University of Tasmania
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An unusual habitat for a common shark : life history, ecology and demographics of the spiny dogfish (Squalus acanthias) in Macquarie Harbour, Tasmania

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posted on 2023-05-28, 09:07 authored by Moreno Iturria, DA
Sharks and their relatives (Elasmobranchii) are commonly understood to have a conservative life history that makes them particularly vulnerable to external pressures. However, species with high mobility potential and generalist diets have a higher ecological flexibility, resulting in an increased adaptive potential. One such species is the small and abundant spiny dogfish, Squalus acanthias. Intense fishing pressure across much of their northern hemisphere distribution has resulted in the collapse of some populations. Some of these populations have been able to recover from near catastrophic declines after regulatory action was set in place. Due to differences in environment, prey and demographics, the life-history parameters of different populations are variable, thus, appropriate management is highly reliant on apposite information. However, despite being abundant in temperate waters of Australia, the life history of S. acanthias in the region has remained largely unstudied. Squalus acanthias is arguably one of the best-understood elasmobranch species, it is widely distributed and has a conservative life history that closely resembles that of large pelagic shark species. These characteristics make it an ideal model species for the group, especially when the environment, ecology or biology of a population differ from what is typical for the species. One such instance potentially exists in Macquarie Harbour, Tasmania. Squalus acanthias inhabits this small (275 km2) ria estuary where adults of both sexes are found throughout the year, an atypical behaviour for this migratory species. The physicochemical characteristics of the system create a strongly stratified environment that differs from the coastal and oceanic habitats where S. acanthias is usually found. Anthropogenic impacts from mine dewatering, changed river flows associated with hydro-electricity production and, more recently, salmonid aquaculture, have been linked to important environmental changes and degradation in the area (e.g. declines in dissolved oxygen levels), which may affect the native fauna. Recognizing the interesting set of circumstances, and the need for current information on the species, the main objective of this study was to conduct a comprehensive investigation into the life-history characteristics, ecology and biology of S. acanthias in Macquarie Harbour to provide information on the status of the species in the area and inform future management. The first component of this study aimed to use passive acoustic telemetry to investigate the spatial ecology and residency patterns of S. acanthias in Macquarie Harbour. Adult male and female S. acanthias remained resident within Macquarie Harbour throughout the entirety of a 1.5-year observation period, with no evidence to indicate emigration out of the estuarine system. Individuals showed continuous movement throughout the harbour, with the potential to transverse the whole extent of the system within a few hours. Male and female ranges overlapped throughout the year, although the upper reaches of the estuary were used exclusively by males during winter. These results suggest that S. acanthias remain resident in Macquarie Harbour for extended periods, independent of sex or season. Other populations of S. acanthias seasonally limit their use of small coastal embayments and typically migrate offshore seasonally, where the spatial distribution of the species is specific to sex and ontogeny. The range-restricted spatial utilisation in Macquarie Harbour will likely increase the population's vulnerability to external stressors, particularly by increasing their susceptibility to environmental degradation. By implementing three distinct approaches to the modelling of telemetry data, this work developed a robust framework for interpretation of the tracking data set. Random walk models, social network analysis and state-transition models were used complementarily to detect complex trends in the movement and behaviour of a species through time. Site selectivity of S. acanthias was found to be non-random, with all individuals showing affinity for the central area of the harbour. Results also showed that although the small area of the harbour prevented sexual segregation, behavioural avoidance resulted in decreased association between sexes during winter. This is the first time such a mechanism has been shown in the species, suggesting alternative adaptive responses through which species may adjust to density dependent or niche constraints when spatial partitioning is not possible. The second component of this study sought to investigate the life history of the population, specifically diet, reproduction, age and growth. Squalus acanthias are typically opportunistic, generalist feeders; however, in the naturally depauperate Macquarie Harbour, prey composition was overwhelmingly dominated by spill-over pellets from the aquaculture operations. Stomach content analysis and stable isotope mixing models produced consistent results, revealing that the dietary trends seen for the population are seasonally invariant. Alternative prey items were consumed during individual sampling events, the most common being escaped Atlantic salmon and mytilid bivalves, with both also being associated with aquaculture operations. Over the observation period, there was an atypically high prevalence of empty stomachs (68%) suggesting there are no viable prey alternatives to the resources provisioned by aquaculture, which may now act as a limiting factor for the population. In contrast with other S. acanthias populations, reproduction in Macquarie Harbour did not conform to a seasonal pattern, with evidence of continuous mating through the year. Females with candled embryos in-utero were observed during all seasons, however, other stages of pregnancy were mostly absent. A high frequency of non-gestational mature females showing no sign of recent pregnancy indicates a resting period between pregnancies. Although the possibility that gestation and parturition occur outside Macquarie Harbour cannot be fully discounted, evidence reported in this study suggests that it is highly unlikely. Furthermore, the sizes and ages at maturation, birth and longevity are some of the lowest reported for the species, contrasted by comparably high juvenile growth rates. Current developments for modelling of life-history characteristics in data-poor stocks were incorporated into the analytical framework implemented in this thesis. The results of which are believed to represent the best available estimates for all demographic parameters while characterizing uncertainty in a way that better resembles biological variability. Results are suggestive of a population that is reproductively compromised, likely reducing their productivity potential and thus, their ability for recovery. Overall, this thesis reveals a unique set of behavioural and life-history characteristics in this common elasmobranch that is living in an unusual environment. Their demographic characteristics and spatial ecology strongly suggest a closed or semi-closed population likely isolated within Macquarie Harbour. Epigenetics, environmental change, density dependent responses and anthropogenic pressures are all potential causes of the peculiar life history of this population, however, the absence of historical data precludes the partitioning of these factors. A demographic modelling framework constructed around the results of this work revealed that under current conditions the population is unlikely to thrive and will have a severely limited ability to cope with further external pressures. Thus, the past and ongoing anthropogenic impacts of mining and aquaculture are of concern, as they may have overwhelming effects on the population. Nonetheless, by removing large portions of waste pellets from the system, S. acanthias provides an important ecosystem service, likely aiding in the partial mitigation of the effects that spill-over aquaculture feed has on other species and nutrient enrichment of the ecosystem. Also, due to the strong interdependence between aquaculture and the species, S. acanthias could serve as an ideal indicator species to evaluate the condition of Macquarie Harbour. Considering the results of this thesis, it is recommended that further monitoring and management action be undertaken to ensure the permanence of this important and historically ignored species in the area.


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