Estimates of biomass, sustainable yield, and harvest: neither necessary nor sufficient for the management of non-commercial urban intertidal shellfish fisheries

Quantitative assessment of the sustainability of non-commercial harvests from urban shellfish populations is rarely possible, as the scale of these fisheries does not usually warrant extensive research. Management is generally based upon a paucity of information, often resulting in the implementation of inappropriate controls. In this study, comparatively comprehensive data are used to assess the likely short-term sustainability of non-commercial cockle (Austrovenus stutchburyi) gathering at three beaches, by comparing harvest, biomass and yield estimates. Although harvest estimates were broadly similar to estimates of yield, these findings were sensitive to assumptions about size at recruitment to the fishery, temporal changes in biomass, and possible changes in the selectivity and behaviour of fishers. Because of this uncertainty, we conclude that management on the basis of sustainable yield is inappropriate given the unpredictable nature of non-commercial gathering effort and shellfish population dynamics. Further, we conclude that the current management regime of daily bag limits per gatherer may be inadequate. Daily bag limits fail to constrain annual non-commercial harvests to appropriate levels when accessibility and interest is high, yet gathering is one of the main determinants of urban shellfish abundance. In the absence of intensive monitoring and management, a precautionary minimum legal size limit based on a species reproductive biology, appears to be the most promising means of ensuring the viability of a shellfish population when its abundance is low. Other management measures such as daily bag limits and beach closures also have their role to play, but are not in themselves sufficient.