Page 11 - Ragonese_Bianchini_1996

S. Rawmese, M.L. Bianchini l Fisheries Research 26 ( 1996) 125-137 135

Concerning the mortality estimates, the steady-state assumption of Heincke' s approach
can be accepted, given the relative stability, in both recruitment an d fishing pattern, reported
for the Sicilian fishery in the last decade (Arena, 1985) and more generally also in the
Catalan Sea ( Demestre and Martin, 1993).
Determining a natura! mortality value is a criticai and always difficult choice (Vetter,
1988), particularly for deep-water shrimps.
The situation is further complicated by the possibility that F is age- or size-dependent,
as females of Aristeus antennatus could be more vulnerable when aggregated to spawn
( Demestre an d Lleonart, 1993).
Moreover, an inverse relationship between F and M has been suspected for short-living
highly predated animals (Cobb and Caddy, 1989); according to this hypothesis, over a
wide range of exploitation regimes, as F increases, M would decrease, resulting in a constant
value of Z. However, deep-water red shrimps should not be considered high1y-predated
ani mals ( Demestre an d Martin, 1993).
Notwithstanding the different approach, the shape of the Y l R curves (Fig. 5) obtained
for the Sicilian population resembles that of Demestre and Lleonart ( 1993) off the Catai an
coasts.
Aristeus antennatus populations are generally considered resi1ient to exploitation, mainly
because on1y the upper frac ti o n of the stock is available to the tra wl ( Demestre and Lleonart,
1993; Demestre and Martin, 1993; Sardà et al., 1993).
Considering the high fishing pressure exerted on the demersal resources of the Mediter-
ranean Sea ( Anonymous, 1989), a condition of underexploitation ( e.g. M h= 0.8 an d
Z = 1.1) is un1ike1y, making the Y l R curves based o n M = 0.5 more realistic. The compar-
1
ison between the two curves in Fig. 5b suggests a condition of growth-overfishing, and
yield-per-recruit should increase substantially by delaying the age at first capture to 1.5
years ( corresponding to CL= 38 mm).
The possibility of achieving this result by concentrating the 45 days of the annua) fishing
ban in summer (Levi, 1991) does not seem to be practicable because the fishing activity
peaks in season. As the mesh size presently in use (16-18 mm, si de) is not selective, the
only feasible management tool would be an increase in the cod-end mesh size.
Moreover, recent selectivity studies suggest that a mesh size up to 28 mm side can be
employed to reduce the juveniles in the catch without any substantial loss of adult (an d
more prized) ani mals ( Ragonese et al., 1994a) .
In conclusion, under any scenario, a delay of 6 months in the age at first capture would
result in an increase of the yield-per-recruit, without any economie harm, even short-term.

References

Anonymous. 1989. Evolution récente des péches méditerranéennes. GFCM/RM/VII/3, 34 pp.
Anonymous. 1992. Programma pesca Mazara. Ass. Liberi Armatori Pesca, Mazara. Italy, III pp.
Arculeo, M .. Baino, R., Abella, A. and Riggio, S., 1992. Accrescimento e riproduzione di Aristeus antennatus
(Crustacea. Decapoda) nel Golfo di Castellammare (Sicilia N/W). Oebalia, 17(suppl. 2): 117-118.
Arena. P .. 1985. Studio sulla possibilità di razionalizzare e rendere più produttiva la pesca a strascico nel Canale
di Sicilia e nel Mediterraneo centro-meridionale. Ente Siciliano Pesca ltticolt., 33 pp.