- Title
- Age, growth and patterns of occurrence in smooth hammerhead sharks (Sphyrna zygaena) off the coast of New South Wales, Australia
- Creator
- Wray-Barnes, Alexander
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2017
- Description
- Masters Research - Master of Philosophy (MPhil)
- Description
- Significant declines in the catch rates of smooth hammerhead sharks (Sphyrna zygaena) have recently been reported off the coast of New South Wales, Australia. Quantitative investigations on the life history and correlates of distribution of exploited marine species is fundamental in providing sound species management, as resulting quantifiable results can help determine how population structures are affected by fishing, and their capacity to recover from reduced stocks. This information is particularly important for commercially targeted animals, such as sharks. This thesis assesses the age, growth and distribution of juvenile smooth hammerhead sharks (Sphyrna zygaena) on the east coast of Australia. Vertebra, along with information on sample sex, size and stage of maturity were collected from 144 sharks. Lengths-at-age and growth rates were estimated from vertebral growth band counts for 82 females (109 – 284 cm total length (LT) and 62 males (120 – 255 cm LT). A multimodal approach (various growth functions) was used on pooled data and then separated by sex. These were compared using Akaike Information Criterion, sorted by Akaike score (Δ) with supporting evidence weighed using Akaike weights (ω). These indicate that a multimodal approach is necessary for growth analysis and that sex specific models are required. Females attained a maximum theoretical total length (L∞) of 302.2 cm and k of 0.06, whereas male L∞ was larger at 340.7 cm with a k of 0.06. Sex ratios were similar, however 96% of samples were not sexually mature, indicating that the coastal population of S. zygaena are mostly immature. These results have important implications in assessing the resilience of S. zygaena to stock depletion in south eastern Australian waters. This is fundamental for management decisions about status listings and allowable fishery interactions. Environmental variables influencing the catch of juvenile S. zygaena within the New South Wales Shark Meshing (Bather Protection) Program (SMP) were investigated to identify potential variables that explain spatial and temporal variability in catches. Using remotely sensed products and spatial conditions, 23 years of daily catch data from the SMP were applied to generalised linear mixed models with random effects to predict capture. The environmental variables assessed included sea surface temperature (SST), rainfall and chlorophyll a, as well as spatial and temporal variables such as distance to estuary mouth, substrate type, moon phase and southern oscillation index. Additionally, the introduction of acoustic deterrent devices were included in the model using year of introduction (1999). Corellative information indicates that juvenile S. zygaena catch were greatest at warmer SSTs, during dry weather, dark moon phases, in primarily sandy surroundings, and in nets closest to estuary mouths. Chlorophyll a concentration and southern oscillation did not help explain variation in catch. However, after adjusting for temporal changes in significant environmental factors, a temporal decline in catch was still present, indicating that the temporal decline in catch was not attributed to the environmental conditions assessed. The best predictor of temporal decline in the model was the introduction of acoustic deterent devices on nets. While a catch decrease after device introduction may be coincidental to an actual decline in the population, the close agreement between the fit of the model and the change point suggests that the temporal decline was associated with the introduction of the acoustic devices. This study suggests that further investigation at a finer level of detail (i.e. satellite tracking) is required into how each of the significant environmental conditions drives movement patterns of S. zygaena. This will also allow for a confirmation of the current studies method in detecting environmental patterns of occurrence and may allow an opportunity to test how acoustic alarms may affect their sensory biology. This thesis can assist decision makers in potential status listings both locally and worldwide. The identification of environmental and demographic catch patterns allow for informed coastal management decisions to take place, complementing future species specific adaptive management strategies.
- Subject
- smooth hammerhead shark; age and Growth; Sphyrna zygaena; acoustic deterrent device; shark meshing program; Australia; environmental; sea surface temperature; rainfall; moon phase; substrate; multimodal; estuary
- Identifier
- http://hdl.handle.net/1959.13/1353450
- Identifier
- uon:31099
- Rights
- Copyright 2017 Alexander Wray-Barnes
- Language
- eng
- Full Text
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