Medits 1998 - Technical and Biological Report for the Region M3 (Italian coasts - South Tyrrhenian Sea and Strait of Sicily)

The results of the Medits survey carried out in the South Tyrrhenian Sea and in the Strait of Sicily (Region M3; Zone 1.3.4) in the period 21st May - 28th June 1998, are presented and discussed. The MEDITSIT 1998 survey (hereby defined as MED'98) was conducted, as the previous years, using the professional stern trawler "S. ANNA" and the MEDITS gear. A total of 141 hauls was carried out according to the calendar reported below: Sub-regionCodeNN° of haulsSurvey period South Tyrrhenian Sea: Volturno River - Cape SuveroM3a5721st May-5th June South Tyrrhenian Sea: Cape Suvero - Cape San VitoM3b286th June-12nd June Strait of SicilyM3c5613th June-28th June Mean abundance were estimated and length frequency distributions (LFD) were computed by IndMed 4 program and analysed for the 31 Medits target species; before analysing data, errors were checked by CheckMed program. According to the MEDITS "Manual of protocols", the overall range of the length frequency distribution are expressed as total length (TL; cm), carapace length (CL; mm) and mantle length (ML; cm) for fish, crustaceans and cephalopods, respectively. Practically, the observed catches, both in number (N) and weight (kg), are referred to a standardised area of 1km2, and hereby expressed as "Density index" (DI= N°/km2) and "Biomass index" (BI= kg/km2) by each stratum (sub-regional and regional level), and for shelf (10-200m; i.e., 1st, 2nd and 3rd stratum combined; hereby defined as shelf-index) and slope (200-800m; i.e., 4th and 5th stratum combined; hereby defined as slope-index) bottoms (at regional level) and for the overall area. Results will be mostly presented and discussed at sub-regional (sub region or, equivalently, sub zone) level; in fact, it is well known that the specific representativeness as well as the abundance indices may be influenced by the sampling period, and may change when species depth and space distribution is considered. For example, catches of Mullus surmuletus and Raja clavata were recorded, almost exclusively, in the Strait of Sicily (Sub zone M3c), being these species rare or lacking at all in the other two sub zones (M3a and M3b). On the other side, Pagellus acarne and Pagellus bogaraveo still represent an important catch in the South Tyrrhenian Sea (especially in M3b), whereas negligible yields generally resulted in the Strait of Sicily. Given the heterogeneity of the three M3 sub regions, and the lack of joint comparative analysis, the abundance estimates and the overall length structure at the regional level must be considered as indicative only. Furthermore, it must be pointed out that the abundance estimates at regional level for those species showing restricted bathymetric distribution (such are the cases of the deep water red shrimps and common octopus) underestimate the true "biological" abundance, given the inclusion in the computations of the strata where the species has never occurred. Splitting abundance between shelf and slope bottoms partially reduces this problem. Nevertheless, inter years comparison can be obviously done given the invariance of the statistical design and strata sampling frequencies. A total of 228 different species (159 fish, 33 crustaceans, and 36 cephalopods) was identified in the catch. Among the first 31 species of the reference list (hereby defined as "target" species, or more simply targets), 30 were represented in the catch, in one sub region at least. It is worth to note that no specimen of Eutrigla gurnardus was caught in the MED'98, whereas only one specimen had been identified in the previous survey (MED'97). Considering the sub regions, only two target species (Solea vulgaris and Raja clavata) were not caught at all in M3a and M3b, in the present survey; furthermore, the common cuttlefish (Sepia officinalis) was not recorded in the catches of the M3b sub region. Considering the new catches in M3, only two records (Diaphus metopoclampus Cocco, 1829, sin. Aethoprora metopoclampa, and Arnoglossus kessleri) were added to the "FM list", following the procedure indicated in the "Manual of protocols". For sake of simplicity, only the numeric values of the mean abundance, both as density index (DI) and biomass index (BI), as previously defined, are hereby presented and discussed; the presence of the decimal in BI (whereas DI is an integer) allows an immediate distinction between the two indices. When some confusion may arise, the figures are followed by the corresponding units, kg or N, but 1/km2 is always omitted. Corresponding coefficient of variation (CV=SD/Mean).