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Case study 8, Approach Approach B2: Implication of engine power compliance (reduction) measure to the fleet operating in Northern Alboran Sea. Bio-economic scenarios assuming engine power compliance with the law.


This case study scenario include a 15-year catch projection of the fishery assuming engine power reduction (by 25%). These scenarios were applied to the trawl fleet in Northern Alboran Sea. We included this scenario because we found it particularly relevant for the Spanish case. Spanish regulations impose certain limits on the engine power of the vessels. New vessels or vessels that have changed their engine power for a new one tend to install more powerful engines than allowed by the regulation. Therefore, they are requested to install some type of torn that prevents the engine to work at full power, in order to respect the regulation. However, most vessels really operate (illegally) with their engines at full power. The inclusion of this scenario deals with the following question: what would happen if the vessels would not use their real engine power but the declared (official) one?. Therefore, in some sense, this scenario is equivalent to the inclusion of enforcement measures in order to guarantee that the law is fulfilled.

Description of the fishery, stocks and management system

See general introduction of CS8

Description of the base case and scenario evaluations

See general introduction of CS8

Data and parameters

Stock data

As in most Mediterranean fisheries, the main problem when analyzing this fishery is the lack of good biological and economic data. It was not possible to find data by size or by individuals. Moreover, the size of the sample is limited to eight years. However, we had daily catch and effort data over this period, which has allowed us to make some estimations based on a large amount of cross sectional observations over the sample period.Therefore assessment of the current hake population in the Alboran Sea was obtained by using surplus production models as the lack of data on catches by fish size forced us to use surplus production models. Specifically, we used the Pella- Tomlimson model which is more general than the traditional Shaeffer model. Results showed that the estimations of the stock seemed to follow a steady decrease over time as can be appreciated in the figure 1.

Figure 1. Evolution of the hake stock biomass.

Concerning the estimation estrategy, we decided not to use equilibrium estimation procedures, but regression methods and minimum error estimators instead. The reason for this is that rarely fisheries are in the equilibrium state which is the basic assumption when using equilibrium estimation procedures. In figure 2, it can be observed the estimated vs the predicted catches

Figure 2. Estimated vs. predicted catches.

Fleet data

We had daily landing data for six spanish south mediterranean port. We used a non-linear production function with three inputs: stock size, effort and engine power of the vessels. Moreover, we relaxed the assumption that the elasticities of the main inputs are equal to 1 (which is usually included in the basic models). Since this turns out to be a very non-linear model, we first carried out the analysis using a more basic model based in the biological Shaeffer model and a Cobb-Douglas function with elasticities equal to one. This model provided us with initial pre-estimates for the parameters, which we used to estimate the more general model described above.


FLR (Operating Model)

The FLR packages used were FLCore for simulations.

Conditioning of Operating Model

Historical Estimates of Time Series

The operational model was based on the assessment we made of the hake population in the Alboran Sea using surplus production models.

Biological Parameters

The values of the parameters found were K =112 (hundred tonnes), r=0.501 and m=1.1.

Fisheries & Fleets

We had landing data for the trawl fleet based in six different Spanish Southern Mediterranean Ports. However as many of the vessels landed their captures in different ports over time we could not distinguished ports in our analysis.


Economic data were considered. Individual engine power for each vessel was available and daily individual amount and value of the catch. Estimates of variable and fixed costs for the trawl fishery in Northern Alboran Sea are currently included in the models. The economic depreciation of the vessels was also estimated. The results also provide estimates of the average monthly fishermen salaries.

Management Procedure

We assumed for this scenario a reduction of 10% and 20% in effort.


Our simulation results predict that, in the benchmark case, for the following 15 years there will be a sharpest decrease in the catches if no policy is implemented and the exploitation pattern keeps at current levels.

What is worthwhile to point out is that the reduction in the engine power produces the higher increase in the catches form year 2015 onwards when compared to the reduction of effort scenarios. From this result, we may conclude that (in principle) it would be more convenient that managers would take into force the engine-power regulation rather than imposing new measures.

Figure 3.Simulated evolution of catches for the engine power reduction scenario

Concerning the evolution of the simulated stock for the following 15 years, our simulation results (Figure 4) suggest that even if the decline of the biomass is slower when we consider the reduction either in effort or in engine power, the biomass is still prone to decline. Only in the case of the reduction in the engine power it seems to decline very slowly getting nearly stabilised. Therefore, in principle, it could be concluded that if managers are interested in recovering the hake stock, they should implement stronger measures than those considered here.

Figure 4.Simulated evolution of the stock biomass for the engine power reduction scenario

In Figure 5, it has been graphed the estimated average values of fishermen’s salaries over the period considered. Monthly salaries are estimated using the accumulated annual salaries and taking into account that fishermen work 10 months per year as the fleet is on subsidised seasonal closures for two months. It can be seen that the salaries have shown a sharp decline and they will keep the decrease in the case of effort reductions. It seems remarkable that the sharpest decrease in average fishermen’s salaries and shipowners’ profits shows up for the case of engine power compliance. This result seems to be contradictory with the stock simulation, which shows a softer decrease for this scenario. Then, it seems that power limitation brings desirable results for the fish stock at the cost of reducing economic benefits. The explanation for this may be that engine power reduction will imply that more vessels would have to operate closer to the coast and, as a consequence, they would catch smaller amounts of other higher-value species (mostly shellfish). Therefore, in order to determine which specific policy alternative is preferable, it can be relevant to take into account which is the situation of the alternative species. If those species happen to be overexploited, then it could be advisable to enforce power limitations together with some decrease in effort, because this will help to recover the stock of both the main species and the secondary species.

Figure 5. Evolution of fishermen’s monthly salaries for the engine power reduction scenario

Figure 6. Evolution of shipowner’s monthly profit for the engine power reduction scenario


The following paper has been produced form to be submitted to Fisheries Research:

Herrero, I., Riesgo, L., Andre, F.J.(2008) “Is it time to impose new regulations or simply to enforce current ones?”.


EFIMAS Contribution to the work

This work has been made under the EFIMAS Project.

Participants: Ines Herrero , Laura Riesgo y Francisco Javier André.

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