Ecology of the eastern rock blackfish Girella elevata across a latitudinal gradient

Abstract

Spatial and temporal variation in growth and latitudinal clines in the reproductive biology of a temperate marine herbivore, the eastern rock blackfish Girella elevata, was examined from 3 regions of the south-east Australian coast. Biological sampling covered 780 km of coastline, encompassing the majority of the species distribution. The sampling range incorporated 3 distinct oceanographic regions of the East Australian Current, a poleward-flowing western boundary current of the Southern Pacific Gyre and climate change hotspot. Latitudinal gradients in life-history characteristics were examined to shed light on expected changes in fish productivity associated with climate and oceanographic shifts off south-east Australia. Girella elevata exhibited great longevity, displaying spatial variation in growth and reproductive fitness. The highest growth rates were observed within the centre of the species distribution. An inverse relationship was observed between growth and reproductive fitness, attributed to variation in energy allocation and trade-offs between life-history traits. Furthermore, analysis of otolith growth chronologies showed decreased growth rates during El Nino events characterised by cooler ocean temperatures in the western Pacific. In addition, spatial metrics of habitat use were investigated to determine the latitudinal connectivity of populations, contribute important data for the development of spatial management measures and provide further insight into the functioning of temperate rocky-reef marine ecosystems. To examine the movements and site fidelity of G. elevata a comprehensive array of acoustic receivers were deployed at two near-shore coastal sites in south-east Australia. Prior to the array deployment a study on the performance of acoustic-tracking technology in near-shore marine environments was conducted. This study demonstrated the importance of in situ range-test studies to array design, transmitter choice and interpretation of acoustic telemetry data. Girella elevata exhibited varying residency periods to the arrays, but all fish had small activity-space sizes. Life-history traits of longevity, slow growth, late maturation, restricted home range, and high residency potentially make G. elevata populations vulnerable to fishing pressure. Suggested management arrangements are proposed to ensure the sustainability of the population taking into consideration life-history traits and spatial metrics of habitat use. Furthermore, latitudinal clines in life-history traits of G. elevata and the associations with climate and oceanographic conditions suggest potential alterations to the latitudinal productivity of G. elevata populations in response to expected climate and oceanographic shifts in south-east Australia.