Thresholds Database > Rock lobster community structure, West Coast, South Africa

Background

Off the west coast of South Africa are two nearby islands, Malgas and Marcus, that lie within a rock lobster (Jasus lalandii) reserve. Rock lobsters, an important commercial fishery in South Africa, are plentiful at Malgas but absent from Marcus Island. Local fishermen report that lobster populations were originally similar at both islands. In addition, there are no discernable differences in ocean temperature, turbidity or nutrients, and rock lobsters confined in cages survive equally well at both places.

Alternate Regimes

1. At Malgas Island the benthic community is dominated by high densities of rock lobsters and seaweeds, but a relative scarcity of other organisms.

2. At Marcus Island, only 4 kilometers away, the community is dominated by beds of mussels and large populations of sea cucumbers, sea urchins, and whelks (Burnupena sp.).

Mechanism

When lobsters are freed into the benthic community at Marcus Island, they are quickly killed by the mass attack of hundreds of whelks. Apparently it is the high densities of whelks around Marcus Island that prevents the re-establishment of lobsters.

Rock lobsters are active predators and scavengers that, although preferring to feed on mussels, will eat any animal matter, including whelks. In areas where lobsters are abundant, they appear to operate as keystone predators, regulating their prey species, including mussels and whelks, at relatively sparse densities, thereby allowing the establishment of other species such as seaweeds. Under these conditions, whelks cannot become numerous enough to mass-attack and kill lobsters.

On occasion, extensive mortality of lobsters has been observed following sharp declines in sea-water oxygen levels. In the event of such a disturbance, whelk populations may be able to increase to such an extent that they can prevent re-establishment of rock lobsters. In fact, a period of low oxygen was observed in the waters off Marcus Island in the early 1970s. High fishing pressure may act in a similar manner.

Management Decisions in Each Regime

There are two important questions relevant to the management of the lobster fishery off the West Coast of South Africa. The first concerns the level of lobster fishing that can be sustained without driving rock lobsters to local extinction by the combined action of harvesting and whelk predation. The second concerns the re-establishment of lobster fisheries in areas dominated by whelks. Hypothetical models provide managers with possible solutions to these questions. For example, an adaptive management strategy, using controlled harvesting and/or stocking experiments on small isolated islands or enclosures, could be used to define the actual basins of attraction of the two equilibrium points and the location of the unstable separatrix.