Mediterranean Fishing Strategies and interactions Between Small Scale Fisheries and Trawlers in the Mediterranean Hake Fishery

Mediterranean fisheries are characterized by a high diversity of fishing strategies that can be differentiated according to a gradient from the coast to the offshore area. Over the gradient, fleets interact through the resource. The paper first provides a description of the fishing strategies and proposes a typology for the French Mediterranean fleet based on this gradient. A focus on the interactions between fleets is then provided through the example of the Mediterranean hake fishery. In this fishery, two main strategies, small scale gillnetters operating in the 3 milles area and trawlers operating more offshore, compete for the resource. A description of these two segments is provided and the management implications of the technical interactions are highlighted.Keywords: Fisheries Economics, Fish and Aquaculture Sectors Development, Small Scale FisheriesKeywords: Fisheries Economics, Fish and Aquaculture Sectors Development, Small Scale Fisherie

Investigating tradeoffs in alternative catch share systems with a vessel-based bio-economic model

A vessel-based bio-economic model (IAM) is presented and applied to the Bay of Biscay sole fishery to investigate alternative quota management systems from a multi-criteria perspective. For this study, the model integrates several institutional arrangements related to catch share management. The current French co-management system with non-transferability of quota is compared to an alternative ITQ system in a context of transition to maximum sustainable yield (MSY). Trade-offs between ecological and socio-economic impacts are highlighted and the effectiveness of governance scenarios is discussed in regard to the challenge of capacity adjustment. Results emphasize that the introduction of ITQ is expected to reduce by 40% the number of vessels in the fishery. While effectively mitigating the economic impacts of the transition phase to MSY, ITQs are also expected to significantly increase the fishing effort by trawlers, which may cause ecological concerns. The scenarios tested also include the simulation of a decommissioning scheme where subsequent decommissioned vessels are significantly different from the vessels that would lease out their quotas in an ITQ system, resulting in differentiated ecological and socio-economic impacts between scenarios

Addressing the Distribution of Bio-Economic Impacts of Management Measures Between Fleets: the Case of the Demersal Fisheries in the Bay of Biscay

Assessing distribution of the expected bio-economic impacts of management measures between fleet segments is a main issue for decision making in fisheries management. This requires as a first step a good description of the system and of the interactions between fleets through stocks. Making this assessment operational also needs flexible and fast capabilities to mobilize appropriate data to perform the impact analysis. Bio-economic modelling of the system allows performing simulations of management scenarios and to analyze expected costs and benefits at short, middle and long terms. The paper addresses the question of differentiated impacts on fleets in the case of the demersal fisheries in the bay of Biscay. As mixed fisheries, they are characterized by high technical interactions mainly derived from targeting or by-catching hake, Nephrops, anglerfish or sole and are relevant illustrations to address this question. The fleets involved in the Nephrops, hake, sole and anglerfish fisheries in the bay of Biscay are first characterized in terms of contributions to fishing mortality of these species and gross revenue dependence and their activity, productions and economic profitability are described. The paper then analyzes the impacts of effort and selectivity measures for these fleets through a multi-fleets, multi-metiers, multi-species bio-economic model. Methodological issues are underlined for operational bio-economic modelling and management plan assessment.Keywords: Fisheries Economics, Bioeconomic Analysis of Fisheries, Fish and Aquaculture Sectors Developmen

Subducting slabs: jellyfishes in the Earth mantle

International audienceThe constantly improving resolution of geophysical data, seismic tomography and seismicity in particular, shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle. This observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle, where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle. The distribution and character of seismicity reveal, however, that slabs undergo vertical extension in the upper mantle and compression near the transition zone. In this paper, we demonstrate that during the subduction process, the shape of low viscosity slabs (1 to 100 times more viscous than the surrounding mantle) evolves toward an inverted plume shape that we coin jellyfish. Results of a 3D numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head. The head is linked to the body of the subducting slab by a thin tail. A complete parametric study reveals that subducting slabs may achieve a variety of shapes, in good agreement with the diversity of natural slab shapes evidenced by seismic tomography. Our work also suggests that the slab to mantle viscosity ratio in the Earth is most likely to be lower than 100. However, the sensitivity of slab shapes to upper and lower mantle viscosities and densities, which remain poorly constrained by independent evidence, precludes any systematic deciphering of the observations