Quick link to: EFIMAS home, WP4, CS7

Case study 7, Approach 8: Evaluation of a progressive capacity reduction by assuming assessment error and an empirically estimated S/R model


This scenario includes a 20-year catch projection of the fishery assuming a progressive capacity reduction from the second to the fifth year (5% each year)

Description of the fishery, stocks and management system

See general introduction of CS7

Scenario evaluations

The scenario evaluated a progressive 4-year capacity reduction reaching 20% of the initial value. The annual recruitment rates are obtained from an empirically estimated S/R model (Hilborn & Walters, 1992).

Data and parameters

Stock data

The required number- at-age indices of abundance to set up the population model were taken from the results of the latest ICCAT assessment (Anonymous, 2007).

Fleet data

Two fleets were assumed: (a) long-line and (b) gillnet. Data on catch by fleet were taken from the ICCAT database. Effort and economic data (prices by age class, fixed and variable costs) for the Greek and Italian long-line fleets were provided by HCMR and IREPA respectively. Since economic data existed only for the longline fleet only this fleet was included in the economic analysis.


FLR (Operating Model)

The FLR packages used are FLCore ver. 1.4-3. Data were modelled by means of the FLFleet and FLBiol classes of FLCore. Additional classes concerning economic data (e.g. FLPrice, FLcost) were added (see R-code). The projection scenario was simulated 100 times.R-code

Conditioning of Operating Model

Historical Estimates of Time Series

The operational model was based on the results of the latest ICCAT assessment which included data from 1985-2005. The initial vector of number and catch by age was considered as that at the beginning of 2005. Hence, projections were run for the 2005-2025 time period.

Biological Parameters

Natural mortality was assumed to be independent of age (M=0.2) and fishing mortality (F) by age equal to the mean of the 2003-2005 period.

Stock Recruitment Relationships

A Beverton-Holt S/R model was assumed. VPA results have not allowed estimation of model parameters due to low contrasts. Consequently they were estimated empirically (Hilborn & Walters, 1992) assuming that (a) current recruitment is 70% of the maximum recruitment and (b) half of the current stock size will produce about half of the maximum recruitment.

Fisheries & Fleets

In this approach it was considered that the long-line fleet (LL) is composed of Greek and Italian vessels. The catch of this fleet is about 50% of the total catch; thus it was assumed that FLL=Ftotal/2. As usual it was assumed that fishing mortality equals age specific catchability times effort. The capacity reduction was assumed to take effect from the second to the fifth year of the simulation and corresponding to 5% annually (20% total). After the fifth year the capacity was simulated to remain constant.


The current estimates of fixed and variable costs for the long-line fleet were obtained from the IREPA and HCMR partners and were considered constant over the projection period. The same was considered for the price/kg by age class for the catch.

Reference Points

No reference points have been established for the stock

Observation Error Model

The observables in the model are the initial vector of abundance by age and the predicted recruitment from the S/R model. Both were modeled by assuming log-normally distributed errors with CV’s equal to 15% and 30% respectively. Thus the corresponding values were drawn randomly from the assumed distributions.

Management Procedure

Since the stock is managed through technical measures and control effort regulations there is not any specific management procedure, such as harvest control rules.


Results indicated a decrease in the production of the first five years to 14000 MT and then an increase reaching the initial levels by 2015 and stabilizing therefater (Fig. 1). Spawning stock biomass (SSB-Fig. 2) showed a slight trend of increase and then stabilized at 30500 MT. Gross revenue and net revenue showed a rate of decrease in the first five years and remain stable at 74 and 39 million € respectively, thereafter.

Left: Figure 1. Landings by year reported for the years 1985-2005 and model estimations for the years 2005-2025. Dashed lines indicate lower and upper quantiles.

Right: Figure 2. Spawning stock biomass by year as estimated for the years 1985-2005 from the ICCAT assessments and model estimations for the years 2005-2025. Dashed lines indicate lower and upper quantiles.

Left: Figure 3. Gross revenue by year reported for the years 1985-2005 and model estimations for the years 2005-2025. Dashed lines indicate lower and upper quantiles.

Right: Figure 4. Net revenue model estimations for the years 2005-2025. Dashed lines indicate lower and upper quantiles.


Preliminary results of the management evaluation scenarios were presented in the 2006 SCRS/ICCAT Species Meeting. The FLR framework has been used for the development of a series of management scenarios during the 2007 ICCAT Assessment of the Mediterranean swordfish stock and at an ICCAT Intersessional meeting on Mediterranean swordfish organized in February 2008.

Tserpes, G. and Peristeraki, P., 2007. Effects of a seasonal closure of the Mediterranean swordfish fisheries on the stock production levels. ICCAT Collective Volume of Scientific Papers, 60: 2059-2062.

Tserpes, G., Tzanatos, E., Peristeraki, P., Placenti, V. and Kell, L., 2008. A bioeconomic evaluation of different management measures for the Mediterranean swordfish. ICCAT SCRS/2008/026.


Hilborn, R. and C.J. Walters. 1992. Quantitative fisheries stock assessment. Chapman and Hall, London, 570 p.

Anonymous, 2007. ICCAT Mediterranean swordfish stock assessment session. SCRS/2007/016.


EFIMAS Contribution to the work

Participants: George Tserpes, Panagiota Peristeraki, Vincenzo Placenti, Evangelos Tzanatos, Paolo Accadia

efimas1/wp4/cs7/appr8/main.txt · Last modified: 2008/11/16 01:11 by admin
Except where otherwise noted, content on this wiki is licensed under the following license:CC Attribution-Noncommercial-Share Alike 3.0 Unported
Recent changes RSS feed Donate Powered by PHP Valid XHTML 1.0 Valid CSS Driven by DokuWiki