عنوان

GCP/RER/010/ITA/MSM-19 (MedSudMed Technical Documents n°19)
Preliminary results on spatial distribution of abundance indices, nursery and spawning areas of Merluccius merluccius and Mullus barbatus in the central Mediterranean

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Preliminary results on spatial distribution of some key variables (abundance indices, recruits and spawner density indices) of Merluccius merluccius and Mullus barbatus populations in central Mediterranean are presented at regional level for the first time. Data were collected during trawl surveys carried out under national programmes (Italy, Libya, Malta and Tunisia) in the MedSudMed Project area (GFCM GSAs 12, 13, 14, 15, 16 and 21). An effort to standardize the data collection and processing was made with the support of the MedSudMed Project. About 290 hauls were made within the depth range 10–800 m in May–August 2003, covering an area of about 150,000 km2. For each haul, spawners were identified as female fish whose total length was greater than the length at 50% maturity (the established common value of 30 cm TL and 15 cm TL for hake and red mullet, respectively). Also for each haul, the proportion of recruits, expressed in terms of young of the year, was estimated by analysis of the length–frequency distribution. In the case of hake, a species that reaches maturity at the age of one year, the spatial distribution of the young fish with a length lower than L50% (juveniles) was also considered. Spawning and nursery areas were identified as areas showing the highest values of spawner and recruit indices, respectively. To avoid bias due to the different sampling gears, all the indices were standardized to the maximum value, keeping separate the areas sampled with different nets. Qualitative comparison of hake and red mullet stocks among the Italian–Maltese, the Tunisian and the Libyan sub-regions, revealed that large differences exist in abundance indices and the demographic structure implicit in these indices. Though differences in the small size-classes may result from differences in gear selectivity and survey season, the contrasting distribution observed in the large size-classes is likely to result from an actual paucity of large specimens in the Italian–Maltese sub-region. Analysis of spatial distribution by life-phase showed that both species are distributed, or migrate, across the boundary of two adjacent GSAs or the boundary of a zone of national maritime jurisdiction and the adjacent high seas. Proposals for future research were made to better understand occurrence of stock units and the effect of environmental factors on them.