Research paperEvidence for the biotic origin of seabed pockmarks on the Australian continental shelf

- Not all pockmarks are necessarily evidence of seabed fluid flow.
- Small pockmarks less than 10 m in diameter may be excavated by fish.
- Such pockmarks have distinctive characteristics and distribution patterns.
- The majority of pockmarks on the Australian continental shelf are considered biotic.
- The proposed agent is E. coioides, but definitive proof of its activity is lacking.

The generally accepted formation mechanism of pockmarks worldwide is the expulsion of fluid at the seafloor, but such a mechanism does not explain the close association between pockmarks and seabed infrastructure such as pipelines and wellheads within the Stag oil field on the North West Shelf of Australia. Furthermore, certain characteristics of the pockmarks, such as conical mounds of sediment positioned around their perimeters, are strongly suggestive of a biotic origin. Pockmarks in this case are typically 5 m in diameter and 1 m deep, excavated within a sandy seabed in 45 m water depth. Inspection of ROV footage acquired during oilfield operations within the Stag field supports but does not entirely confirm without doubt the proposition that the pockmarks are created by fish of the genus Epinephelus.Having determined the characteristic features of pockmarks within the Stag field which mark them as biotic excavations, data from commercial seabed surveys at 11 other sites on the North West Shelf, all of which reveal numerous pockmarks, was reviewed for evidence of similar pockmark characteristics. Based on the review, it appears likely that the majority of pockmarks on the shallow North West Shelf (between 40 m and 130 m water depth) are representative of biological rather than geological activity. The probability that pockmarks less than approximately 10 m in diameter throughout the remainder of the Australian continental shelf are also the result of purely biological activity is high, as demonstrated by the analysis of data from three further sites.Close inspection of seabed survey data from further afield could extend the findings of this paper throughout not only the tropical Indo-West Pacific (the range of the particular fish species implicated on the North West Shelf), but potentially worldwide if other species can be shown to display similar behaviour.