Quantifying Shark Distribution Patterns and Species-Habitat Associations: Implications of Marine Park Zoning

Abstract

Quantifying shark distribution patterns and species-specific habitat associations in response to geographic and environmental drivers is critical to assessing risk of exposure to fishing, habitat degradation, and the effects of climate change. The present study examined shark distribution patterns, species-habitat associations, and marine reserve use with baited remote underwater video stations (BRUVS) along the entire Great Barrier Reef Marine Park (GBRMP) over a ten year period. Overall, 21 species of sharks from five families and two orders were recorded. Grey reef Carcharhinus amblyrhynchos, silvertip C. albimarginatus, tiger Galeocerdo cuvier, and sliteye Loxodon macrorhinus sharks were the most abundant species (>64% of shark abundances). Multivariate regression trees showed that hard coral cover produced the primary split separating shark assemblages. Four indicator species had consistently higher abundances and contributed to explaining most of the differences in shark assemblages: C. amblyrhynchos, C. albimarginatus, G. cuvier, and whitetip reef Triaenodon obesus sharks. Relative distance along the GBRMP had the greatest influence on shark occurrence and species richness, which increased at both ends of the sampling range (southern and northern sites) relative to intermediate latitudes. Hard coral cover and distance across the shelf were also important predictors of shark distribution. The relative abundance of sharks was significantly higher in non-fished sites, highlighting the conservation value and benefits of the GBRMP zoning. However, our results also showed that hard coral cover had a large effect on the abundance of reef-associated shark species, indicating that coral reef health may be important for the success of marine protected areas. Therefore, understanding shark distribution patterns, species-habitat associations, and the drivers responsible for those patterns is essential for developing sound management and conservation approaches.

Evidence for rapid recovery of shark populations within a coral reef marine protected area

“…Although our modelling showed that habitat explained 30% of the variance for shark abundance recorded by BRUVS at Ashmore Reef ( Figure A.3) we found no significant differences in habitat categories between surveys. In both 2004 and 2016 areas that had reef structure (coral, rock, algae), generally had higher relative abundances of sharks than areas of sand, which is consistent with numerous studies (Barnett et al 2012;Economakis and Lobel 1998;Espinoza et al 2014;Field et al 2011;McKibben and Nelson 1986;Speed et al 2016). It also seems unlikely that habitat quality and prey availability was a driver of movement between Ashmore Reef and Cartier Reef, as average cover of hard coral (ca.…”

“…Visual estimates of coral cover (0-100% rounded to the nearest 5%), complexity (Low, Medium, and High), and habitat type (Sand, Reef, Other) were estimated by eye, similar to other studies (Espinoza et al 2014;Malcolm et al 2011;Tickler et al 2017). All visible habitat was included in the classification process.…”

“…Similarly, the mean time to arrival in 2004 (16.6 min ± 3.8 Se) was faster than in 2016 (24.3 ± 3.0 Se), although this was also a non-significant relationship (t = -1.58, df = 14.47, p-value = 0.13). Espinoza et al (2014).…”

“…Examples of increasing (Espinoza et al 2014) or even stable (Bradley et al 2017) populations of reef sharks in these environments are very rare. Over the study period of 12 years we found that abundances of the most common species, C. amblyrhynchos, increased four-fold on Ashmore Reef.…”

“…To address differences in deployment times of BRUVS, 'soak time' was used as an offset within the model structure (e.g. Espinoza et al 2014). A data exploration phase was used to determine whether there was any collinearity among variables.…”