Date of Award
Program or Major
Natural Resources and Environment
Doctor of Philosophy
An extensive search of historical data sources and publications has been carried out in different countries of the Mediterranean. This lead to the construction of the largest compilation of historical fisheries information existing in the Mediterranean region. The goal first here was to quantify historical trawling effort. This shows that Mediterranean demersal communities underwent a much longer and more systematic exploitation than previously thought, very likely the longest known exploitation by means of trawls in Europe and North America. Analysis of the data available for the Catalonian, Italian and French areas showed a clearly emerging pattern: fishing capacity increased in Mediterranean EU countries up to and through the 20th Century until the 1980s-1990s, depending on the area. From that period on, fleet size has been decreasing steadily. However, it is unclear whether this decrease in vessel numbers in the last 20 years has been accompanied by a decrease in fishing power and fishing mortality.
Trawl gear was reconstructed with the goal of deriving qualitative and quantitative estimates of increase in fishing power and improved gear performance. The rate of adoption of new technology (synthetic nets, hydraulic winches, navigation equipment, etc.) was reconstructed by area and the effect of these improvements on catch rates was discussed. Analysis of the change in the horizontal opening in trawl nets over time, parameter A1, proved that, with the adoption of new net material and net rigging, the actual size of the net, for the same vessel HP, almost doubled over 40 years.
Reconstructing relative trends in demersal species abundance was one of the primary goals of this project and the intention was to go as far back in time as possible. A first set of analyses was carried out by individual fishing areas/countries with consistent data going back only to 1950. In Blanes, France, the Adriatic Sea and the Sicilian Channel, the drop in biomass was extremely large. In Tuscany the temporal trend since the mid 1960s appears flat but in this analysis the historical data are likely underestimates and a fishing power correction was not used. The second set of analyses pooled all available data together, including LPUEs from sail and steam trawlers from the beginning of the 20th Century and covered the entire western Mediterranean. When LPUE kg/fishing day was modeled, the highest relative biomass was identified in the 1920s with a second lower peak in the 1960s and contemporary biomass even lower. The further back the series was reconstructed, the larger the decline in demersal biomass. This is a quantification of the shifting baseline syndrome: today we are assessing stock solely based on data from the past 20 years, which correspond to the lower part of the trends in all models, so that we have no knowledge of the extent of the decline. A case study was built with data from Catalonia for individual species. Results showed steep declines for red shrimp and blue whiting and important declines for hake and mullets, although for the latter, residual patterns are not optimal.
The overall depleted status of demersal stocks in most West Mediterranean calls for serious management and implementation of credible recovery plans for most demersal stock via adequate reductions of F paired with the establishment of large MPAs. The latter will allow the recovery of vulnerable species that have life history traits that would not be unresponsive compatible with even reduced fishing mortality levels. The incorporation of historical data will be of crucial importance for proper assessment of demersal stocks given the exploitation history as well as for the constructing rebuilding plans.
Osio, Giacomo Chato, "The historical fisheries in the Mediterranean Sea: A reconstruction of trawl gear, effort and trends in demersal fish stocks" (2012). Doctoral Dissertations. 678.