Understanding predation risk in fisheries : octopus depredation in the southern rock lobster (Jasus edwardsii) fishery in Australia

Depredation within fisheries occurs when a predator preys upon target species caught within fishing gear. In many cases predators can be habituated to commercial catches from fisheries, especially when prey are too large or too fast for the predator to forage successfully in the natural system. Depredation in fisheries has received much attention over the last few decades in light of conservation and economic implications. Current knowledge is still focused, however, on the direct impacts of depredation on fisheries (e.g. financial losses), or predator populations, without consideration of the implications for prey populations. Recent advances in predator‚-prey theory have demonstrated that predation not only generates direct impacts on prey populations via consumption, but also has indirect impacts via causing changes in prey activity, behaviour, physiology and development, i.e. 'non-consumptive' effects of predation or predation risk. For example, prey under predation risk are forced to make behavioural choices between vital activities, such as feeding and predator avoidance, by reducing activity or seeking out shelters. Hence, individual-level decision making (e.g. sheltering vs feeding) can have profound implications on prey life history traits such as survival, growth and reproduction, which can in turn impact populations and ecosystems. In regards to the prey, consumptive and non-consumptive effects have not yet been thoroughly examined in a fishing context, particularly on fishing grounds where predator habituation to fishing gear occurs and the implications may be extended to the prey population. In this thesis, the interaction between the Maori octopus (Pinnoctopus cordiformis) and the southern rock lobster (Jasus Edwardsii) is used as a predator‚-prey case study, with particular emphasis on octopus depredation within the southern rock lobster fishery (SRLF) in Australia. A multi-level approach examines predation risk at the population level from fishery-dependent information on fishing grounds, and at the individual level from laboratory experiments designed to elucidate the physiological responses of lobsters under predation risk. In-pot lobster mortality is used as a proxy of predation risk for the fishery (i.e., 'in-pot predation risk'), and its examination across spatial and temporal scales and with a range of life history traits forms the basic understanding of this case study. At a finer-scale individual level, information from experiments provides new insights on prey decision-making processes under predation risk, elucidating the non-consumptive effects of octopus predation on lobster. Specifically, this study comprises four linked, but stand-alone, chapters addressing the following: understanding spatio-temporal patterns of octopus predation in the SRLF (Chapter 2) and the identification of lobster life history traits associated with octopus depredation in the SRLF (Chapter 3); and the examination of physiological responses of adult J. Edwardsii under predation risk (Chapter 4); and how such responses can be altered under the combination of temperature and predation risk in key lobster life stages such as sub-adults (Chapter 5). Different spatial and temporal patterns in octopus depredation were found in the SRLF, demonstrating the dynamic nature of this predator‚-fishery interaction. Using fisheries information from Tasmania, it was evident that while octopus depredation reached maximum levels during winter on the east coast, predation risk increased during summer time on the west coast. Such spatial- temporal variability in predation risk was examined further with time series modelling using fishing and environmental variables as explanatory variables of in-pot lobster mortality within stock assessment areas. Most eastern areas showed a stronger effect of fishing variables, such as lobster CPUE and fishing effort, than sea surface temperature, with the opposite pattern along the west coast. A classification of SRLF stock assessment areas based on predation risk levels was developed and implications for fisheries management highlighted. In-pot predation risk increased with lobster size and was elevated for male lobsters (i.e., size- and sex-dependent mortality). The effect of individual traits of J. Edwardsii such as body length and sex on in-pot predation risk was examined in the fishing zones of South Australia, using mixed- modelling techniques. Predation risk was also associated with the catch rate of lobsters (used as a proxy of lobster abundance), revealing a density-dependent rate of mortality that varied among fishing zones. Findings from these two components of the thesis provide new insights on spatial and seasonal components of predation risk in the SRLF, how predation risk was related to lobster life history traits, and the ecological and economic implications associated with octopus depredation on the lobster fishing grounds. Individual-level information from physiological experiments revealed that lobster metabolism was strongly affected under predation risk scenarios. Adult individuals exposed to predator cues (kairomones) during the nocturnal-active phase of lobster activity reduced their metabolism as a predator avoidance mechanism. Low metabolism is suggested to reflect lobster inactivity or sheltering. Although such an anti-predator response may serve as a strategic mechanism to survive, deleterious effects on other vital functions such as feeding are expected to influence rates of growth and reproduction and affect population-level dynamics. In defining lobster decision making under predation risk (e.g. feeding vs. sheltering), environmental changes associated with temperature (e.g. warming waters) may force individuals to be more active during high-risk scenarios in order to satisfy metabolic demands. This was examined in sub-adults J. Edwardsii using an experimental approach combining predation risk and thermal acclimation scenarios; the latter was used as a proxy of projected temperatures for the south-east Australian region under climate change. Individuals acclimated at warming temperatures (e.g 23¬∞C) did not exhibit anti-predator responses such as a reduction of metabolic rates observed in sub-adults at ambient temperatures (20¬∞C). Instead, lobsters at warmer temperature increased their activity once energetic requirements were elevated. This suggests that anti-predator responses such as sheltering may be cancelled under projected temperatures for the region. Such findings can be used as a proxy to understand how climatic variability associated with climate change can generate direct impacts on the survival of individuals, and indicate the longer-term implications for local ecosystems and fisheries. This study provides insights on depredation in fisheries by articulating information at different organizational levels. Hence, it provides an ecological framework to examine fishing grounds as high-risk environments for lobsters, serving as a baseline for future studies on predation risk and its implications for lobster populations. At the same time, findings from this thesis will provide a better understanding of predator‚-fishery interactions, which can enhance fishing management especially in the context of implementing ecosystem-based fishery management.