Quick link to: EFIMAS home, WP4, CS8

Case study 8: Approach A1: Implication of a reduction in fishing Effort to different fleet segments (Aegean Sea: bio-economic scenarios A1.1 & A1.2 with 10% and 20% Effort reduction).

Introduction

This case study scenarios include a 20-year catch projection of the fishery assumming a 10% and 20% reduction in fishing effort. These scenarios ware applied to both fleet segments of the trawl fishery of hake in the Aegean Sea.

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

The assessment of the current hake population in the Aegean Sea was obtained by running a pseudo-cohort VPA on the trawl total catches (landings and discards) of 2004, using the average total catch at age distribution of the last three years, with 7 as the ‘plus’ group. The results of this assessment were used in the current case study. Growth and natural mortality rates were obtained, mostly from the bibliography (Aldebert et al. 1993; Aldebert & Recasens, 1998; Bozzano et al. 1997; Recasens et al. 1998, Papaconstantinou and Stergiou 1995).

Fleet data

Both segments of the Aegean trawl fleet targeting hake were considered. These segments involved: a) vessels with total length between 12-24 and b) vessels with TL 24-40 meters.

Modelling

FLR (Operating Model)

The FLR packages used were FLCore ver. 1.3-3 and FLStock. Data were modelled using the FLStock class of FLCore. The projection scenario was simulated 1000 times.

Conditioning of Operating Model

Historical Estimates of Time Series

The operational model was based on the results of the latest assessment of the hake population in the Aegean Sea using a pseudo-cohort VPA on the trawl total catches (landings and discards) of 2004.

Biological Parameters

Natural mortality was assummed constant at M=0.2. The fishing mortality (F) estimates by age equal the 2004 derived values.

Stock Recruitment Relationships

Recruitment was considered independent of stock size. Based on the assessment estimates, annual recruitment rates for the projection period were randomly obtained from a normal distribution with mean and coefficient of variation (CV) equal to those estimated for the assessment period (including random variability 10% in recruitment)

Fisheries & Fleets

Both segments of the Aegean trawl fleet targeting hake were considered. These segments involved: a) vessels with total length between 12-24 and b) vessels with TL 24-40 meters.

Economics

Economic data have also been considered. Estimates of variable and fixed costs for the trawl fishery in the Aegean Sea are currently included in the models.

Reference Points

Reference points for hake in the Aegean Sea are yet to be advanced and thus were not considered.

Observation Error Model

The observables in the model are the initial vector of abundance index by age group. Observation error model was not assumed here.

Management Procedure

The stock is managed through technical measures and effort control rules, thus there is no specific management procedure (e.g. harvest control rules, TAC, quotas)

Results

Scenario A1.1: 10% reduction in Effort scenario

The results of the current runs of the model suggested a relatively stable production pattern in the medium term. The 10% reduction of effort was applied to 2004 recorded fishing effort of the two fleet segments and was implemented in the next year’s total effort, i.e. 2005. The model revealed a respond of the Aegean hake fishery to this effort restriction measure. More precisely a 6% reduction in the total hake landings was evident. As both fleet segments were characterised by similar fishing effort in terms of total fishing days this drop in landings was observed to both segments. However, it took only one year (2005) for the fishery to level up (2006) to the starting values of the landings (2004). After that and throughout the projection period the production stabilises around a 6% higher level than the 2004 (prob > 50%).

Figure 1. Projected Box-plots by year, for the production estimates and the net revenues obtained from 100 simulations with a 10% reduction in effort.

Scenario A1.2: 20% reduction in Effort scenario

The results of the current runs of the model suggested a relatively stable production pattern in the medium term. The 20% reduction of effort was applied to 2004 recorded fishing effort of the two fleet segments and was implemented in the next year’s total effort, i.e. 2005. The model revealed a respond of the Aegean hake fishery to this effort restriction measure. More precisely a 20% reduction in E induces a 13% decrease in the following year’s catch (2005). However, it took only one year (2005) for the fishery to level up (2006) to the starting values of the landings (2004). In 2006 production is by 14% higher than that of 2005. Hereafter the catches stabilise at 13.5 % higher values compared to the 2004 (prob > 50%).

Figure 2. Projected Box-plots by year, for the production estimates and the net revenues obtained from 100 simulations with a 20% reduction in effort.

Dissemination

The following paper has been produced from CS work carried out so far. This paper will also be presented in the 38th COMMISSION INTERNATIONALE POUR L’EXPLORATION CIENTIFIQUE DE LA MER MÉDITERRANÉE (CIESM) Congress to be held in Istanbul Turkey in April 9-13th :

J. Haralabous, CD Maravelias, G. Tserpes & C. Papaconstantinou. 2007. Developing an FLR operational model for evaluation of fisheries management strategies: an application to hake fishery. 38th CIESM Congress, 9-13 April 2007, Istanbul, Turkey.

References

Aldebert, Y. & C. Carries. 1988. Problèmes dans l’exploitation du merlu en Golfe du Lion. Rapport 5ème Consultation Technique du Conseil Général des Pêches de la Méditerranée. FAO Fish. Rep., 395: 87-91.

Aldebert, Y., L. Recasens & J. Lleonart. 1993. Analysis of gear interactions in a hake fishery: The case of the Gulf of Lions (NW Mediterranean). Sci. Mar. 57(2-3): 207-217.

Aldebert, Y. & L. Recasens. 1996. Comparison of methods for stock assessment of European hake, Merluccius merluccius, in the Gulf of Lions (north-western Mediterranean). Aquat. Living Res. 9(1); 13-22.

Cheret, Y., H. Farrugio, A. Jadaud, P. Lespagnol, C. Mellon, M. Gaza, E. Massutí, G. Pomar, P. Pereda & J. Lleonart. 2002. Stock assessment of the French-Spanish shared stock of hake (Merluccius merluccius) in the Gulf of Lions. Working document Nº 6 to the GFCM SAC Working group on the assessment of demer-sal stocks, Rome 20-22 March 2002.

Fiorentino, F., A. Zamboni & G. Relini. 1998. La selettività della rete a strascico in Merluccius merluccius sulla base delle esperienze riportate in letteratura. Biol. Mar. Med. 5(2): 465-474.

Lleonart, J. & J. Salat. 1992. VIT: Software for fishery analysis. FAO, Computerized In-formation Series – Fisheries, Nº 11. Rome.

Lleonart, J., F. Maynou, L. Recasens & R. Franquesa. 2003. A bioeconomic model for Mediterranean Fisheries, the hake off Catalonia (Western Mediterranean) as a case study. Sci. Mar., 67 (suppl. 1): 337-351. In: Fisheries stock assessments and predictions: Integrating relevant knowledge, Ø. Ulltang, G. Blom [eds.].

Oliver, P. & E. Massutí. 1995. Biology and fisheries of western Mediterranean hake (M. merluccius). In Hake: Biology, fisheries and markets. Alheit, J. & T. J. Pitcher (ed.) London: Chapman & Hall, pp. 181-202.

Maravelias, C. D. & C. Papaconstantinou 2006. Geographic, seasonal and bathymetric distribu-tion of demersal fish species in the eastern Mediterranean. Journal of Applied Ichthy-ology 22: 35-42.

Papaconstantinou C. & K.I. Stergiou. 1995. Biology and Fisheries of Eastern Mediter-ranean hake (M. merluccius). In: Hake: Biology, fisheries and markets. Edited by J.Alheit and T. Pitcher. Chapman & Hall, London, 1995.

Acknowledgements

EFIMAS Contribution to the work

This work has been conducted under the EFIMAS Project.

Participants: Costas Papaconstantinou, John Haralabous, Christos Maravelias, George Tserpes

 
efimas1/wp4/cs8/appr3/main.txt · Last modified: 2008/11/16 01:22 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