Summer and winter plankton fish assemblages around offshore oil and gas platforms in south-eastern Australia

Abstract

Opportunistic plankton surveys were conducted within a 5-nmi radius of nine offshore oil and gas platforms in Bass Strait, south-eastern Australia, in February 1998 and 1999 (summer) and August 1998 (winter). The 108 day-night samples collected alongside (vertical tows) and nearby (surface and oblique tows) platforms yielded 1526 larval and early juvenile fishes representing 55 taxa from 45 families. Epipelagic/mesopelagic taxa dominated the catches, whereas hard/soft habitat-associated taxa were uncommon. Carangidae (36.2%) and Myctophidae (31.5%) dominated in summer and winter, respectively. The most abundant taxon was Trachurus declivis (Carangidae, 35.1%), followed by Bovichtus angustifrons (Bovichtidae, 8.7%), Scomberesox saurus (Scomberesocidae, 3.7%), Centroberyx affinis (Berycidae, 3.0%) and Arripis trutta (Arripidae, 1.7%). Fish concentrations (nos. per 100 m 3) alongside platforms did not differ significantly between day and night across all surveys. Likewise, concentrations nearby platforms in February 1999, including those of T. declivis, did not vary significantly by tow type (surface vs. oblique) or day vs. night. The far greater diversity and abundance recorded in February 1999 are likely the result of upwelling conditions over the eastern Bass Strait shelf during the sampling period, and which were not detected in February 1998. In the absence of data on adult fishes associated with the Bass Strait platforms, and given the limited availability of reefs directly around the area, it could be argued that some of the taxa caught may originate from spawning around neighboring natural reefs, particularly those off the Gippsland coastline and the south-east corner of mainland Australia. However, the prime position of the platforms almost right in the center of a productivity “hotspot” would have a confounding effect on the potential source(s) of larval fishes in that region of south-eastern Australia. The role of platforms as potential de-facto reefs for juvenile fishes in Bass Strait, as well as spawning areas, is discussed based on the findings of this study, the first on early stages of fishes around oil and gas platforms in Australia.