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Approach 2: Evaluation of sensitivity of the current Nephrops stock assessment tools to varying parameter inputs

Introduction

The FLR framework was used to evaluate the effect of alternative nephrops gears on the commercially exploited gadoid stocks and fisheries (i.e. cod, haddock and whiting). Within commercial fisheries, particularly mixed fisheries, both target and non-target species are often discarded. Discarding represents a potentially significant loss to the productivity of fish stocks; it can have damaging ecological consequences, and is a potential cause of the failure of recovery plans. The Nephrops fishery in the North Sea is classified as a mixed fishery. Nephrops trawls are constructed with smaller meshes than trawls used to target whitefish; consequently, the bycatch of juvenile fish can be substantial. Several new Nephrops trawl designs have been tested in the North Sea. The data from these trials are used to investigate the potential impact of their implementation on cod, haddock and whiting stocks in the North Sea (including the Kattegat and Skagerrak).

This work and the simulation tool development is carried out in cooperation with the corresponding work and simulation model development under Case Study 2, Approach 3.

Description of the fishery, stocks and management system

See main page under Case Study 4.

Description of the base case and scenario evaluations

See main page under Case Study 4.

Data and parameters and Modelling

See paper publication: T.L. Catchpolea, , , A.N. Tidda, L.T. Kella, A.S. Revilla and G. Dunlinb The potential for new Nephrops trawl designs to positively effect North Sea stocks of cod, haddock and whiting Fisheries Research Volume 86, Issues 2-3, September 2007, Pages 262-267.
Catchpole et al 2007

EFIMAS Irish Sea Nephrops Report

EFIMAS-NECESSITY Changes in Gear Selectivity_Nephrops_Portugal

EFIMAS_Nephrops case study_Portugese Report

EFIMAS_Nephrops_case_study_Bay of Biscay_France

Results

Stock assessments have been run on affected whitefish stocks, with and without discards to determine effects of discarding. The process is repeated with modified discard data that comes from the using the modified fishing gears.

==== North Sea ====

Within commercial fisheries, particularly mixed fisheries, both target and non-target species are often discarded. Discarding represents a potentially significant loss to the productivity of fish stocks; it can have damaging ecological consequences, and is a potential cause of the failure of recovery plans. The Nephrops fishery in the North Sea is classified as a mixed fishery. Nephrops trawls are constructed with smaller meshes than trawls used to target whitefish; consequently, the bycatch of juvenile fish can be substantial. Several new Nephrops trawl designs have been tested in the North Sea. The data from these trials are used to investigate the potential impact of their implementation on cod, haddock and whiting stocks in the North Sea (including the Kattegat and Skagerrak).

The model developed examines five trawl designs, and also the scenarios of a cessation of discarding in all North Sea fisheries and in just the Nephrops fishery. The model is deterministic, and evaluates the relative differences between scenarios assuming all other variables remain constant. If discarding of cod, haddock and whiting in the North Sea fisheries were eliminated, stocks would increase by 41%, 14% and 29% respectively within 10 years. Eliminating discarding in the Nephrops fishery alone would increase stocks by 2%, 1% and 13% respectively, reflecting the relative proportion of catches of these species in the Nephrops fishery. For cod and haddock, the introduction of the Nephrops trawl with a grid with a square-mesh codend was the only scenario in which a notable increase in stock number was observed. This trawl design facilitates the escape of fish of all ages/sizes from the trawl, effectively making the Nephrops fishery a single-species fishery. For whiting, stock numbers and landings increased under all scenarios, but forecasted landings were lower than if current discard patterns continued in all except the no-discards scenario. The dependency of the results on the validity of the assumptions and on the accuracy of the input data is discussed. Full details of this work has been peer-reviewed and is published in Fisheries Research (Catchpole et al, 2007). Catchpole et al 2007

==== Irish Sea ====

An Irish case study has been undertaken which modelled the potential effects upons stocks of whitefish which could result from the introduction of the coverless (cutaway) trawl in selected Irish Nephrops fisheries. Full details on this work is contained in the pdf attachment . EFIMAS Irish Sea Nephrops Report

==== Portuguese Coast ====

A case study has been undertaken pertaining to the nephrops fisheries located off the Portuguese coast. In this model the Portuguese workers successfully identified the immediate and long term effects on landings and spawning stock biomass of nephrops Norwegicus from the southwest and south of Portugal as a result of changing trawl cod end mesh size, configuration and introducing a sorting grid. Full details on this work are detailed in the pdf attachment (document and powerpoint presentation). EFIMAS-NECESSITY Changes in Gear Selectivity_Nephrops_Portugal
EFIMAS_Nephrops case study_Portugese Report

== Bay of Biscay ==

A case study has been undertaken by french Colleagues who examined the effect upon stocks of Nephrops norwegicus in the bay of Biscay in relation to various fishing gear changes. full details are contained in the attached pdf document. EFIMAS_Nephrops_case_study_Bay of Biscay_France

Dissemination

T.L. Catchpolea, , , A.N. Tidda, L.T. Kella, A.S. Revilla and G. Dunlinb The potential for new Nephrops trawl designs to positively effect North Sea stocks of cod, haddock and whiting Fisheries Research Volume 86, Issues 2-3, September 2007, Pages 262-267.
Catchpole et al 2007

EFIMAS Irish Sea Nephrops Report

EFIMAS-NECESSITY Changes in Gear Selectivity_Nephrops_Portugal

EFIMAS_Nephrops case study_Portugese Report

EFIMAS_Nephrops_case_study_Bay of Biscay_France

Links to Other Work

Case Study 2 Approach 3

References

T.L. Catchpolea, , , A.N. Tidda, L.T. Kella, A.S. Revilla and G. Dunlinb The potential for new Nephrops trawl designs to positively effect North Sea stocks of cod, haddock and whiting Fisheries Research Volume 86, Issues 2-3, September 2007, Pages 262-267.

EFIMAS Irish Sea Nephrops Report

EFIMAS-NECESSITY Changes in Gear Selectivity_Nephrops_Portugal

EFIMAS_Nephrops case study_Portugese Report

EFIMAS_Nephrops_case_study_Bay of Biscay_France

Acknowledgements

EFIMAS Contribution to the work

This work was made as a cooperation under the EU FP6 EFIMAS Project and the EU FP6 NECESSITY Project.

Participants

Participants: See CS4 Main page.

J. Rasmus Nielsen 2008/11/04 17:04

 
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