sharks Carcharhinus plumbeus within (Rechisky and Wetherbee 2003) and between McCandless et al 2007) adjacent bays in the north-west Atlantic may have been influenced by spatial heterogeneity in hydrodynamics, prey distributions and predation risk (McCandless et al 2007). These results suggest that, in response to biotic and abiotic conditions, young sharks may adopt different tactics in different bays or avoid certain bays altogether (Yates et al 2015b). The temporal distribution of neonates corroborates the general predominance of summer parturition in the study species across tropical Australia (Last and Stevens 2009).…”

“…In particular, over larger geographic scales, some bays may provide nursery habitat for these species while others may not (Yates et al 2015b). Heupel et al (2007) provided three criteria for the identification of a shark nursery: (1) relatively high abundance of neonates or young juveniles, (2) site fidelity and (3) Rockingham and Repulse bays, these locations may meet the first nursery area criterion and therefore be particularly important for these species.…”

“…Temporal variations in occurrence were detected for pigeye and spot-tail sharks whereas spatial variations characterised the occurrences of blacktip and scalloped hammerhead sharks. Although tropical coastal environments are typically occupied by a diverse group of young sharks (Compagno 1984;Harry et al 2011;Yates et al 2015b), differences in occurrence patterns may be associated with interspecific resource competition (White and Potter Confidence intervals were not plotted for groups with zero encounters 2004) and variable susceptibilities to anthropogenic impacts (Chin et al 2010;Knip et al 2010). Such varied utilisation of coastal ecosystems has important implications for the conservation of shark populations and the management of coastal habitats.…”

“…Up to two longlines were deployed simultaneously for c. 40 min sets (72 % between 40 and 60 min). Additional information on the survey methodology can be found in Yates et al (2015b). Eight rounds of surveys, spanning a minimum of 5 weeks each, were undertaken between January 2012 and March 2014.…”

Nearshore movement ecology of a medium-bodied shark, the creek whaler Carcharhinus fitzroyensis

“…For example, the mediumbodied spottail shark Carcharhinus sorrah exhibits high site fidelity to single nearshore embayments with individuals using small home ranges and consistent habitats [22], similar to large-bodied juvenile populations. However, at a population level, C. sorrah use a wide range of coastal habitats and juvenile and adult spottails are known to occupy the same coastal areas simultaneously [22,23].…”