Most of the major effects of fishing recorded around the world occur in Mediterranean ecosystems too. They vary from local effects on the sea bottom caused by damaging trawler gears to large-scale impacts on cetacean populations arising from the entanglement of the animals in long driftnets. This variety is due to three principle factors: the huge diversity of fishing gears and practices (most of them artisanal), the very high intensity of fishing, and an important biological diversity. The latter is demonstrated by the Mediterranean presence of a vast array of vulnerable species, including emblematic seals, whales, turtles and sharks.
The case by case approach adopted in this document notwithstanding, it should be emphasized that the impact of fishing goes far beyond the mere effects caused on single populations of target and bycatch species, or the degradation of the physical support system. Fishing profoundly affects the complex structure of ecosystems, altering their internal functioning. A measure of human appropriation of marine biological production, the percentage of the primary production required to sustain a given fishery (%PPR), has been estimated on a global basis by Pauly and Christensen (1995). The results obtained indicated a much higher ecological footprint for fishing than expected: up to 35.3% in the case of world non-tropical shelves. Another ecological index, the average trophic level of the fishery (TL), indicates fishing impact on the structure of marine food webs over time. Pauly et al. (1998) described the existence of a global fishing down marine food webs effect (FDMFW) based on the steadily decreasing trend of TL values of catches recorded for the period 1950-1994, also verified in the Mediterranean. The lack of correspondence between the harvesting on lower TLs and the expected increase of catches points to the disruption of major energy pathways and the subsequent decrease in yields, resulting from the structural and functional degradation of the ecosystem. Well-structured ecosystems maintain healthy predator population levels, tend to be energetically more efficient and more resilient to external perturbations, and are the bases for sustainable fisheries.
Some recent attempts to evaluate the overall ecosystem effect of fishing in specific areas of the Mediterranean appear to arrive at the same conclusions as those above. Claims concerning the role of the increase in biological production due to the anthropic nutrient enrichment of waters add some uncertainty to the interpretations.
Tudela (2000) estimated a %PPR for the mixed pelagic/demersal fishery operating off the central Catalan coast (northeastern Spain) on the shelf and the slope down to 1 000 m as slightly more than 40% of the total primary production. This figure takes both discards and misreporting of catches into account. Such a huge level of human appropriation, one of the highest ever recorded, together with the stagnation of landings despite the growth of fishing capacity and the fact that the fishery works at moderately low trophic levels implies the full, and ecologically unsustainable, exploitation of the ecosystem. The author warned about the possible loss of ecosystem resilience in these conditions. A further assessment by Tudela, Coll and Palomera, (2004) of the overall pressure on the Mediterranean ecosystem due to fisheries demonstrates that the threshold for ecosystem overfishing (as defined by ecological indicators on ecosystem structure and functioning) has already been exceeded. Stergiou and Koulouris (2000), using official landing statistics in the eastern Mediterranean basin, studied the evolution of mean TLs of catches during the last 30 years, looking for a local FDMFW effect. The results showed that at least in the main part of the Aegean Sea the mean trophic level has decreased in recent years, and the authors concluded that the present pattern of harvesting in not sustainable. In any case, such a high level of ecosystem exploitation is liable to disrupt food-webs, and prevent the ecosystem from supporting healthy populations of apex predator species. This phenomenon may underlie many of the conflicts reported in previous sections and point to the need for combining both conservation policies for specific threatened species (i.e. monk seal) and sustainable fishery policies, allowing ecosystems to rebuild themselves.
This reduction of the mean TL of an exploited community may be intentional from the start, as exemplified by fishermen in southern Sicily: they customarily clean the sea by repeatedly trawling a new fishing ground to eliminate sharks and other undesirable species (Badalamenti, pers. comm.). Conversely, the creation of marine protected areas (MPA) in which fishing is banned has proved useful for rebuilding the diversity of exploited communities: the mean TL of fish assemblages in seagrass beds has risen following protection along the French Mediterranean coast (Harmelin-Vivien, pers. comm.).
It has been suggested that the increase in primary production in the northwestern Mediterranean in recent years may have been having positive effects on fisheries production in the region (Caddy, 1997; 2000). This hypothesis, if confirmed, could provide a mechanism to counter the reduction in fishery production due to ecosystem overexploitation, as explained above. In the Mediterranean, the relationship between the overall increase in fishery production and the decrease in the mean TL of catches would be compatible with such a bottom-up effect, as demonstrated by the upward trend of the FIB index relating both parameters, although there are alternative explanations (Pauly, pers. comm.). However, meta-analyses of data from mesocosm-based experiments and natural marine ecosystems from all over the world show a general weak coupling of N loading and phytoplankton productivity with higher trophic levels, implying that anthropogenic nutrient loading is unlikely to result in increased fish biomass, regardless of local conditions and the magnitude of nutrient enrichment (Micheli, 1999). In the absence of a specific study, these conclusions seem to challenge the validity of the former hypothesis.
It could be inferred from the evidence presented above that the effects of fishing in the Mediterranean go far beyond the isolated impacts on overfished target species, vulnerable non-commercial groups or sensitive habitats. The ecosystem effects of fishing in the Mediterranean are also conspicuous at the systemic level, as highlighted by the massive ecological footprint of fishing or the marked effects on the food-web structure. A holistic approach should therefore be adopted if the overall changes to the structure and the functioning of marine ecosystems caused by fishing are to be remedied. These changes directly affect important ecosystem properties such as its resilience to human interference.
There is growing consensus on the potential use of marine reserves or marine protected areas (MPAs) as a precautionary tool for the systemic management of fisheries (Roberts, 1997; Hall, 1998; Lauck et al., 1998; Hastings and Botsford, 1999). The use of this approach in the Mediterranean appears to be promising, given the preliminary results of some limited experiments with marine reserves (see above). The idea of rebuilding degraded ecosystems, mostly through MPAs is gaining support in the scientific community (Pitcher and Pauly, 1998). These authors think the goal should consist not of the conservation of ecosystems in their current states, but rather be to reconstruct past, healthier states that existed prior to their extensive modification by man. This approach would be of particular interest in the Mediterranean given the profound transformation of the marine ecosystems due to centuries of intense human exploitation. As suggested in the respective sections of this document, these precautionary ecosystem-based measures should be accompanied by general improvements in both intra- and interspecific selectivity of gears and fishing practices, the minimization of the physical damage they cause to the supporting environment, and parallel educational programmes for fishermen. Public subsidies diverted to these measures, which in some cases would involve the eradication or tight restrictions on especially harmful fishing practices, would probably result in the improvement of fisheries and their related ecosystems.
In conclusion, a reductionist approach alone may not prove sufficient to tackle the issue of the conservation of Mediterranean ecosystems and their biological diversity satisfactorily. Furthermore, conservation policies targeting vulnerable species or habitats shouldn't be separated from fisheries management policies, given that they have essentially the same goal. From the cases reported in this document it becomes clear that apart from the issues linked to technical aspects, such as those concerning harmful gears or fishing practices, overfishing is a central problem underlying many of the other problems. Many instances have been reported of how intensive fishing exacerbates interactions between vulnerable groups and fisheries. The development and enforcement of integrated precautionary policies appears to be highly necessary, which appeals to the need to develop an operational framework aligned with a truly ecosystem-based fisheries management in the Mediterranean.