Basic HTML Version
Int'l Journal of Marine Science 2012, Vol.2, No.8, 57
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61
http://ijms.sophiapublisher.com
58
For these reasons, this work comes to confirm the
necessity to incorporate environmental parameters to
explain variability of common octopus abundance and
the importance of these results in the assessment and
management advice. In fact, many controversies about
fitting and specifically surplus models that do not
include environmental effect have, presumably, a
strong effect on the recruitment of the short-lived
organisms.
1 Results
The estimation of R² for both models (Scheaffer and
Fox) was acceptable and statistically significant (Table
1). We use the Fox model, because of the importance
of estimated R² for exponential model.
The analysis of graphic shows globally an
under-fishing situation (Figure 1). In the latter years
(2006, 2005, 2004, 2003) the maximum sustainable
yield, MSY, which is the highest theoretical
equilibrium yield, was not attained yet.
Table 1 Estimation of R² for both models
Fox model
Schaeffer model
R²
0.582
0.528
P (α=0.05)
0.007*
0.020*
Note: *Statistically significant
Figure 1 Fox model illustrated by the common octopus
fisheries in Monastir
2 Discussions
The use of surplus production approach permits to
calculate the maximum sustainable yield MSY and the
optimal fishing effort (F
opt
) without catchability,
requires only catches and effort data and do not needs
age structure.
The application of the surplus production models
constitutes a privileged way of the cephalopod stock
assessment (FAO, 1985). Analytical assessment is
currently impractical (ICES WGCEPH, 2010).
Our results showed an under-fishing state. It is
recommended to increase the effort until 30 000 trips
which corresponds to a maximum sustainable yield of
206 tons. But these value still dependant on fisheries
conditions, economic situation and particularly
environmental factors
.
In fact, catches are decreasing;
stock may be in depleted state without necessarily
being over fished. This situation, however, may be
explained by the effect of climatic parameter as
demonstrated by Jabeur et al (2010) on this stock.
Besides, the effect of the quality of data may not be
avoided.
Surplus production approach does not tell us much
about the mechanism affecting the population
dynamics and assumes that the stock has stabilized at
current rate of fishing.
Trophic linkages and interaction between several
species, which are captured simultaneously and may
be differently affected by gears, are excluded from
mono-specific models. For this reason, these models
are insufficient to explain different exploitation
scenarios (Guerra et al., 2010).
Furthermore, some studies prove that integration of
environmental parameters in models should be
developed. As it is demonstrated by authors,
fluctuation of cephalopod population is explained by
the influence of various environmental factors (Boyle
and Rodhouse, 2005; Caddy, 1983; Pierce et al., 2008)
and confirmed by Jabeur et al (2010) on stock of the
east coast of Tunisia. In fact, according to this last
study, the decrease of yield is related to the effect of
temperature and rainfall. Monastir, for instance, some
years were particularly influenced by climatic
parameters from 1999 to 2006; these years were
characterized by weak yield. For these last seven years,
temperature was increased during larval period
corresponding to the hot season from August to
October (for example mean temperature was 21.1
3
℃
in 1995 and 23.14
℃
in 2006, INM, National Institute