Localization of nursery areas of Parapenaeus longirostris (Lucas, 1846) in the Central-Southern Tyrrhenian Sea by Geostatistics

Abstract

Studying the spatial correlations of the distribution of juveniles and their relative abundance is the ® rst step in assessing and localizing the nursery areas of demersal resources.

This information might comprise a distinctindication for management. Thus, the protection of nursery areas, through limitation of the ® shingpressure throughout the year or in ® xed periods, could be considered as an effective, complementaryregulation tool for a short-lived species likeP . longirostris.Studying the spatial correlations of the distribution of juveniles and their relative abundance isthe ® rst step in assessing and localizing the nursery areas of demersal resources. These data canprovide useful information both for a better understanding of the biology of the deep-water roseshrimpParapenaeus longirostrisand for suggesting advice to ® sheries management.Data were collected in the central-southern Tyrrhenian Sea during bottom trawl surveys carriedout since 1994, using a random, strati® ed sampling design. In order to describe the nursery areas,the<20 mm fraction of the length frequency distributions (carapace length) of the species wasemployed. The experimental variograms were constructed on the variable namedªabundanceindex of recruitsº(individuals/km2).

The model best describing the spatial continuity was detectedand validated by the jackknife technique. An exponential model was ® tted with ranges describingsmall 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 estimatedby non-linear geostatistics, namely the so-called disjunctive kriging technique. Two thresholdvalues were adopted (2000 and 3000 individuals/km2) and four nursery areas were localized at thelower 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 distinctindication for management. Thus, the protection of nursery areas, through limitation of the ® shingpressure throughout the year or in ® xed periods, could be considered as an effective, complementaryregulation tool for a short-lived species likeP . longirostris.