Even when fishing effort data are not available in a developing fishery, it is often still possible to develop a simple fishery indicator to obtain information about the status of the stock. Using catch data only, Grainger and Garcia, (FAO Fish. Tech. Pap. 359, 1996) showed that the changes over time in the relative rate of catch increase (RRCI) can be used to detect when a stock reaches its over-fishing level, i.e., the year when RRCI falls to zero. The method presupposes that fishing effort increased steadily over the period concerned. We propose a generalization of this method that consists in obtaining a crude estimate of the maximum sustainable yield (MSY) by plotting the trend in catches against the smoothed RRCI. The yellowfin tuna (Thunnus albacares) fishery in the Eastern Atlantic was used to show the bold relationship between MSY estimates obtained from standard equilibrium production models and from this method. Given that it is very difficult to estimate effective fishing effort for skipjack tuna (Katsuwonus pelamis), we show how this simple fishery indicator can be used to obtain proxies of MSY for skipjack fisheries located in the Eastern Atlantic Ocean and in the Indian Ocean.

A new fishing method has been developed during the last twelve years by the tuna baitboats fishing seasonaly (from July to December) in the area of Senegal and Mauritania (West Africa, 12 to 21° N). Three species of tunas, yellowfin, skipjack and bigeye tunas are caught in similar proportions in this fishery. This method has produced a dramatic increase in the baitboat catch per unit of effort (CPUE). The paper describes this new fishing method where the boat is permanently running associated with a very large tuna school, night and day, during several months. A team of two baitboats exploits each school, operating one after the other. The biological and environmental characteristics of this associated-school fishing are analyzed from detailed log book data. The daily positions of each school and the catches by species and sizes are analyzed for each boat and school. The dynamics of the association between the tunas and the baitboat and the dynamics of this exploitation (i.e. input and output of tunas in each school) are discussed in relation to the peculiarities of the local environment (mainly sea surface temperature and bathymetry). The reasons for the increased CPUE in this associated-school fishing are analyzed. Further researchesshould be conducted in order to better understand how fish behaviour relates to this fishing mode and in order to estimate the potential local yield of the fishery, if increased fishing effort is exerted in the future.