Broadscale Mapping of Strangford Broadscale Mapping of Strangford Lough’s Subtidal Habitats: The Lough’s Subtidal Habitats: The

Application of an Evolving Application of an Evolving Technology Technology

Annika J. MitchellAnnika J. MitchellThe Queen’s University of BelfastThe Queen’s University of Belfast

Mega | Meso | Macro | Micro

Evolving technologies used in sampling the marine benthosEvolving technologies used in sampling the marine benthos

| RoxAnn | Video/Diver |

Broadscale mapping methods: 1993 - Broadscale mapping methods: 1993 - 20032003

• Vessel-based ‘remote’ acoustic sensing : RoxAnnVessel-based ‘remote’ acoustic sensing : RoxAnnTMTM acoustic ground acoustic ground discrimination system (AGDS): uses the first and second return discrimination system (AGDS): uses the first and second return echoes from an echosounder transducer to compute roughness (E1 echoes from an echosounder transducer to compute roughness (E1 – echo1) and hardness (E2 – echo2) of the seafloor.– echo1) and hardness (E2 – echo2) of the seafloor.– used to discriminate different benthic habitats used to discriminate different benthic habitats when coupled with when coupled with

adequate ground-truthingadequate ground-truthing

• Ground-truthing – what do the different ground-types shown by Ground-truthing – what do the different ground-types shown by RoxAnn actually represent?RoxAnn actually represent?

Methods:Methods:– Towed underwater video sledge with stills cameraTowed underwater video sledge with stills camera– Diver surveys with camerasDiver surveys with cameras– Grab samplesGrab samples

Please see Broadscale Mapping methods poster on displayPlease see Broadscale Mapping methods poster on display

Spatial coverage and habitat Spatial coverage and habitat detectiondetection

• Data spatial coverageData spatial coverage::– RoxAnnRoxAnn provides point data along survey tracks. provides point data along survey tracks.

Points cover an area that is determined by the Points cover an area that is determined by the acoustic footprintacoustic footprint of the echosounder. An average of the echosounder. An average of 20mof 20m22 per data point is used for Strangford Lough. per data point is used for Strangford Lough. • Within the footprint, all habitats are averagedWithin the footprint, all habitats are averaged

• Data requires interpolation to give 100% cover of Data requires interpolation to give 100% cover of seafloor.seafloor.

– Accuracy of interpolation affected by heterogeneity of Accuracy of interpolation affected by heterogeneity of ground and track spacingground and track spacing

– Towed videoTowed video gives continuous cover along narrow gives continuous cover along narrow tracks, with medium-large positional uncertainty. tracks, with medium-large positional uncertainty.

– Diver surveysDiver surveys and and grab samplesgrab samples provide discrete provide discrete point data (only use diver entry position as no point data (only use diver entry position as no acoustic tracking device)acoustic tracking device)• Diver, grabs and towed video may detect smaller scale Diver, grabs and towed video may detect smaller scale

variation in habitats that cannot be detected by RoxAnnvariation in habitats that cannot be detected by RoxAnn

History of Broadscale Mapping in History of Broadscale Mapping in Strangford Lough: 1975 – 1989Strangford Lough: 1975 – 1989

Diving surveysDiving surveys

• Roberts (1975): Diving survey revealed the general Roberts (1975): Diving survey revealed the general distribution of two communities of distribution of two communities of Modiolus modiolusModiolus modiolus ((Chlamys varia Chlamys varia association in north of the lough; association in north of the lough; Ophiothrix Ophiothrix fragilis fragilis association in south of the lough).association in south of the lough).

• Ulster Museum Diving Survey (Erwin, 1977) : Identification of Ulster Museum Diving Survey (Erwin, 1977) : Identification of communities and substrates at over 250 sites : Production of communities and substrates at over 250 sites : Production of broad distribution map for ten communities (later referred to broad distribution map for ten communities (later referred to as ‘Erwin 1977 Communities’).as ‘Erwin 1977 Communities’).

• Northern Ireland Sublittoral Survey (NISS; Erwin Northern Ireland Sublittoral Survey (NISS; Erwin et alet al., 1986): ., 1986): 300 dive records between 1968 and 1985.300 dive records between 1968 and 1985.

• Brown / National Trust (1989) : Produced a map of the Brown / National Trust (1989) : Produced a map of the distribution of distribution of Modiolus modiolusModiolus modiolus within the lough, based upon within the lough, based upon extrapolations between the existing extensive dive records.extrapolations between the existing extensive dive records.

Brown Brown (1989) (1989) Modiolus Modiolus distribution distribution map, as map, as published in published in Service Service (1990)(1990)

History of Broadscale Mapping in History of Broadscale Mapping in Strangford Lough: 1990 – 2003Strangford Lough: 1990 – 2003Remote and direct survey methodsRemote and direct survey methods

• Industrial Science Division (of the Industrial Science Division (of the then Dept. of Economic then Dept. of Economic Development) survey to assess Development) survey to assess impact of commercial fishing impact of commercial fishing (Service, 1990):(Service, 1990):– Remotely operated vehicle (ROV) Remotely operated vehicle (ROV)

video survey and grab sampling of video survey and grab sampling of 44 stations, coupled with sidescan 44 stations, coupled with sidescan sonar survey of the central sonar survey of the central channel.channel.

• Trawling impact scale:Trawling impact scale:0 = non-0 = non-ModiolusModiolus community community1 = intact 1 = intact ModiolusModiolus beds beds2 = largely intact 2 = largely intact Modiolus Modiolus beds, beds,

some trawl markssome trawl marks3 = some trawl damage, less surface 3 = some trawl damage, less surface

epifauna but largely intact clumpsepifauna but largely intact clumps4 = moderate trawl damage, a few 4 = moderate trawl damage, a few

clumps of clumps of ModiolusModiolus5 = heavily trawled, very few 5 = heavily trawled, very few

Modiolus, Modiolus, much dead shellmuch dead shell

0

1-2

3

4-5

History of Broadscale Mapping in History of Broadscale Mapping in Strangford Lough: 1990 – 2003Strangford Lough: 1990 – 2003

Use of RoxAnn Acoustic Ground Discrimination Use of RoxAnn Acoustic Ground Discrimination System, towed video and diving surveysSystem, towed video and diving surveys

• 19931993: Queen’s University / DARD Survey as part of PhD project on : Queen’s University / DARD Survey as part of PhD project on the impact of commercial fishing (Magorrian the impact of commercial fishing (Magorrian et al.et al., 1995): Trialling of , 1995): Trialling of RoxAnn (50 kHz) in the mapping of the central channel of Strangford RoxAnn (50 kHz) in the mapping of the central channel of Strangford Lough, coupled with extensive towed video ground-truthing.Lough, coupled with extensive towed video ground-truthing.

• 20002000: DARD follow-up mapping survey: RoxAnn (200 kHz) survey : DARD follow-up mapping survey: RoxAnn (200 kHz) survey covering wider area of the lough coupled with towed video ground-covering wider area of the lough coupled with towed video ground-truthing.truthing.

• 20032003: Queen’s University / DARD / EHS mapping survey: Extensive : Queen’s University / DARD / EHS mapping survey: Extensive RoxAnn (200 kHz) survey coupled with comprehensive diver survey RoxAnn (200 kHz) survey coupled with comprehensive diver survey (commissioned for the SLECI Project). (commissioned for the SLECI Project).

• Analysis Analysis

1993199320002000

20032003

Year: 1993 2000 2003Frequency (kHz) : 50 200 200Save rate (s) : 5 5 1Datapoints (common area) : 9399 4530 29405

Erwin 1977 Community Acoustic Habitat JNCC Habitat Classification

Boulder community Rubble (cobbles and boulders) RU

CR.LCR.BrAs.AmenCio.Ant; CR.LCR.BrAs.LgAsSp; CR.LCR.BrAs.AntAsH

Cobbles community Rubble (cobbles and boulders) RU SS.IGS.ScupHyd

Coarse sand community Coarse sediments (sand and gravel) CS SS.CGS; CR.MCR.EcCr.FaAlCr.Bri (pebbles, no cobbles)

Muddy sand community Muddy sand MS SS.CMS.AfilEcor

Clean sand community Coarse sediments (sand and gravel) CS SS.CGS.Ven.Neo

Fine mud/sand community Mud (not burrowed) MU

SS.CMS.VirOph; SS.IMX.FaMx.An; SS.IMU.AreSyn; SS.IMU.PhiVir (no Philine aperta )

Fine mud community Mud with burrows MU_Burrows SS.CMS.SpMeg

Mud and shell community Modiolus community 1 (dense clumps) MM_1 CR.SCR.ModCVar; CR.SCR.ModHAs

Modiolus community 2 (sparse, scattered clumps) MM_2 SS.CMX.ModHo, damaged CR.SCR.ModCVar & CR.SCR.ModHAs

2003 only - Mud with shell (>70% shell) MU_Sh scattered SS.CMX.ModHo; SS.IMX.An

Subtidal community classificationsSubtidal community classifications

MudMudMud with Mud with burrowsburrows

Muddy sandMuddy sand Modiolus Modiolus clumpsclumps

Sparse Sparse ModiolusModiolus Mud with dead shellMud with dead shell Coarse sedimentCoarse sediment Rubble (cobbles)Rubble (cobbles)

Predictive Habitat MapsPredictive Habitat Maps

Species known Species known to be fished in to be fished in 1990 1990 (from (from Service, 1990)Service, 1990)

‘‘Stripped down’ Stripped down’ ModiolusModiolus distribution distribution maps for the same central channel maps for the same central channel

areaarea

 

0.00

5.00

10.00

15.00

20.00

25.00

Area (km2)

1993 2000 2003

Year

Area coverage of habitats between survey years in central channel

MUSh

Ru

CS

MM_2

MM_1

MS

MUburrows

MU

Habitat discrimination issues: Map Habitat discrimination issues: Map accuracy assessmentaccuracy assessment

Which habitats does RoxAnn readily confuse Which habitats does RoxAnn readily confuse in the 2003 map?in the 2003 map?

The Internal Error Matrix: an assessment of classification The Internal Error Matrix: an assessment of classification accuracy:accuracy:

MU_sh MU CS MM_2 MM_1 Ru Mu_burrows MS Total % CorrectMU_sh 107 0 21 25 0 13 1 0 167 64.07MU 7 18 26 0 0 2 6 5 64 28.13CS 4 0 18 1 0 2 0 0 25 72MM_2 28 0 10 20 3 7 0 0 68 29.41MM_1 37 0 13 17 23 2 0 0 92 25Ru 34 3 15 25 0 22 1 5 105 20.95Mu_burrows 5 15 0 0 0 0 62 0 82 75.61MS 0 0 3 0 0 0 2 66 71 92.96

Total 222 36 106 88 26 48 72 76 674

% Correct 48.2 50 16.98 22.73 88.46 45.83 86.11 86.84 50%

Ground-truth data

Map

ped

units

Low discriminatory ability between:Low discriminatory ability between:

MU_Sh – mud with thick dead shellMU_Sh – mud with thick dead shell

MM_2 – scattered clumps of MM_2 – scattered clumps of ModiolusModiolus

MM_1 – dense clumps of MM_1 – dense clumps of ModiolusModiolus

MU_Sh

MM_2

MM_1

Habitat: E1min E1max E1mean E2min E2max E2meanMU_Sh 0.212 1.079 0.774 0.051 1.43 0.384MM_2 0.6 1.1 0.805 0.155 1.064 0.535MM_1 0.793 0.97 0.885 0.402 0.754 0.551

Summary Statistics (standardised E1 and E2)

Future work: Can broadscale maps Future work: Can broadscale maps contribute to monitoring of Strangford contribute to monitoring of Strangford

Lough?Lough?

• How can acoustic signatures of habitats be How can acoustic signatures of habitats be improved?improved?– Stratified / targeted ground-truthing by diver, grab samples Stratified / targeted ground-truthing by diver, grab samples

and towed videoand towed video– Nested survey design – tight track spacing over Nested survey design – tight track spacing over

heterogeneous groundheterogeneous ground

• Maps allow the appropriate stratification of regular Maps allow the appropriate stratification of regular diving surveys; the two feed into each otherdiving surveys; the two feed into each other

• Maps allow broadscale substrate changes to be Maps allow broadscale substrate changes to be monitored with a relatively low survey effortmonitored with a relatively low survey effort

SummarySummary• Broadscale surveys from 1975 to 2003 show a degradation in the quality of Broadscale surveys from 1975 to 2003 show a degradation in the quality of

Modiolus Modiolus habitat, notably between dive surveys of c.1985 and ROV video habitat, notably between dive surveys of c.1985 and ROV video surveys in 1990. surveys in 1990.

• RoxAnn-based surveys completed in 1993, 2000 and 2003 RoxAnn-based surveys completed in 1993, 2000 and 2003

– Cannot be directly compared: differing methodologies, BUT:Cannot be directly compared: differing methodologies, BUT:– Broad substratum patterns in agreement between maps and with local Broad substratum patterns in agreement between maps and with local

knowledge.knowledge.– The fragmented distribution of The fragmented distribution of ModiolusModiolus beds shown in 2003 was also present in beds shown in 2003 was also present in

1993.1993.– Acoustic discrimination between different Acoustic discrimination between different ModiolusModiolus habitat quality classes is poor habitat quality classes is poor

• Acoustic signatures could be improved with further survey effortAcoustic signatures could be improved with further survey effort

• The 2003 habitat map forms a baseline for future survey workThe 2003 habitat map forms a baseline for future survey work• Good overall coverage of RoxAnn dataGood overall coverage of RoxAnn data• Excellent ground-truthing by diving surveyExcellent ground-truthing by diving survey• Discriminates habitats that are recognised by diving surveysDiscriminates habitats that are recognised by diving surveys

ReferencesReferences• Brown, R.A. 1989. Bottom trawling in Strangford Lough: Problems and Brown, R.A. 1989. Bottom trawling in Strangford Lough: Problems and

Policies. 3Policies. 3rdrd North Sea Seminar Proceedings, Rotterdam. 117-125. North Sea Seminar Proceedings, Rotterdam. 117-125.• Erwin, D.G. 1977. A diving survey of Strangford Lough: The benthic Erwin, D.G. 1977. A diving survey of Strangford Lough: The benthic

communities and their relation to substrate – A preliminary account. communities and their relation to substrate – A preliminary account. Biology of Benthic Organisms. 11Biology of Benthic Organisms. 11thth Symposium on Marine Biology, ed. Symposium on Marine Biology, ed. B.F. Keegan, P. O’Ceigigh, P.J.S. Boaden. Pergamon Press, 215-224.B.F. Keegan, P. O’Ceigigh, P.J.S. Boaden. Pergamon Press, 215-224.

• Erwin, D.G., Picton, B.E., Connor, D.W., Howson, C.M., Gilleece, P. and Erwin, D.G., Picton, B.E., Connor, D.W., Howson, C.M., Gilleece, P. and Bogues, M.J. 1986. The Northern Ireland Sublittoral Survey. Ulster Bogues, M.J. 1986. The Northern Ireland Sublittoral Survey. Ulster Museum, Belfast: HMSO.Museum, Belfast: HMSO.

• Magorrian, B.H., Service, M. and Clarke, W. 1995. An acoustic bottom Magorrian, B.H., Service, M. and Clarke, W. 1995. An acoustic bottom classification survey of Strangford Lough, Northern Ireland. Journal of classification survey of Strangford Lough, Northern Ireland. Journal of the Marine Biological Association of the United Kingdom 75, 987-992.the Marine Biological Association of the United Kingdom 75, 987-992.

• Roberts, C.D. 1975. Investigations into a Roberts, C.D. 1975. Investigations into a Modiolus modiolusModiolus modiolus community in Strangford Lough, Northern Ireland. Report of the community in Strangford Lough, Northern Ireland. Report of the Underwater Association 1 (NS), 27-49.Underwater Association 1 (NS), 27-49.

• Service, M. 1990. The Impact of Commercial Trawling on the Benthos Service, M. 1990. The Impact of Commercial Trawling on the Benthos of Strangford Lough (Interim Report). Industrial Science Division of Strangford Lough (Interim Report). Industrial Science Division TI/3160/90. TI/3160/90.