Case study 1: North Sea Demersal Flatfish Fisheries

For this case study two stocks are considered; North Sea plaice and North Sea sole.

The total value of landings of the North Sea was 976 million euro in 2001 (Anon. 2002¹⁾). Flatfish fisheries (mainly plaice and sole) accounted for approximately 40% of this value and is regarded as a major fishery in the North Sea.

North Sea flatfish are mainly taken in a mixed demersal flatfish fishery by beam trawlers in the southern and south-eastern North Sea. Although plaice and sole are the main targets in the mixed flatfish fishery, important by-catches are often taken of other flatfish species (e.g. dab, turbot, brill) and some roundfish species (cod, whiting). Directed fisheries for flatfish are also carried out with seine and gill net, and by beam trawlers in the central North Sea. Due to the minimum mesh size (80 mm in the mixed beam trawl fishery), large numbers of (undersized) plaice and other species are discarded.

Fleets exploiting North Sea flatfish have generally decreased in number of vessels in the last 10 years, partly due to the MAGP policy and decommissioning schemes. However, in some instances these reductions have been compensated by reflagging vessels to other countries.

The state of the plaice and sole stocks are annually assessed by the ICES Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak (e.g. ICES 2005²⁾). Both plaice and sole have experienced relatively high fishing mortalities over a period of at least 20 years. Both stocks tend to produce occasional (very) strong year classes. The plaice stock has benefitted from the strong 1981 and 1985 year classes which led to high SSB levels (400,000 t.) in the late 1980s. In the early 1990s the stock decreased strongly, reaching a historic minimum in 1997 and has been seen to increase since then. Sole has experienced a similar high SSB level in the late 1980s but has generally been more stable than plaice.

Other flatfish stocks are not assessed because historic data have been lacking. Under the current data regulation (EC 2001³⁾), essential catch data for these species is being collected so that they can likely be assessed in the near future. A specific case study on the state of the turbot and brill stocks has been completed in 2000 (Boon and Delbare 2000⁴⁾) and could provide a starting point for further analysis.

The economic state of the fishing fleets is well described in the Annual Economic Reports that have been issued since 1998. The value of landings of the Dutch beam trawlers, the most important fleet segment in the North Sea flatfish fisheries, was rather stable in the years 1996-2001, but has decreased since then. Profits have also decreased and turned to losses over this period mainly due to increasing fuel prices. The number of vessels and employment declined, mainly due to lower catches. Similar trends occurred in the Belgian beam trawl segment. A significant part of the Belgian and U.K. flatfish fleet is owned by Dutch fishermen.

The North Sea flatfish fisheries have been managed by means of single species TACs, by technical measures and by fleet capacity measures. The TAC for North Sea plaice is agreed between Norway and the EC, all other TACs are set by the EC only. The technical measures (notably mesh size) applicable in the flatfish fishery are largely determined by the catching opportunities of sole, which is a highly valued species with a relatively slender build. Therefore, the minimum mesh size for beam trawl gears is set to 80 mm (and north of 55° N, where sole does not occur, 100 mm). Other technical measures include the “plaice box”, a closed area on the Danish, German and Dutch coasts. The plaice box has been implemented in 1989 and is closed for fishing with towed gears by vessels with engine powers exceeding 300 HP. In 2006 the EC has proposed a long term flatfish management plan which is still under evaluation. In this plan management of sole and plaice is linked, and the plan aims to reduce fishing mortality to long term sustainable target levels while TAC changes should not be too large. The plan aims to restrict fishing through TACs and days at sea restrictions.

Several studies of scenario evaluations have been undertaken using FLR as evaluation frame.

1. The first (chapter 7.3 in Grift et al. 2005⁵⁾) is a spatially explicit simulation model for the evaluation of the effects of five management scenarios: a. Increasing the mesh size; b. Effort reduction; c. Efficiency reduction of Euro cutters; d. Move 30% of the effort below 55° to the North; e. The integration of several management measures. This study was commissioned by the Dutch ministry of LNV (Agriculture, Nature, and Food Quality) and the Dutch Fish Product Board.
2. The second study (Poos et al. 2006⁶⁾; Pastoors et al. 2006⁷⁾; Pastoors et al. 2007⁸⁾), also commissioned by the Dutch ministry of LNV (Agriculture, Nature, and Food Quality), is an evaluation of a management plan for North Sea flatfish that was advised by the North Sea RAC in 2005. The evaluation has been restricted to the effects of proposed effort measures. Limitations owing to TACs and the proposed 15% limit in annual TAC changes have not (yet) been used in this evaluation.
3. The third study is an evaluation of the long term management plan for North Sea plaice and sole proposed by the EC in January 2006. The biological part of the evaluation Machiels et al. 2006⁹⁾) was initially commissioned by the Dutch ministry of LNV (Agriculture, Nature, and Food Quality). The study was subsequently extended with economic models by STECF in an impact study of the plan (STECF 2006¹⁰⁾). In the EC plan, management of sole and plaice is linked, and the plan aims to reduce fishing mortality to long term sustainable target levels through TACs and days at sea restrictions, while TAC changes should be within 15% limits.
4. The third study resulted in a fouth study when the actual implementation of the flatfish management plan was evaluated in a study by Machiels et al. in 2008¹¹⁾.
5. A study on the effect of economic dynamics on the management of the mixed flatfish fisheries has resulted in a publication that will be published in Aquatic Living Resources. This study extends the base case by using field estimate relations between catches and effort (Oostenbrugge et al. 2008).
6. Furthermore, one more approach, initiated under EU FP5 TECTAC project, was developed using the TEMAS evaluation frame software. It is described under Case Study 1, Approach 2

Each of these studies considers the mixed fishery of North Sea plaice and sole. The scenario evaluations are presented below and described in detail on the respective links.

The base case of the North Sea flatfish Case Study treats the two species – North Sea sole and North Sea plaice – independently (as is currently done in ICES). The Base Case will be a biological-economic model consisting of two unlinked single species models for plaice and sole respectively. The model will be as close as possible to the assumptions of the ICES stock assessment. The base case will look as follows, for each of the two species (sole and plaice).

The biological OM

• M, Stock Weight, Catch Weight, Maturity as in ICES WGNSSK
• One fleet, with historical exploitation pattern from ICES WGNSSK; future exploitation pattern constant as the average of last five years.
• Historical recruitment from ICES WGNSSK, future recruitment, e.g. a Shephard model with error.
• Historical yield from ICES WGNSSK
• 15 age groups (age 15 years and older in the plus group)

The biological MP

• Perceived M and Maturity equal to “true” values (of OM)
• Historical perceived Stock and Catch weight equal to “true” values (OM), for future equal to the averages of the “true” values (OM) of the previous three years, as is done in ICES WGNSSK.
• Perceived Catch-at-age, equal to “true” (OM).
• CPUE series; Catchabilities and errors from existing tuning series from ICES WGNSSK.
• XSA assessment, as in ICES WGNSSK.
• Advice: TACs based on catch forecasts, with the assumption for the intermediate year being status quo F (as in ICES WGNSSK), with Fpa as target F as long as SSB is above Bpa, and otherwise the highest F that brings SSB above Bpa in one year.

The economic model

• Non-linear effort/F relationship.
• Prices based on landings and price elasticity.
• Revenues = Sum(Landings*Price) by species.
• Costs: Proportional to landings and effort.
• Profits (= net revenues) = revenues – costs.

Case Study 1, Approach 1

See also Case Study 1, Approach 2, TEMAS-based modelling with fleets behaviour

The results of each of the studies are described in the respective documents (Grift et al. 2005, Poos et al. 2006, Pastoors et al. 2006, 2007, Machiels et al. 2007, STECF 2006, Machiels et al. 2008). The economic dynamics of the proposed management plan were described in (Oostenbrugge et al. 2008). Each of these documents describes the effects of different management proposals to the North Sea Demersal flatfish fisheries. Because the different management proposals interact differently with the fisheries system, different processess have been incorporated in the operating models of the simulation studies.

The first study, described in (Grift et al. 2005), studies several scenarios, with a focus n plaice.It should be noted that the results of the simulations should be interpreted with caution. The exact values are strongly dependent on the parameterisation of the model. Landings and stock biomass trends will be very sensitive for the assumption of the incoming recruitment as well as the growth rate and the spatial distribution. Nevertheless, the simulation studies can be used to compare the various scenarios to explore which management measure will contribute to the goal of rebuilding the stock. The first scenario was an increase of the mesh sizes for the beam trawl fleet in the southern region. In this region, the current minimum mesh size for trawled gears targetting flatfish is 80 mm. The results indicated that an increase in mesh size will hardly result in a decrease in the landings as compared to the status quo scenario (80 mm). An increase in mesh size will reduce the fishing mortality of the youngest age groups, which dominate the discards. Already one year after the mesh increase, the landings will be higher than with the 80 mm status quo scenario. Discard percentages will be substantially reduced, whereas the SSB shows a substantial increase. With an increase in mesh size to 100 mm SSB will increase to a level above Bpa in two years. A second scenario involved reducing a reduction of the fishing effort. This scenario explored the effect of a reduction in fishing effort of 10 to 30% in all fleets. Effort reduction was simulated as a reduction in the days at sea and was applied evenly during all quarters of year. In this scenario, the landings temporarily drop for 2 years and then increase to a level exceeding the level of the status quo scenario. Discard percentage decreases from about 45% at status quo, to about 35% at a 30% reduction in effort. Already at an effort reduction of 10%, SSB will increase to the precautionary reference level by 2012. With an effort reduction of more than 10%, SSB will increase beyond the Bpa level in 2008. For the other tested scenario's in this study we refer to the report.

Figure 1. summary of mesh size reduction scenario's with respect to landings, SSB and discard fraction of plaice

Figure 2. summary of effort reduction scenario's with respect to landings, SSB and discard fraction of plaice

The second study, described in detail in Poos et al. 2006, Pastoors et al. 2006 and 2007 deals with 4 issues regarding the management of North Sea plaice and sole: an evaluation of a management plan advised by the North Sea RAC in 2005, the relation between fishing effort and fishing mortality, the effects of TAC size on fishing mortality and biomass and the TACs needed to ensure stock sizes above Bpa. The evaluated management plan was supplemented by measures and objectives by the fisheries department of the Dutch Ministry of Agriculture, Nature Conservation and Food Quality. The evaluation was restricted to the effects of effort measures. Limitations owing to TACs and the 15% limit in annual TAC changes have not been used in the evaluation. The management plan did not always clearly state the objectives and measures. This leaves room for interpretation, and results from analyses evaluating this plan are dependent on this interpretation.

The simulation consists of an operating model and a management procedure. The operating model is expected to mimic the true stock and fishery dynamics. The management procedure consists of the process of acquiring data, doing stock assessments and implementing a harvest control rule. The operating model consists of two species (plaice, sole), two areas (north, south) and two fleets. All the relevant processes in the stocks and the fishery have been modelled but at different levels of detail. The operating model has been conditioned on the data from the ICES Working Group on demersal stocks in the North Sea and Skagerak (WGNSSK). The operating model was “fitted” using a very simple iterative process to estimate catchability, distribution over areas and recruitment levels. The final operating model generates approximately equal stock sizes in both observed and fitted SSB but the predicted landings for sole and plaice appear to be lower than the observed landings. The results indicate that the probability of achieving the plaice Bpa target in 2010 is between 98% and 62% depending on the type of stock-recruitment relationships and whether or not additional measures are taken when a stock is below Blim. The probability of reaching the sole Bpa in 2010 is between 82% and 48%. Owing to the low recruitment in 2003 and 2004 that has been measured in the surveys, the SSB in all runs is expected to fall below Bpa in the first years after the implementation of the management plan. The assumption on the shape of the stock recruitment relation has a strong impact on the expected success of the plan. The Ricker curve can be interpreted as a favourable environmental scenario and the Beverton-Holt as an unfavourable scenario. For plaice, the expected landings associated with the management plan remain stable between 65 000 and 75 000 tonnes. For sole, the landings initially decrease in all runs. When a favourable environmental regime is assumed (Ricker curve) the landings are expected to increase again after the initial decline.

The success of effort management is dependent on a clear relation between fishing effort and fishing mortality. At present that relationship is poorly known, and surrounded by a large amount of variance, especially for North Sea plaice. Observed changes in the relation between fishing effort and fishing mortality may be the result of shifts in the spatial distribution of fishing effort. Such shifts are observed during the last 10 years, with fishing effort being allocated closer to the coastal areas. The model used to evaluate the effects of the NSRAC management plan assumes a much more “responsive” effort-F relationship than observed in recent history, which could give the impression that changing effort will be directly detectable whereas in practice these changes in effort have often not lead to changes in fishing mortality. The analysis of the effect of the size of the TACs on the spawning stock biomass of North Sea sole and plaice on the short-term indicates that a reduction of the plaice TAC to 55 000 tonnes is needed In 2006 or 2007 to ensure SSB exceeds Bpa. For North Sea sole, reductions of the TACs to levels below 15 000 tonnes are needed in both 2006 and 2007 to ensure spawning stock biomass does not fall below Bpa in 2008.

Figure 3. summary of the different scenario's tested in Poos et al. 2006, with respect to landings of plaice (far left panels), landings of sole (center left panels), SSB of plaice(center right panels) and SSB of sole (far right panels). Boxplots connected through median by black line indicate model fit and prediction. Blue line indicates WG estimates of landings and SSB from WGNSSK 2005. Horizontal lines indicate Bpa (black) and Blim (red).

Finally, there were the two studies on the long term management plan proposed and imlpemented by the EC. hese are described in detail inMachiels et al. 2007, STECF 2006 and Machiels et al. 2008. Here we describe the first of these studies, which deals with the first proposal of the management plan. For the purpose of evaluating the plan, a simulation model was developed, which contains several modules. The operating module simulates the true stock and dynamics of the fishing fleet. An observation module mimics the indices generated by fisheries-independent surveys and the observed catches and catch at age composition from the commercial catches. Based on this information a stock assessment using the XSA procedure is executed, which results in perceived stock numbers at age and fishery mortality rates per age group. The assessment results are inputs to calculate the TAC's and the maximum number of days at sea following the rules in the management plan.

Spatial and seasonal differentiation in stock abundance and fleet effort allocation were not included. Also the fleet structure was simplified, the two stocks are exploited by a beam trawl fleet, which consists of the combined Dutch and UK fleet. In practice these fleets contribute most to the international catch. The operating model has been conditioned using data from the ICES Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak (WGNSSK), by calibrating catchability and recruitment levels from the historical data. The behaviour of the fishing fleet was simulated using a number of options on the fisher’s response to the annual management measures. This fleet behaviour is uncertain and therefore several scenarios were formulated.

Results show that through the plan proposed by the EC, F target levels have been reached in 2015. At the same time the effort allowed (maximum number of days at sea) reduces to about 50% of its current (=2006) level. SSB of both species are expected on average to increase and the risk that SSB is below Bpa in 2012 is less then 20%. Under the assumption of a Ricker type stock recruitment relationship, average recruitment until 2015 shows no trend. Assuming a Beverton and Holt stock recruitment function results in a positive trend for the recruitment. Average TAC’s and landings vary depending on the scenario used for a run. TAC’s and landings for sole seem to level of at 14000-15000 tons. For plaice TAC and landings increase on average with 4000 tons per year at the end of the simulation period (2014).

Figure 4. summary of the different scenario's tested in Machiels et al. 2007, The TAC, landings (in thousand tonnes) and SSB and recruitment over time (plaice: far left and center left panels, sole center right and far right panels). Red: scenario 1. Black: scenario 2. Triangles: medians. Thick lines end at the 25th (red) and the 75th (black) percentile respectively. Thin lines end at the 5th (red) and 95th (black) percentile respectively. For scenario 1 only the downward variation and for scenario 2 only the upward variation are shown, but the uncertainty is expected to be similar between the scenarios.

The economic dynamics of the first proposal of the long term management plan were studied in Oostenbrugge et al. (2008). This study extended the management plan evaluation of Machiels et al. (2007) by estimating the relation between fishing effort and fishing mortality and using this relation in a simulation of the management plan. The methodology introduces a simple and practical algorithm for a nonlinear catch - input relationship based on accepted theory in economics (decreasing returns to effort). The basic assumption is that when subject to effort restrictions fishermen will skip those trips from which they expect the lowest earnings per unit of effort. The methodology indicates a convex catch – input relationship in case of effort management, opposed to a concave relationship when applied in TAC driven scenarios. The catchability was defined as: qt=q0(Et/E0)β Where q = catchabilty and E = Effort and β is an estimated parameter. Estimation of the parameter β is based on cross sectional analysis of variation in results by vessel and by trip. The catchability equation and other economic equations fit into stock dynamics models based on standard procedures applied by ICES. The functions themselves can be “plugged into” into stock dynamics models as needed. Both effort and TAC functions are available and can be applied depending on the type of restriction prevailing in the management plan. In addition, an accounting function can be used in combination with either of the above functions to calculate typical accounting identities such as gross revenues and profits. The equations used in the model are described shortly in Appendix .. of ECONOWS (2008). A more detailed description of the model is found in Oostenbrugge et al. (2008). The following diagrams show non-linear relationship between the cumulative catch of plaice (left) and sole (right) versus cumulative effort for Dutch large beam trawlers for 2006.

Fig. 5 Non-linear relationship between the cumulative catch of plaice (left) and sole (right) versus cumulative effort for Dutch large beam trawlers for 2006.

Non-lineair catch-effort relation ship evidently causes a larger catchability than intended by the management plan. This has both biological and economic implications. In comparison to the linear case, non-linear cases result in larger fishing mortality rates and hence smaller SSBs. However the model demonstrates that a higher catchability, all else equal, means that the chance that an individual fish will be caught by gear will increase, so that a lower SSB doesn’t necessarily translate into a lower catch. In fact, the economic results predicted by the model show that the economic implications continue to be positive; the results for the non-linear cases are better than predicted by the linear case because the increase in catchability for a given amount of effort will more than offset the reduction in SSB. The diagrams present a summary of economic and biologic results following from annual reduction of effort by 10% following from the multi-annual management plan, with different assumptions for the effort-F relation.

Figure 6. Summary of the different scenario's tested in (Oostenbrugge et al. 2008), with respect to landings, net revenue and SSB. Each line in a panel indicates a scenario with a different assumption for the curvature of the effort-F relation.

The results of the TEMAS-based modelling and simulations of fleets behaviour under CS1 approach2, which was initiated under the EU FP5 TECTAC project, are described under Case Study 1, Approach 2 (TEMAS Evaluation Frame evaluation).

Where the work was commissioned by external clients, reports have been distributed accordingly: Grift et al. (2005), Poos et al. (2006), and Machiels et al. 2007 to the Dutch ministry of LNV (Agriculture, Nature, and Food Quality), and STECF 2006 to the EC. In addition, the work of |Poos et al. (2006) and Pastoors et al. (2006) was circulated in the North Sea RAC, and was used in the ICES SGMAS in January 2006. This work was subsequently presented at the ICES symposium on Fisheries Management in Galway in June 2006 and has been published as a peer-reviewed article (Pastoors et al. 2007). Machiels et al. 2007 was in addition used in the ICES WGNSSK in September 2006, and served as a basis for the work by STECF 2006. Machiels et al. 2008 was used in the ICES WGNSSK in May 2008. The economic study will be published in Aquatic Living Resources and was presented at the EAFE conference (2007). Further details are available in ECONOWS (2008) ECONOWS: Report from the Economic Workshops of EFIMAS.

Dissemination in relation to CS1, approach2: This work was preented to the ICES symposium on Fisheries Management Strategies, 27-30 june 2006. See the presentation slides.ppt. Ulrich, C., Andersen, B.S., Sparre, P.J., and Nielsen, J.R. 2007. TEMAS: fleet-based bioeconomic simulation software to evaluate management strategies accounting for fleet behaviour. ICES J. Mar. Sci., 64: 647-651. Ulrich et al. (2007).

All the work discussed here is carried out in close connection with the EC-funded COMMIT project. In addition, the work by Poos et al. (2006) and Pastoors et al. (2006) is linked to work carried out by the ICES SGMAS. And the work by Machiels et al. 2007 is linked to work carried out by the ICES WGNSSK and STECF 2006. The work on the economic model is linked to the EC-funded CAFÉ and CEVIS projects.

In relation to CS1 Approach 2 this work has been initiated under and conducted as a cooperation with the EU FP5 TECTAC Project.

All of the work has been funded by the EU EFIMAS project. A number of studies done within this context have been additionally been funded by other EU projects, like COMMIT, or though national funding programs. In relation to CS1 Approach 2 this work has been initiated under and conducted as a cooperation with the EU FP5 TECTAC Project.

Anon. (2002). Annual Economic Report 2002. Economic performance of selected European fishing fleets.

Boon, A. R. and D. Delbare (2000). By-catch species in the North Sea flatfish fishery (data on turbot and brill) preliminary assessment (DATEBRAS). Final report, EC 98/078, RIVO. C020/00.

EC (2001). COUNCIL REGULATION (EC) No 1639/2001 of 25 July 2001 establishing the minimum and extended Community programmes for the collection of data in the fisheries sector and laying down detailed rules for the application of Council Regulation (EC) No 1543/2000. No. 1639/2001.

Grift, R., W. Dekker, O. van Keeken, S. Kraak, B. van Marlen, M. Pastoors, J.J. Poos, F. Quirijns, A. Rijnsdorp, I. Tulp. 2005. Evaluation of management measures for a sustainable plaice fishery in the North Sea. RIVO report C019/05.

Hoff, A., Frost, H. 2006. Economic response to harvest and effort control in fishery. FOI report.

ICES (2005). Report on the Assessment of Demersal Stocks in the North Sea and Skagerrak. Bergen, Norway, 7-16 September 2004. ICES CM 2005/ACFM:07.

Machiels, M.A.M., Kraak, S.B.M., van Beek, F.A. 2007. Evaluation of a management plan as proposed by the European Commission in 2006 for fisheries exploiting stocks of plaice and sole in the North Sea. IMARES report C011/07

Machiels, M.A.M. Kraak, S.B.M., Poos, J.J. 2008. Biological evaluation of the first stage of the management plan for fisheries exploiting the stocks of plaice and sole in the North Sea according to Council Regulation (EC) no 676/2007 IMARES report C031/08

Netherlands Directorate of Fisheries, Netherlands Institute for Fisheries Research, LEI B.V. (Institute of Agricultural Economics) (2006). Technical Report of Activity 2005 – THE NETHERLANDS, detailing the state of completion of the aims set at the time of the drawing-up of the minimum programme and of the extended programme of the Data Collection Regulation. (http://datacollection.jrc.cec.eu.int/NP/2005/Technical%20Report%20NL%202005.doc)

Oostenbrugge, J.A.E. van, Powell, J.P., Smit, J.P.G., Poos, J.J., Kraak, S.B.M., Buisman, E.F.C. 2008. Linking catchability and fisher behavior under effort management. Aquatic Living Resources (in press)

Pastoors, M.A., Poos, J.J., Machiels, M.A.M. 2006. Evaluation of a proposed management plan for Northsea flatfish. http://flr-project.org/doku.php?id=applications:nsrac

Pastoors, M.A., Poos, J.J., Kraak, S.B.M., Machiels, M.A.M. 2007. Validating operating models in simulations of management plans and the implications for how the results can be communicated. ICES Journal of Marine Science. 64:818-823

Poos, J.J., Machiels, M.A.M., Pastoors, M.A. 2006. Investigation of some management scenarios for North Sea sole and plaice in 2006 and beyond. CVO report 06.004.

STECF 2006. Impact assessment of long-term management plans for sole and plaice. http://old-stecf.jrc.it/meetings/sgeca/0605/stecfreportanannex.pdf

Ulrich, C., Andersen, B.S., Sparre, P.J., and Nielsen, J.R. 2007. TEMAS: fleet-based bioeconomic simulation software to evaluate management strategies accounting for fleet behaviour. ICES J. Mar. Sci., 64: 647-651. Ulrich et al. (2007).

Coordinator: Sarah Kraak, Marcel Machiels

Participants (approach 1): Laurence Kell, Ayoe Hoff, Hans Frost, Charlotte Deerenberg, Clara Ulrich Rescan, Graham Pilling, Hans van Oostenbrugge, Jan Jaap Poos, Marcel Machiels, Martin Pastoors, Sarah Kraak, Trevor Hutton, Wim Demaré

Participants (approach 2): Bo Sølgaard Andersen, Youen Vermard, Clara Ulrich, Holger Hovgaard, Dave Bromley, Simon Mardle, Jan Jaap Poos, Per Sparre, Hans van Oostenbrugge