Rapp. Comm. int. Mer Médit., 36,2001
316
Introduction
In the western Mediterranean Scorpaena scrofais an important
commercial species captured mainly in trammel-net artisanal fisheries.
It is a slow growing1 benthic predator that inhabits mainly hard sub-
strates between 20 and 200 m depth. The rather limited mobility of S.
scrofaand its slow growth make this species particularly vulnerable to
overfishing. For the same reasons, fishing restrictions are expected to
be particularly effective in rebuilding populations of S. scrofa although
to date no studies exist that evaluate this response. This study assess-
es for the first time the responses of S. scrofato fishing restrictions in
terms of population abundance, size structure and mortality.These
changes are the most widely documented effects taking place in
marine populations positively affected by protection2.
Material and Methods
Study areas. The study was conducted in the Columbretes Islands
Marine Reserve (MR) and in two areas open to fishing (OF) : the
Subarra area (OF-I) (40º 03’30’’N – 00º 40’50’’E) and the area off
the NE coast of Mallorca island (OF-II) (39º 52’00’’N – 03º 29’
00’’E). The MR was established in 1990, extends over an area of
14km
2
and is located 30 miles off the eastern coast of the Iberian
peninsula (39º 52’50’’N – 00º 40’20’’E). All fishing is forbidden in
the MR with the exception of purse seine and pole-and-line fishing in
restricted areas.
Sampling design. Trammel net fishing was carried out in the MR (sur-
veys) and in the open fishing areas (commercial) between 50 and 80 m
of depth during the summers of 1998, 1999, and 2000. Trammel nets
were 600 m long and had a drop of 1.7 m. The mesh size of the inner
and outer panels were 80 mm and 300 mm respectively. Given the
complex morphology of S. scrofa(spines and appendices), trammel
net selectivity for this species may be approximated by a logistic
model where all animals over a certain size are entangled3. Twelve
sets were done in the RM and a larger number were sampled in the
open fishing areas (OFI and OF II) each year.Abundance indices were
estimated from catch rates (numbers caught per set) standardised to
one night soak time.
Statistical analysis. To assess temporal and spatial differences in relati-
ve abundance, catch rate data were analysed by orthogonal analysis of
variance with year and area as fixed factors. Multiple comparisons after
A
N
OVA were done by SNK tests4. Data were log-transformed to com-
ply with the assumptions of the analysis 4. To obtain equal samples
sizes in the three areas, random subsets of 12 replicates per year were
d
r
awn from the larger pool of samples available from the OF areas.
For the analysis of length data, samples of the three years were
pooled to increase sample size. Spatial differences in mean size were
assessed by one factor ANOVA.
Mortality rates. Total mortality (Z) was estimated for the protected and
exploited populations from the size frequency distributions using the
Beverton and Holt’s Z equation for length data 3. The growth parame-
ters were taken from Bradai and Bouain 1.
Results and discussion
The abundance of S. scrofadid not vary significantly over time in
any of the study areas (F
0.05 (1) 2,99
= 0.361, p = 0.70). However, abun-
dance was significantly higher in the marine reserve than in the open
fishing areas at all times (F
0.05 (1) 2,99
= 10.08, p <0.0001), while it was
similar in the two exploited areas (Fig. 1).
S. scrofacatches were comprised mainly of specimens between 26-
40 cm TL in the exploited areas and between 32-46 cm TL in the MR
(Fig.2). The mean size of S. scrofavaried significantly among areas
(F
0.05 (1), 2, 294
= 46.70; p< 0.0001) with a high proportion of large
(hence old) specimens in the MR. The modal and mean sizes in the
MR (39.5 ± 4.7 cm TL) were larger than in any of the OF areas which
showed similar modal and mean sizes (OF-I : 33.7 ±4.2 cm TL; OF-
II: 33.0 ± 4.3 cm TL) (Fig. 2).
Figure 1. Abundance index (catch in number per standard set) (± stan-
dard error) of S. scrofa.
MR : Columbretes Islands Marine Reserve; OF-I : Subarra open fishing area;
OF-II : NE Mallorca Island open fishing area.
Figure 2. Size frequency distributions of S. scrofain the three study
areas.
MR : Columbretes Islands Marine Reserve; OF-I : Subarra open fishing area;
OF-II : NE Mallorca Island open fishing area. LT : total length.
The total mortality rate estimated in the protected population pro-
vides a first approximation to the natural mortality rate (M) of the
exploited phase of S. scrofa.The value of M ranges between 0.09 and
0.14 per year depending on the choice of first size group under full
exploitation. Given the similarity of size frequency distributions
obtained in the two OF areas, Z was calculated for both areas com-
bined. The fishing mortality (F) in the OF areas, estimated by the dif-
ference between Z and M, ranged between 0.18 and 0.33 per year.
The results of this study suggest a swift response of S. scrofato the
cessation of fishing and a progressive recovery of the population in the
marine reserve after 8 years of protection. On the basis of the estimat-
ed mortality rates, annual survival of the exploited phase of S. scrofa
in the Columbretes Islands Marine Reserve is 15-24% higher than in
the exploited areas.
References
1- Bradai M.N. and Bouain A., 1988. Age et croissance of Scorpaena
scrofadu Golfe de Gabès. Bull. Inst. Nat. Scient. Tech. Océanogr. Pêche
Salammbô, 15 : 13-38.
2- Roberts C.M. and Polunin N.V.C., 1991. Are marine reserves effective
in management of reef fisheries? Rev. Fish Biol. Fish. 1 : 65-91.
3- Sparre P. and Venema S.C., 1992. Introduction to tropical fish stock
assessment. Part I. FAO Fish. Tech. PaperN. 306.1, Rev.1, Rome FAO,
376 p.
4- Zar J.H., 1984. Bioestatistical analysis,2nd Edition, Prentice Hall
Inter. 717 p.
EFFECTS OF FISHING RESTRICTIONS ON THE ABUNDANCE, SIZE STRUCTURE AND MORTALITY
RATE OF AWESTERN MEDITERRANEAN POPULATION OF SCORPAENA SCROFA(LINNAEUS, 1758)
Olga Reñones, Antoni Quetglas and Raquel Goñi*
IEO-Centre Oceanogràfic Balears, Palma de Mallorca, Spain - olga.renones@ba.ieo.es
Abstract
This study evaluates the changes occurred in a population of Scorpaena scro
f
aafter eight years of protection (fishing ban) in the Columbretes
Islands Marine Reserve (western Mediterranean). Relative abundance, size structure and mortality rates of the protected stock are compared
with those of exploited stocks in the region. Higher abundance and a greater number of large-size individuals characterise the protected stock.
A first estimate of the natural mortality rate, together with estimates of the exploitation rates in the fished stocks, is also given.
Keywords : fishing ban, marine reserve, Scorpaena scrofa, western Mediterranean.
