Yield per recruit and spawning stock biomass models for the management of four Mugilidae species in Mesolonghi –Aitoliko lagoon (W. Greece)
George Katselis, Katerina Koukou, Dimitrios Moutopoulos
Department of Aquaculture and Fisheries Management, TEI of Mesolonghi, Nea ktiria 30200, Greece
The fishery exploitation of the most of Mediterranean lagoonal systems is a common extensive culture, based on the seasonal fish migration from sea to the lagoon and the summer-to-winter offshore fish migration. The Mugilidae group of species represents more than 50% of the total fishery landings in the Mesolonghi-Aitoliko lagoon system. Spawning stock biomass and yield per recruit models were developed for the four Mugilidae species (Liza saliens, L. aurata, L. ramada and Mugil cephalus) to estimate their exploitation level. Growth parameters (asymptotic length, L∞ and growth rate, K, W-L relationships) and fecundity estimations were gathered from past surveys in the study area. Moreover, length frequencies data from four species caught from fish traps during 1992, 1994, 1998 and 2004 were also used. The results indicated that: (1) the exploitation of all species was occurred at first capture lengths that related to maximum yield per recruit, (2) for the most important fish species (L. aurata, L. ramada and M. cephalus) high values of exploitation rates (ranged from 0.66 to 0.87) was imposed and (3) the spawning stock of L. aurata and L. ramada were out of safe levels. At first view the results can be explained by the continuously decreasing landings of the exploited species in the Mesolonghi–Aitoliko lagoon during the last two decades. However, in the case of lagoon the models embodied both real and phenomenal total mortalities which could be explained an overestimate of total mortality in present study. Despite the above uncertainties a part of the above results could be attributed on a long-term effect of human actions (i.e. increasing of marine aquaculture) on coastal zone which modify the spatial fish distribution and increase fishing mortality in the sea species populations.
Keywords: Yield per recruit, Spawning potential ratio, Management, Lagoon, Mullet, Mesolonghi