University of Tasmania
Schmidt_Roach_whole_thesis.pdf (2.54 MB)

Stock structure and critical habitats for a key apex predator : the broadnose sevengill shark Notorynchus cepedianus

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posted on 2023-05-28, 09:27 authored by Schmidt-Roach, ACJ
Sharks are ecologically important marine animals and currently among the most threatened. Gaps in knowledge about the ecology and behaviour of many species continue to hinder our ability to effectively conserve and manage these animals. Understanding the population structure and movement patterns can provide valuable information on their stock structure and connectivity, behaviour and identify key habitats. This information can then be used to identify possible threats and devise effective management and conservation strategies. The broadnose sevengill shark (Notorynchus cepedianus) is an ecologically important apex marine predator. It is commonly found in temperate coastal areas worldwide, nevertheless, similar to other shark species there are gaps in knowledge about its ecology and behavior, as is listed as Data deficient‚ÄövÑvp on the IUCN red list. In Australia, gaps in knowledge persist with respect to stock structure, identification of key habitats such as nursery/pupping areas, and the understanding of early life-stage behavior and movement for this species. This thesis aims to expand the currently available data on this species in South Eastern Australian waters in a more comprehensive, multi-method approach, and identify levels of population structure and connectivity from global to local scales. Using genetic and telemetric analyses this study aims to elucidate stock structure, movement patterns and identify key habitats in order to provide vital information to develop appropriate management strategies. The majority of research has been focused on regional or local spatial scales, with little information currently available on this species global population structure. To determine if this species is panmictic or an assemblage of distinct subpopulations across its global distribution, chapter 2 assessed the genetic phylogeny using mitochondrial (mtCR) and nuclear (ITS2) markers sequencing a total of 249 individuals from three oceanic regions (six locations) across N. cepedianus' global distribution. Moderate levels of genetic diversity compared to other shark species were observed, with low diversity within oceanic regions. Significant levels of genetic divergence were observed among oceanic regions, indicative of little to no mixing between the populations. Overall, three genetically distinct populations of N. cepedianus were identified across its global distribution, disproving any suggestions of global panmixia. My findings emphasise the necessity for further taxonomical review of this species to determine if regional lineages may be categorised as separate species, particularly the Eastern Pacific population. In chapter 3, the genetic population structure of N. cepedianus across south-eastern Australia was investigated using genotyping by sequencing (GBS). This revealed an overall moderate genetic diversity with little genetic structuring across its Australian distribution. This is indicative of high levels of mixing and genetic connectivity within Australia. This strong intraregional connectivity complements those observed in the previous chapter and stresses the need for multi-jurisdictional management of this species within coastal waters of New South Wales, South Australia, Victoria and Tasmania. At local scales, key habitats such as feeding, nursery/pupping, and mating areas are essential for the health and proliferation of species. By observing and monitoring the spatial and temporal movement as well as behaviour of individuals, key habitats can be identified and their importance to population stability and propagation revealed. Notorynchus cepedianus has been shown to exhibit seasonal movement, in to (spring ‚Äö-summer) and out of (autumn ‚Äö- winter) sheltered coastal bays. This has generally been thought to be associated with feeding behaviour but some evidence suggests the use of coastal bays, as nursery areas may also be important. To identify potential nurseries, seasonal movement and habitat use, I used a threefold methodological approach combing genetic, telemetric and tagging tools. Chapter 4 used acoustic telemetry to elucidate N. cepedianus neonate movement patterns and behavior in a Victorian coastal bay, as well as to identify key habitats for this species in Australia. Results revealed the presence of neonates (<80 cm TL) within Port Phillip Bay (PPB) during the autumn-winter months. The majority of tagged neonates were not detected in the bay after July 2015 and remained undetected within the bay for the rest of the study period (2 years). Neonate movement pattern revealed a preference for deeper areas of the bay (>15 m). Long distance movement revealed the connectivity between the different coastal waters of south-eastern Australia, with both neonate and other life-stages moving between the State jurisdictions. Though, a pupping area for N. cepedianus was identified in PPB, neonates only spent a few months within the coastal bay. Thus, while Port Phillip Bay is important for the first few months of neonate development, monitoring and management strategies will need to consider anthropogenic pressures both within, and outside the bay where the majority of their early development occurs. In summation, this thesis is an important contribution to the further understanding of the ecology of N. cepedianus, particularly population structure, identifying key habitats and understanding early life-stage behaviour. Additionally, this thesis also demonstrates the importance of multidisciplinary approaches for providing a more comprehensive assessment of ecological process.


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