Assessment of bioresource sustainability in marine coastal waters: An attempt based on retrospective studies in the area of Ohta River [Japan] catchment and Hiroshima Bay

Many enclosed coastal seas are environmentally deteriorated with concomitant decrease in fisheries production and need a proper management to aim the sustainability of bioresources therein. This study has two objectives: one is to establish proper criteria to indicate the sustainability of bioresources and the other is to actually assess the sustainability for Hiroshima Bay. We examined the historical changes in environment of the catchment area of Ohta River, a major river flowing into Hiroshima Bay, in particular, nutrient (i.e. N and P) loads into the bay were considered. We also collected long-term data on the diversity of inter- and sub-tidal invertebrates and fisheries and aquaculture production in Hiroshima Bay. Due to increased anthropogenic activity, both N and P loads increased from the 1960s to 1980s, and accordingly the eutrophication of Hiroshima Bay was proceeded. Reduction of P load was effective since the 1980, but there is no sign of decrease in N load. The water quality (i.e. transparency and nutrient concentrations) and sea bottom environment (i.e. ignition loss and sulfide content) in Hiroshima Bay were the worst during the 1970s, and there has been no significant recovery since then. Annual landings from fisheries and aquaculture decreased temporarily in the 1970s, and receded further in the 1990s. Both the diversity in feeding habit of fisheries animals and in species composition of inter- and sub-tidal invertebrates decreased steady from the 1970s. Based on such retrospective data, we proposed three criteria for the assessment of bioresource sustainability: l) productivity, 2) efficiency, and 3) stability of bioresources or ecosystem. An integration of these criteria revealed that the sustainability of bioresource in Hiroshima Bay was highest in the middle and late 1960s. Then, the sustainability was gradually deteriorated and the worst in the l990s.