Depestele, J.

Polet, H.

Van Craeynest, K.

Van de Walle, S.

Vincx, M.

Open Marine Archive

  
 
 
SUBSAMPLING ACCURACY IN BEAM TRAWL CATCHES 
 
Introduction 
In commercial beam trawling data on fish catches is routinely 
available, while composition of invertebrate discards is ignored. 
However, from an ecosystem perspective such data is equally 
important yet is highly labor intensive to acquire. Subsampling the 
discards, and accepting the error this inherently imposes, is the only 
option. Here we investigate these error rates, which will allow for 
more accurate data or estimates in discard composition.  
 
 
Samuel Van de Walle 1, 2, Jochen Depestele1,2 Hans Polet1, Kris Van Craeynest1 & Magda Vincx2 
1Institute for Agricultural and Fisheries Research, Animal Sciences – Fisheries 
ILVO, Biological Environmental Research, Ankerstraat 1, 8400 Oostende, Belgium  
2Ghent University, Biology Research Group, Marine Biology Section 
Krijgslaan 281 (Campus Sterre – S8), B-9000 Gent, Belgium 
 
Corresponding author: 
 +32(0)472 31 94 88  samuel.vandewalle@outlook.com 
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0 20 40 60 80 100
F1 (48,6kg) F2 (86,3kg)
F3 (57,7kg) F4 (28,0kg)
F5 (28,8kg) A1 (110,0kg)
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N1 (122,0kg) N2 (154,0kg)
N3 (19,0kg) N4 (65,0kg)
N5 (53,0kg) N6 (125,0kg)
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N1 (122,0kg)
95% CI
  Species Individuals % 
  Asterias rubens  24068 47,19 
  Echinidea sp. 21324 41,81 
  Buccinum undatum  901 1,77 
  Pagurus bernhardus  675 1,32 
  Liocarcinus  
  depurator  
636 1,25 
  Liocarcinus holsatus  608 1,19 
  Aphrodita aculeata  416 0,82 
  Palaemoninae  sp. 318 0,62 
  Necora puber  247 0,48 
  Ophiura ophiura  236 0,46 
  … … … 
  Totaal 51001   
Materials & Methods 
Twelve trawls were performed, in February 2009(F1-F3), February 2010(F4&F5), April 
(A1) and November2011 (N1-N6) at the location in Fig. 2. Every species x trawl 
combination was assigned to an abundance category(1) based on an abundance  
index (n). Hauls were subdivided into 10L buckets(2) and individuals were identified. 
Different numbers of buckets(from 1 to all) from a trawl were used to simulate 
different subsample sizes. For a large number of random recombination's of buckets, a 
sampling error (S) was calculated(3) and from all these a mean error and a confidence 
interval   was   derived(4). A  similar  approach  was then  used  to  calculate  the mean  
 
 
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 Rare 
 Common 
Abundant 
Dominant 
Future Prospects 
Currently most results are represent all trawls individually. In the next step, we aim 
to produce overall results for all trawls combined. Interpolation and/or a general 
linear mixed model approach should allow for this, while also accounting for the 
complex interdependence of all data points. 
 
 
Figure 2: Trawling Location 
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-4 -2 0 2 4 6 8 10
N° 
E° 
N° 
W° 
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Sa
m
p
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Er
ro
r 
(S
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(%
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Proportion of Catch Analysed (%) 
 Rare 
 Rare 95% CI 
 Common 
 Common 95% CI 
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Sa
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p
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(S
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(%
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Proportion of Catch Analysed (%) 
 Abundant 
 Abundant  95% CI 
 Dominant 
 Dominant 95% CI 
N
u
m
b
e
r 
o
f 
C
as
e
s 
(S
p
e
ci
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s 
x 
Tr
aw
l)
 
 
Categories of Abundance Index (n) 
Proportion of Catch Analysed (%) 
N
u
m
b
e
r 
o
f 
Sp
e
ci
e
s 
R
e
co
rd
e
d
 
 
Table 1: Top Abundant Species 
C 
B 
A 
Discussion 
 
(A) Species Abundances 
All hauls were highly dominated by 
starfish and sea urchins. A majority of 
species was found to be rare. 
Figure 1: Catch Sorting in Progress 
S S S S S 
S 
number of  species in a 
subsample of a certain 
size. 
 
(1) 
(2) 
(3) 
(4) 
 Mean Sampling 
error for 3/8 of 
haul analyzed 
= 
p= proportion of  
catch analyzed 
a= Species abundance 
 in subsample 
T= Species abundance 
 in haul 
W= Total weight of haul 
(B) Species Richness 
An average of 23 species per trawl was found. Despite large differences in catch 
weight there was comparatively little variation in number of species. Most trawls 
follow a similarly shaped curve regardless of their weight or number of species. For 
N1 the confidence interval showed a constant range over al subsample sizes. 
(C) Sampling Error 
Results for N1 are given as example, other trawls revealed similar patterns. Average 
error decreased with increasing subsample size. Rare species showed the highest 
error rates while common species had intermediate error rates. For abundant & 
dominant species errors were up to 10 times lower and surprisingly similar to each 
other. This indicates that after a certain abundance threshold is reached(around 
5ind./10kg), error rate is unlikely to drop much further, even at highest abundances. 
 
 
 
Trawl N1 


Subsampling accuracy in beam trawl catches

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