Interannual variation in summer habitat utilization by juvenile southern bluefin tuna (Thunnus maccoyii) in southern Western Australia

Abstract

The spatial habitat utilization of juvenile southern bluefin tuna in southern Western Australia was investigated using automated acoustic receivers with acoustic transmitters implanted in tagged fish during three austral summers (2004/2005, N = 79 fish, 2005/2006, N = 81, 2006/2007, N = 84). Seventy acoustic receivers were deployed at three cross‐shelf lines and three coastal topographic features (lumps) between December and May. We observed markedly different patterns of habitat utilization between the three seasons: (i) aggregation at lumps in 2004/2005 and 2006/2007, and (ii) wide distribution over the continental shelf (i.e., few occurring at lumps) in 2005/2006. Vertical profile by conductivity‐temperature‐depth casts showed these spatial shifts were caused by interannual changes in the presence of sub‐Antarctic water. The sub‐Antarctic water was present in the subsurface layer close to the continental slope only during 2005/2006, and the area had higher chlorophyll‐a concentrations than the coastal areas, including at the lumps. These environmental characters, related to the nutrient rich sub‐Antarctic water, appear to have a strong influence on fish distributions in 2005/2006, and may occur generally during La Niña events. Interannual fluctuations in habitat utilization will influence detection of fish in recruitment monitoring surveys and thus bias the resulting juvenile abundance indices.

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

“…These data are then used to calculate the rate of progression through the area and also to examine swimming speed based on movement past known points. Similar SBT cross-shelf distribution and habitat utilization in southern Western Australia are described in previous papers [3,4]. However, these studies did not explicitly address movement patterns and differences in residence time.…”

“…The reliability of these survey indices may depend on the changes caused by spatial and temporal distribution of SBT. A recent study suggests an interannual difference in the spatial and temporal distribution [4]. Thus, there is need to examine what causes declines in indices relative to the spatial and temporal distribution (especially the timing of the departure from the ARS area, migratory pathways, and residence times in and out of the ARS area) of summer-resident 1-year SBT in coastal waters of southern Western Australia [3].…”

“…Some applications using acoustic tracking data have examined migration patterns of individual animals [4,6,11]. These studies typically look for individuals that pass a specific point or through a line of receivers to define migration metrics.…”

“…The same protocol used for the capture and selection of SBT for conventional tagging was followed for the acoustic tagging [3,4]. Briefly, fish were caught by polling or trolling at the stern of the vessel (F/V Quadrant) and immediately placed in a tagging cradle.…”

“…Recent advances in acoustic tagging technology have made available low-cost, submersible receivers that can automatically detect and identify passing fish, such as cod and tuna [4][5][6][7]. The use of acoustic tracking systems has provided valuable insight into migration pathways [6], residence time on small and large seamounts [8,9] and habitat usage [10].…”