Localization of nursery areas of Parapenaeus longirostris (Lucas, 1846) inthe central-southern Tyrrhenian Sea by geostatistics

Studying the spatial correlations of the distribution of juveniles and thei r relative abundance is the first step in assessing and localizing the nurs ery areas of demersal resources. These data can provide useful information both for a better understanding of the biology of the deep-water rose shrim p Parapenaeus longirostris and for suggesting advice to fisheries managemen t. Data were collected in the central-southern Tyrrhenian Sea during bottom trawl surveys carried out since 1994, using a random, stratified sampling design. In order to describe the nursery areas, the < 20 mm fraction of the length frequency distributions (carapace length) of the species was employ ed. The experimental variograms were constructed on the variable named "abu ndance index of recruits" (individuals/km(2)). The model best describing th e spatial continuity was detected and validated by the jackknife technique. An exponential model was fitted with ranges describing small scale aggrega tions (16.03 km in 1996 and 19.94 km in 1997). Studying the nursery areas, the conditional probability of exceeding a given threshold value of the var iable was estimated by non-linear geostatistics, namely the so-called disju nctive kriging technique. Two threshold values were adopted (2000 and 3000 individuals/km(2)) and four nursery areas were localized at the lower level of threshold value. When the more restrictive cut-off value (3000 individu als/km(2)) was used, only one nursery area remained dominant. This informat ion might comprise a distinct indication for management. Thus, the protecti on of nursery areas, through limitation of the fishing pressure throughout the year or in fixed periods, could be considered as an effective, compleme ntary regulation tool for a short-lived species like P. longirostris.