Discussion and conclusions
The extended data set that has been analysed offers, for the first time, a coherent picture of Cuban fisheries over the last 60 years. The landing stat-istics show sustained growth from the mid-1950s until the end of the 1970s, a diminution of the rate of increase during the 1980s and a decrease, coinciding with the decline of some important fisheries, from the beginning of the 1990s.
Analysis of the dynamics of the 21 most important species or species groups also shows that, in 1995, approximately 38.9 percent of the resources were in the senescent phase (with declining catches), 48.7 percent were in the mature phase at a high exploitation level and only 12.4 percent were still in the developing phase with some possibility of increased landings. None of the fisheries was still in the undeveloped phase. This means that 87.6 percent of fisheries resources are in a critical stage and, therefore, there is an urgent need to implement fishery management measures such as the control and reduction of the fishing effort.
The assumption that the exploitation of fish communities has the effect of changing the relative abundance of the different functional groups of the ecosystem (Froese, Torres and Pauly, 1998) can be considered as confirmed for the data examined here. In addition, the "fishing down marine food webs" theory (Pauly et al., 1998), as well as the reduction of the average maximum size of catches, have also found correspondence in this study of Cuban fisheries. The mean trophic level and the average maximum size of the 21 species and groups analysed here had both negative and significant trends throughout the period being considered.
According to Baisre (FAO, 1993), most of the Cuban commercial species are at high risk of overexploitation, as they are in other tropical countries. This is the result of one or more of the following factors, some of which are typical of tropical fisheries:
Analysis of the resources and of the sequence of peak landings shows that a historical loss of 20 000 tonnes has occurred. This means that, if the landings of individual species or groups could be re-established, about 20 000 additional tonnes of yield would be obtained. Adding together the landing peaks of all the species examined here, produces a total yield of about 50 800 tonnes (Table 1), which becomes 79 870 tonnes if the total landings of all Cuban marine resources are considered. This figure is very close to previous estimates (80 000 tonnes) of Cuban fisheries potential (Baisre and Páez, 1981; Baisre, 1985). On the other hand, potential maximum production, according to the generalized fishery model, corresponds to about 57 000 tonnes (Table 2), 12.1 percent higher than the sum of landing peaks and 87 percent higher than 1995 landings.
The immediate question is: Are there any real possibilities for recuperating these losses? The decreases in landings of Nassau grouper and mullets, about 95 percent and 88 percent respectively, seem to be particularly dramatic. Although earlier landing statistics of the Nassau grouper might have been overestimated (they included the catches from Bahamian waters until 1976, when that country prohibited fisheries within its territorial waters), the continued decrease of catches is very significant and corresponds with the drop observed for similar species in other regions (FAO, 1997). There are probably several causes that have contributed to this striking decrease. The species is particularly vulnerable because it gathers in spawning aggregations during the reproductive season (Colin, Shapiro and Weiler, 1987; Olsen and La Place, 1979; Tucker, Bush and Slaybaugh, 1993), when it is caught with non-selective methods; it is a hermaphrodite species and this provokes a differential mortality between the sexes; and it has a slow growth rate. The fact that there is also a general decrease in the landings of grouper species in the Caribbean area as a whole might indicate that there are few possibilities for recovery in the short term.
Although overfishing is undoubtedly one of the main causative forces, not all of the observed changes can be attributed to this single factor, and some of them are probably irreversible. In the case of mullets, the indiscriminate use of set nets, even during spawning runs out of the coastal lagoons (Baisre, 1985), and the impact of the construction of dams in the main rivers have probably both helped to provoke the dramatic drop in the catches of this group of species, whose life cycle is fairly dependent on the good health of the estuarine environment. Their recovery depends, therefore, on the restoration of their nursery habitats, but there is strong evidence that the damming of numerous rivers flowing into southeastern Cuba, where the most important fishing grounds for mullets and other estuary-associated species are located, has resulted in both a shortage of nutrients in the coastal waters and a reduction of the nursery habitats, resulting from the filling of many coastal lagoons (González and Aguilar, 1984). Therefore, the recovery of these fisheries does not seem feasible for the next few years.
According to Caddy (1991), there has been little documentation on the effects provoked by various terrestrial inputs on fisheries in open-sea areas. Caddy also discussed how increased nutrient discharges can play a role in semi-enclosed seas, such as the Mediterranean and the Black seas, increasing biological production and fishery yields by cultural eutrophication. In Cuba, the opposite effect seems to be occurring, as a result of intensive river damming, which has produced a considerable shortage in nutrient discharges in open seas, augmented by the drastic reduction of fertilizer imports and production during the last ten years. The magnitude of this reduction as well as its impacts on fishery resources must be a research priority in the