Ko Fujioka

Spatial and temporal variation in the distribution of juvenile southern bluefin tuna Thunnus maccoyii: Implication for precise estimation of recruitment abundance indices

Acoustic tags were used to examine the spatial and temporal distribution of southern bluefin tuna (SBT) in southern Western Australia, which is in a region where fishery-independent acoustic surveys of the recruitment abundance index of SBT have been historically undertaken. We investigated patterns of SBT distribution within and inshore of the acoustic survey area during three summer seasons. Annual differences in distribution patterns were characterized by two distinctive migration pathways. An inshore-migrating pathway was observed in two seasons (2004/2005 and 2006/2007), with a relatively high proportion of tagged SBT (84.5, 65.0%) migrating inshore of the acoustic survey area. The other pathway was concentrated along the shelf (2005/2006 season), with an estimated 63.3% of tagged SBT moving within the survey area. These variable migration patterns may bias the interannual fluctuations in abundance indices. Current survey methods can be modified to include both inshore and continental shelf areas. This contribution shows that the accuracy of acoustic surveys can be improved by including ecological patterns.

The influence of body size on the intermittent locomotion of a pelagic schooling fish.

There is a potential trade-off between grouping and the optimizing of the energetic efficiency of individual locomotion. Although intermittent locomotion, e.g. glide and upward swimming (GAU), can reduce the cost of locomotion at the individual level, the link between the optimization of individual intermittent locomotion and the behavioural synchronization in a group, especially among members with different sizes, is unknown. Here, we continuously monitored the schooling behaviour of a negatively buoyant fish, Pacific bluefin tuna (N = 10; 21.0 ∼ 24.5 cm), for 24 h in an open-sea net cage using accelerometry. All the fish repeated GAU during the recording periods. Although the GAU synchrony was maintained at high levels (overall mean = 0.62 for the cross-correlation coefficient of the GAU timings), larger fish glided for a longer duration per glide and more frequently than smaller fish. Similar-sized pairs showed significantly higher GAU synchrony than differently sized pairs. Our accelerometry results and the simulation based on hydrodynamic theory indicated that the advantage of intermittent locomotion in energy savings may not be fully optimized for smaller animals in a group when faced with the maintenance of group cohesion, suggesting that size assortative shoaling would be advantageous.

Evaluation of multipath effects on depth measurements provided by acoustic transmitters in shallow water

Measurement errors caused by multipath effects are one of the problems of acoustic biotelemetry, especially in shallow waters because both the sea surface and bottom can be major boundaries reflecting ultrasonic pulses. We conducted an experiment using transmitters with a pressure sensor to examine occurrence probabilities of multipath effects in a shallow sea area. We deployed seven transmitters with a 1.28-s signal transmitting interval in the same location, and two receivers located 18.5 m (R1) and 38.0 m (R2) horizontally from the transmitters. In order to solve the problems derived from multipath effects, we applied a state-space model to the depth data to estimate the correct values. Then we compared data availabilities, which were percentages of the number of correct values to the number of actual transmissions, between the acquired data and the model estimation. In 15 min recording, the mean proportions of the correct values for all transmitters were 88.4% for R1 and 86.3% for R2. The data availabilities became significantly larger from 65.7% to 96.7% with R1, and from 64.6% to 93.3% with R2 by applying the model. This suggests that we can reduce the measurement errors caused by multipath effects by post processing.