Page 7 - Froglia1982

Biology of Parapenaeus lonKirostris 167

In the Golfo di Patti no specimens smaller than 20 mm c. I. were present in cat-
ches obtained frorp depths of 250 m or more. Off the Egadi islands specimens
smaller than 20 mm c. I. were found to depths of 400 m and in a haul made at depths
of 260-380 m they represented by number 230Jo of the total catch of P. longirostris.

These differences between the two areas probably can be explained by the dif-
ferent geomorphology of their trawling grounds.

Similarly for the trawling grounds of the Gulf of Cadiz MASSUTI (1959)
recorded an increase in the mean size of both sexes with increase in depth.

When specimens smaller than 20 mm c.l. were present in the catch, two peaks
could be evidenced in the histograms of length distribution in both sexes.

In all areas investigated, at depths greater than 450 m no srecimens smaller
than 20 mm c.l. could be found and a single peak is shown in the histograms.

At these depths, the catch of P. longirostris was not of commercial importance
and Red Shrimps (Aristaeomorpha joliacea and Aristeus antennatus) were more
abundant.

Putting together data available for all depth strata within each area, a diagram
was obtained that can be considered a rough representation of the length distribu-
tion of Parapenaeus longirostris populations (Fig. 2).

Comparing figures thus obtained, the same trend was found f, •r both sexes and
for all three areas considered.

Age determination in Crustaceans is difficult because all hard structures are re-
jected by animals at moulting. Growth and intermoulting times obtained in aquaria
cannot be transfered "tout court" to natural populations and tagging experiments
present many practical problems.

Estimates of age composition of a population can be obtained indirectly by
analysis of length distributions derived from trawl samples. This is especially true
for species with a short spawning season.

The technique of anaiysis by means of probability paper developed by CASSlE
(1954) was applied to present data.

For the Golfo di Patti and the area off the Egadi islands it was possible to
distinguish two age classes in both sexes.

The first one had its central value at about 13 mm c.l.; the second one showed
small differences between areas and between sexes in the same area and its central
values were between 23 and 29 mm c.l. (Tab. 2).

TAB. 2 - Age classes/length for P. longirostris as obtained in June 1974 on dif-
ferent trawling grounds around Sicily (mean and standard deviation).

Age class 0 + 0' Patti Egadi Pantelleria
Age class 1+ 0'
Age class 0+ 9 12,5 ± 1,69 13,5 ± 1,16 23,9 ± 2,35
Age class 1+ 9 22,5 ± 1,91 24,5 ± 1,72 28,1 ± 3,3
12,5 ± 1,67 14,0 ± 1,55
25,5 ± 2,82 29,5 ± 2,58