Intrapopulation variations in diet and habitat use in a marine apex predator, the broadnose sevengill shark Notorynchus cepedianus

Abstract

Intrapopulation differences in diet and/or movement are important for understanding the role mobile predators play in different systems. However, ecological studies traditionally overlook individual differences. δ13C and δ15N were used in conjunction with diet and movement information to identify intrapopulation differences in diet and movement patterns of the apex predator broadnose sevengill shark Notorynchus cepedianus in southeast Tasmania. Sevengill samples from 3 inshore and 3 offshore sites were collected, and δ13C and δ15N compared between sites, sizes and sexes. Individuals captured offshore had lower δ15N than those captured inshore, indicating some degree of spatial segregation. Sevengills also had variable δ13C and δ15N within coastal habitats, suggesting intrapopulation differences in diet or migration schedules. In comparison to their main prey, most individuals had δ15N lower than expected for a top predator, also suggesting that they do not reside permanently in these areas, as their tissue was not in isotopic equilibrium with their known prey. This is in agreement with tracking data that showed seasonal use of coastal areas, with most animals leaving for the colder months but returning the following year. There was also a group of females with relatively high δ13C that suggests greater association to coastal habitats, again in agreement with tracking data, as some tagged females remained in the coastal areas over winter. Overall, together with diet and tracking information, results indicate that there are differences in movement and possibly diet in this sevengill population. This multimethods approach allowed a better understanding of sevengill ecology than the use of any one of the techniques alone.