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

Nephrops burrow density 2008 - 2009 Exploratory surveys by Denmark Add Presentation Title in Footer via ”Insert”; ”Header & Footer” 24 November 201824 November 201824 November 2018

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 Add Presentation Title in Footer via ”Insert”; ”Header & Footer” 24 November 201824 November 201824 November 2018

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) Add Presentation Title in Footer via ”Insert”; ”Header & Footer” 24 November 201824 November 201824 November 2018

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 Add Presentation Title in Footer via ”Insert”; ”Header & Footer” 24 November 201824 November 201824 November 2018

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 Add Presentation Title in Footer via ”Insert”; ”Header & Footer” 24 November 201824 November 201824 November 2018

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 Add Presentation Title in Footer via ”Insert”; ”Header & Footer” 24 November 201824 November 201824 November 2018

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 Add Presentation Title in Footer via ”Insert”; ”Header & Footer” 24 November 201824 November 201824 November 2018

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 Add Presentation Title in Footer via ”Insert”; ”Header & Footer” 24 November 201824 November 201824 November 2018

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) ? Add Presentation Title in Footer via ”Insert”; ”Header & Footer” 24 November 201824 November 201824 November 2018

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 24 November 201824 November 201824 November 2018