بهره برداری غیر تعاونی از شیلات همگروهی، نقش انتخاب چرخ دنده در شیلات کاد شمال شرق قطب شمال

North-East Arctic cod is shared by Russia and Norway. Taking its multi-cohort structure into account, how would optimal management look like? How would non-cooperative exploitation limit the obtainable profits? To which extent could the strategic situation explain today’s over-harvesting? Simulation of a detailed bio-economic model reveals that the mesh size should be significantly increased, resulting not only in a doubling of economic gains, but also in a biologically healthier age-structure of the stock. The Nash equilibrium is close to the current regime. Even when effort is fixed to its optimal level, the non-cooperative choice of gear selectivity leads to a large dissipation of rents.

The Barents Sea is a rich and productive ecosystem. North-East Arctic cod (Gadus morhua) is by far the most valuable biological resource of this ocean. The fish stock, which is shared by Russia and Norway, is one of the world’s largest populations of Atlantic cod. It is considered to be within safe biological limits ( ICES, 2008) and the Joint Russian–Norwegian Fisheries Commission manages the exploitation of the resource by agreeing on an annual catch quota and on several technical regulations. In spite of this, the resource appears to be over-exploited. Scientific analysis has repeatedly shown that the harvesting pattern is “hugely inefficient” (Arnason et al., 2004, p. 531). Not only have catches and quotas been consistently above scientific advice ( Aglen et al., 2004), but catch by age has also been shifted towards younger age classes with industrial exploitation ( Ottersen, 2008). Here we identify prospective gains from improved management practice and contrast these to the result of a non-cooperative game. How does an optimal management regime look like and how is it limited by non-cooperative exploitation? To which extent could such a strategic international situation explain today’s over-harvesting? In order to answer these questions, three scenarios have been simulated: 1. A continuation of the current harvesting pattern. 2. Optimal management of a hypothetical sole owner who maximizes economic gain. 3. Exploitation from two agents unable to make binding agreements. The first scenario may be interpreted as the outcome where Russia and Norway face constraints from the political process and the behaviour of fishermen. The second scenario represents the first-best outcome that a social planner would employ and where the rents from fishing are divided by some unspecified transfer mechanism. The third scenario constitutes an intermediate case where both Russia and Norway are able to control perfectly their own exploitation but fail to jointly manage the fish stock in an efficient manner. This could be an appropriate description of the strategic situation as cooperative agreements are not enforceable in international relations and the actual harvesting decision is difficult to observe.1 There exists a large literature on the North-East Arctic (NEA) cod fishery (e.g. Hannesson, 1975, Steinshamn, 1993, Sumaila, 1997b, Armstrong and Sumaila, 2001, Sandal and Steinshamn, 2002, Arnason et al., 2004 and Kugarajh et al., 2006). The Russian–Norwegian interactions have been analyzed by Armstrong and Flaaten, 1991, Sumaila, 1997a, Stokke et al., 1999, Hannesson, 1997, Hannesson, 2006 and Hannesson, 2007, but mainly in a cooperative setting. In general, game theory has been fruitfully applied to fishery economics (see Kaitala and Lindroos (2007) for an overview). Although the multi-cohort structure of the stock is taken into account by many analyses, there is, to the best of our knowledge, not any application of a non-cooperative differential game to an age-structured resource. For a general survey of age-structured optimization models in fisheries bioeconomics, see Tahvonen (forthcoming). This is especially relevant as our work shows that the choice of gear selectivity is of paramount importance for the outcome. In fact, that the minimum size of fish could be a control dimension of great consequence has generally been overlooked so far, in spite of the early result from Turvey (1964, p. 74), that “either mesh regulation or the control of fishing effort is better than nothing but that regulation of both is still better.” Another important feature of this study is that it rests upon an ecological model which has been derived through statistical analysis of time-series data from the Barents Sea system (published in Hjermann et al., 2007). The economic model is essentially a simplified version of the one employed in Diekert et al. (2009). In order to highlight the effects of non-cooperative exploitation, we have concentrated on one gear type (trawl) and have made the players Russia and Norway symmetric. Because the state of the fish stock and the agent’s exploitation decisions are only imperfectly observable, we postulate an open-loop information pattern and aim for Nash equilibria of this kind. A procedure that finds stable equilibria by iteratively updating best responses has been designed. By this interdisciplinary approach, we are able to point out that the gains from optimal management could be substantial. In particular the choice of a larger mesh size than currently employed is taking the individual growth potential of the fish into account. However, the agents fail to do precisely this in a non-cooperative game. Rather, the nets are tightened to catch the fish before the respective opponent does. The outcome of the non-cooperative game is indeed close to the simulation of the current harvesting pattern. The article proceeds as follows: Section 2 develops the bio-economic model, Section 3 discusses the simulation approach, results are presented in Section 4, and Section 5 concludes.

Optimal management of the North-East Arctic cod, which takes the age- and gear-specific effects of harvesting decision into account, would lead to more than a doubling of the current economic gains while at the same time resulting in a much healthier fish stock. In contrast, a situation where two nations, each completely controlling their harvest, exploit the resource non-cooperatively would lead to a large loss of resource rents. Instead of a NPV of 116 billion Kroner, only a NPV of 67 billion Kroner could be earned over the next 50 years by each agent. An effort which is too high, and in particular a mesh size which is too small, implies a serious overuse of the resource. Its replenishing potential and the individual fish growth is not taken into account properly, a result which is remarkably similar to the current harvesting regime. Viewed in this light, it seems fair to conclude that today’s inefficiency is largely due to the strategic structure in the Barents Sea. Long-term forecasts of different management options are sensitive to a complex web of environmental (biological and economic) factors whose changes cannot be predicted for all practical purposes. Hence, these results are not to be taken as actual predictions of the future state but as comparisons of alternative management scenarios, provided all other things remain equal. Table 5 summarizes the results, where the steady-state values for the respective choice and state variables are reported for one of the two symmetric fleets. Table 5. Summary of simulation results. Status Quo SoleOwner-Em Game-Em NPV (billion NOK) 55 116 67 Harvest (thousand tons) 392 647 418 Effort (million units) 11 9.7 10.8 Mesh size (mm) 135 206 139 Stock biomass (thousand tons) 2493 7468 2751 Table options Note however, that the result that the Joint Commission agrees on what would have been the outcome even in absence of any channel of communication does not mean that the existence of the Joint Commission is superfluous. Quite to the contrary, the Commission serves many other purposes as well. It provides stability in an essentially unstable environment and most importantly, it establishes a platform from which measures that improve on the current situation might be taken. The age-structured modeling revealed that a significantly enlarged mesh size is key to enlarging the economic gain from the fishery. Focusing on this relatively simple measure might be more rewarding than trying to come to an agreement about fishing effort (Turvey, 1964). In general, the analysis highlights the importance of age- and gear-specific modeling in fishery economics. The large gains of optimal management were possible because essentially the right fish were targeted while a non-cooperative and seemingly today’s harvesting regime fail to do exactly this. An analytic understanding of the role of age-structure and gear selectivity for optimal and non-cooperative exploitation should shed new insights into the possibilities and limits to the management of today’s marine resources. Exploring this potential will be the theme of work to come.