Final Report Period covered by Report: 05/01/2018 - 4/30/2019
Sea Scallop Research NOAA Grant Number: NA18NMF4540018 Award Date: 5/1/2018 End Date: 4/30/2019
Project Title: High resolution drop camera survey examining sea star dynamics in extremely dense scallop beds of the Nantucket Lightship Closed Area
Principal Investigators: Kevin D. E. Stokesbury, Ph.D., N.David Bethoney, Ph.D., Craig A. Lego, M.S. candidate Address: School for Marine Science and Technology, University of Massachusetts Dartmouth, 836 S. Rodney French Blvd. New Bedford, MA 02744 Phone: (508) 910-6373 Fax: (508) 910-6374 Email: [email protected]
Amount: We were allocated 38,288 lbs. ($421,171) for research and compensation.
Project Summary: The goal of this project was to investigate sea scallop and sea star predator- prey interactions and produce a 2018 biomass estimate of scallops to aid in management of the area. We surveyed the Nantucket Lightship Closed Area (NLCA) with a centric systematic design resulting in 509 stations sampled in 2018. We produced spatial specific estimates and associated error of scallop size as well as maps of exploitable and juvenile scallop distributions at the time of the survey. This information was supplied to the New England Fisheries Management Council and the National Marine Fisheries Service and included in the annual scallop allocation setting process. To investigate predator-prey interactions station and quadrat level data from 2010-2018 were utilized. Scallop and sea star density, ratio, and size trends at the spatial scale of the NLCA indicate an extremely high scallop recruitment event in this area linked to decreases in sea star densities. Additionally, Morisita’s indexes show similar increases and decreases in aggregation of scallops and sea stars following this extreme recruitment event. At smaller spatial scales scallops and sea stars were found clustered within 130 cm or less of each other. This increase in proximity could lead to more encounters and possibly captures of sea scallops by sea stars in densely populated beds created after extremely high recruitment events. 2
Project Goals: The goal of this project was to investigate the effects of extreme recruitment of juvenile sea scallops in relation to the distributions, abundance and predator-prey interactions of predatory sea stars in the Nantucket Lightship Closed Area (NLCA) on Georges Bank and produce a biomass estimate of scallops to aid in management of the area.
Objectives and Deliverables: • Identify if predatory sea stars are aggregating and persisting in extreme recruitment areas in the Nantucket Lightship Closed Area: Results will include 1) spatial and size specific estimates of sea stars and scallops, 2) maps of adult and juvenile scallop distributions in addition to sea star distributions within the Nantucket Lightship and 3) densities of individuals in m2.
• Examine spatial relationships between sea stars, juvenile and adult scallops in the Nantucket Lightship Closed Area: Results will include 1) nearest neighbor analysis 2) correlation analysis of sea scallops and sea stars on multiple different spatial scales.
• This project will also produce an additional year of estimates of total, exploitable sea scallop biomass and mean meat weight (g) at the time of the survey. Mean meat weight (g) will be derived from shell height(mm) frequencies and shell height to meat weight regressions used in the 50th Scallop Stock Assessment Workshop (SAW) or as specified by the New England Fisheries Management Council Scallop Plan Development Team. Estimates of total and exploitable biomass of scallops will be derived from area-specific shell heights and meat weight relationships, and commercial dredge selectivity equations (NEFSC 2010) and presented to the Scallop PDT on 1 August 2018. Habitat characteristics will also be recorded.
Methods: We surveyed the Nantucket Lightship Closed Area on a 2.8 km (1.5 nmi) grid totaling approximately 509 stations. These stations were sampled as part of two research cruises (Figure 1). During each survey, the SMAST sampling pyramid, supporting cameras and lights was deployed from a commercial fishing vessel (Stokesbury 2002, Bethoney and Stokesbury 2018, Figure 2). A mobile studio including monitors, computers for image capturing, data entry, and survey navigation (software integrated with the differential global positioning system) was assembled in the vessel’s wheelhouse. The vessel stopped at each pre-determined station and the pyramid was lowered to the sea floor. Two downward facing cameras mounted on the sampling pyramid provided 2.3 m2 and 2.5 m2 quadrat images of the sea floor for all stations (Figure 2). Additionally, a third camera providing a 0.6 m2 view of the seafloor was deployed. Quadrat images from all cameras and video footage from the 2.5 m2 quadrat view of the first quadrat were saved and then the pyramid was raised, so the seafloor was longer seen. The pyramid was then lowered to the seafloor again to obtain a second quadrat; this was repeated four times at each station. Onboard the survey vessel, scallop counts, station location, and depth was recorded and saved through a specialized field application for entry into a SQL Server Relational Database Management System.
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Figure 1. SMAST drop camera stations in 2018 displayed by vessel with survey dates.
Figure 2. University of Massachusetts Dartmouth, School for Marine Science and Technology drop camera survey pyramid including cameras and lights used for data collection with the area of each quadrat. 4
After the survey, the 2.3 m2 high resolution digital still images were used as the primary data source. Within each quadrat, scallops and other macrobenthos were counted and the substrate was identified (Stokesbury 2002, Stokesbury et al. 2004, Bethoney and Stokesbury 2018). Scallop shell heights were also measured at this time. After the images were digitized, a quality assurance check was performed on each image for accuracy of counted and identified species. Sediments in digital still images were visually identified following the Wentworth particle grade scale from images, where the sediment particle size categories are based on a doubling or halving of the fixed reference point of 1 mm; sand = 0.0625 to 2.0 mm, gravel = 2.0 to 256.0 mm and boulders > 256.0 mm (Lincoln et al. 1992). Gravel was divided into two categories, granule/pebble = 2.0 to 64.0 mm and cobble = 64.0 to 256.0 mm (Lincoln et al. 1992). Shell debris was also identified.
Analysis for abundance estimates included increasing the camera view area to account for counting scallops that lie on the edge of the image. This expansion was reviewed and accepted in the 50th SAW and is based on the average shell height of scallops in the area. The length and width of each image were increased by the mean shell height of measured scallops within the survey area using the equation:
(1) 퐸푥푝푎푛푑푒푑 푉푖푒푤 퐴푟푒푎 = (푙 + 푆퐻̅̅̅̅) × (푤 + 푆퐻̅̅̅̅) where l and w are quadrat length and width and 푆퐻̅̅̅̅ is mean shell height (O’Keefe et al. 2010).
Mean densities and standard errors of scallops were calculated using equations for a two-stage sampling design where the mean of the total sample is (Cochran 1977): n xi (2) x = i=1 n where n is the number of stations and xi is the mean of the 4 quadrats at station i. The SE of this 2-stage mean was calculated as:
1 2 (3) S.E.(x) = (s ) n n 2 2 where: s = (xi − x) /(n −1) . According to Cochran (1977) and Krebs (1999) this simplified version of the 2-stage variance is appropriate when the ratio of sample area to survey area (n/N) is small. In this case, thousands of square meters (n) were sampled compared with thousands of square kilometers (N) in the study areas. All calculations used number of scallops per square meter.
The number of scallops in the survey areas was calculated by multiplying scallop density by the total area surveyed (Stokesbury 2002). Estimates of scallop meat weight in grams were derived from shell height (mm) frequencies collected during each survey and shell height to meat weight regressions used in the 65th SAW or as specified by the NEFMC Scallop PDT. The mean meat weight for each 5 mm size bin was multiplied by the total number of scallops in the survey 5 area to estimate the total biomass of scallop meats. Exploitable biomass was calculated using the commercial scallop dredge selectivity equation determined by Yochum & Dupaul (2008).
To investigate the effects of extreme recruitment of juvenile sea scallops in relation to the distributions, abundance and predator-prey interactions with predatory sea stars 1,888 stations surveyed within the NLCA over from 2010-2018 were utilized. Scallop counts and measurements in all images had already been quantified and quality controlled. For this project, sea star arm lengths and distance between nearest neighbors were measured. Due to interannual differences in locations of stations within the NLCA and total area surveyed (km2), only stations that were in areas surveyed more than 4 years in a row were utilized for density and scallop-sea star ratio results. To identify these stations the NLCA was divided into a 5.6 km grid, matching the maximum distance between stations within a year. Densities and associated error for sea stars were calculated following equations (2) and (3). Yearly ratios for sea scallop counts to sea star counts were calculated by dividing the cumulative total number of sea scallop counts by the cumulative total number of sea star counts for all stations in that year. Ratios for sea scallops to sea star maps were calculated by dividing the total number of counts of sea scallops per station by the total number of sea stars at that same station.
To examine spatial relationships between sea stars, juvenile and adult scallops Morisita’s Standardized Index and nearest neighbor distances were calculated on stations and quadrat level data. Morisita’s Standardized Index was calculated following Krebs (1999). First an unstandardized index (MI) was calculated:
푆(∑ 푛2−푁) (4) 푀퐼 = 푁(푁−1)
Where n is the total number of individuals in a quadrat or station, N is the total number of all individuals and S is the total number of quadrats or stations. Then index’s for uniform (Mu) and clumped (Mc) distributions were calculated:
푋2 −푛+∑ 푥푖 (5) 푀 = .975 푢 (∑ 푥푖)−1
푋2 −푛+∑ 푥푖 (6) 푀 = .025 푐 (∑ 푥푖)−1
2 2 Where 푋.975 and 푋.025 are the chi-squared values with (n-1) degree of freedom that have 97.5% or 2.5% of the area to the right respectively. Morisita’s Standardized Index (MIS) was then calculated. One of the following equations was used depending on the unstandardized, uniform or clumped index values: 푀퐼−푀푐 (7) When MI ≥ 푀푐 > 1.0 then: 푀퐼푆 = 0.5 + 0.5 ( ) 푛−푀푐
푀퐼−1 (8) When 푀푐 >MI ≥ 1.0 then: 푀퐼푆 = 0.5 ( ) 푀푢−1
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푀퐼−1 (9) When 1.0 > MI >Mu then: 푀퐼푆 = −0.5 ( ) 푀푢−1
푀퐼−푀푢 (10) When 1.0 > Mu > MI then: 푀퐼푆 = −0.5 + 0.5 ( ) 푀푢
The closer the MIS is to +1, the more aggregated the sea scallops and/or sea stars are, the closer is it to zero, the more randomly distributed and the closer it is to -1 the more uniform the distribution.
To determine if changes occurred between nearest neighbor distances from sea stars to sea scallops under different densities of scallops, distance frequency distributions were created from the distance in millimeters from each sea star to its closest sea scallop neighbor. Nearest neighbor distances were allocated to quadrat density based on quantiles of the data used: (1) 0 – 0.36, (2) 0.37 – 0.87, (3) 0.88 – 2.14 and (4) >2.14 individuals per meter2. Nearest neighbor distance distributions were then compared using Kolmogorov-Smirnov tests with the test statistic critical level adjusted to the number of independent samples (number of quadrats) taken from each comparison. The Kolmogorov-Smirnov equation used is as follows:
(10) 퐷푛,푚 = 푠푢푝푥|퐹1,푛(푥) − 퐹2,푚(푥)|
Where F1 and F2 are the empirical distribution functions of the first and second set of densities being compared. Supx is the the largest gap between the two functions. The critical value to accept or reject the null hypothesis was calculated by:
푛+푚 (11) 퐷 > 푐(훼)√ 푛,푚 푛푚
Where 훼 = 0.05 and n and m are the number of samples in the first and second set of densities being compared respectively. If the value for D is greater than the critical value then the null hypothesis that the data are from the same distribution is rejected. The alternative hypothesis is that the two functions are from different continous distributions, or in this case that the distance distributions are different for each density level.
To test if sea stars were closer to juvenile scallops (≤75 mm in shell height) we used a contingency table similar to Carey et al. 2013 to compare observed nearest neighbor interactions to expected values from a random distribution:
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Nearest Neighbor Base Individual Adult Juvenile Sea Scallop Scallop Star Total Adult Scallop AA' JA' SSA' Na' Juvenile Scallop AJ' JJ' SSJ' Nj' Sea Star ASS' JSS' SSSS' Nss' Total Na Nj Nss N
Chi-square analysis was used to determine the significance of these interactions (ɑ = 0.05). Expected frequencies of nearest neighbor interactions were:
푁푗 푥 푁푗′ 푁푎 푥 푁푗′ 푁푠푠 푥 푁푗′ 푁(퐽퐽′) = 푁(퐴퐽′) = 푁(푆푆퐽′) = 푁 푁 푁 푁푗 푥 푁푎′ 푁푎 푥 푁푎′ 푁푠푠 푥 푁푎′ 푁(퐽퐴′) = 푁(퐴퐴′) = 푁(푆푆퐴′) = 푁 푁 푁
푁푗 푥 푁푠푠′ 푁푎 푥 푁푠푠′ 푁푠푠 푥 푁푠푠′ 푁(퐽푆푆′) = 푁(퐴푆푆′) = 푁(푆푆푆푆′) = 푁 푁 푁
Residuals were examined to determine which interactions were contributing the most to the significance.
Results and Discussion
Results from the 2018 survey of the Nantucket Lightship show continued high densities and biomass of scallops in the west and southern portions of the area (Figures 3-6, Tables 1 & 2). Densities in the northern part of the area, NLS-AC-N, have been considerably reduced since high recruitment was observed in the area due to fishing. Similarly, lower density in the NLS-EXT was observed, but this may be due to sampling intensity and the extremely concentrated aggregation in the northwest of this area (Figure 3). No substantial recruitment was observed and the prevalence of recruit size scallops in the NLS-AC-DEEP is due to stunted growth of existing scallops rather than incoming year-classes to the fishery (Figures 4 & 5). Most scallops in all SAMS zones appear to be great than 75 mm in shell height (Figures 6-11, Table 1). These results were presented to the SMAST Fishermen Steering Committee on 15th August at SMAST and to the Scallop Plan Development Team on 28th and 29th August 2018 in Falmouth, MA. All data associated with these results were provided to the Northeast Fisheries Science Center (NEFSC) and NEFMC by August 1st. All survey images were also made available on the SMAST Survey Image Browser (http://webserver.smast.umassd.edu/lab_stokesbury/ssib.html).
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Figure 3. Overall scallop density at 2018 SMAST drop camera survey stations in the Nantucket Lightship Scallop Area Management Simulator zones (red lines) based on digital still camera observations.
Figure 4. Pre-recruit scallop density at 2018 SMAST drop camera survey stations in the Nantucket Lightship Scallop Area Management Simulator zones (red lines) based on digital still camera observations. 9
Figure 5. Recruit scallop density at 2018 SMAST drop camera survey stations in Nantucket Lightship Scallop Area Management Simulator zones (red lines) based on digital still camera observations.
Figure 6. Scallops over 75 mm density at 2018 SMAST drop camera survey stations in the Nantucket Lightship Scallop Area Management Simulator zones (red lines) based on digital still camera observations. 10
Table 1. SMAST digital still camera data from the 2018 survey of the Nantucket Lightship Scallop Area Management Simulator zones. Included in the table is the quadrat area sampled (Quad), mean shell height of scallops observed (mm), number of scallop shell heights measured, mean number of scallops per m2, number of stations sampled, standard error, coefficients of variance, and an estimate of the number of scallops in millions. Area Quad SH #Measured Sc. Per m2 #Stations SE CV% Scallops NLS-AC-N 2.6 102.1 137 0.12 137 0.02 16 127 NLS-EXT 2.6 100.3 92 0.22 54 0.19 84 93 NLS-W 2.6 96.6 3,232 2.45 184 0.53 21 3,482 NLS-AC-S-SHALLOW 2.6 88.8 325 1.30 33 0.67 51 330 NLS-AC-S-DEEP 2.5 75.4 4,728 6.85 103 1.28 17 5,442
Table 2. SMAST digital still camera 2018 estimates of total and exploitable biomass for the Nantucket Lightship Scallop Area Management Simulator zones. Meat weights were estimated using equations from the Virginia Institute of Marine Science, as specific by the Scallop Plan Development Team. Biomass estimates were rounded to the nearest 50 tons. (MW = mean scallop meat weight (g), total weight of scallops in millions of pounds (LBS) and metric tons (MT) and the standard error in metric tons). Estimation of Total Biomass Estimation of Exploitable Biomass Area MW LBS MT SE MW LBS MT SE NLS-AC-N 30.3 9 3,850 600 42.3 7 3,200 500 NLS-EXT 19.4 4 1,800 1,500 23.2 3 1,150 950 NLS-W 16.8 129 58,500 12,550 20.4 66 29,800 6,400 NLS-AC-S-SHALLOW 12.5 9 4,100 2,100 20.6 5 2,150 1,100 NLS-AC-S-DEEP 7.5 90 40,700 7,600 11.1 17 7,900 1,450
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Figure 7. Shell height distribution of scallops in NLS-AC-N from SMAST drop camera digital still images. The average shell height was 102.1 mm and 137 scallops were measured.
Figure 8. Shell height distribution of scallops in NLS-EXT from SMAST drop camera digital still images. The average shell height was 100.3 mm and 92 scallops were measured.
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Figure 9. Shell height distribution of scallops in NLS-W from SMAST drop camera digital still images. The average shell height was 96.6 mm and 3,232 scallops were measured.
Figure 10. Shell height distribution of scallops in NLS-AC-S-SHALLOW from SMAST drop camera digital still images. The average shell height was 88.8 mm and 325 scallops were measured. 13
Figure 11. Shell height distribution of scallops in NLS-AC-S-DEEP from SMAST drop camera digital still images. The average shell height was 75.4 mm and 4,728 scallops were measured.
Refinement of density and ratio analysis to areas surveyed in more than half of the years focused results on the central and eastern parts of the NLCA (Figure 12). Examination of scallop and sea star trends from 2010-2018 show the large scallop recruitment coinciding with a decline in sea star abundance and density (Figure 13). This density change explains the sharp increase in the ratio of sea scallop counts to sea star counts from 2012-2013 (Figure 14). Additionally, the mean sea scallop shell heights in the NLCA are much smaller from 2012-2013 than the rest of the time period (Figure 15). Mean shell heights for sea scallops in the NLCA were the largest in 2010 reaching 113.87 mm (SD = +/- 26.78 mm). Shell heights then decreased from 2010 reaching a low in 2013 of 28.6 mm (SD = +/- 23.94 mm) (Figure 15). From 2013 until 2018 sea scallop mean shell heights increased steadily reaching approximately 84.86 mm in 2018. Average sea star diameters have slightly increased throughout the 9 years ranging from 36.41 mm in 2010 to 49.35 mm in 2018. Sea star diameter was the lowest in 2012 at 30.28 mm (SD = +/- 8.32 mm) and the highest in 2017 at 65.81 mm (SD = +/- 24.77 mm). Combined, the density and shell height results indicate an extreme recruitment event in this area starting sometime in 2012 when predator densities lessened perhaps allowing more pre-recruits than usual to survive.
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NAD 1983 UTM 19N
Figure 12. Consistency of survey spatial coverage within the Nantucket Lightship Closed Area from 2010-2018. Area classifications are based on stations from different years (colored dots) within 5.6 km2 grid cells. Stations in red areas were used to calculate mean densities.
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Figure 13. Mean densities (individuals per m2) for sea scallops (solid line) and sea stars (dashed line) in the Nantucket Lightship Closed Area. Standard error bars included for each years mean density.
Figure 14. Ratio of total cumulative sea scallop counts to sea star counts per survey year for all stations.
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140.00
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MeanLength(mm) 40.00
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Figure 15. Mean shell height in mm for sea scallops (solid line) and mean sea star diameter in mm (dashed line) in the Nantucket Lightship Closed Area. Standard error bars included for each year’s mean length.
Sea scallop and sea stars were distributed heterogeneously throughout the NLCA (Figures 16 & 17). On average from 2010 to 2018 the distribution of high-density areas of sea scallops were in the southern and west portions of the NLCA with moderate densities occurring in the northern NLCA (Figure 16). Sea scallops had relatively low density from 2010 to 2012 and were distributed in the northern portion of the NLCA (Figure 18-20). From 2013 to 2018 there were multiple areas of high density in the NLCA with most scallops concentrated in the south and west portions of the NLCA (Figure 21-26). On average from 2010 to 2018 the distribution of high-density areas of sea stars were in the southern and west portions of the NLCA with moderate densities occurring in the northern NLCA (Figure 17). High densities of sea stars from 2010 to 2012 were distributed in the south and west in 2010, in the north, south and west in 2011 and mostly in the west in 2012 (Figures 27-29). From 2013 to 2018 sea stars had lower densities with distributions occurring mainly in the north and south (Figures 30-34). The spatial distribution of scallop to sea star ratio reflects density results (Figures 35-43). A consistent higher ratio of sea scallop counts compared to sea star counts in the southern portion of the NLCA was apparent from 2013-2018 (Figures 38-43). Sea scallops and sea stars were both found at stations the most in year 2016 while they occupied similar stations the least in 2014 (Figures 39 & 41). In 2010 there were more stations with higher counts of sea stars than scallops (i.e. ratios below 1), while 2016 had more stations with higher scallop counts than sea stars (i.e. ratios higher than 1) (Figures 35 & 41).
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NAD 1983 UTM 19N
Figure 16. Average sea scallop density (individuals per m2) from 2010 to 2018 based on a 5.6 km grid.
NAD 1983 UTM 19N
Figure 17. Average sea star density (individuals per m2) from 2010 to 2018 based on a 5.6 km grid. 18
NAD 1983 UTM 19N
Figure 18. Densities of scallops (individuals per m2) in the NLCA during 2010 based on a 5.6 km grid. Majority of scallops concentrated in northeast.
NAD 1983 UTM 19N
Figure 19. Densities of scallops (individuals per m2) in the NLCA during 2011 based on a 5.6 km grid. Majority of scallops concentrated in northeast. 19
NAD 1983 UTM 19N
Figure 20. Densities of scallops (individuals per m2) in the NLCA during 2012 based on a 5.6 km grid. Majority of scallops concentrated in northeast.
NAD 1983 UTM 19N
Figure 21. Densities of scallops (individuals per m2) in the NLCA during 2013 based on a 5.6 km grid. Majority of scallops concentrated in southeast. 20
NAD 1983 UTM 19N
Figure 22. Densities of scallops (individuals per m2) in the NLCA during 2014 based on a 5.6 km grid. Majority of scallops concentrated in southeast.
NAD 1983 UTM 19N
Figure 23. Densities of scallops (individuals per m2) in the NLCA during 2015 based on a 5.6 km grid. Majority of scallops concentrated in southeast. 21
NAD 1983 UTM 19N
Figure 24. Densities of scallops (individuals per m2) in the NLCA during 2016 based on a 5.6 km grid. Majority of scallops concentrated in southeast.
NAD 1983 UTM 19N
Figure 25. Densities of scallops (individuals per m2) in the NLCA during 2017 based on a 5.6 km grid. Majority of scallops concentrated in southeast. 22
NAD 1983 UTM 19N
Figure 26. Densities of scallops (individuals per m2) in the NLCA during 2018 based on a 5.6 km grid. Majority of scallops concentrated in south and west.
NAD 1983 UTM 19N
Figure 27. Densities of sea stars (individuals per m2) in the NLCA during 2010 based on a 5.6 km grid. Majority of sea stars concentrated in south and west. 23
NAD 1983 UTM 19N
Figure 28. Densities of sea stars (individuals per m2) in the NLCA during 2011 based on 5.6 km grid. Majority of sea stars concentrated in north, south and west.
NAD 1983 UTM 19N
Figure 29. Densities of sea stars (individuals per m2) in the NLCA during 2012 based on a 5.6 km grid. Majority of sea stars concentrated in the west. 24
NAD 1983 UTM 19N
Figure 29. Densities of sea stars (individuals per m2) in the NLCA during 2013 based on a 5.6 km grid. Majority of sea stars concentrated in south and north.
NAD 1983 UTM 19N
Figure 30. Densities of sea stars (individuals per m2) in the NLCA during 2014 based on a 5.6 km grid. Majority of sea stars concentrated in south and west. 25
NAD 1983 UTM 19N
Figure 31. Densities of sea stars (individuals per m2) in the NLCA during 2015 based on a 5.6 km grid. Sea stars concentrated in the south.
NAD 1983 UTM 19N
Figure 32. Densities of sea stars (individuals per m2) in the NLCA during 2016 based on a 5.6 km grid. Majority of sea stars concentrated in the south. 26
NAD 1983 UTM 19N Figure 33. Densities of sea stars (individuals per m2) in the NLCA during 2017 based on a 5.6 km grid. Majority of sea stars concentrated in west, with some to the north and south.
NAD 1983 UTM 19N Figure 34. Densities of sea stars (individuals per m2) in the NLCA during 2018 based on a 5.6 km grid. Majority of sea stars concentrated in south and north. 27
Figure 35. Ratio of sea scallop to sea star counts for the 2010 NLCA survey.
Figure 36. Ratio of sea scallop to sea star counts for the 2011 NLCA survey. 28
Figure 37. Ratio of sea scallop to sea star counts for the 2012 NLCA survey.
Figure 38. Ratio of sea scallop to sea star counts for the 2013 NLCA survey.
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Figure 39. Ratio of sea scallop to sea star counts for the 2014 NLCA survey.
Figure 40. Ratio of sea scallop to sea star counts for the 2015 NLCA survey.
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Figure 41. Ratio of sea scallop to sea star counts for the 2016 NLCA survey.
Figure 42. Ratio of sea scallop to sea star counts for the 2017 NLCA survey.
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Figure 43. Ratio of sea scallop to sea star counts for the 2018 NLCA survey.
Morisita’s standardized indexes suggested significant aggregation of scallops and sea stars at the station (10’s of meters) and quadrat (meters) levels for all years. Both sea scallops and sea star indexes were above 0.5, which is consistent with significant aggregated distribution (Figure 45). However, sea stars and sea scallops were more aggregated at the station level than the quadrat level as indicated. At the station level, scallops were aggregated in 2015 more than any other year, while in 2017 sea stars were aggregated more than any other year (Figure 45). Sea stars aggregated more than scallops during 2 of the 9 years (2012 and 2018) while all other years sea scallops were aggregated more. At the quadrat level, 2015 had the highest level of aggregation for both sea scallops and sea stars. Sea stars were more aggregated than scallops during 3 of the 9 years (2010, 2012 and 2018) while all other years sea scallops were aggregated more (Figure 45). After 2013, trends of increases and decreases of scallops and sea stars are similar.
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Figure 45. Morisita’s standardized index of dispersion for station and quadrat level from 2010 to 2018 in the NLCA. Scallops (sc) are black bars and sea stars (ss) bars are dashed. Standardized indexes above 0.5 suggest significant aggregation and the closer the standardized index is to one the more aggregated the animals are.
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Under higher sea scallop densities (>2.14 m2) sea stars were closer to sea scallops then in lower sea scallop densities. Based on Kolmogorov-Smirnov test values, distance frequency distributions for densities (1) 0 – 0.36, (2) 0.37 – 0.87, (3) 0.88 – 2.14 scallops per m2 were all 2 significantly different than (4) >2.14 scallops per m (D1,4 = 0.46, p < 0.05, D2,4 = 0.42, p < 0.05, D3,4 = 0.37, p < 0.05; Figure 46). All other comparisons were not significant. All distributions were right skewed favoring smaller distances of 130 cm or less, with >90% of distances between sea scallops and sea stars from all densities within 130 cm away from each other. Distances at densities >2.14 scallops per m2 had ~ 67% of all distances within 300 mm as compared with ~ 21%, 25%, 30% for the other densities, respectively. The first set of densities had the largest variability of distances from sea scallops to sea stars, while the second and third set were less variable than the first and they were almost identical. Juvenile-juvenile and adult-adult associations were higher than expected while sea star-adult and sea star-juvenile were not significantly different in nearest neighbor contingency tables (X2 = 188.58, DF = 4, p<0.05; Table 3). Sea stars were more frequently nearest neighbors with adults compared with juveniles.
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Figure 46. Distance frequency distributions for varying sea scallop densities. Distances are defined as the length from each sea star to its closest scallop nearest neighbor. Densities are: 0 – 0.36 (red), 0.37 – 0.87 (green), 0.88 – 2.14 (purple), and > 2.14 (blue) scallops per m2.
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Table 3. Nearest neighbor contingency tables of adult-juvenile-sea star interactions with observed and expected (in parenthesis) counts from years 2010 to 2018; 354 quadrats were used for this analysis. (X2 = 188.575, DF = 4, p<0.05) Neighbor Base Individual Adult Juvenile Sea star Total
Adult 188 25 294 507 Na' (142.10) (69.01) (295.89)
Juvenile 14 90 134 238 Nj' (66.71) (32.40) (138.90)
Sea star 321 139 661 1121 Nss' (314.19) (152.59) (654.22)
Total 523 254 1089 1866 Na Nj Nss N
These analyses provide insight into the species interactions between sea scallops and sea stars on multiple levels. Though varying, sea stars and sea scallops are both aggregated on the station and quadrat level. After the extreme recruitment event of scallops we do see gradual increases and decreases of the level of sea star aggregation reflecting the pattern observed for scallops. Within the quadrat, sea scallops and sea stars are usually found clustered within 130 cm or less of each other and at higher densities above >2.14 scallops per m2 the majority of distances between sea stars and scallops are even less. This close proximity to its prey could lead to more encounters and possibly more successful captures of sea scallops in densely populated beds. However, sea stars do not appear to prefer being close to juveniles (scallops ≤ 75 mm shell height) versus adult scallops. This work part of Craig Lego’s ongoing master’s thesis project who is projected to graduate in fall of 2019.
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