Assessment of the Exploitation Status of the North Aegean Sea (Greece) Using A Mass-Balance Model

Mediterranean fisheries are multispecies and multi-gear in nature with numerous small-to-medium size boats, creating difficulties in management.Ecosystem models are useful tools for the study of the interactions among ecosystem compartments, including fisheries.  Meta-analysis of model results has contributed in the development of novel exploitation indices and proposition of reference points.

An Ecopath mass-balance model was developed for the North Aegean Sea ecosystem (Eastern Mediterranean, Greece) in order to assess fishing impacts and exploitation status of the ecosystem. The model area, despite the fact that it is an oligotrophic region, is one of the most productive areas in the Eastern Mediterranean which is reflected in the high relative fishing catch. The ecosystem model was built for the period 2003-2006 and was restricted in the continental shelf where most fishing vessels operate. In total, 40 functional groups (FGs) were defined including pelagic and demersal fishes, several benthic invertebrates, dolphins, turtles, seabirds, detritus and discards. Five fleets were included: trawls, purse seines, static nets, longlines and pots.  Fisheries data was collected with high regional and temporal detail. Species-specific and fleet-specific discards/marketable ratios were used to estimate the amount of discards generated in an annual basis.

Fisheries were responsible for 22% of the consumption of fished production, excluding plankton and benthic invertebrates, similar to the other Mediterranean regions. The primary production required to sustain the fisheries, taking into account a. only the primary production and b. the primary and detrital production, was lower than other Mediterranean regions and than the mean global average for shelf ecosystems. The mean trophic level of the catch was higher than other regions. Despite the fact that these two metrics suggest that the ecosystem is sustainably fished, a series of novel exploitation indices point that the ecosystem is under a regime of high exploitation. The probability of the fisheries being sustainably fished (P_sust), calculated based on the Loss in secondary production index (Lindex), was low (a. 70,5%, b. 44,7%), similar to S. Catalan and N.C. Adriatic Seas. In order to achieve 75% and 95% probabilities of the exploitation to be sustainable, catches should be reduced 23% and 72% respectively.

The N. Aegean Sea compared to the S. Catalan Sea and N.C. Adriatic Sea is slightly less exploited. A further standardization and comparison of the different ecosystems inside and outside the Mediterranean could be useful for the exploration of gradients in exploitation patterns and the proposition of reference points.