LOCALIZATION OF NURSERY AREAS OF PARAPENAEUS LONGIROSTRIS (LUCAS, 1846) IN THE CENTRAL-SOUTHERN TYRRHENIAN SEA BY GEOSTATISTICS

Studying the spatial correlations of the distribution of juveniles and their relative abundance is the first step in assessing and localizing the nursery areas of demersal resources. These data can provide useful information both for a better understanding of the biology of the deep-water rose shrimp Parapenaeus longirostris and for suggesting advice to fisheries management. 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 employed. The experimental variograms were constructed on the variable named ''abundance index of recruits'' (individuals/km2). The model best describing the spatial continuity was detected and validated by the jackknife technique. An exponential model was fitted with ranges describing small scale aggregations (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 variable was estimated by non-linear geostatistics, namely the so-called disjunctive kriging technique. Two threshold values were adopted (2000 and 3000 individuals/km2) and four nursery areas were localized at the lower level of threshold value. When the more restrictive cut-off value (3000 individuals/km2) was used, only one nursery area remained dominant. This information might comprise a distinct indication for management. Thus, the protection of nursery areas, through limitation of the fishing pressure throughout the year or in fixed periods, could be considered as an effective, complementary regulation tool for a short-lived species like P. longirostris. L'etude des correlations spatiales de la distribution des juveniles et de leur abondance relative represente le premier pas dans l'estimation et la localisation des aires de nursery des ressources demersales. Ces informations peuvent etre tres utiles pour comprendre la biologie de la crevette rose d'eau profond Parapenaeus longirostris et pour fournir des suggestions dans la gestion des pecheries. Les donnees ont ete collectees dans le centre-sud de la mer Tyrrhenienne au cours des campagnes de chalutage demersal realisees depuis 1994 en utilisant un plan aleatoire stratifie. Pour decrire les aires de nursery, la fraction des frequences de taille (longueur du cephalothorax) inferieure a 20 mm a ete employee. Les variogrammes experimentaux ont ete construits sur la variable ''indice d'abondance des recrues'' (specimens/km2). Les modeles de variogramme decrivant le mieux la continuite spatiale ont ete determines et valides par la technique du ''jackknife''. Un modele exponentiel a ete adapte, aux extensions [''ranges''] (16,03 km en 1996 et 19,94 km en 1997) qui decrivent des agregations a petite echelle. Pour etudier les aires de nursery, la probabilite conditionnelle de depasser un seuil donne de la valeur de la variable a ete estimee par la geostatistique non-lineaire, en utilisant la technique dite du krigeage disjonctif. Deux seuils de valeurs ont ete utilises (2000 et 3000 specimens/km2) et quatre aires de nursery ont ete localisees au niveau inferieur de la valeur du seuil. Quand le seuil plus restrictif (3000 specimens/km2) a ete utilise, une seule aire de nursery a ete dominante. Cette information peut comporter une indication distincte pour la gestion des pecheries. Ainsi la protection des aires de nursery, grace a la reduction de l'effort de peche pendant l'annee ou pendant des periodes determinees pourrait etre consideree comme un outil efficace et complementaire de la regulation de la peche pour les especes a courte duree de vie, comme P. longirostris.