Hammerhead sharks (Sphyrnidae) of southeast Queensland: habitat and movements

Abstract

Hammerhead sharks (Sphyrnidae) are iconic and charismatic species that have received little attention until more recently, resulting in knowledge gaps about life histories, habitats, behaviours and migratory drivers, mostly in the southern hemisphere such as Australian waters. Globally, shark populations are declining as many species extinction risk has increased under the threat of fishing and habitat degradation. Hammerhead shark (Sphyrnidae) populations are highly susceptible to human-induced pressure such as long lines and are currently undergoing severe declines, especially in Australia. Recently, hammerhead sharks have been added to the International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species, with scalloped hammerhead listed as critically endangered. Therefore, further information is needed to address the current shortfalls regarding hammerheads in Australia, for the management and conservation of these species. Earlier research using catch data from the Queensland Shark Control Program (QSCP) showed a decline in numbers of caught hammerhead sharks since the start of the program in 1960. A review of the literature (CHAPTER 1) revealed that hammerhead sharks were understudied in Australia, with information shortfalls on movement, resource use, nursery areas and habitat use. The general aim of this thesis was to provide a better understanding of the movement, distribution, habitat use and resource use of scalloped hammerhead shark (Sphyrna lewini) in Queensland, Australia to improve conservation and management strategies. I used multiple methodologies to determine (i) the distribution of suitable habitat for juvenile scalloped hammerhead sharks and the percentage of overlap with marine protected areas (CHAPTER 2); (ii) fine-scale movement of juvenile scalloped hammerheads within these habitats and determine behavioural states using high-resolution acoustic tracking (CHAPTER 3); (iii) thermal tolerance range “thermal niche” of hammerhead shark using historical catch records from the QSCP (CHAPTER 4); and (iv) resource overlap between hammerhead sharks species and with other co-existing large sharks using carbon and nitrogen stable isotopes (CHAPTER 5). This allowed me to build a solid baseline framework of the ecology of scalloped hammerhead sharks in Queensland. The species distribution model identified 10,082.87 km2 of suitable habitat characterised by low current velocities ( 20°C), estuarine/oceanic salinities (25-35 PSU) and shallow bathymetries (< 20 m). Suitable habitats occurred in coastal areas within wetland and seagrass habitats. Highly suitable areas accounted for 1,784.33 km2 of the total distribution and ii occurred around highly developed areas, such as Moreton Bay, Hervey Bay, Townsville and Cairns. Coastal beach areas of the Gold Coast were also uncovered as highly suitable habitats and may be due to close proximity of the Gold Coast seaway and Tweed River connections to more sheltered estuarine habitats. The majority of the predicted suitable distribution occurred within the lower protection multi-use zones (6,291.20 km2) and outside marine protected areas (3,791.67 km2); where no-take zones protected only 11 % and 8.56 % of high and medium modelled suitable habitats. Within these habitats, continuous tracking of two juvenile scalloped hammerhead sharks uncovered highly active diving behaviours where both sharks continuously dove from water surface to the bay floor over 12- and 3-hour tracks. Two diving patterns were observed in both sharks and described as A-type: one long dive with multiple small dives at depth, and B-type dives: one long dive without extra dives at depth. The two-state behavioural model uncovered low activity (forage) and high activity (direction) states, which were influenced by distance to seagrass and coral habitats as well as habitat depth. Quantile regression modelling determined catches of hammerhead sharks in relation to changes in seas surface temperatures changed with latitude along the Queensland coast. Scalloped hammerhead catches in the southern areas were most likely to occur between 20℃ - 25℃ and likely to occur throughout the year in the northern areas, as sea surface temperatures remained above 22℃. Co-existing bull (Carcharias leucas) and tiger (Galeocerdo cuvier) sharks also occurred throughout this thermal range while white shark (Carcharodon carcharias) only occurred below 23℃. Stable isotope analysis revealed scalloped and great hammerhead sharks overlapped 69% in isotopic space and with several large shark species. Both hammerhead species undertook a seasonal dietary shift, and scalloped hammerheads showed a wider δ13C and lower δ15N than the more specialised great hammerhead. Tiger sharks feed at a lower trophic level overlapping mostly with both hammerhead sharks. White and bull sharks competed with hammerhead sharks at higher δ15N. Overall, results from my research make original contributions to the knowledge of hammerhead sharks by identifying previously unreported distributions of suitable juvenile habitats of the scalloped hammerhead, which occurred in developed areas and within multi-use protection zones. Additionally, the research described previously unreported diving patterns and state-switching of juvenile hammerhead sharks within highly developed areas, highlighting the importance of wetland and estuarine habitats for shark conservation. Furthermore, the research showed high overlap in resource use between two endangered hammerhead species with several large predatory shark species. In iii summary, this thesis highlights the relevance of movement and habitat use information in the conservation and management of endangered iconic shark species