Settlement and recruitment processes in the southern rock lobster, Jasus edwardsii : the influence of oceanographic features, pueruli behaviour and kelp habitat
Creators: name: Hinojosa Toleda, IAE
Publisher: name: University Of Tasmania
Year: 2023

description: Settlement processes and subsequent post-settlement survival influence the demographic structure of marine populations and also their resilience to human exploitation, or other perturbations. Therefore, better understanding of these two periods of the lifecycle could assist in the management and conservation of economically important species. The rock lobster, Jasus edwardsii, supports valuable fisheries throughout southern Australia and New Zealand. Settlement levels of pueruli (late larval-stage) have been monitored on artificial crevice collectors throughout the geographical range with a long-term program that commenced in the early 1990's. Examination of the settlement patterns indicated that most areas of the Australian fishery suffered declines in settlement at some time between 2000 and 2010. Patterns in settlement at some sites appear to be partially driven by environmental processes that move pueruli inshore by onshore advection. However, a lack of correlation with environmental variables at other sites, and the strong swimming ability of the pueruli, suggested that the process of settlement inshore is complex and is influenced by the ability of pueruli to orientate onshore, although no behavioural experiments demonstrating this have been reported for J. edwardsii. Settlement behaviour is also of interest in the Tasmanian jurisdiction because climate-change has led to the loss of giant kelp (Macrocystis pyrifera) forests, which previously provided dense cover across large areas of the east coast. This change in habitat in areas important to the fishery has also raised the question of whether there were effects on puerulus settlement and post-settlement survival. This thesis used a combination of data analyses, and laboratory and field experiments to assess the influence of environmental variables and puerulus behaviour on settlement and post-settlement survival. To assess the contribution of environmental variables on settlement magnitude, time series analyses were conducted over different spatial scales (Chapter 2). In addition, laboratory and field experiments were conducted to examine whether the pueruli used chemical cues from coastal waters and from the giant kelp (Chapter 3), and underwater reef noise (Chapter 4) for orientation to find settlement habitat. The effect of kelp forest depletion on settlement and survival of pueruli and early juveniles was examined with field experiments (Chapter 5). Settlement was found to be affected by regional scale oceanic processes (100-500's km) measured by the Southern Oscillation Index (SOI), Dipole Mode Index (DMI) and Southern Hemisphere Annular Mode (SAM) although outcomes varied between regions. At a local scale, waves, wind strength and current appeared to have some influence on pueruli transport, with sea surface temperatures being less important, but much of the settlement variability was not explained and the effect of environmental factors was not consistent between sites. In a field experiment it was found that underwater reef noise was detected by puerulus and appeared to be an attractant for settlement during calm sea conditions. Laboratory experiments showed that pueruli were attracted to chemical cues from coastal waters, but they were not attracted to chemical cues from the giant kelp. The effect of kelp was also examined in field experiments where kelp habitats increased settlement into crevice collectors and the subsequent survival of the early juveniles, indicating that decline in kelp habitat could reduce recruitment at a local scale. Overall, these results highlight the complexity of settlement and recruitment where larval behaviour and oceanographic process interact at different scales. The research helps to explain settlement processes and also has value for including puerulus behaviour into future dispersal models for exploring spatial patterns and testing dispersal hypotheses in this species which supports a valuable fishery. The apparently complex processes affecting settlement strength showed that environmental conditions that reduce settlement strength in one region of the fishery often increase settlement strength in other regions. This suggests resilience to climate change at the scale of the entire fishery. However, local habitat changes would also be expected to affect future recruitment of this valuable species.