Modelling the population dynamics of a benthic octopus species : exploring the potential impact of environment variation and climate change

Cephalopods are increasingly targeted by fisheries, yet their population dynamics are generally poorly understood due to their intrinsically complex nature and their great sensitivity to environmental factors. As a consequence, population structure and biomass can change rapidly over short time-scales, with currently no means of predicting future recruitment or the consequences of climate change on these species. The aim of this study was therefore to develop a mechanistic model to predict the population dynamics and the potential impact of environmental variability and climate change on a cephalopod species. The benthic octopus Octopus pallidus was the main focus of this research and laboratory rearing of juveniles allowed the relationships between early growth and the significant factors affecting growth to be examined (i.e. food intake, food conversion and fluctuating environmental temperatures). Results indicated high intra- and inter-individual variability in feeding rates, conversion rates and growth rates, with no indication of periodicity for any of the variables. Based on the concept that growth is bi-phasic (a rapid exponential growth phase followed by a second slower growth phase) and using results from captive studies on O. pallidus and Octopus ocellatus, a temperature-dependent bioenergetic model describing growth in octopus was developed. Model projections were consistent with laboratory data and sensitivity analyses suggested that metabolic rate has the greatest influence on the growth threshold at which the switch from fast to slower growth occurs. In order to simulate juvenile growth trajectories in the wild, the bioenergetic model was further developed to include dynamic seasonal temperatures and individual variability in growth and hatching size. Results indicated that hatching size was secondary to inherent variation in growth rates in explaining size-at-age-differences, and that size-at-age distributions in some cohorts tended to become bimodal under certain food intake levels. Predictions from the individual-based bioenergetic models were integrated into a matrix population model, which was used to project the population under the predicted temperature conditions generated by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) from the emission scenarios of the Intergovernmental Panel for Climate Change (IPCC). Simulations suggested that increasing water temperatures might not be as beneficial to octopus as previously thought. Survivorship and incubation time were found to drive the population dynamics and while O. pallidus has the potential to survive under climate change conditions, the population structure and dynamics are likely to change substantially with a potential decrease in average generation time, a streamlining of the life cycle, and a possible loss of resilience to catastrophic events. Mechanistic models relating cephalopod biology to the environment, like the one presented in this thesis, constitute a valuable way forward to elucidate population dynamics in these highly plastic animals.