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Case study 2: Approach 3: Evaluation of discard focusing on mixed fisheries aspects

Introduction

This approach evaluate scenarios addressing management issue two with respect to discard evaluation using the R/FLR framework. The approach use the OM developed under Approach 1 as well as the OM developed under Case Study 4 (in cooperation with CS4 and the EU FP6 Necessity Project) to evaluate different discard scenarios in the mixed fisheries context in relation to application of different gear technological selectivity properties and regulations.

The approach has incorporated results from a scientific study that utilises a gear selectivity formula, coupled with sorting ogive and length distribution at age data to generate discard numbers. This has been used in relation to conditioning (calibrating) the operating models, as well as to perform sensitivity analyses of the operating models, in relation to use of different selectivity properties.

Description of the fishery, stocks and management system

See description of this under the Case Study 2 Main Page.

Description of the base case and scenario evaluations

See description of this under the Case Study 2 Main Page.

Discard module using empirical net selectivity and sorting ogive

The proposed approach is to generate discard numbers using gear selectivity data coupled with sorting ogive and length distribution at age data as proposed by Casey (1996). Because numbers and lengths at age are known with certainty in the biological model it is possible to simulate a ‘true’ number of discards at age for any defined gear selectivity and sorting ogive.

Empirical data to parameterise net selectivity is available for cod, haddock and whiting. For the same species sorting data is available from the Scottish demersal fleets.

Data and parameters

Stock data

See case study 2, approach 1.

Fleet data

See case study 2, approach 1.

Modelling

FLR (Operating Model)

Conditioning of Operating Model

Historical Estimates of Time Series

See case study 2, approach 1.

Biological Parameters

See case study 2, approach 1.

Discards

= The Model =

Proportion of fish caught that enter a fishing gear

The following assumes all age/length groups are equally susceptible to exploitation. Therefore the proportion of each age group a entering the gear and being retained (Ra) is the sum over all lengths within an age group of the product of the proportions at length retained by the gear (Sl) and the proportions at length available for capture Pl|a

(1)

The distributions of length within a given age group can be obtained by direct observation through research vessel survey. In general lengths at age are considered normally distributed with density function

(2)

where μa is the mean length at age and σa gives the standard deviation.

Sorting ogive and proportion of fish at age landed

Commercially important demersal species are regulated with respect to minimum landing size. There may also be commercial reasons why smaller fish from a given species are discarded. The discarding practice of vessels, i.e. the proportions at each length retained and discarded can be obtained using observations at sea or by assuming a sorting ogive, similar in form to the gear selectivity ogive. If the proportion of fish at length that have been captured and are landed is given by PLl then the proportion of fish of age group a that entered gears and is landed (PLa) is given by

(3)

If the number landed at age (NLa) is known then the number of fish caught at age can be calculated from

(4)

The operating model of this case study simulates recruitment and includes a growth model component. Therefore in that part of the simulation model that deals with the real system the exact number of fish at age and their length distribution is known. Therefore if the entire population were fished the number of fish of age a landed is given by

(5)

and the number of age a fish discarded (NDa) by

(6)

where Na is the number of fish at age a. If total allowable catches (TACs) are assumed to represent landings and unallocated landings assumed zero then the proportion of the stock subjected to fishing can be considered equal to

                                               {{efimas1:wp4:cs2:appr3:eqn7.png?150|}}(7)
                       

where Wa represents mean weight at age a of the stock. The number of discards at any age is given by

                                               {{efimas1:wp4:cs2:appr3:eqn8.png?150|}}(8)

= Model Parameterisation =

Gear selectivity

The selectivity parameters for diamond mesh codends have been estimated from a dataset of 512 hauls by Fisheries Research Services, Aberdeen (FRS), (Anon, 2003). These parameters are the length of 50% retention (L50) and the length (or selection) range between 25% and 75% retention (SR). Most data was available for haddock and the selectivity of haddock was determined using a fixed and random effect model. Selectivity results were also determined for cod and whiting. Less data was available for these species so L50 values were found using an unweighted linear regression upon haddock L50, while SR values were determined assuming the ratio SR/L50 to be constant.

If considering all possible fleets/fisheries operating in the North Sea the number of possible gear types allowed under EU or national laws is extensive. For illustrative purposes we define here eight generic gear types based on the main mesh size ranges used to set effort limits within the latest regulation governing fishing opportunities (EC 2006). The gear definitions are given in Table 1 and are based on minimum mesh size within each mesh size category. Within each mesh size category specifications conforming to EU and UK legislation are defined with respect to fleets targeting Nephrops for gears with mesh <100mm and targeting demersal fish species for mesh sizes ≥100mm. Figure 1 shows the proportions at length retained by the mesh (Sl) for those gear types defined.

a) U.K. legislation

Mesh size Twine thickness Open meshes in Use of Square Mesh Panel
codend circumference lifting bag mesh size distance from codline
80mm 4mm ≡ 2.828mm Tnom 120 YES 90mm 18m
90mm 4mm ≡ 2.828mm Tnom 100 YES 90mm 18m
100mm 5mm ≡ 5mm Tnom 100 NO 90mm 12m
120mm 5mm ≡ 5mm Tnom 100 NO NA NA

b) E.U. legislation

Mesh size Twine thickness Open meshes in Use of Square Mesh Panel
codend circumference lifting bag mesh size distance from codline
80mm 5mm ≡ 5mm Tnom 120 YES 80mm NA
90mm 5mm ≡ 5mm Tnom 100 YES 80mm NA
100mm 5mm ≡ 5mm Tnom 100 YES 90mm NA
120mm 5mm ≡ 5mm Tnom 100 YES NA NA

Sorting ogive

Records of numbers of fish at length discarded and landed by the Scottish fleet are collected by the FRS observer programme. Usually sub-samples are taken from the baskets of fish discarded and from the fish retained for landing. Total numbers at length discarded were raised according to the proportion of baskets discarded that were sampled. Total numbers of fish at length landed were raised according to the ratio of landed weight to sampled weight. Taking cod for the purpose of illustration Figure 2 shows proportion of cod at length retained for landing if brought on board (PLl) obtained from sampling trips conducted between 1978 and 2003. Results have been aggregated over all years but results from the west of the UK have been separated from results from the North Sea (and to the north east of 4ºW). The records have been split further into those classified as coming from Nephrops trawls and those from other categories.

If there are sufficient samples the data conforms to the idea that proportions retained at length form a logistic curve (Figure 2 data from the North Sea, non-Nephrops gears). If data is more limited and the sorting ogive is not so clearly logistic it is still possible to use the data to provide the parameters necessary to define a suitable logistic curve. Logistic curves fitted to data from the west coast of Scotland are shown in red in Figure 2.

Gear selectivity at length (Sl) for cod using specifications for two sets of four generic gear types related to mesh size and targeted species. One set of specifications is based on minimum requirements under E.U. legislation, the other based on Scottish regulations. Vertical line denotes minimum legal landing size for cod.

Sorting ogives for cod using data from FRS observer sampling programme. NTR= vessels recorded as using a Nephrops trawl; other = vessels recorded as using either motor trawl, light trawl, seine or pair trawl gear types. Vertical line denotes minimum legal landing size. Red lines represent logistic curve formed using the L50 and L25 values from the west coast NTR and non-NTR gears.

Stock Recruitment Relationships

See case study 2, approach 1.

Fisheries & Fleets

See case study 2, approach 1.

Economics

See case study 2, approach 1.

Reference Points

See case study 2, approach 1.

Observation Error Model

See case study 2, approach 1.

Management Procedure

See case study 2, approach 1.

Results

Codend selectivity versus proportion landed

If values for proportions at length retained by the mesh (Sl) are multiplied by proportion at length retained for landing if brought on board (PLl) the result is an ogive of proportion at length both caught and landed. For a given gear this can be contrasted to the curve for Sl to gain an indication of the proportion at length discarded. Figure 3 shows results for cod for the generic gear types considered. Each gear has been matched to what is considered the most appropriate sorting ogive. For the 80 mm and 90 mm codend gears this was the smooth logistic curve parameterised by data from the North Sea Nephrops fishery. For the 100 mm and 120 mm codend gears a smooth logistic curve parameterised by data from gears other than Nephrops gears was used. The space between the Sl and Sl*PLl curves has been filled by a solid colour. At each length the distance of the shaded area along the vertical axis can be interpreted as the expected proportion discarded at that length.

Filled polygons showing the difference between the proportion of cod at length selected by a given gear and the proportion (of those entering the net) expected to be landed given the combination of gear and sorting ogive selected. At each length the distance of the shaded area along the vertical axis can be interpreted as the expected proportion discarded at that length. Vertical dotted line denotes minimum legal landing size. Legend: 70mm etc. codend meshsize of gear specifications found in table 1. Sorting ogives: NTR= vessels recorded as using a Nephrops trawl; other = vessels recorded as using either motor trawl, light trawl, seine or pair trawl gear types.

Dissemination

A report on the discard estimation methodology will form one part in a series of reports to be published by Fisheries Research Services (FRS), Scotland in relation to an internal research project investigating Harvest Control Rules (HCRs) and using R/FLR in a manner similar to this case study.

Links to Other Work

It is anticipated the method of calculating discards will be incorporated as a module within the R/FLR Management Strategy Evaluation (MSE) model developed for approach 1 of case study 2 in the first instance, (see also under case study 2, approach 1 'Links to other work').

This work is linked to EFIMAS Case Study 4 as well as work under ICES and the National Research Institutes involved in the case study.

References

Anon, (2003). Appendix 5 of Report of Expert Meeting on Cod Assessment and Technical Measures. Brussels April/May 2003. DGFish, European Commission, Brussels.

Casey, J. 1996. Estimating discards using selectivity data: the effects of including discard data in assessments of the demersal fisheries in the Irish Sea. J. Northw. Atl. Fish Sci., 19, 91-102.

Council Reg. (EC) No 51/2006 of 22 December 2005 fixing for 2006 the fishing opportunities and associated conditions for certain fish stocks and groups of fish stocks, applicable in Community waters and, for Community vessels, in waters where catch limitations are required, 183 pp.

Acknowledgements

EFIMAS Contribution to the work

This work is done as an cooperation between National Research Institutes, ICES WGNSSK, and the EU FP6 EFIMAS Project.

Participants

Participants: Steven Holmes, Coby Needle (FRS), J. Rasmus Nielsen, Clara Ulrich-Rescan, Katell Hamon (DIFRES), Laurence Kell, Chris Darby (CEFAS), Aoye Hoff, Hans Frost (FOI)

Discard modelling: Steven Holmes

 
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