Case study 7, Approach 5: Evaluation of a 4-month fishery closure in the peak of the recruitment period by assuming recruitment independent of stock size

This scenario includes a 20-year catch projection of the fishery assuming a 4 month closure in the peak recruitment months (winter).

See general introduction of CS7

The scenario evaluated the closure of the swordfish fishery for 4 months during the recruitment period (winter months). The annual recruitment rates vary randomly around a constant value (average of the years 1990-1999).

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).

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.

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

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.

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.

A constant recruitment value varying at random was assumed. The mean recruitment was equal to the average of the years 1990-1999.

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 F_LL=F_total/2. As usual it was assumed that fishing mortality equals age specific catchability times effort. The 4 months closure of the fishery was assumed to have two effects: (a) a reduction of the total fishing effort by 20% for both fleets, (b) a catchability decrease of 90% for the 0-age class.

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.

No reference points have been established for the stock

The observables in the model are the initial vector of abundance by age and the predicted recruitment. 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.

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 production pattern stabilizing after 7-8 years with annual catches reaching ~16000 MT (Fig. 1). Spawning stock biomass (SSB-Fig. 2), revenue (Fig. 3) and net revenue (Fig. 4) show a similar pattern. SSB reaches ~38000 MT. Revenue and net revenue stabilize around 85 and 50 million € respectively.

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.

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