مدیریت پرتفولیو سهام ماهی وحشی

Managing fish stocks in terms of a portfolio of economic assets is likely to significantly increase benefits for society relative to single-species approaches. A portfolio framework systematically combines fish stocks that are joined by ecology (e.g., predation, competition) and unspecialized fishing technologies (e.g., mixed-species trawls) into a portfolio which balances expected aggregate returns against the risks associated with stock-attribute and other uncertainties. To be productive, however, this framework must be combined with property rights institutions that clearly state management objectives, create long-run time-horizons among harvesters, internalize spillovers caused by ecological and technological jointness, and reduce uncertainty through research and adaptive management. Although the cost of reducing scientific uncertainty about ecological interactions may limit the portfolio approach to intensive management of relatively few species, its scope can be broadened to integrate tradeoffs among more types of marine resources, such as nature preserves and oil and gas deposits.

The predominant approach that governments use to regulate harvests of finfish and invertebrate resources treats species in isolation from each other. In the United States, the Magnuson-Stevens Fishery Conservation and Management Act (Magnuson-Stevens Act) hints at multi-species management of stock complexes and the prey of managed species, but, in practice, overfishing, bycatch, and optimum yield are defined in terms of maximum sustainable yield (MSY) from individual stocks. The so-called single-species approach to fisheries management conflicts with policy advice since the 1960s, however. Primary production probably is not sufficient to support all fish stocks in an ecosystem at estimated MSY levels (Brown et al., 1976). Further, there can be an infinite number of MSYs for each stock which are contingent on the populations of other species in the community and environmental states Larkin, 1966, May et al., 1979, Pontecorvo, 1986 and Steele, 1998, making MSY and its economics counterpart, MEY, short-run policy objectives. Unspecialized fishing technologies that harvest several species jointly similarly affect MSY and MEY steady states Quirk and Smith, 1970, Anderson, 1975 and Huppert, 1979. As a result of these considerations, many prominent biologists, mathematicians, and economists have spoken strongly against using MSY in policy, including Larkin (1977) who mocked the concept while it was being institutionalized in the statutory laws of several countries. May et al. (1979, p. 275) concluded from the results of multi-species models “that simple considerations of MSY, species by species, are insufficient for enunciating management principles”. Clark (1976, p. 1) began his popular book about mathematical bioeconomics stating that “the MSY concept is, in many respects, far too simplistic to serve as a valid operational objective for the management of most living resource stocks”. Finally, Arnason (1998, p. S151) commented “that in virtually all fisheries, single-species analysis is liable to lead to serious mistakes in the interpretation of the observed data, not to mention in policy recommendations and predictions.” Recent promising research on system models of fish communities and fisheries (e.g., Pope, 1991, Walters et al., 1997 and Arnason, 1998) underscores difficult questions that have been sidestepped by single-species policies, namely what is the appropriate mix of species and population sizes in a managed ecosystem, how do we manage environmental and stock-attribute uncertainty affecting species complexes, and what sorts of institutional arrangements can operationalize multispecies management. There is no reason to expect biological yield objectives to coincide with society's preferences for food or economic growth (e.g., Clark, 1976), including variations on the single-species approach which avoid extinctions of slow-growing species and top predators while maximizing assemblage yield (see Tyler et al., 1982). This view is widely held by social scientists and harvesters, and is shared by some fisheries biologists (e.g., Larkin, 1977 and Hilborn et al., 2001). Indeed, Gulland (1982, p. 8) was blunt: “[T]he scientific community, as represented by ICES, has no special responsibility or competence in deciding what objective should be chosen”. Likewise, Beddington et al. (1984, p. 236) stated “that efforts to partition and reduce the ‘fishery problem’ to its biological essentials are misguided. And, by extension, the ‘solutions’ thereby determined and presented by biologists are too often suboptimal—if not irrelevant—to the actual needs of those trying to make the best use of the resource.” The notion of optimum yield as defined in statutory laws such as the Magnuson-Stevens Act does not help to resolve the question of objectives because in addition to being conditional on MSY, it is too vague to be operational and evaluated (e.g., Clark, 1976). Our paper builds on Hanna's (1998) thought to reorient fisheries management from single species to portfolios of species. Unlike the related diversification of harvest opportunities for individual fishermen recently advocated by Hilborn et al. (2001) as a response to random recruitment and serial depletion of fish stocks, our use of portfolio theory explicitly recognizes fishery resources as risk-bearing capital assets that can provide society with benefits indefinitely. That is, strategies that accommodate depletion of valuable fish stocks are counterproductive under the portfolio approach. Our application of portfolio theory to multi-stock management also differs from the small literature in fisheries economics which has to do with harvesting a multicohort stock (Baldursson and Magnusson, 1997) and seafood product diversification (Larkin et al., in press). The portfolio approach to multi-species fisheries management has two complementary and necessary parts. One is a framework which systematically evaluates tradeoffs in fishery benefits that result from ecology (e.g., predation, competition) or unspecialized fishing technologies (e.g., mixed-species catches), and balances the expected aggregate benefits from manipulating stock levels against the risks associated with various natural, market, and institutional uncertainties. The second part designs property rights institutions that create long time-horizons among harvesters which are necessary for fish stocks to become economic assets, that resolve common pool spillovers (e.g., “your fish eats my fish”) cost-effectively, and that protect investments in information which reduce uncertainty. These two components of the portfolio approach are described in turn below and followed by a discussion of factors that will affect implementation.

Dramatic shifts in community biomass and species composition occur naturally in marine ecosystems, but they are accentuated in rule-of-capture fisheries whereby harvesters sequentially discover, develop, deplete, and abandon the most valuable fish stocks (Hanna, 1997). Many important fish stocks have collapsed at times, such as Atlantic herring during the 1960s and 1970s and, remarkably, cod more recently in Canadian, U.S., and European waters. Other stocks, such as Atlantic halibut (Hippoglossus hippoglossus), have become economically extinct. The scope of similar losses has been documented recently by Pauly et al. (1998) who described a trend of “fishing down the food web” worldwide—i.e., the gradual transition from stocks of long-lived, high trophic level, piscivivorous bottom fish toward short-lived, low trophic level invertebrates and planktivorous pelagic fish. Annual returns from communities of finfish and invertebrates are too variable and risky when the most valuable assets are depleted opportunistically. Regulatory institutions that are founded on the single-species approach have exacerbated the economic problems as discussed above. In response to this state of marine fisheries, we recommend the portfolio approach as a conceptual model to optimally combine stocks of wild fish species that interact ecologically, are caught jointly, and are characterized by stock-attribute uncertainty. Unlike financial portfolio analysis where the decision concerns which assets to add to the set, the portfolio approach to multi-species management balances the biomass and other attributes of species through subtraction. That is, populations of low-value or high-risk species are candidates for selective depletion in order to balance the expected aggregate returns of a fishery portfolio against return variability, given clearly defined and measurable objectives and biological and ecological safeguards. The portfolio framework requires compatible institutions to be implemented, however. Property rights institutions should be designed to create the necessary incentives for harvesters (or other claimants) to value fish stocks as economic assets over long time-horizons, to internalize spillovers caused by joint ecological interactions and harvest technologies, and to reduce environmental and stock-attribute uncertainties. The dearth of examples of adaptive multispecies management worldwide—such as removal of large carnivorous fish off Spain to create a shrimp fishery (May, 1984) and experimental management of an Australian multispecies fishery (Sainsbury, 1991)—is indicative of extant management institutions that are not compatible with integrated management approaches. Wild fish stocks are not fully developed economic assets in government regulatory regimes because the wedge between the harvest rights and privileges which are granted to producers and the stock rights which typically are reserved by governments causes fishermen to discount long-term benefits from most stock-attributes, including predation or other ecological interactions. The portfolio approach addresses this problem systematically, but adopting it is problematical from a public policy standpoint. It is not likely that governments could legislate the portfolio approach into existence due to the very high contracting and political costs of redefining harvest arrangements among the many competitive stakeholders in fisheries. Commons (analogous to a mutual fund) or corporate (private stock) institutional arrangements are a society's best bet to unify stock management decisions in beneficial ways because the opportunity costs of ignoring spillovers and other interactions are felt personally (Edwards, 2003). The likelihood of these alternative governance and property rights arrangements coming about are akin to the Berlin Wall being dismantled—i.e., very small at any time, but not negligible under the right circumstances. A weak theoretical foundation under the single-species approach and the potential for quite large economic gains favor valid alternatives such as the portfolio approach some day if we prepare for it. Being a different way to think about fish stock management, the portfolio approach cannot, of course, be completely developed and illustrated in one paper. There are many areas that need investigation, including the data and methods of environmental accounting applied to wild fish stocks (e.g., Nordhaus and Kokkelenberg, 1999 and Repetto, 2002), the empirical application of portfolio theory and techniques to the choice of fishery assets and their attribute levels (i.e., estimating efficient frontiers; see Elton and Gruber, 1995), and the evolution of compatible property rights and governance arrangements from government regulatory regimes (e.g., comprehensive assignment of secure harvest rights followed by contracting into centralized harvest arrangements; Edwards, 2003). Case studies that build on Gulland's (1982) inquiry into North Sea fisheries (e.g., simulations of actual fisheries, management experiments) can bring these elements together. In ending, we consider the likely scope of a multi-species portfolio of wild fish stocks which is confronted with complex trophic structure and poorly understood ecosystem dynamics. Any institution which is designed to undertake the portfolio approach would weigh the costs of new information against its potential benefits, manifest in greater returns and risk reduction. On this basis, we would expect portfolio managers to intensively manage fewer species than are currently exploited, and to modify the environment in ways (e.g., containment pens, artificial habitat, predator control, etc.) that capitalize on more stock-attributes than biomass and size structure (e.g., population genetics). This result was foreseen by Silvert and Smith (1977) and Mendelssohn (1980) who discussed the public policy implications of results from complex multi-species models, although not in a portfolio context. It implies an evolution towards ocean aquaculture and sea ranching (Anderson, 2002) as now found in the coastal waters of Japan (Ruddle, 1987). Simplifying ecosystem structure through intensive management appears to be in conflict, however, with peoples' complex preferences for nature preserves as well as income and with concerns about ecosystem functions and resilience (Arrow et al., 1995). There is currently a movement to create a network of reserves in the world's oceans (Sala et al., 2002), including within the U.S. Extended Economic Zone (Airame et al., 2003). Portfolio theory could be applied here, too, though, with the objectives measured in terms of integrated ecological attributes (see Airame et al., 2003). The sizes and locations of protected areas would be selected on the basis of expected ecological benefits and risks. At some point, however, tradeoffs between possible preserves and economic portfolios should be evaluated without jeopardizing the safe minimum standard protections against risk of extinction for individual species. Indeed, we can imagine an overarching ocean portfolio that includes oil and gas reserves, hard minerals, ocean space (e.g., shipping), and other assets besides living marine resources. The property rights and governance institutions that could decide the tradeoffs in a society's best interest is an interesting question that will not be tackled here. This general scenario is mentioned only to highlight the versatility and utility of the portfolio approach in providing a systematic method for integrating the work of scientists, economists, and policy makers. It is entirely consistent with the observations and conclusions stated in the U.S. Commission on Ocean Policy's April, 2004 preliminary report on the state of the oceans.