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Ecological analysis

Trophic levels and average maximum sizes

It has been suggested (Froese, Torres and Pauly, 1998) that the exploitation of multispecies communities has the effect of changing the relative abundance of the different functional groups in the ecosystem that supports these communities. The most common effect expected is that large, long-lived species with high trophic levels would be replaced in the catches by smaller, short-lived species with lower trophic levels. Pauly et al. (1998) have analysed the FAO capture database, which covers 45 years of global capture production, and found a so-called "fishing down marine food webs" trend. The same methodology has been applied here (for the first time to a single country) in studying the trends of the mean trophic level and the average maximum size of landings for the 21 most important species or species groups in Cuban fisheries.

The data of the mean trophic level and of the average maximum size for each species or species group were obtained from FishBase (ICLARM, 1998). To calculate the trophic level of a species, both the diet composition and the trophic levels of their food items were taken into account applying the formula:

trophic level = 1 + mean trophic level of the food items

where the mean is weighted by the contribution of the different food items (Pauly and Christensen, 1998).

FIGURE 8: Linear regression of the mean trophic levels of Cuban fishery landings

FIGURE 9: Linear regression of the average maximum size of Cuban fishery landings

Figures 8 and 9 show the linear regressions (highly significant, P < 0.001) of the mean trophic level and of the average maximum size for the 1955 to 1995 period. The negative trends are clearly visible and indicate a gradual transition from landings of large piscivorous fishes to small fishes and invertebrates feeding on smaller organisms (plankton and bottom invertebrates). These results are in line with those obtained by Pauly et al (1998) on the global scale.

Demersal versus pelagic fishery resources

Each species or group of species was classified as pelagic or demersal and the proportion of total landings to fall into each of the two categories was calculated. The pelagic fish species include: small tunas, sharks, sardines, thread herring, jacks and Spanish mackerels. The demersal species include: spiny lobster, shrimps, lane snapper, yellowtail snapper, mutton snapper, gray snappers, mangrove oyster, Nassau grouper, turkey wing clam, grunts, mullets, mojarras, porgies, blue crab and land crab. Figure 10 shows the linear regression of the ratio between the landings of demersal and pelagic resources over the last 40 years. As can be noted, the demersal species have far higher landings than the pelagic species, but this trend is decreasing (r2 = 0.195, P < 0.001) along the time series. Although some of the pelagic species are not intensively exploited because there are no advanced fishing technologies available (e.g. the small tunas fishery) or because they are not suitable for industrial processing (e.g. scaled sardines), the coefficient of variation of pelagic species landings (27.7) is higher than that of demersal species (19.2).

FIGURE 10: Ratio of demersal to pelagic landings

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