The role of apex predators in coastal ecosystems: A case study examining the Broadnose Sevengill Shark Notorynchus Cepedianus

Large apex predators are believed to play key roles in ecosystem structure and dynamics. However, for the vast majority of large predators their ecological roles have not been defined or quantified. In particular, there is a lack of information on large mobile marine species. The ecology of the Broadnose sevengill shark, Notorynchus cepedianus was examined to determine their role in coastal systems of Tasmania. This was addressed by conducting studies on the different aspects of N. cepedianus ecology in two coastal areas of south-east Tasmania; the Derwent Estuary and Norfolk Bay. Both these areas are designated shark protected areas. Seasonal longline sampling showed that adult and sub-adult N. cepedianus (size 105-295cm) are highly abundant in summer declining to near absence in winter. The absence of smaller size classes (<80cm) from the catches suggests that N. cepedianus are not using these coastal habitats as nursery areas. Both longline sampling and acoustic movement analysis demonstrated that N. cepedianus showed site fidelity in the use of the coastal habitats. The general pattern was for sharks to exit coastal areas over winter and return the following spring or summer. However, sexual segregation was evident with females abundant in spring and to a lesser extent occurring in winter; conversely males appear in coastal areas later in summer. Both satellite and acoustic tracking methods showed that males can make northerly migrations during winter to distances of at least 1000km. Individuals tagged in both coastal areas (Derwent Estuary and Norfolk Bay) showed low spatial (Piankas index: O = 0.34) and dietary overlap (O = 0.45), suggesting localised site fidelity and fine spatial scale resource partitioning. In general, individuals from both locations consumed the same broad dietary categories (sharks, batoids, teleosts and mammals). However, there were differences in species composition within these categories for each location. The simultaneous tracking of five chondrichthyan prey species and N. cepedianus showed similar seasonal use of coastal areas by all species. Predator and prey also showed high spatial overlap and similar habitat use patterns once they were within the coastal system. These similar movement patterns of predator and prey combined with the additional ecological information (diet, relative abundance) suggests that N. cepedianus display feeding site fidelity, moving into coastal systems following their main chondrichthyan prey. These combined approaches demonstrate that N. cepedianus probably exert significant predation pressure on prey using these protected coastal areas during summer. Natural mortality is an important, but difficult parameter to estimate in assessing commercially fished populations. However, given the common occurrence and high abundance of Notorynchus cepedianus in the Tasmanian coastal waters gazetted as shark nursery areas and the prevalence of gummy shark Mustelus antarcticus in their diet, it is likely that N. cepedianus exert significant predation pressure on this commercially important species. Therefore, Notorynchus cepedianus is competing with humans (fisheries) as the top predators for common food resources, and as mortality from predation can exceed that from fisheries, this information is important for fisheries management. Overall this study is an important contribution to the ecology of N. cepedianus and marine apex predators in general, but it also illustrates the value of simultaneously recording and integrating multiple types of information to better understand predator-prey relationships, the likelihood of interactions, and to build a clearer picture of ecosystem dynamics.