Loughs Agency Foyle, Carlingford and Irish Lights Commission Pre-Fishery Stock Assessment Lough Foyle Native Oyster Fishery Summary Report Autumn 2017
1
Aquaculture & Shellfisheries
____________________________________ Report Reference LA/0Y/0217
Version No. 1 2
Author CMG CMG
Date Issued 06/10/17 13/10/17
Amendments
Approved by
1st draft for comment BF Format and minor BF amendments
Prepared by: Ciarán McGonigle/Cass Bromley/Tom Sheerin/David McClean
The Loughs Agency (Foyle Carlingford and Irish Lights Commission)
For further information contact: Loughs Agency 22, Victoria Road Prehen Derry/Londonderry BT47 2AB Tel: 028 71 342100 Fax: 028 71 342720 email:general@loughs-agency.org
Loughs Agency D’arcy Magee Court Dundalk Street Carlingford Co Louth Tel: 042 93 83 888 Fax: 042 93 83 888 email:carlingford@loughsagency.org
www.loughs-agency.org
1
Contents 1.
Executive Summary ................................................................................................................ 5
2.
Introduction ............................................................................................................................ 6
3.
Methods.................................................................................................................................. 6
4.
Oyster Density ........................................................................................................................ 7
5.
Biomass Estimate ................................................................................................................... 8
6.
Spatfall .................................................................................................................................. 10
7.
Mortality Levels .................................................................................................................... 12
8.
Population Structure ............................................................................................................ 14
9.
Oyster Bed Review ............................................................................................................... 17
9.1
The Perch .............................................................................................................................. 17
9.2 The Flat Ground ....................................................................................................................... 19 9.3 Quigley’s Point ......................................................................................................................... 21 9.4 Middle Bed South .................................................................................................................... 23 9.5 Southside North ....................................................................................................................... 25 9.6 Southside South ....................................................................................................................... 27 10.
Conclusions ........................................................................................................................... 29
2
List of Figures Figure 1 Harvestable Biomass (t) on Foyle oyster beds autumn 2017 ................................................... 5 Figure 2 Methods used during survey .................................................................................................... 6 Figure 3 Interpolated oyster density Lough Foyle Autumn 2017 ........................................................... 7 Figure 4 Mean Density Lough Foyle 2008-2017...................................................................................... 8 Figure 5 Historic Biomass and Landings Lough Foyle ............................................................................. 9 Figure 6 Observed and Estimated Landings from Lough Foyle............................................................. 10 Figure 7 Length Frequency of oysters in Lough Foyle 2016-2017 ........................................................ 11 Figure 8 Oyster Spat Density in Lough Foyle Autumn 2017.................................................................. 11 Figure 9 Historic Spat Density and Presence Lough Foyle .................................................................... 12 Figure 10 Overall mortality recorded in Lough Foyle Autumn 2017 .................................................... 13 Figure 11 Historic Mortality levels Lough Foyle .................................................................................... 13 Figure 12 Length Frequency of Lough Foyle oysters 2007 & 2017 ....................................................... 14 Figure 13 Cumulative Frequency across all oyster beds in Lough Foyle 2016-2017 ............................ 15 Figure 14 Length classes on oyster beds Lough Foyle 2017 ................................................................. 15 Figure 15 Size class on each bed in Lough Foyle Autumn 2017 ............................................................ 16 Figure 16 Proportional Length Frequency The Perch 2015-17 ............................................................. 17 Figure 17 Cumulative Frequency: The Perch 2016-2017 ...................................................................... 18 Figure 18 Percentage of length classes: The Perch 2017...................................................................... 18 Figure 19 Proportional Length Frequency: Flat Ground 2015-17 ......................................................... 19 Figure 20 Cumulative Frequency: Flat Ground 2016-2017 ................................................................... 20 Figure 21 Percentage of each length class: Flat Ground 2016.............................................................. 20 Figure 22 Proportional Length Frequency Quigley's Point 2015-17 ..................................................... 21 Figure 23 Cumulative Frequency: Quigley's point 2016-2017 .............................................................. 22 Figure 24 Percentage of length classes: Quigleys Point 2017............................................................... 22 Figure 25 Proportional Length Frequency Middle bed 2015-17........................................................... 23 Figure 26 Cumulative Frequency Middle Bed 2016-2017..................................................................... 24 Figure 27 Percentage of length classes Middle Bed 2017 .................................................................... 24 Figure 28 Proportional Length Frequency Southside (N) 2015-17 ....................................................... 25 Figure 29 Cumulative Frequency Southside (N) 2016-2017 ................................................................. 26 Figure 30 Percentage of length classes: Southside (N) Autumn 2017 .................................................. 26 Figure 31 Length Frequency Southside (S) 2016-17 ............................................................................. 27 Figure 32 Cumulative Frequency Southside (S) 2016-2017 .................................................................. 28 Figure 33 Percentage of length classes: Southside (S) 2017................................................................. 28
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List of Tables Table 1 Oyster biomass per oyster bed in Lough Foyle Autumn 2017 ................................................... 9 Table 2 Summary Information: The Perch ............................................................................................ 17 Table 3 Summary Information: The Flat Ground .................................................................................. 19 Table 4 Summary Information: Quigley's Point .................................................................................... 21 Table 5 Summary Information: Middle Bed South ............................................................................... 23 Table 6 Summary Information: Southside North .................................................................................. 25
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1. Executive Summary A pre-fishery stock assessment was conducted on oyster beds in Lough Foyle during autumn 2017. The results of this survey are in keeping with the results of the post-season survey with the population skewed towards larger size classes and continued evidence of the dominance of the cohort of oysters that settled in Lough Foyle in late summer 2014. The harvestable biomass of oysters on all oyster beds is estimated to be 315 tonnes (Figure 1). Landings from the fishery in the 2016/17 season amounted to just over 185 tonnes. The total biomass of oysters on these beds has improved relative to the spring situation thanks largely to a very good growing season in 2017. There was no evidence of a widespread spatfall in 2017. It is recommended that the fishery does not remove the total harvestable biomass in the 2017/18 season. This will help to prevent the loss of an effective spawning stock for the 2018 spawning period and help retain sufficient stock to support a sustainable fishery for 2018/19.
Figure 1 Harvestable Biomass (tonnes) Foyle oyster beds autumn 2017
5
2. Introduction A pre-fishery survey of the native oyster beds in Lough Foyle took place in August and September 2017. This survey was conducted by Agency staff on board a local fishing vessel.
3. Methods A dredge survey was carried out in Lough Foyle using a local fishing vessel. Scientific staff from the Loughs Agency assessed the population distribution and abundance by analysing dredge contents from each sample location (figure 2A-B). Oyster density is calculated by dividing the area dredged (m2) by the total number of oysters after adjusting for dredge efficiency (25%) and any sub-sampling. Dredge efficiency has been taken from the work done in Lough Foyle during the Baseline Survey of Shellfish Resources conducted by Cefas in 2006. Oyster density is interpolated across the oyster beds using a six point Inverse Distance Weighting (IDW) method and this allows a biomass figure to be calculated based on the area (m2) of the oyster bed within each density category. This follows the approach of similar work in Irish oyster fishery stock assessments conducted by the Marine Institute. The average weight of the oysters on each bed is used to factor up to an overall biomass based on mean density within each bed. Any fresh dead oysters are recorded as a measure of recent mortality, these exhibit no fouling on the inner surface of the shell valves. Oyster spat (<30mm) presence or absence was recorded at each sample point as was total numbers of spat per dredge sample. A
B
Figure 2 Methods used during survey: (A) Analysing and recording dredge contents. (B) Oysters are measured for length frequency analysis
6
4. Oyster Density Oyster density was calculated for each of the sample stations (figure 3). The mean density for all beds is 0.2 oysters/m2. This figure is in keeping with results from previous years (figure 4). The maximum density recorded was 3 oysters/ m2 which was observed on the Middle Bed. 15% of the sampled stations had an oyster density of over 1 oyster/m2, this is represented by the green areas in Figure 3. Oyster density recorded from each dredge tow has been represented as point data and then interpolated using a 6 point IDW methodology. The interpolation has been performed only in areas of the oyster beds that were surveyed and have point data present. A barrier feature has been placed within the interpolation between the oyster beds in or to the north of the navigation channel and all beds on the east of the navigation channel to help replicate the impacts of a natural barrier (a sandbank) between the beds in this area.
Figure 3 Interpolated oyster density Lough Foyle Autumn 2017
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Mean Adult Density Lough Foyle Oysters 2008-2017 0.6 0.5 0.4 0.3 0.2 0.1 0 2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
Figure 4 Mean Density Lough Foyle 2008-2017
5. Biomass Estimate The estimated total oyster population biomass is 684 tonnes (table 1). This is an increase from the estimated biomass of 615 tonnes in autumn 2016. Total stock biomass above 80mm is currently estimated to be 315 tonnes. This is the highest estimated fishable biomass figure on record and bodes well for a productive fishery for the coming season. The total stock biomass is high relative to previous years (figure 5) and this is likely due to good growth in the younger cohort of oysters that settled in late summer 2014 and the carryover of stock from 2011/2012 settlements as a result of the reduced fishing activity since 2014. These oysters have now attained a length of around 60-90mm. Mean weight was higher than previous years with a mean of 60g for all oysters sampled during the survey (n=1386). The previous biomass estimates have correlated well with the observed landings in the fishery (figure 6). Note that on this graph the survey estimate is that of the pre-season (autumn) survey and the landings figure is taken from logbook returns made by fishermen on a continuous basis throughout the season. On one occasion (2013/14) the stock assessment was not completed on all beds prior to the commencement of the fishery and the landings made prior to the survey being completed were separated for the comparison.
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Table 1 Oyster biomass per oyster bed in Lough Foyle Autumn 2017
Bed name Barney's Bank Black Ghee Drumskellan Flat Ground Great Bank McGhee Middle North Middle South Moville Peak Perch Quigley's Pt Redcastle Sandy Ridge Shooting Range Southside North Southside South Total
Mean Length (mm) 85.0 79 81.5 80.18 81.6 77 85.5 79.2 75 82 78 75.8 83 86.7 74 79.1 79.3 80.1
Area (ha) 200 236 91 970 827 167 228 531 250 208 276 141 164 474 215 288 319 5584
Bed Total (t) 0.13 4.68 12.38 205.92 60.04 7.48 0.66 133.98 1.17 10.64 107.98 20.62 2.71 6.28 5.75 47.63 56.73 684.78
Total (t) >80mm 0.10 2.34 6.19 96.78 33.62 0.75 0.49 66.99 0.58 5.32 41.03 6.39 1.36 4.40 0.57 22.86 26.10 315.88
Biomass Estimates and Landings Biomass (tonnes)
800
698
700
Landings (tonnes) 725 684 615
Biomass (tonnes)
600 500
556 467 413 376
400
295
294
300
258 221
200
200
158 119
95
100
113
185
168
148
69
0 2006
2008
2009
2010
2011
2012 Year
Figure 5 Historic Biomass and Landings Lough Foyle
9
2013
2014
2015
2016
2017
Observed and Estimated Landings 2009-2017 350
Landings (tonnes)
Survey estimate (tonnes)
Landings (t) pre survey 315
300
Tonnes
250 200
176 158 150
150
177 165
148 127 119
100
111
185
167 158
174
138
98 69 44
50 0 2009/10
2010/11
2011/12
2012/13
2013/14
2014/15
2015/16
2016/17
2017/18
Season
Figure 6 Observed and Estimated Landings from Lough Foyle
6. Spatfall Oyster spawning and larval survival is thought to be heavily dependent on good environmental conditions throughout the spring and summer and the threshold water temperature values being exceeded for a minimum period of time, usually 16⁰C for 2-4 weeks. If these conditions do not occur it is expected that spawning will be limited to a small percentage of the stock on each bed and the numbers of juveniles will be minimal. Spawning is also limited by oyster density and proximity of each individual oyster to another has an impact on fertilisation success. Removal of oysters during the fishing season reduces the oyster density. Poor spawning and settlement was observed in the 2013 and 2015 spawning seasons and no widespread spatfall was observed in 2016. Spat settlement appears to have taken place on the highest density beds in 2017 with 11.9% of the stations sampled having spat present (figure 7 and 8). Mean spat density was 0.018 spat/m2 which is much lower than previous years in which widespread heavy spatfall was observed (figure 9). The number of sampled stations with spat present has increased by 50% from 2016, however this was less than that observed in years with widespread spatfall when 50% or more than the stations had spat present.
10
% Frequency
Length Frequency Foyle Oysters 2016-2017 20 18 16 14 12 10 8 6 4 2 0
Autumn 2016
Spring 2017
Autumn 2017
Length Class Midpoint (mm) Figure 7 Length Frequency of oysters in Lough Foyle 2016-2017
Figure 8 Oyster Spat Density in Lough Foyle Autumn 2017
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Mean Spat Density and Spat Presence 2008-2017 Mean Spat Density/m2
% stations spat present
0.2
Landings 250
221
185
0.16 158
0.14
200
168
0.12
150 119
148
0.1 0.08 0.06 0.04
100
113
62 51
50
30
0.02
69
50
23
20
19
6
5
3
% Stations Positive/Landings (t)
Mean Spat Density (m2)
0.18
12
0
0 2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
Year
Figure 9 Historic Spat Density and Presence Lough Foyle
7. Mortality Levels There was very limited evidence of recent mortality on the oyster beds (figure 10). Mortality was recorded in 11.9% of the samples taken and the majority of these appeared to be due to natural causes. The latest Bonamia test results show infection rates of between 0-30% on the beds and this may result in elevated mortality levels if the intensity of the infection is high. Historic data on mortality show the gradual reduction in mortality levels since the first major reported incident in 2011 (figure 11). It may be prudent to continue to monitor this situation closely if the total biomass of the stock increases and if mean densities on the oyster beds increase due to natural recruitment or through any stocking or enhancement programmes in the future.
12
Figure 10 Overall mortality recorded in Lough Foyle Autumn 2017
% Stations with Mortality 30 25.8
25
% Stations +
20.3
20 15.3
15
11.9
11 8.8
10 5.5
5
4.7
3.3
0 Spring 2011
Spring 2012
Spring 2013
Spring 2014
Spring 2015
Survey Figure 11 Historic Mortality levels Lough Foyle
13
Spring 2016
Autumn 2016
Spring 2017
Autumn 2017
8. Population Structure The length frequency of the oyster population shows that the proportion of larger size classes has increased since the 2007 stock survey. The 2007 results show there was poor representation of oysters 75mm and above in the population. In the years since the regulation of the fishery in 2008 there has been greater evidence of larger oysters within the population (see figure 12). The larger oysters are an important resource within the population. These make up the majority of the spawning stock biomass. Although oysters as small as 35mm are capable of spawning, the larger oysters are capable of producing large quantities of larvae for the fishery, and are therefore of great importance. If there is a higher percentage of large oysters present in the population along with suitable environmental conditions there is a greater chance of spawning success. The overall sampled population consists of 83% above 71mm. Figure 12 highlights the bias towards larger length classes as a result of poor recruitment in 2015 and 2016.
Length Frequency Foyle Oysters 2007 & 2017 25 Autumn 2007
Autumn 2017
% Frequency
20 15 10 5 0
Length Class Midpoint (mm) Figure 12 Length Frequency of Lough Foyle oysters 2007 & 2017
The cumulative length frequency (figure 13) shows the gradual growth in smaller size oysters with a shift of the line to the right in all the length classes as a result of good growing conditions and the absence of fishing removal of larger length classes since December 2016.
14
Cumulative Length Frequency 2016-2017 120
Autumn 2016
Spring 2017
Autumn 2017
% Frequency
100 80 60 40 20 0
Length Class Midpoint (mm) Figure 13 Cumulative Frequency across all oyster beds in Lough Foyle 2016-2017
The pie chart in figure 14 shows a bias towards larger size classes within the stock as a result of limited recruitment of juveniles into the stock in the 2015 and 2016 spawning seasons. This is likely to lead to a reduction in the fishable stock size in the 2019/20 and 2020/21 seasons.
Length Classes Lough Foyle Oysters Autumn 2017 5%
0%
4%
0% 0%
2% 1-20 (mm)
12% 14%
21-30 (mm) 31-40 (mm) 41-50 (mm) 51-60 (mm) 61-70 (mm) 71-80 (mm) 81-90 (mm)
30%
91-100 (mm)
33%
101+ (mm)
Figure 14 Length classes on oyster beds Lough Foyle 2017
Figure 15 illustrates the percentage of each length class on the oyster beds in Lough Foyle. This chart highlights the differences between beds in terms of the health of their stock structure. The beds which consistently produce commercially important quantities of oysters (Middle Bed, Perch, Flat Ground) have good representation of length classes from 61mm-101mm+.
15
Length Classes on Oyster Beds Lough Foyle Autumn 2017 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Flatground
Quigleys
Perch
Southside (north)
Southside (south)
1-20mm
21-30mm
31-40mm
41-50mm
51-60mm
61-70mm
71-80mm
81-90mm
91-100mm
101mm+
Figure 15 Size class on each bed in Lough Foyle Autumn 2017
16
Middlebed
Great bank
9. Oyster Bed Review 9.1 The Perch Table 2 Summary Information: The Perch
Bed Name
Perch
Area (h)
276
Total Biomass (t)
107.9
Biomass (t) >80mm
41
The Perch is one of the 5 main commercially fished oyster beds with the highest density in Lough Foyle and it covers an area of 276 hectares. The overall biomass estimated for this bed is 107.9 tonnes, 41 tonnes of that figure is currently above market size of 80mm and is available to the fishery. 19.2 tonnes of oysters were landed from this bed during the 2016/17 fishing season. The length frequency for the Perch shows there is a notably higher percentage of 80mm+ oysters recorded during the autumn 2017 survey than the autumn 2016 survey; this shows the growth progression during 2017. There was only a small percentage of oysters <30mm recorded during the survey due to a relatively unsuccessful spawning season (Figure 16).
25
Proportional Length Frequency Perch Autumn 2015
Autumn 2016
Autumn 2017
Frequency (%)
20
15
10
5
0
Length Class Midpoint (mm)
Figure 16 Proportional Length Frequency The Perch 2015-17
17
Cumulative Length Frequency 2016-2017 120
Autumn 2016
Spring 2017
Autumn 2017
51-60 (mm)
71-80 (mm)
Frequency (%)
100 80 60 40 20 0 1-20 (mm)
21-30 (mm)
31-40 (mm)
41-50 (mm)
61-70 (mm)
81-90 (mm)
91-100 (mm)
101+ (mm)
Length Classes
Figure 17 Cumulative Frequency: The Perch 2016-2017
Length Classes Autumn 2017 1.85 6.17
0.62
0.00
0.00
1.23
1-20 (mm) 21-30 (mm)
6.17
31-40 (mm) 17.28
41-50 (mm) 51-60 (mm)
27.78
61-70 (mm) 71-80 (mm) 38.89
81-90 (mm) 91-100 (mm) 101+ (mm)
Figure 18 Percentage of length classes: The Perch 2017
18
9.2 The Flat Ground Table 3 Summary Information: The Flat Ground
Bed Name
Flat Ground
Area (h)
970
Total Biomass (t)
205.9
Biomass (t) >80mm
96.78
The Flat Ground covers an area of 970 hectares. Total stock biomass for this bed is estimated to be 205.9 tonnes. 96.7 tonnes is above market size therefore harvestable biomass. 34 tonnes of oysters were landed from the Flat Ground during the 2016/17 season. The length frequency (figure 19) shows evidence of limited spatfall (<30mm) was recorded during the survey. The growth progression of the 2014 spat is observed in the data with an increase from an average of 12.5mm to an average of 35mm in autumn 2015 with another increase again to over 67mm in autumn 2016. This cohort is now recruiting into the fishery at 80mm+. In keeping with the overall length frequencies there is good representation of older age classes which would not have been recorded in the stock assessments during the period 2007-2010.
Proportional Length Frequency Flat Ground 25 Autumn 2015
Autumn 2016
Autumn 2017
Frequency (%)
20
15
10
5
0
Length Class Midpoint (mm)
Figure 19 Proportional Length Frequency: Flat Ground 2015-17
19
The cumulative frequency (Figure 20) for the stock on the Flat Ground shows the growth in the smallest length classes since the spring survey with the majority of the population now above 70mm on this bed.
Cumulative Length Frequency Flat Ground 120
Autumn 2016
Spring 2017
Autumn 2017
Frequency (%)
100 80 60 40 20 0 1-20 (mm)
21-30 (mm)
31-40 (mm)
41-50 (mm)
51-60 (mm)
61-70 (mm)
71-80 (mm)
81-90 (mm)
91-100 (mm)
101+ (mm)
Length Classes
Figure 20 Cumulative Frequency: Flat Ground 2016-2017
The pie chart (Figure 21) also draws attention to the fact that the smaller size classes are not represented on this bed and the vast majority of the population on the bed is more than 70mm in size.
Length Classes Flat Ground Autumn 2017 4.81
0.48
4.81 5.29
10.10
1-20 (mm) 0.96
1.92
21-30 (mm) 31-40 (mm) 11.54
41-50 (mm) 51-60 (mm) 61-70 (mm) 71-80 (mm)
27.40
81-90 (mm) 91-100 (mm) 32.69
Figure 21 Percentage of each length class: Flat Ground 2016
20
101+ (mm)
9.3 Quigley’s Point Table 4 Summary Information: Quigley's Point
Bed Name
Quigley’s Point
Area (h)
141
Total Biomass (t)
20.6
Biomass (t) >80mm
6.3
Quigley’s point covers an area of 141 hectares. The total biomass has been estimated to be 20.6 tonnes on this bed with 6.3 tonnes above the market size of 80mm. 14.2 tonnes were landed from Quigley’s bed during the fishing season of 2016/17. The length frequency for Quigley’s Point (figure 22) shows limited evidence of any spatfall on the bed. The cohort that settled in 2014 has now attained a size of 80mm+ on average.
Propotional Length Frequency Quigleys Point 25 Autumn 2015
Autumn 2016
Autumn2017
Frequency (%)
20 15 10 5 0
Length Class Midpoint (mm)
Figure 22 Proportional Length Frequency Quigley's Point 2015-17
The cumulative frequency (Figure 23) again highlights the reduction in the proportion of the stock in the smaller size classes on this bed.
21
Cumulative Length Frequency Quigley's Point 120
Autumn 2016
Spring 2017
Autumn 2017
31-40 (mm)
51-60 (mm)
71-80 (mm)
Frequency (%)
100 80 60 40 20 0 1-20 (mm)
21-30 (mm)
41-50 (mm)
61-70 (mm)
81-90 (mm)
91-100 (mm)
101+ (mm)
Length Classes
Figure 23 Cumulative Frequency: Quigley's point 2016-2017
Figure 24 shows the proportion of each length class on the bed. It is clear that smaller size classes are missing from this bed as is the case on many other beds in 2017. The market sized stock makes up 31% of the total population.
Length Classes Quigleys Point Autumn 2017 1.31
0.00 0.00 0.00
0.65 5.23
4.58
1-20 (mm) 21-30 (mm) 31-40 (mm) 24.84
25.49
41-50 (mm) 51-60 (mm) 61-70 (mm) 71-80 (mm) 81-90 (mm) 91-100 (mm)
37.91
101+ (mm)
Figure 24 Percentage of length classes: Quigley’s Point 2017
22
9.4 Middle Bed South Table 5 Summary Information: Middle Bed South
Bed Name
Middle Bed South
Area (h)
531
Total Biomass (t)
133.9
Biomass (t) >80mm
66.9
The south section of the Middle Bed covers an area of 531 hectares. There is an estimated total biomass of 133.9 tonnes on this bed and 66.9 tonnes of that figure is of market size. The length frequency for the Middle Bed South (figure 25) shows that the majority of oysters on this bed are now over 50mm indicating poor juvenile recruitment in 2015 and 2016. There is some limited evidence of a spatfall in 2017 with a proportion of the stock in the 20-30mm size classes.
Proportional Length Frequency Middle Bed 20 18
Autumn 2015
Autumn 2016
Autumn 2017
Frequency (%)
16 14 12 10 8 6 4 2 0
Length Class Midpoint (mm)
Figure 25 Proportional Length Frequency Middle bed 2015-17
The cumulative frequency (Figure 26) shows an increase in the proportion of larger length classes since the spring 2016 survey. This movement reflects the good growing conditions observed in 2017. The pie chart (Figure 27) shows evidence of a skew towards larger age classes once again with over 40% of the stock above the market size of 80mm.
23
Cumulative Length Frequency Middle Bed 120 Autumn 2016
Spring 2017
Autumn 2017
Frequency (%)
100 80 60 40 20 0 1-20 (mm)
21-30 (mm)
31-40 (mm)
41-50 (mm)
51-60 (mm)
61-70 (mm)
71-80 (mm)
81-90 (mm)
91-100 (mm)
101+ (mm)
Length Classes
Figure 26 Cumulative Frequency Middle Bed 2016-2017
Length Classes Middle Bed Autumn 2017 4.09
0.58 6.43
2.92 0.00
9.94
1-20 (mm) 21-30 (mm)
1.17
31-40 (mm) 41-50 (mm) 14.62
51-60 (mm) 61-70 (mm)
28.65
71-80 (mm) 81-90 (mm) 91-100 (mm) 31.58
Figure 27 Percentage of length classes Middle Bed 2017
24
101+ (mm)
9.5 Southside North Table 6 Summary Information: Southside North
Bed Name
Southside North
Area (h)
288
Total Biomass (t)
47.6
Biomass (t) >80mm
22.8
The North area of the Southside bed covers an area of 288 hectares. The overall biomass on this bed is estimated to be 47.6 tonnes, 22.8 tonnes of this figure represent oysters over 80mm in length. The length frequency (figure 28) shows limited evidence of any settlement in 2017 with a very small proportion of the stock less than 30mm. The spatfall from 2014 has now progressed into the 80-90mm age classes.
Proportional Length Frequency Southside North 18
Autumn 2015
Autumn 2016
Autumn 2017
16
Frequency (%)
14 12 10 8 6 4 2 0
Length class Midpoint (mm)
Figure 28 Proportional Length Frequency Southside (N) 2015-17
25
Cumulative Length Frequency Southside North 120
Autum 2016
Spring 2017
Autumn 2017
31-40 (mm)
51-60 (mm)
71-80 (mm)
Frequency (mm)
100 80 60 40 20 0 1-20 (mm)
21-30 (mm)
41-50 (mm)
61-70 (mm)
81-90 (mm)
91-100 (mm)
101+ (mm)
Length Classes
Figure 29 Cumulative Frequency Southside (N) 2016-2017
Length Classes Southside North 0.60 5.95
0.60
4.17 0.60
1-20 (mm)
3.57
10.71
21-30 (mm) 31-40 (mm) 14.88
41-50 (mm) 51-60 (mm) 61-70 (mm)
29.76
71-80 (mm) 29.17
81-90 (mm) 91-100 (mm) 101+ (mm)
Figure 30 Percentage of length classes: Southside (N) Autumn 2017
26
9.6 Southside South Bed Name
Southside South
Area (h)
319
Total Biomass (t)
45.7
Biomass (t) >80mm
26.1
The south section of the Southside bed covers 319.0 hectares in total. The overall biomass of oysters calculated during the autumn 2017 survey is 45.7 tonnes and 26.1 tonnes of this figure is currently above 80mm therefore available to the fishery for the 2017/18 season. Again, like the length frequency for the northern section of the Southside there was limited evidence of spat (<30mm) recorded for this survey. The stock on this bed is skewed towards the larger size classes with the majority above 75mm in length.
Proportional Length Frequency Southside South 18
Autumn 2015
Autumn 2016
Autumn 2017
16
Frequency (%)
14 12 10 8 6 4 2 0
Length Class Midpoint (mm)
Figure 31 Length Frequency Southside (S) 2015-17
27
Cumulative Length Frequency Southside South 120
Autumn 2016
Spring 2017
Autumn 2017
31-40 (mm)
51-60 (mm)
71-80 (mm)
Frequency (%)
100 80 60 40 20 0 1-20 (mm) 21-30 (mm)
41-50 (mm)
61-70 (mm)
81-90 (mm)
91-100 (mm)
101+ (mm)
Length Classes
Figure 32 Cumulative Length Frequency Southside (S) 2016-2017
Length Classes Southside South Autumn 2017 4.86
0.00 5.41
3.24 0.54
1-20 (mm) 4.32
15.14
21-30 (mm) 31-40 (mm)
11.35
41-50 (mm) 51-60 (mm) 61-70 (mm)
22.70 32.43
71-80 (mm) 81-90 (mm) 91-100 (mm) 101+ (mm)
Figure 33 Percentage of length classes: Southside (S) 2017
28
10. Conclusions The total biomass estimated for the stock is an increase on the estimates made in spring 2017 and autumn 2016. There has been evidence of a limited spatfall in 2017. The biomass above the minimum landing size of 80mm is estimated to be 315 tonnes. This is the highest fishable stock estimate ever produced for this fishery. The overall length frequency for the surveyed stock shows the length classes are skewed towards the larger size classes and the majority of the stock is more than 75mm. If a percentage of this stock is left behind and environmental conditions are favourable in summer 2018 there is a greater potential for a successful spawning event and recruitment into the fishery. It is recommended that the fishery does not remove the total harvestable biomass in the 2017/18 season. This will help to limit the loss of an effective spawning stock for the 2018 spawning period and help to retain enough stock to support a sustainable fishery in 2018/19.
11.Recommendations Consider partial removal of the harvestable stock biomass to help alleviate pressure on the stock from fishing activity and help sustain an effective spawning broodstock and a viable fishable stock for 2018/19.
29
Appendix I. Raw data from interpolated density data and biomass estimates Bed name
Density Class Range
N
Polygon Area (m2)
Mean Weight (g)
No. of Oysters
Total Biomass (t)
Barneys
0 - 0.01000
12
1664112
70
1664
0.12
Barneys
0.01001 - 0.05000
1
13700
70
230
0.02
Barneys
0.05001 - 0.10000
0
0
70
0
0.00
Barneys
0.10001 - 0.50000
0
0
70
0
0.00
Barneys
0.50001 - 1.00000
0
0
70
0
0.00
Barneys
1.00001 - 2.50000
0
0
70
0
0.00
Barneys
2.50001 - 5.00000
0
0
70
0
0.00
Drumskellan
0 - 0.01000
4
18500
64
18
0.00
Drumskellan
0.01001 - 0.05000
1
66100
64
1066
0.07
Drumskellan
0.05001 - 0.10000
0
114548
64
5727
0.37
Drumskellan
0.10001 - 0.50000
3
567945
64
172749
11.06
Drumskellan
0.50001 - 1.00000
3
22600
64
13878
0.89
Drumskellan
1.00001 - 2.50000
0
0
64
0
0.00
Drumskellan
2.50001 - 5.00000
0
0
64
0
0.00
Flat Ground
0 - 0.01000
16
889460
63
889
0.06
Flat Ground
0.01001 - 0.05000
8
1135126
63
32601
2.05
Flat Ground
0.05001 - 0.10000
6
624376
63
40950
2.58
Flat Ground
0.10001 - 0.50000
23
4369718
63
1084481
68.32
Flat Ground
0.50001 - 1.00000
16
2227909
63
1514271
95.40
Flat Ground
1.00001 - 2.50000
6
452388
63
595321
37.51
Flat Ground
2.50001 - 5.00000
0
0
63
0
0.00
Black Ghee Black Ghee Black Ghee Black Ghee Black Ghee Black Ghee Black Ghee Great Bank Great Bank Great Bank Great Bank Great Bank Great Bank Great Bank
0 - 0.01000 0.01001 - 0.05000 0.05001 - 0.10000 0.10001 - 0.50000 0.50001 - 1.00000 1.00001 - 2.50000 2.50001 - 5.00000 0 - 0.01000 0.01001 - 0.05000 0.05001 - 0.10000 0.10001 - 0.50000 0.50001 - 1.00000 1.00001 - 2.50000 2.50001 - 5.00000
9 14 3 4 0 0 0 9 10 8 11 2 0 0
759166 818275 702034 67200 0 0 0 1062683 1875092 1354824 3827792 136200 0 0
58 58 58 58 58 58 58 64.3 64.3 64.3 64.3 64.3 64.3 64.3
759 22863 48610 8437 0 0 0 4494 42344 92789 705322 88757 0 0
0.04 1.33 2.82 0.49 0.00 0.00 0.00 0.29 2.72 5.97 45.35 5.71 0.00 0.00
30
Bed name
Density Class Range
N
Polygon Area (m2)
Mean Weight (g)
No. of Oysters
Total Biomass (t)
Middle north Middle north Middle north Middle north Middle north Middle north Middle north Middle south Middle south Middle south Middle south Middle south Middle south Middle south Moville Moville Moville Moville Moville Moville Moville Peak Peak Peak Peak Peak Peak Peak Perch Perch Perch Perch Perch Perch Perch Quigley'sPt Quigley'sPt Quigley'sPt Quigley'sPt Quigley'sPt
0 - 0.01000 0.01001 - 0.05000 0.05001 - 0.10000 0.10001 - 0.50000 0.50001 - 1.00000 1.00001 - 2.50000 2.50001 - 5.00000 0 - 0.01000 0.01001 - 0.05000 0.05001 - 0.10000 0.10001 - 0.50000 0.50001 - 1.00000 1.00001 - 2.50000 2.50001 - 5.00000 0 - 0.01000 0.01001 - 0.05000 0.05001 - 0.10000 0.10001 - 0.50000 0.50001 - 1.00000 1.00001 - 2.50000 2.50001 - 5.00000 0 - 0.01000 0.01001 - 0.05000 0.05001 - 0.10000 0.10001 - 0.50000 0.50001 - 1.00000 1.00001 - 2.50000 2.50001 - 5.00000 0 - 0.01000 0.01001 - 0.05000 0.05001 - 0.10000 0.10001 - 0.50000 0.50001 - 1.00000 1.00001 - 2.50000 2.50001 - 5.00000 0 - 0.01000 0.01001 - 0.05000 0.05001 - 0.10000 0.10001 - 0.50000 0.50001 - 1.00000
9 2 0 0 0 0 0 19 4 5 9 8 2 1 16 3 0 2 0 0 0 3 5 2 8 0 0 0 7 2 3 12 10 7 1 14 5 3 13 12
2120111 102044 41959 17252 0 0 0 1052933 822952 41959 4214121 864923 225200 15600 1752968 637143 86100 19500 0 0 0 309693 522191 379484 859393 0 0 0 87100 283477 131529 898950 965043 382150 2500 49822 117678 227901 808507 224419
91 91 91 91 91 91 91 59 59 59 59 59 59 59 55 55 55 55 55 55 55 50.6 50.6 50.6 50.6 50.6 50.6 50.6 64 64 64 64 64 64 64 56.4 56.4 56.4 56.4 56.4
2120 1278 2098 1725 0 0 0 1053 25379 3155 1341277 526158 326781 47010 1753 12549 4305 2600 0 0 0 310 11089 30568 168317 0 0 0 87 8391 10585 314589 743828 602893 6838 50 2432 14883 195772 150782
0.19 0.12 0.19 0.16 0.00 0.00 0.00 0.06 1.50 0.19 79.14 31.04 19.28 2.77 0.10 0.69 0.24 0.14 0.00 0.00 0.00 0.02 0.56 1.55 8.52 0.00 0.00 0.00 0.01 0.54 0.68 20.13 47.60 38.59 0.44 0.00 0.14 0.84 11.04 8.50
31
Bed name
Density Class Range
N
Polygon Area (m2)
Mean Weight (g)
No. of Oysters
Total Biomass (t)
Quigley'sPt Quigley'sPt Redcastle Redcastle Redcastle Redcastle Redcastle Redcastle Redcastle Roof Isles Sandy Ridge Sandy Ridge Sandy Ridge Sandy Ridge Sandy Ridge Sandy Ridge Sandy Ridge Southside N Southside N Southside N Southside N Southside N Southside N Southside N Southside S Southside S Southside S Southside S Southside S Southside S Southside S McGhee Bank McGhee Bank McGhee Bank McGhee Bank McGhee Bank McGhee Bank McGhee Bank Firing Range Firing Range
1.00001 - 2.50000 2.50001 - 5.00000 0 - 0.01000 0.01001 - 0.05000 0.05001 - 0.10000 0.10001 - 0.50000 0.50001 - 1.00000 1.00001 - 2.50000 2.50001 - 5.00000 0 - 0.01000 0 - 0.01000 0.01001 - 0.05000 0.05001 - 0.10000 0.10001 - 0.50000 0.50001 - 1.00000 1.00001 - 2.50000 2.50001 - 5.00000 0 - 0.01000 0.01001 - 0.05000 0.05001 - 0.10000 0.10001 - 0.50000 0.50001 - 1.00000 1.00001 - 2.50000 2.50001 - 5.00000 0 - 0.01000 0.01001 - 0.05000 0.05001 - 0.10000 0.10001 - 0.50000 0.50001 - 1.00000 1.00001 - 2.50000 2.50001 - 5.00000 0 - 0.01000 0.01001 - 0.05000 0.05001 - 0.10000 0.10001 - 0.50000 0.50001 - 1.00000 1.00001 - 2.50000 2.50001 - 5.00000 0 - 0.01000 0.01001 - 0.05000
1 0 9 8 4 3 0 0 0 0 46 8 3 2 0 0 0 4 5 3 8 3 1 0 2 4 2 10 5 1 0 3 0 3 5 0 0 0 4 0
1600 0 886669 497605 152634 85819 0 0 0 0 2853507 1272659 541816 66600 0 0 0 174924 906865 436703 1023427 161676 183707 0 11100 187629 407444 2189815 345114 54704 0 66633 81917 171314 430000 0 0 0 826749 190430
56.4 56.4 70.9 70.9 70.9 70.9 70.9 70.9 70.9 67.3 77.6 77.6 77.6 77.6 77.6 77.6 77.6 64.4 64.4 64.4 64.4 64.4 64.4 64.4 60.8 60.8 60.8 60.8 60.8 60.8 60.8 60 60 60 60 60 60 60 50 50
1766 0 887 12006 11667 13731 0 0 0 0 2854 28564 41366 8208 0 0 0 175 29099 29588 213406 96845 370502 0 11 4229 29172 547766 273012 78853 0 67 819 12683 111142 0 0 0 827 1904
0.10 0.00 0.06 0.85 0.83 0.97 0.00 0.00 0.00 0.00 0.22 2.22 3.21 0.64 0.00 0.00 0.00 0.01 1.87 1.91 13.74 6.24 23.86 0.00 0.00 0.26 1.77 33.30 16.60 4.79 0.00 0.00 0.05 0.76 6.67 0.00 0.00 0.00 0.04 0.10
32
Bed name
Density Class Range
N
Polygon Area (m2)
Mean Weight (g)
No. of Oysters
Total Biomass (t)
Firing Range Firing Range Firing Range Firing Range Firing Range
0.05001 - 0.10000 0.10001 - 0.50000 0.50001 - 1.00000 1.00001 - 2.50000 2.50001 - 5.00000
1 2 0 0 0
489739 314438 0 0 0
50 50 50 50 50
36846 75338 0 0 0
1.84 3.77 0.00 0.00 0.00
33