Studies some years ago suggested that yields of Jasus lalandii in the northern Cape and Namibia had declined as a result of severe overfishing and progressive reductions of minimum size limits, especially between 1960 and 1970. Although catches were temporarily boosted to artificially high levels by reductions in minimum size, evidence is presented which suggests that sustainable yields may have declined largely as a result of environmental changes during and after the 1960s. It is postulated that a progressive expansion of oxygen-deficient shelf water may have forced lobsters to occupy a much-reduced habitat in shallow waters, where overcrowding has resulted in reduced growth and survival, and production and yields have declined accordingly. It is suggested that increased competition for food and space in the better-oxygenated shallows has led to a diminution in the size at sexual maturity of female lobsters and reduced adult growth rates. In addition, low levels of dissolved oxygen may have direct physiological effects on rates of feeding, growth and mortality. Possible reasons for the likely change in oxygen concentrations after the mid 1960s are discussed. Reduced grazing of phytoplankton by planktonic herbivores (zooplankton and clupeoid fish) as well as increased phytoplankton production per se are considered to be possible causative factors leading to the carbon-overloading prevalent in the central and northern Benguela system during the past two decades.