The Ministry of EL&I asked IMARES to do an ex ante evaluation of temporal / spatial closures based on effort and cod catches by the Dutch demersal, TR1 

Beare, D.J.

Helmond, A.T.M., van

Machiels, M.A.M.

English

Wageningen Yield 

  Ex-ante evaluation of 
Seasonal,  Real Time and 
Move-on Closures. 
  
   
   
 Doug Beare, Edwin van Helmond and Marcel Machiels   
   
 Report number C084/11   
   
 
 
  
 
 
   
   
   
    
 
IMARES Wageningen UR 
(IMARES - Institute for Marine Resources & Ecosystem Studies) 
 
  
    
 Client: ir. H.R. (Henk) Offringa, Directie Visserij (DVIS), EL&I 
Bezuidenhoutseweg 73 
2594 AC Den Haag  
 
    
    
    
 Publicatiedatum: 19/07/11  
 
2 of 9 Report number C084/11 
 
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A_4_3_2-V12.1  
Report number C084/11 3 of 9 
 
Contents 
 
 
 Contents .................................................................................................................. 3 
 1. Summary & Conclusions ......................................................................................... 5 
2. Introduction .......................................................................................................... 5 
3. Overall Dutch Cod-CpUE by year .............................................................................. 6 
4. Effect of spatio-temporal closures ............................................................................. 6 
5. Effects of move-on measures ................................................................................... 7 
Justification ............................................................................................................. 10
 
4 of 9 Report number C084/11 
 
1. Summary & Conclusions 
 
An ex ante evaluation of the effect of closures on cod LpUE is executed by modelling spatial and temporal 
closures of two fleets (TR1 and TR2), based on previous year(s) landing and effort data. The change of 
cod CpUE is proportional to the change of cod LpUE and depends on the discarded fraction of the cod 
catches. 
 
 Reductions in LpUE can be substantial when the current year is used to inform the choice of 
closures. 
 
 Reductions in cod LpUE are much less, and can remain unchanged (TR1) on average, when the 
previous year is used to inform the choice of closures. 
 
 Some reductions (15%) in cod LpUE by TR2 can be achieved even when closures are informed 
with previous year’s data. Using the previous two year's data show similar reductions 
 
 Reductions in TR1 (<120mm mesh) cod LpUE will not be achieved when choice of closures is 
informed with the previous year’s data but would be substantial in the situation of real-time 
closures. 
 
 The move-on measures seem to be potentially the most useful , leading to substantial LpUE 
reductions, but it all depends on the actual implementation of the measure. 
 
 Setting a maximum cod-LpUE of 20-40 kg/2 hours, to trigger a move-on measure, has on 
average similar effects as limiting the cod fraction in the (2 hour) landings to 5%, but the 
variation is less. 
 
 The effects of applying both closure and move-on measures are additive. 
 
 
2. Introduction 
The Ministry of EL&I asked IMARES to do an ex ante evaluation of temporal / spatial closures based on 
effort and cod catches by the Dutch demersal,  TR1 (<120mm mesh) and TR2, fleets 1. The background 
to the project is the planned transfer of approximately 6.6×106 KW-days of BT2 effort to TR1 and TR2 
gear categories. This is because fishing with TR gears is believed to have less damaging environmental 
impacts. To facilitate the effort transfer a 'Cod Avoidance Plan' was developed which combined 'technical 
measures' such as gear modifications with various types of  spatio-temporal closure.  The ideas was that 
these measures should be able to reduce the Cod-CpUE, Catch per Unit of Effort, of TR1 and TR2.  It was 
IMARES' job to ascertain whether the intended reductions in CPUE were actually achievable using the 
measures suggested by the Ministry   
 
3. Overall Dutch Cod-CpUE by year 
 
                                                 
1 . Gear grouping is defined in Counsel Regulation(EU) No 1342/2008: 
 TR1 Bottom trawls of mesh equal to or larger than 100 mm and less than 120 mm 
 TR2 Bottom trawls of mesh equal to or larger than 70 mm and less than 100 mm 
 BT2 Beam trawls of mesh equal to or larger than 80 mm and less than 120 mm  
Report number C084/11 5 of 9 
 
The estimation of overall Cod-CpUE’s by TR1, TR2 and BT2 , between 2006 and 2010, is based on 
landing- and effort information from logbooks and our best estimates of cod-discard fractions by weight 
and presented in Table 1. Discard fractions by weight are smaller than fractions by number because 
average size of discarded cod is smaller than the size of landed cod. The discard fraction of TR2 (0.21) is 
based on estimates of cod-catches of twinning trawl vessels presented by Helmond and Overzee (2009)2 
and is lower than (STECF) estimates. STECF discard estimates are used for the fractions of TR2 and BT2. 
To meet the requirement of an effort transfer from BT2 to TR of 3 to 1, the cod-CpUE [cod-landing ( kg ) / 
 effort ( Kwdays )] should be less than 0.18 kg/Kwdays in 2011. The 2010 CpUE of the TR1 fleet is 1.5 
times less and the 2010 TR2 CpUE is slightly lower (0.163 kg/Kwdays). 
 
 
4. Effect of spatio-temporal closures 
The decision on potential candidate areas and -periods for closure starts with identifying the area/period 
combinations with highest cod concentrations. The ex-ante evaluation is done by model calculation based 
on a landing database using the VISSTAT logbook database (2006-2010) combined with the VMS 
(geographical position) database of the individual vessel-trips from 2006 to 2010. The landings, 
partitioned to VMS positions every 2 hours of a trip, are assigned to subareas by dividing each ICES 
rectangle into (4x4=16) sub-squares. Since model calculations are based on landings, results in this 
report are presented as effects on LpUE values. Relative changes in LpUE's and CpUE's are equal in case 
one discard fraction per year and gear category is used. It should be noted that the model-data lack 
landing- and effort information from small vessels (<15m). Total effort is between 80 and 90 % of the 
total VISSTAT effort and total cod-landing is approximately 95%. LpUE estimates from model calculations 
are therefor higher then LpUE estimated from logbook data. 
 
Cod-landings and fishing-effort are aggregated to month/sub-area combinations. These aggregated 
records are sorted on the absolute level of landings, revealing the areas with the highest cod-landing per 
month. Depending on the selected boundaries of the closure period and the number of sub-areas to be 
closed in each month of the period, the actual months/area closures are defined and merged with the 
landings database. Database records that meet the closure criteria are selected and the cumulative 
fishing effort of this selection is re-distributed among the records that are outside the closure criteria 
during the same trip. The cod-landings realized in that particular location  are reduced to 0 while  the 
effort is redistributed over the non-closures. The landings that might be realized in the non-closure part 
of a trip are 'corrected' by multiplying the cod-landings with the ratio of effort, after and before, the 
redistribution. This correction, based on the increase of effort, assumes that cod-LpUE of non-closures 
remains the same. The savings reached by the selected closure scenario is expressed as a (relative) 
value (%) of the average cod-LpUE after modeling the scenario. 
 
Different scenarios are possible with respect to merging the closures with the landing database. The 
most realistic procedure is to aggregate cod-landings and fishing-effort in year Y and merge the selected 
closure areas-months with the landings-database of year Y+1 because then closure selection is based on 
the best available information. The Ministry asked us to extend this scenario, in conformity with the 
English system, by aggregating cod-landings and fishing-effort over 2 years to determine closure 
selection, so information gathered during years Y & Y-1 is applied in year Y+1. 
 
In the case of Real Time Closures (RTC's), where fishers supply managers directly with information on 
their actual cod landings per haul, showing temporal and spatial cod concentrations, a scenario where 
closure criteria are gathered and applied to the landings-database in the same year Y is applied. 
                                                 
2[Helmond, A.T.M. Van Overzee, H.M.J. Van (2009) Discard sampling of the Dutch Nephrops fishery in 
2007-2008 IJmuiden : Centre for Fishery Research, (CVO report 09.007)] 
6 of 9 Report number C084/11 
 
  
When 120 closures are selected, by defining 10 closed areas for each month of the year, the largest 
LpUE reductions are attained when closure selection is based on aggregated landings in the current year. 
TR1 LpUE's reduced on average to 22% (SD=11%) of its original value, which is a reduction of 78%. TR2 
LpUE was reduced to 48% (SD=1%). When previous years' data are used, the mean LpUE of TR1 
remains unchanged (average=102% and SD=11%) while  the LpUE of TR2 reduced to 85%  of its 
original value (SD=8%). 
In case the closure (sub-)area-month matrix is reduced to fewer month and/or areas combinations, the 
reduction of LpUE is proportionally lower. A scenario of applying closures in winter (4 months, Dec-Mar) 
and 9 areas per month the (TR2) cod-LpUE is reduced to approximately 95% of the original value 
(SD=7%), using previous years' data. The use of two years of aggregated data results in a slightly lower 
reduction to 96%. 
5. Effects of move-on measures 
 
 
Additional to the closure scenarios described above possible move-on measures are modelled by using 
the options of maximizing cod landings per hour and applying a limitation on the percentage of cod in the 
landings to 5% of the total 2 hours landings. 
 
The impact of these cod limits are modelled by using levels of 20, 40 and 80 kg cod/2hours fishing. 
In case on of these  trigger values is reached in a landing database record, the next sequential record is 
removed from the database. 
 
 
Figure 1. Reductions of cod-LpUE (fraction of the original values) after applying move on measures 
based on cod landings limitation from 20 to 80 kg/2hours (lines) or 5% rule (scatter). Error bars 
are SD's. 
 
Results of applying move-on measures are presented in Fig 1. Cod-LpUE is reduced   to between 71 and 
88% of its original value, depending on the gear category and the cod-landing  trigger used. In case TR1 
is used as gear, the LpUE  reduction is higher, ranging between  71 and 84% of its original values. The 
Report number C084/11 7 of 9 
 
result of applying the 5% rule results in reductions of LpUE to 80 %(TR1) and 78%(TR2), similar as 
applying a cod landing limitation of 40 kg for both gears but the variation of TR1 results is higher 
(SD=15%). 
When combining two measures, closures based on last years aggregated data and move on measures, 
results show that the effect of the measures is additive. The additional reduction of applying the 5% rule 
was an average CpUE of 79 % of the original values for TR2 
 
Discarding of Cod occurs for reasons related to fishing regulations e.g. the cod caught are below the 
minimum landing size or the fisher holds insufficient quota for cod. Since information of discard fractions 
on a temporal and spatial scale is lacking, discards are assumed to be proportional to registered 
landings. Estimated LpUE reductions based on a temporal and spatial differences of cod landings are 
equal to CpUE reductions 
 
 
8 of 9 Report number C084/11 
 
 Table 1. Summary of Effort, Landings and Catch realized by gear categories BT2, 
TR1 (mesh-sizes<120mm) and TR2. 
 Effort 
(year) 

 BT2 
 Kwdays/106 
 TR1 
 (Kwdays/103
) 
 TR2 
 (Kwdays/103) 
 2006  35.54  375.20  1322.03 
 2007  34.98  467.80  1530.12 
 2008  25.54  1027.19  2061.82 
 2009  26.46  768.81  1740.72 
 2010  26.39  684.39  1808.36 
        
 Landings  (tons) 
    
 2006  921  5.42  121.8 
 2007  923  12.18  218.7 
 2008  1056  48.51  234.8 
 2009  1421  52.60  268.9 
 2010  1365  56.26  244.0 
        
 Discard   (fraction) 
    
 2006  0.16  0.21  0.21 
 2007  0.11  0.39  0.21 
 2008  0.26  0.55  0.21 
 2009  0.17  0.38  0.21 
 2010  0.17  0.38  0.21 
        
 Catch  (tons) 
    
 2006  1070  6.54  147 
 2007  1011  16.88  265 
 2008  1335  75.28  284 
 2009  1667  72.68  325 
 2010  1602  77.73  295 
        
 CpUE 
 kg 
cod/Kwdays 
    
 2006 ◦ 0.03  0.017  0.111 
 2007  0.03  0.04  0.173 
 2008  0.05  0.07  0.138 
 2009  0.06  0.1  0.187 
 2010  0.06  0.11  0.163 
  
 
Report number C084/11 9 of 9 
 
Justification 
 
Report number:             C084/11 
Project Number:  4308101024 
 
 
 
 
 
The scientific quality of this report has been peer reviewed by the a colleague scientist and the head of 
the department of IMARES. 
 
 
 
Approved: Stijn Bierman 
Function: researcher 
 
Signature:  
  
 
 
Date: 19 juli 2011  
 
  
 
  
Approved: Tammo Bult 
 Head of the department Fisheries 
 
 
Signature: 
 
 
 
Date: 19 juli 2011 
 
 
 
  
 


Ex-ante evaluation of Seasonal, Real Time and Move-on Closures

The Ministry of EL&amp;I asked IMARES to do an ex ante evaluation of temporal / spatial closures based on effort and cod catches by the Dutch demersal, TR1 

Ex-ante evaluation of Seasonal, Real Time and Move-on Closures

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