An ecosystem dynamic model for the valuation of externalities of fishing activities

There is a growing interest in summarizing, in economic terms all the variables that are referred to as ecological goods and services. The challenge of economic valuation of environmental goods, services and function relies on the understanding of the environmental feedbacks that markets do not account. In this study a mass-balance model and an energy based methodology for economi c valuation are applied to the case study of the Venice Lagoon, in order to quantify some of the externalities of fishing activity. Artisanal fishery in the Venice lagoon is a multitarget activity with a long tradition. It was the main fishing activity till the late ’80s when, after the introduction and spread of the Manila clam (Tapes philippinarum), the mechanical clam harvesting started. A mass-balance model of the lagoon ecosystem was developed using the Ecopath with Ecosim software. 80 scenarios were obtained by changing the fishing effort of the two different types of fishery with the use of Ecosim dinamic model. Those scenarios were used to explore the impact of the fishing activity on the ecosystem. A set of indicators was applied in order to compare the two fishing activities. The results obtained showed that the two activities are strongly interlinked, even if they don’t exploit the same resources. The mechanical clam harvesting could reasonably be considered the driving force; it is capable of determi ning the state of the lagoon ecosystem. The possibility of reporting those eclogical linkages in economic term can represent a step towards a sustainable management of the fishery resources in the Venice Lagoon. Introduction From ancient times, fishing has been a major source of food for humanity, and a provider of employment and economic benefits to those engaged in this activity (FAO, 1995). But the exploitation of a common-property, like fish, is revealing to be unsustainable as is shown, on a global scale, by the phenomena of stock depletion (Botsford et al., 1997), reduction of the mean Trophic Levels in the catches (Pauly et al., 1998), and marine habitat disturbances (Hall, 1999). Notwithstanding all the above, when the correct procedures are not in place, the fishing industry is driven to search for new technologies, thus producing an intensification of the fishing effort. Consequently fishing vessels are becoming larger and faster, are using more expensive types of technology and are catching fish in shorter periods of time, thus enhancing the gap between sustainability and fishing activity (Mathew, 2001). These factors are producing an increasing number of people involved in the exploitation of marine biological resources, who lack training, experience and skills. This gap can result in a lack of ‘traditional ecological knowledge’ leading to an unsustainable exploitation of the living resources. On the contrary, artisanal fishery requires more experience and is usually based on a strong link between fishermen and the ecosystem, all these drive to a sustainable exploitation activity. The coexistence of technological and artisanal fisheries can also generate conflicts concerning space and resources, (Allison and Ellis, 2001; Mathew, 2001). In coastal areas, where small-scale and artisanal activities are particularly rooted (FAO, 2000), conflicts between new and old types of fishery can even be bigger. These modifications in fishery structures drive changes at an economic and social level (FAO, 2000; Ruttan et al., 2000; Sumaila et al., 2001) other than at an ecological level; they can result in conflicts between “old” and “new” activities. The artisanal sector is particularly vulnerable, as it often depends on the use of gears, which are incompatible with towed gears, such as those used by industrial trawlers. The international framework of policy regulations is giving greater interest to coastal resources and fishery conflicts in order to enforce the sustainable development of human activities. Coastal communities and their customary practices are accorded special recognition by the Code of Conduct for Responsible Fisheries (FAO, 1995) where explicit suggestions are also given in order to obtain the protection and rehabilitation, nursery and spawning areas, in so far as it is possible. The importance of artisanal and small-scale fishery to employment, income and food security is also recognised in the above mentioned Code. Therefore, fishery management has to take into account not only sound research concerning the ecosystem, but also the socio-economic components of the system. Indicators for the ecological, economic and social effects of fishery are demanded, and a new interest in environmental changes, rather than merely in stock changes, is required (Anonymous, 2000). These indicators could be used as a basis for the evaluation of fishing pressure, and applied in fishery management, in order to create integrated policies, characterised by the combinination of the principles of fisheries and ecosystem management, under the shield of sustainability. The indicators have, therefore, to include a reference direction, allowing for the prediction of whether the indicator will increase or decrease due to exploitation (Rochet and Trenkel, 2003). In such a complex framework, a potential core set of indicators have been developed within many national and international organisations, with the aim of describing the driving forces, pressures, states, impacts on and responses of the ecosystem to fishing activity (Zenetos et al., 2002). However, the economic and social patterns also have to be clarified, in order to achieve an understanding of all these pressures and for the correct management tools. Fishery in the Venice Lagoon – case study The Venice Lagoon is a sensitive area subjected to different kinds of anthropogenical pressures, from industrial activity to resource exploitation. With regard to this last aspect, small scale fisheries have a long tradition also in terms of management (Granzotto et al., 2001) but the introduction, in the middle of the 80s, of the Manila clam (Tapes philippinarum) induced major changes concerning ecological, economic, and social dimensions of the lagoon. From the perspective of sustainable exploitation, a greater effort has to be made to define and apply management strategies which can assure the sustainable development of fishing activities, and the coexistence of the two types of fishing activities, in an environment as crucially important as the Venice Lagoon. According to Mathew (2000), the definition of artisanal fishery can be based on different categories: the social, environmental, and technological features, the boat size, and the fishing ground size. As regards the Venice Lagoon, the small-scale fishery can be defined as artisanal, because of the strong link between the fishermen and the environment, resulting from centuries of traditions rooted in the past. This traditional knowledge led, to the utilization of more than 25 types of fishing technique up to the middle of the 20 century, (Granzotto et al., 2001). At present, only two kinds of static fishing gear are still used. Artisanal fishermen target a wide range of species, including both residents and migrants, depending on the seasons, the fishing grounds and the tide (Mainardi et al., 2002). On the contrary, mechanical clam harvesting is a monospecific fishing activity, carried out using of small boats with 25HP engines, positioned outboard amidships (Fig. 1). The fishing grounds are shallow water areas, where the propeller can reach the bottom, resuspending the sediment and the clams, which are then collected in the net. This type of boat is also equipped with a 300HP engine, for the purposes of reaching the fishing grounds located in the whole lagoon. According to Sacchi (2001), the Mediterranean fisheries are described as mainly small-scale type. They are comprised of small enterprises, with little capital, run by artisans who often own the production tools (vessel plus fishing gear) and to a certain extent they also control the commercialisation network regarding this product. Both the fishing techniques considered here can be classified as small-scale. However, the mechanical clam harvesting can also be classified as ‘industrial fishery’, as this classification is used when other factors, such as the fact that these vessels catch only one target species, the high level of technology used in the process are taken into account. Fig. 1 – The location, distribution and estension of the fishing grouds relatively to artisanal and mechanical clam fisheries in the Venice Lagoon. These two different fishing activities could be located at the two extremities of more than 45 types of fishing techniques described for the Mediterranean Sea (Sacchi, 2001). Nevertheless, these two methods are those principally used here: i.e. passive (fyke net) and active (clam dredge). However, major differences can be seen regarding potential impact of the gear, e.g. the interaction with the bottom morphology is the highest possible regarding the clam dredge (which produces a 7-10 cm deep track) and is totally absent when using a fyke net. As stated by Link (2002), it is doubtful whether we are “attempting ecosystem management in a fisheries context or fisheries management in an ecosystem context”. At present, in the Venice Lagoon, given the complete absence of a real fishery management (mainly with respect to mechanical clam harvesting), the first hypothesis has been assumed to be realistic. But, as is also highlighted in Pranovi et al. (2003a), clam harvesting can be seen to be totally unsustainable, and recent evidence, such as a sharp reduction in clam production (about 40%, Boatto et al., 2001), seems to confirm this hypothesis. In this context, a change in perspective is needed, which will introduce an ecosystem-based type of management, capable of ensuring the maintenance of the ‘ecosystem health and sustainability’ (NMFS, 1999) or the ‘ecosy