1. Nephrops UWTV surveys in the Skagerrak and Kattegat (FU 3-4)
Kai Wieland 1, Mats Ulmestrand 2, Jordan Feekings 1 & Sven Koppetsch 2
1: DTU Aqua, Hirtshals, Denmark
2: SLU IMR, Lysekil, Sweden
SLU (Sveriges landbruksuniversitet) Swedish University of Agriculture Science
WGNEPS
5-8 Nov 2013
Barcelona

2. Nephrops burrow density 2008 - 2009
Exploratory surveys by Denmark
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3. Nephrops burrow density 2010
Denmark
2 subareas defined (based on the distribution of the commercial trawl fishery)
Random selection of sampling locations from a 2  2 nautical mile grid
Subarea 1 not covered as planned
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4. Nephrops burrow density 2011
Denmark and Sweden
4 additional subareas defined (based on the distribution of the commercial trawl fishery)
Random selection of sampling locations from a 2  2 nautical mile grid
Subareas 1 and 2 covered almost as planned (DK)
New subareas not well covered (SWE)
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5. Nephrops burrow density 2012
Denmark and Sweden
6 subareas (as in 2011)
Random selection of sampling locations from a 2  2 nautical mile grid
Subareas 1 and 2 covered almost as planned (DK)
Better coverage in subareas 3 to 6 than in 2011 (SWE) but still no sampling in the western part of subarea 3
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6. Nephrops burrow density 2013
Denmark and Sweden
6 subareas (as in 2011 and 2012)
Random selection of sampling locations from a 22 nautical mile grid
Subareas 1 and 2 covered almost as planned (DK)
Still insufficient sampling in the western part of subareas 3 to 5 (SWE)
Modification of station allocation and better coordination has been agreed for 2014
(Workshop held in Sept ’13)
Workshop 3-6 Sep 2013, Hirtshals
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7. Nephrops survey abundance
2010
Subarea
Number of stations
Bias corrected mean density (n/m2)
Standard deviation
Abundance (n * 106)
CV SD (%) 1 29
0.327
0.184
669
376
56.24 2 43
0.351
0.177
573
289
50.38 3 - 4 5 6
Abundance SA 1 to 2:
1243
OECV:
6.65
SA 1 to 6:
2011 52
0.369
0.152
754
311
41.20 50
0.249
0.160
407
262
64.31 10
0.377
0.111
822
243
29.51
0.381
0.113
230 68
29.77
0.418
0.120
247 71
28.65 24
0.496
0.147
721
214
29.75
1160
4.89
3180
3.58
2012
0.225
0.131
460
268
58.33 47
0.092
261
151
57.64 27
0.273
0.117
596
256
43.04 9
0.350
0.132
211 79
37.62 8
0.378
0.119
223 70
31.59 23
0.366
0.203
532
296
55.55
5.99
2283
4.25
Subarea
Number of grid cells
Survey area (km2) 1
149
2044 2
119
1633 3
159
2181 4 44
604 5 43
590 6
106
1454
total:
620
8506
Overall error coefficient of variation (OECV, relative standard error):
 𝑂𝐸𝐶𝑉= 𝑆𝐷 2 𝑖 𝑛 𝑖 / 𝐴 𝑖 ∗100
where SD, n and A denote standard deviation, number of tows and abundance in subarea i
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8. Optimizing station allocation to subareas
For 2014 (based on previous years contribution of the 6 subareas to the overall error coefficient of variation):
Number of stations
Subarea
based on OECV
based on area 1 49 46 2 37 36 3 40 4 12 13 5 11 6 41 32
Needs update when 2013 data have become available
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9. Station selection for 2014 based on
previous years contribution of the subareas to overall error coefficient of variation, and
random selection of 190 stations from the 2  2 nautical mile grid
Share between the two countries (?):
Number of stations
Subareas
DK: 97
1, 2 and 5 S: 93
3, 4, and 6
Needs update when 2013 data have become available
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10. Conclusions for future work
Area coverage has improved and an objective method of allocating stations to subareas has been established to avoid an unbalanced sampling
Random selection of stations (within a subarea) may result in clusters of sampling sites and no sampling in other parts in the subarea, buffered (random) sampling maybe considered as an alternative
A more even spacing of stations in the subareas (based on buffered sampling) may decrease the sampling efficiency (numbers of stations per day) on the survey which is conducted with small vessels (< 15 m), but the actual low overall relative standard error could justify a reduction the total number of stations
The boundaries of the current subareas are a bit arbitrarily defined and might be changed (but on which basis ? Sediment or VMS map ?)
Note: The distribution of the fishery (VMS data) may change from year to year independently from the distribution of the stock (e.g. due to by-catch problems of small gadoids)
The current subareas do not cover the entire distributional range of Nephrops in the Skagerrak/Kattegat. How to extrapolate / raise the survey results or just use the survey as an index based on the covered area (which then should be the same in all years) ?
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11. Independent vs. buffered random sampling: Example from the West Greenland bottom trawl survey
Minimum distance
Independent random: 3.2 km
Buffered random: 7.6 km
Source: Kingsley et al. 2004, JMS 61:12-24
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