Overfishing catastrophes

A 2004 Government report showed that most of Australia’s ocean fisheries are either over-fished or have uncertain status - there is not enough information to determine their status. Of 74 species, 14 were over-fished, 40 were uncertain and 17 were not over-fished. Last year the Australian Government set aside $220 million to help downsize the nation’s fishing effort.

Preventing the collapse of fisheries to extinction levels or to levels which remain low for very long periods is difficult because controlling catch levels can be difficult because of property right and open access issues. It is not true that a stable catch means that a fishery is stable and sustainable because the factors driving the natural regrowth of a fish population vary randomly – for example a change in seasonal wind patterns has been claimed to cause a collapse in the Australian gemfish fishery. The minimum scale of a fishery that permits the continued existence of a fishery – the critical depensation level – may therefore vary randomly. If stocks go below this level then the stock will not regenerate.

These issues are examined in a global context in a paper by Christian Mullon, Pierre Fréon and Phillippe Cury which examines the reasons for collapse of a fishery resource. An edited version of the abstract of their paper summarises the issues well:

The fear of a rapid depletion of world fish stocks because of over-exploitation is increasing. Analysis of 1519 main series of the FAO world fisheries catch database over the last 50 years reveals that 366 fisheries’ collapses occurred, that is nearly one fishery in four. The number of collapses has been stable through time since 1950s indicating no improvement in overall fisheries management. Three typical patterns emerge from the analysis of catch series during the period preceding the collapses:

(i) 33% of collapses are smooth with catch numbers declining regularly over a long period,
(ii) 45% of collapses are erratic with wild fluctuations in catches prior to eventual collapse,
(iii) 21% are plateau collapses where numbers drop abruptly after long, stable, high level of catches.

Using a simple model the authors relate the plateau-shaped collapses (these are the most difficult to predict) to surreptitiously increasing exploitation and a depensatory mechanism at low population levels. Thus, a stable catch over several years is shown to conceal the risk of a sudden collapse. This jeopardizes the common assumption that considers the stability of catch as a goal for fisheries sustainability.

This is important research. Fish are an important source of protein around the world – but particularly in developing countries. The current state of the world’s fisheries – and of the Australian fisheries in particular – is not good. Fish are a renewable resource that can help feed the world. But stocks of fish can be more stressed than current rates of yield suggest and fishery collapses can result in irreversible losses.

Thanks to Greg Price in The Weekend Australian Financial Review (subscription required) provided two of the three references cited above plus valuable discussion.