Page 4 - Ragonese_Bianchini_1996

128 S. RaRonese, M.L. Bianchini l Fisheries Research 26 ( 1996) 125-137

The yield-per-recruit ( Yl R) formulation according to the incomplete beta function can
be written as follows:
YIR=FIK W. e-z(l,-lo) o. (u '')
x. JJX.Xi 'L-
where F and Z represent the instantaneous fishing and total mortality, K ìs the Brody's
coefficient, W" is the asymptotic weight, te is the mean age at first capture, t is the location
0
parameter of the VBGF mode!,
Q . (u tJ) = JXI t''-J ( 1-t)"-Jdf
JJ:t..\] '
x
is the incomplete beta function, u and u being its parameters, i.e., u = Zl K, u, the estimated
allometric coefficient plus one ( specific case v= 3.5), and the integration Iimits: x=
e- K(,, -lo) an d x =e- K(l•- ro) ( with t A= maximum age of contrìbution t o the fishery).
1
The tables of Wilimovsky an d Wicklund ( 1963) were used to compute the discrete Y l R
values for different levels of F and two values of M (0.5; 0.8) and te (l; 1.5 year); the
curves w ere constructed after smoothing ( LOWESS procedure, SYST A T, 1992).
As a rule. mathematical and statistica! computations were performed using the SYSTA T
( 1992) package.

3. Results

Aristeus antennatus is caught between depths of 500 m and 820 m, the latter representing
the Iowest explored an d exploited boundary. The distribution area of the blue-and-red
shrimp, according to tra wl surveys data, is presented in Fig. l, but i t is abundant only in the
north-western sector, around the Egadi Islands; in any case mean yields are generally lower

10° E l5°E

200~
~ ... 7, ,'::!

Fig. l. Delimitation of the Strait of Sicily and of the investigated area with, evidenced, the distribution of Aristeus
l111tennatus.