REPORT OF THE STANDING COMMITTEE ON RESEARCH ... - Iccat

EXECUTIVE SUMMARY SKJ
divided by 10°N latitude, which were established on the basis of their low amount of mixing (only 0.9% of the
tagged fish crossed this latitudinal limit).
A new bayesian method, using only catch information (under a Schaefer-type model parameterization),
estimated the MSY at 143,000-156,000 t, a result which agrees with the estimate obtained by the modified
Grainger and Garcia approach: 149,000 t.
In addition, two non-equilibrium surplus biomass production models (a multi-fleets model and a Schaefer-based
model) were applied for 8 time series of CPUEs, and for a combined CPUE index weighted by fishing areas. To
account for the average increase in catchability of purse seine fisheries, a correction factor of 3% per year was
applied to the CPUE series. As for the bayesian model application that only uses catches, different working
hypothesis were tested on the distribution of the priors of the two surplus production models (i.e., the growth
rate, the carrying capacity, the catchability coefficient of each fleet, etc.). In general, the range of plausible MSY
values estimated from these models (155,000-170,000 t) were larger than in the bayesian model based on
catches. The Committee stated the difficulty to estimate MSY under the continuous increasing conditions of the
exploitation plot of this fishery (one-way of the trajectory to substantially weaker effort values) and which as a
result, the potential range distribution of some priors needs to be constrained (e.g., for growth rate, or for the
shape parameter of the generalized model).
While caution is needed as regards to the generalization of the diagnosis on the stock status of the overall
components of this stock in the East Atlantic, due to the moderate mixing rates that seem to occur among the
different sectors of this region, it is unlikely that skipjack be exploited in the eastern Atlantic (SKJ-Figure 15).
Western stock
The standardized CPUEs of Brazilian baitboats remain stable while that of Venezuelan purse seiners and USA
rod and reel decreased in recent years (SKJ-Figure 16). This decrease, also observed in the CPUE time series
for Venezuelan purse seine, could be linked to specific environmental conditions (high surface temperatures,
lesser accessibility of prey). The average weight of skipjack caught in the western Atlantic is higher than in the
east (3 to 4.5 kg vs. 2 to 2.5 kg), at least for the Brazilian baitboat fishery.
The assessment model from catches estimated MSY at around 30,000 t (similar to the estimate provided by the
Grainger and Garcia approach) and the Bayesian surplus model (Schaefer formulation) at 34,000 t.
The Group attempted several sensitivity analyses for values of natural mortality with Multifan-CL. For this stock
only the three fisheries mentioned above were considered. The final estimate of MSY converges also at about:
31,000-36,000 t. It must be stressed that all of these analyses correspond to the current geographic coverage of
this fishery (i.e., relatively coastal fishing grounds due to the deepening of the thermocline and of the oxycline to
the East).
For the western Atlantic stock, in the light of the information provided by the trajectories of B/B MSY and F/F MSY ,
it is unlikely that the current catch is larger than the current replacement yield (SKJ-Figure 17).
SKJ-4. Effects of current regulations
There is currently no specific regulation in effect for skipjack tuna.
However, with the aim of protecting juvenile bigeye tuna, the French and the Spanish boat owners voluntarily
decided to apply a moratorium for fishing under floating objects between November and the end of January for
the 1997-1998 and 1998-1999 periods. The Commission implemented a similar moratorium from 1999 to
January 2005. This moratorium has had an effect on skipjack catches made with FADs.
On the basis of a comparison of average catches between 1993-1996, prior to the moratoria, and those between
the 1998-2002 period, the average skipjack catches between November and January for the purse seine fleets that
applied the moratoria, were reduced by 64%. During that period (1998-2002), the average annual skipjack
catches by purse seine fleets that applied the moratoria decreased by 41% (42,000 t per year). However, this
decrease is possibly a combined result of the decrease in effort and the impact of the moratoria (the average
annual catch per boat decreased only 18% between these two periods).
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