estimating similarity in benthic communities over decades and in … · (asci 2016, bethoney et al....

MARINE ECOLOGY PROGRESS SERIESMar Ecol Prog Ser

Vol. 595: 15–26, 2018https://doi.org/10.3354/meps12563

Published May 14

INTRODUCTION

Mobile fishing gear (i.e. scallop dredges, beamtrawls, otter trawls) can cause habitat degradation,specifically in sediment composition and benthiccommunity structure, which can result in the reduc-tion of benthic habitat complexity and affect trophicinteractions (reviewed in Stewart & Howarth 2016).Quantifying these fishing impacts is difficult, how-ever, due to the minimal availability of control areas,where fishing is clearly restricted. This makes it diffi-

cult to estimate the degree of impact commercialfishing has on benthic communities as well as therecovery time for key species to return to their un -fished abundance (Jennings & Kaiser 1998, Sciberraset al. 2013, Stewart & Howarth 2016).

Short-term and long-term area closures, or marineprotected areas (MPAs), are effective tools as fish-eries management moves from a single species- to anecosystem-based approach that acknowledges theimpacts of fishing on trophic interactions and benthichabitat conditions (Brodziak & Link 2002, Link 2005,

© The authors 2018. Open Access under Creative Commons byAttribution Licence. Use, distribution and reproduction are un -restricted. Authors and original publication must be credited.

Publisher: Inter-Research · www.int-res.com

*Corresponding author: [email protected]

Estimating similarity in benthic communities overdecades and in areas open and closed to fishing in

the central Gulf of Maine, USA

Samuel C. Asci1,3,*, Richard W. Langton2, Kevin D. E. Stokesbury1

1School for Marine Science and Technology, University of Massachusetts Dartmouth, 836 South Rodney French Boulevard, New Bedford, MA 02744, USA

2Northeast Fisheries Science Center, Maine Field Station, 17 Godfrey Drive, Suite 1, Orono, ME 04473, USA

3Present address: New England Fishery Management Council, 50 Water Street, Mill 2, Newburyport, MA 01950, USA

ABSTRACT: The ledges and banks of the central Gulf of Maine (i.e. Fippennies Ledge, JeffreysLedge, Cashes Ledge, and Platts Bank) have supported groundfish and Atlantic sea scallop fish-eries for centuries. The benthic community of Fippennies Ledge was evaluated and comparedduring 2 time periods separated by more than 2 decades, the first based on a series of photographscollected during manned submersible dives conducted in 1986 and 1987 and the second usingphotographs collected during drop camera video surveys conducted in 2009 to 2014. Further, Fip-pennies, Jeffreys, and Cashes ledges were permanently closed to fishing in 2002, while PlattsBank remained open, enabling a spatial comparison of benthic communities in open and closedareas along with the examination of changes in the benthos over the 28 yr period on FippenniesLedge. In the absence of commercial fishing, the scallop population on Fippennies Ledgeappeared to be unchanged between 1986 and 2014, while observed differences in other dominantspecies were attributed to predator−prey relationships and long-term shifts in benthic conditions.Benthos of scallop habitat in the open and closed areas between 2009 and 2014 were similar inspatially dominant species composition and species richness, likely a result of the study area beinga high-energy environment and therefore resilient to both natural and fishing perturbations.Understanding change in benthic communities over decades and between open and closed areashelps better define the dynamic nature of the central Gulf of Maine and may inform fisheries management in this region as the ocean climate changes.

KEY WORDS: Benthic imaging · Population dynamics · Placopecten magellanicus

OPENPEN ACCESSCCESS

Mar Ecol Prog Ser 595: 15–26, 2018

Stewart & Howarth 2016). MPAs are created to pro-tect and preserve resident target and non-target spe-cies as well as the benthic ecosystems which supporttheir existence. Perhaps MPAs will offer an option ascontrols for studies on benthic perturbation, but gen-erally such areas have been established for conserva-tion rather than experimentation.

In July 1986 and June 1987, 8 dive transects weremade on Fippennies Ledge, Gulf of Maine, USA,using the manned submersible ‘Delta’. High-resolu-tion photographic still frames of the seafloor takenduring each dive were used to quantify mean densi-ties of dominant benthic invertebrates in 1986 andagain in 1987 after 1 yr of heavy scallop fishing(Langton & Robinson 1990). The dominant species,the Atlantic sea scallop Placopecten magellanicus,cerianthid anemone Cerianthus borealis, and sabel-lid myxicolid worm Myxicola infundibulum, declinedsignificantly (70% in scallops, 25 to 27% in ceri-anthids and myxicolids) between 1986 and 1987,suggesting that the benthic community was severelyimpacted by fishing operations (Langton & Robinson1990).

Fippennies Ledge, Jeffreys Ledge, Cashes Ledge,and Platts Bank were surveyed by the School forMarine Science and Technology (SMAST) using adrop camera in early August of 2009, 2010, 2011,2013, and 2014 (henceforth denoted as 2009 to 2014).Three of the areas surveyed (Fippennies Ledge, Jef-freys Ledge, and Cashes Ledge) were closed to fish-ing as of 2002, while the fourth area surveyed (PlattsBank) remained open (Fig. 1). The initial 2009 surveyinvestigated popular commercial fishing groundsthat had been untargeted by the New Bedford scal-lop fleet since the mid-1980s to early 1990s. A strongrecruitment of juvenile scallops was detected in boththe 2009 (Stokesbury et al. 2010) and 2013 surveys(Asci 2016, Bethoney et al. 2016). In addition to esti-mating scallop density, abundance, and size distri -bution, the SMAST video survey characterizes thebenthic habitat including densities per square meterof macroinvertebrates, spatial dominance of mega -fauna, and detailed information on substrate com -position (Stokesbury 2002, Stokesbury et al. 2004,Stokesbury & Harris 2006).

We hypothesize that the benthic community of Fippennies Ledge changed between 1987 and 2009as a result of post-fishing recovery and factors otherthan fishing. The 2 benthic photographic surveys ofFippennies Ledge, in 1986 and 1987 and again from2009 to 2014, presented an opportunity to determineif there was a change in the dominant invertebratefauna over the past several decades.

We also hypothesize that the benthic communitiesin areas open and closed to scallop fishing in the cen-tral Gulf of Maine differed. Benthic communities inthe open and closed areas, which were surveyed inthe 2009−2014 period, were compared to examineimpacts of continual commercial fishing.

These hypotheses were tested by comparing densi-ties of the dominant invertebrate species and com-paring biodiversity measures for the benthic com -munities. Langton & Robinson (1990) monitored thedominant benthic invertebrate community of Fippen-nies Ledge over 2 yr, and Stokesbury et al. (2010)used 1 yr of data to describe the scallop populationswithin the central Gulf of Maine. Using these studiesas a foundation, we were able to synthesize the asso-ciated data sets, which are both based on measuresper unit area from high-resolution photography, toextend this examination temporally and spatially ona scale few other studies have been able to achieve.

MATERIALS AND METHODS

‘Delta’ dive survey

The 1986 and 1987 ‘Delta’ dive transects on Fip-pennies Ledge covered from 1.9 to 3.7 km2 each, and150 to 250 benthic photographs (35 mm) were col-lected per dive. Diver-scientists on board ‘Delta’recorded observations of fauna, substrate type, andother information regarding the benthic habitat on atape recorder. Photographs were examined on a lighttable; animals large enough to be resolved (2 to 5 cm)were identified, and the number of individuals ofeach species category was recorded (Langton &Robinson 1990). For this study, the original raw countsper photograph (1.24 m2 quadrat area) from the 1986and 1987 ‘Delta’ dives were analyzed to estimate thedensity per square meter of benthic invertebrates,where mean densities per square meter and SEs ofthe total sample were calculated using standard sta-tistical methods summarized in Cochran (1977) andKrebs (1989), similar to the methods utilized by Lang-ton & Robinson (1990).

SMAST video survey

Observations of the benthic communities on Fip-pennies Ledge, Jeffreys Ledge, Cashes Ledge, andPlatts Bank were made in 2009 to 2014 using theSMAST drop camera survey (Fig. 1). This surveyemployed a centric systematic sampling design, in

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Asci et al.: Similarity of open and closed areas

which the first station location was chosen randomlyand subsequent stations were then located on a1.0 km2 grid (Stokesbury et al. 2010). Upon arrival ateach station, a commercial scallop fishing vesseldeployed the drop camera pyramid system and as thevessel drifted, the pyramid was lowered to the seafloor and raised 4 times to collect 4 video quadratsamples separated by approximately 40 m. To main-tain the integrity of the sample design, the initial tar-get station locations were the same each year. It isimpossible, however, to drop the pyramid on exactlythe same location of the sea floor each year due tocurrent, wind, and the vessel’s navigational ability;the vessel aims to begin within a 75 m radius of thetarget station location. The station depth rangedbetween 11 and 130 m, but 95% of sampled stationswere at depths between 25 and 100 m, with an over-all average of approximately 71 m.

Fixed to the pyramid were 8 lights, 2 live-feedvideo cameras, and a high-resolution digital stillcamera (Carey & Stokesbury 2011). Prior to survey-ing, the downward-facing live-feed cameras andhigh-resolution digital still camera were calibrated ina test tank to calculate lens height and quadrat size;the downward-facing live-feed cameras providedquadrat images of 2.84 and 0.60 m2, and the digital

still camera provided high-resolution quadrat imagesof 1.04 m2 (Carey & Stokesbury 2011). For densityestimates of scallops on Fippennies Ledge, thequadrat sizes were increased based on the meanscallop shell height in the area to adjust for partiallyvisible scallops counted along the edge of the image(Table 1) (O’Keefe et al. 2010).

Live video feed of the sea floor was recorded in thewheelhouse, and a custom computer field applicationallowed technicians to record station and quadratnumber, date, time, latitude, longitude, depth, andnumber of scallops and other macroinvertebratesobserved in each quadrat as well as substrate com -position. Video recordings were later viewed in the

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Year Mean shell Adjusted quadrat height (mm) area (m2)

2009 50.51 3.022010 55.84 3.032011 62.51 3.062013 66.01 3.072014 72.05 3.09

Table 1. Mean shell height of scallops observed on Fippen-nies Ledge and adjusted quadrat areas used to estimate

scallop density in 2009, 2010, 2011, 2013, and 2014

Fig. 1. Extent of the central Gulf of Maine study area. The Western Gulf of Maine Closure Area and Cashes Ledge ClosureArea boundaries are shaded in grey. Also shown are the locations of ‘Delta’ dive transects from 1986 and 1987 (black lines) onFippennies Ledge and School for Marine Science and Technology video survey stations (black dots) from 2009, 2010 2011,

2013, and 2014 on Fippennies Ledge, Jeffreys Ledge, Cashes Ledge, and Platts Bank


laboratory, and a still image of each quadrat was ana-lyzed. Sediments were visually identified from thevideo images following the Wentworth particle gradescale, where the sediment particle size categorieswere 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 dividedinto 2 categories: granule/pebble = >2.0 to 64.0 mmand cobble = >64.0 to 256.0 mm (Lincoln et al. 1992,Stokesbury 2002, Harris & Stokesbury 2010). Shellheights (mm) of scallops completely within the imagewere measured from umbo to outer shell marginusing ImagePro® image analysis software (Carey &Stokesbury 2011). Density per square meter was calculated for scallops, anemones, sea stars, variousother benthic invertebrates, and fish.

Mean densities and SEs of counted benthic inver-tebrates were calculated using equations for a 2-stage sampling design (Cochran 1977, Krebs 1989,Stokesbury 2002, Carey & Stokesbury 2011). Thissimplified version of the 2-stage variance is possiblewhen the sampling fraction n/N is small (Cochran1977). This is the case for the video survey, wherethousands of square meters are sampled compared tomillions of square meters in the study area (Stokes-bury & Harris 2006). Sponges, hydrozoans/ bryo -zoans, and sand dollars were re corded as present orabsent within each quadrat (Stokesbury 2002, Stokes -bury et al. 2004, Stokesbury & Harris 2006).

Despite the 2-decade difference between the ‘Delta’and SMAST collections, the field sampling methodsand image quality of quadrat still frames were similar(Fig. 2), allowing for the benthic invertebrate speciesto be quantified on the same spatial scale (i.e. ind.m−2). It was therefore possible to compare density persquare meter estimates of the dominant benthic in -vertebrate species observed in 1986 and 1987 (Fig. 3)on Fippennies Ledge with the more recent densityestimates from 2009 to 2014. Significant shifts in den-sity were tested using a 1-way ANOVA (α = 0.05)over the duration of survey years, and then a Tukeypost hoc (α = 0.05) test was employed if significantdifferences were noted. This comparison was repeatedexcluding estimates from 1987, as appreciable dredg-ing had occurred that year (Langton & Robinson1990). Comparing density per square meter of At -lantic sea scallops Placopecten magellanicus, ceri-anthid anemones Cerianthus borealis, sea stars Aste-rias spp., and sabellid myxicolid worms Myxicolainfundibulum, among each of the 7 years FippenniesLedge was surveyed, as well as the 6 years where nocommercial fishing occurred, provided statistical evi-

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Fig. 2. Example of quadrat still frames used to characterizethe benthic community of Fippennies Ledge between the(A) 1986 and 1987 ‘Delta’ dive surveys and (B) 2009, 2010,2011, 2013, and 2014 School for Marine Science and Tech-nology drop camera surveys. Note that ‘Delta’ images (A)

included a time-stamp on the left side of the quadrat

Fig. 3. Digital still images of the dominant invertebrate spe-cies on Fippennies Ledge. (A) Atlantic sea scallop Pla-copecten magellanicus, (B) cerianthid anemone Cerianthusborealis, (C) myxicolid worm Myxicola infundibulum, (D)northern sea star Asterias vulgaris. The combination of ahigh-resolution camera and light-emitting diode lighting isrequired to accurately identify M. infundibulum; this combi-nation was only used in the 2009 School for Marine Science

and Technology video survey


dence of changes in benthic community structure ona scale of almost 3 decades. The SMAST drop camerasamples for each year (2009 to 2014) were treated asindependent due to the variation in vessel position-ing at each station and the effect of 1 yr between sur-veying the benthic community; many of these speciesare annual, winter storms in this area are severe, andtidal currents are strong, causing shifts in sea floorstructure (Thompson 2010). Comparing scallop sizedistributions between 1986−1987 and 2009−2014 wasnot possible because ‘Delta’ surveys did not measurescallops.

The SMAST video surveys covered areas bothopened and closed to fishing; stations surveyed onFippennies Ledge, Jeffreys Ledge, and CashesLedge fell within the boundaries of the Western Gulfof Maine Closure Area and Cashes Ledge ClosureArea, while the surveyed area on Platts Bank was notclosed to fishing (Fig. 1). A preliminary test of inde-pendence was performed to determine if substratecomposition differed between areas (chi-square, α =0.05). At each station, the most abundant coarse sub-strate type (i.e. sand, gravel, cobble, rock) was deter-mined (Harris & Stokesbury 2010). Then, data werecompiled by area for all years surveyed by SMASTand statistically compared. To reduce uncertainty inthe scenario where substrate composition varied sig-nificantly between areas, analysis was conductedusing only stations that were representative of suit-able scallop habitat; this allowed for comparisons ofthe total area surveyed to be made, as well as com-parisons of scallop habitat within each respectivearea.

Scallops aggregate in areas with coarse sand−granule−pebble substrate (Langton & Robinson 1990,Thouzeau et al. 1991, Stokesbury 2002, Stokesbury &Harris 2006). These areas are known to fishermen,and are where the majority of fishing effort isdirected, although fishing effort will shift if the scal-lops are below exploitable size. Therefore, a compar-ison between scallop substrates within the closedand open areas not only captures the highest areas ofimpact but also reduces the effect of different sub-strates on benthic community structure, since thesubstrates analyzed are homogeneous. The dropcamera survey was designed to define exploitablescallop beds, and 1 scallop station−1 is roughly a min-imal commercial density; therefore, a station with 1or more scallops represents suitable scallop habitat(Stokesbury 2002).

A percent similarity index (Renkonen 1938) wasused to measure similarity between benthic commu-nity structures of the open and closed areas over the

entirety of the survey period, in both total habitat andscallop habitat. This index compared relative propor-tions of taxonomic categories present in each area,where relative proportions were standardized as apercentage of the total categories observed. The per-centage similarity ranges from 100% for very similarcommunities to 0% for those that are completely dif-ferent. Although straightforward, this percent simi-larity index is robust to sample size and speciesdiversity (Krebs 1989) and serves as a quantitativecoefficient of community structure (Stokesbury &Harris 2006). The percent similarity index uses spe-cies occurrence (standardized by quadrat) as opposedto abundance (no. of ind. station−1), which means thespatial dominance of species categories was com-pared as opposed to the true number of individualsobserved. This allowed for a more comprehensivemodel of the benthic communities, as rarer specieswere not completely excluded due to the extraordi-narily high abundance of the few dominant species.The percent similarity index included only speciesthat are sessile or exhibit locally mobile behavior.Though the SMAST drop camera survey has the ability to identify highly mobile species, it is de -signed to observe sessile and locally mobile species(MacDonald et al. 2010); consequently, it is possiblethat observations (or lack thereof) of highly mobilespecies (i.e. roundfish, flatfish, and other demersalfish) would misrepresent the actual abundance ofsuch species in any given quadrat.

The frequency of species present at each stationwas used as a metric of species richness (Krebs1989); estimates of species richness were calculatedfor each area, and then open and closed areas werecompared statistically. Because this richness meas-ure was broad and designed to encompass as manyspecies categories as possible, all species identifi-able by the SMAST video survey were included. Allyears of data were compiled by specific area, andthe frequency of species present at each station wasrecorded. Observed species frequencies per stationwere compared using a 2-sample Kolmogorov-Smirnov test, where the test statistic critical levelwas adjusted to the number of collective samples(total stations surveyed) for each area (Siegel &Castellan 1988). Cumulative frequency distributionsof number of species observed per station wereplotted, and sample mean (x ), SE (x ), and mode(Mo) were calculated to further compare open andclosed areas.

Rarefaction, an estimation of expected speciesrichness based on the number of individuals sam-pled, was employed to further compare benthic

19


invertebrate communities. For each area, counts ofindividuals observed per station were compiled byspecies, and rarefaction curves were producedusing the R package vegan (Oksanen et al. 2008).Because this method estimates the expected num-ber of species observed as a function of individualssampled, rarefaction curves included only benthicinvertebrate species which were counted by thedrop camera survey (i.e. scallops, sea stars, crabs,hermit crabs, whelks, lobsters, moon snails, seamice).

A test of heterogeneity was employed which com-pared frequencies of quantifiable species observedby the SMAST video survey between the open andclosed areas. The counts of individual speciesobserved were combined for all years surveyed byeach area, and compared using chi-square (α = 0.05).To avoid violating chi-square assumptions, speciesfrequencies were grouped so that


between 2009 and 2014. No variation in density persquare meter was apparent for scallops (F(4,411) =0.855, p = 0.491) and anemones (F(4,411) = 1.542, p =0.188); however, sea star density per square meterdid vary significantly (F(4,411) = 22.734, p =


Platts (approximately 6000 ind.), and was roughlysimilar on Jeffreys (approximately 3000 ind.) andCashes (approximately 2500 ind.).

Grouped species frequencies were significantlydifferent among all 4 areas (χ2(18,22474) = 3434.9, p <0.001) (Table 5). Expected grouped species frequen-cies were determined for each area, affording theopportunity to compare differences between ob -served and expected frequencies; the number of

individuals expected to exist in a givenarea was based on the number of individ-uals ob served across the entire studyregion as a function of the size of the specific area. The difference betweenobserved and expected values indicatedmore scallops on Fippennies and Jeffreysthan ex pected and fewer scallops onCashes and Platts than expected. Therewere fewer sea stars on Fippenniesand Jeffreys and more sea stars onCashes and Platts than expected. CashesLedge had more bryozoans/hydrozoansand sponges, while the other 3 areashad fewer bryozoans/ hydrozoans thanexpected.

DISCUSSION

Seldom in marine ecology is there theopportunity to examine temporally dis-tinct generations of the benthic inverte-brate community of an offshore ledgeusing comparable sampling units. Here,the Fippennies Ledge macrobenthiccommunity separated by 28 yr was com-pared. Given this extended time period,the minimal variation in scallop densityper square meter between the unfishedyears was striking. The fishing effort forthis area is unknown before 1986 andbetween 1988 and 2002, but after 2002the area was closed to fishing. This sug-gests a stable population structure forscallops occurred shortly after the CashesLedge Closure Area was established. Thebenthos of heavily fished areas can takeover 10 yr to fully recover (Collie et al.2000, 2009, Lambert et al. 2014), whereaspopulations of relatively fast-growingspecies with high fecundity, such as scal-lop populations on Georges Bank, haveappeared to recover in


Between 1986−1987 and 2009−2014, ane mone andmyxicolid density per square meter decreased 67 and95%, respectively, in sharp contrast to the stability ofthe scallop population. As Fippennies Ledge had notbeen fished since 2002, and in accordance with the10 yr recovery time horizon (Collie et al. 1997, 2009,Lambert et al. 2014), it is unlikely that long-termeffects of commercial dredging prevented these spe-cies from recovering. Anemones and myxicolidspotentially feed on scallop larvae (Shumway et al.1985, 1988, Langton & Robinson 1990), and scalloplarvae were clearly abundant given the 2009 and2013 recruitment events (Stokesbury et al. 2010, Asci2016, Bethoney et al. 2016); therefore, food availabil-ity was most likely not limiting. Other factors, such asthe Gulf of Maine water temperature increasingmore rapidly in the past de cade than in other regionsglobally (Pershing et al. 2015), may be driving ashift in the benthic community, for example, andtherefore impacting or even redefining what consti-tutes a recovery.

The marked increase of sea stars between 2013and 2014 (x = 0.16 ± 0.02 to x = 0.51 ± 0.08) correlated

with high scallop abundance on Fippennies in 2014(≈1.81 × 108 ind.). Sea stars are a major predator ofscallops, particularly smaller juvenile scallops (Marinoet al. 2007); sea stars respond to increased prey abun-dance by increasing individual consumption ratesand by increasing in number (Barbeau & Scheibling1994, Barbeau et al. 1996, Marino et al. 2007), andthey tend to aggregate in high density in closed areas(Marino et al. 2007). Interestingly, Platts Bank, theone continuously open area, had fewer scallops thanexpected but a higher concentration of sea stars(Table 5). This is possibly due to the scallop preda-tor−prey relationship, and may relate directly to fish-ing activity; commercial catch is processed at sea,where scallops are shucked for their adductor muscleand all other body parts are discarded overboard,which in turn may attract starfish, as do high den -sities of juvenile scallops (Langton & Bowman 1980,Caddy 1989, Orensanz et al. 1991, Barbeau et al.1996, Marino et al. 2007).

No direct relationship between benthic communitystructure and commercial fishing was apparent. Fip-pennies, Jeffreys, and Platts had the highest percentsimilarity, and were only different in the ranking ofthe most spatially dominant species; scallops weremore spatially dominant than sea stars on Fippenniesand Jeffreys, and sea stars were more spatially dom-inant than scallops on Platts. Fippennies and Jeffreyshad more scallops observed than expected and fewersea stars observed than expected; Platts had fewerscallops than expected and more sea stars. Casheswas the outlier; bryozoans/hydrozoans and spongeswere spatially more dominant than scallops, andwere also observed more frequently than expected,re flecting the difference in substrate composition be -tween Cashes and the other areas, as these speciesinhabit bouldered areas which are common on Cashes(Bell & Barnes 2003, Brusca & Brusca 2003, Weaver etal. 2007, Bell 2008). When benthic community struc-tures of scallop habitat (assumed to have homoge-neous substrate across areas) were compared, eacharea was highly similar, further suggesting that over-all variation in benthic community structure betweenareas was dependent on substrate as opposed to fishing intensity.

Species richness and commercial fishing intensitywere unrelated. Besides Jeffreys and Platts, cumula-tive frequency distributions of species observed perstation were similar. Descriptive statistics for eacharea showed only slight differences between great-est and least sample mean (0.46 species station−1)and mode (1.0 species station−1). Large differences insample sizes, in particular for Jeffreys (n = 277) and

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Fippennies Jeffreys Cashes Platts

n 416 277 316 454

ScallopsObserved 8828 2438 990 4107Expected 7593 2239 2127 4403

Sea starsObserved 867 354 1240 1291Expected 1741 513 488 1010

CrustaceansObserved 22 8 18 91Expected 65 19 18 37

GastropodsObserved 12 2 1 13Expected 13 4 4 8

Sand dollarsObserved 4 10 3 11Expected 13 4 4 8

Bryozoans/hydrozoansObserved 262 51 131 159Expected 280 83 78 162

SpongesObserved 434 212 539 376Expected 724 214 203 420

χ2 3434.9df 18

α = 0.05 p


Platts (n = 454), reflect a common problem encoun-tered with biodiversity measures. Increasing numberof observed species correlates directly with anincreasing sample size (Krebs 1989, Chao et al. 2005,Colwell 2009, Gotelli & Colwell 2011); this phenome-non most likely explains why cumulative frequencydistributions of Jeffreys and Platts were found to besignificantly different. This point is illustrated whencomparing expected species richness based on thenumber of individuals sampled (i.e. rarefaction curves)of benthic invertebrate species across the surveyareas. For example, there were twice as many indi-viduals observed on Platts as on Jeffreys, yet onlyone less species was observed on Jeffreys comparedto Platts. Furthermore, Platts and Jeffreys rarefactioncurves followed a similar slope, suggesting thatincreased sampling on Jeffreys may have led to anincrease in species observed.

Fishing impact was not great enough to noticeablyalter the benthic community of Platts Bank. This iseither because minimal fishing occurred on PlattsBank during the survey period or because the ben-thic community of Platts Bank is very resilient. Thefirst scenario is unlikely; Platts is a favored target ofcommercial scallop fleets, both of which have beenrising in ex-vessel value and annual landings for several years. The second scenario, however, is themore likely explanation; fished benthic communitieswhich experience high natural disturbance are verysimilar to unfished areas with high natural distur-bance (Hiddink et al. 2006, Sciberras et al. 2013, vanDenderen et al. 2015), suggesting that certain ben-thic communities are more resilient to fishing activity(Stokesbury & Harris 2006, Diesing et al. 2013,Szostek et al. 2016).

Benthic communities are both complex and diverse,and vary in disturbance recovery times; the post-fishingmegafaunal community of the offshore region of thenortheastern USA, for example, may take up to 10 yrto fully recover (Collie et al. 1997, Lambert et al.2014). However, the diversity in sediment types andother environmental drivers that exist throughout theGulf of Maine region defy a general characterizationin terms of how a specific ledge or bank respondsto fishing perturbations. In the case of our studyarea, the central Gulf of Maine, sediment types aregenerally more coarse, which reflects the complexoceanography and tidal structure in this region(Townsend et al. 2006) and suggests that it is a high-energy environment, and is subject to strong waveaction and currents. Because these ledges and bankslikely withstand strong natural perturbation, theymay be more resilient to fishing than the surrounding

deep muddy basins, for example, depending of courseon the frequency and intensity of fishing. Our moni-toring of the sea floor found an unchanged scalloppopulation after roughly 2 decades, and describedgreat similarity between an evidently resilient fishedbenthic community and its unfished analogs withinthe central Gulf of Maine. Thus, we must considerthe possibility that commercial scallop fishing is notdetrimental to the overall benthic community of thishigh-energy environment, and that the central Gulfof Maine is capable of maintaining a healthy macro -benthic community over decadal time scales.

Acknowledgements. We thank W. Robinson, I. Babb, and P.Auster for their great contributions to the ‘Delta’ dive pro-gram and to the content of the work done here. Thanks tothe students, staff, captains, and crews that sailed on theSMAST research trips. Special thanks to scallop fishing ves-sel owner-captains D. Eilertsen and M. Vieira for their un -wavering support of this largely unfunded project. Thanksto T. Jaffarian for the development of the SMAST surveysoftware and database. Thanks to G. Fay for his contributingrole as a Master’s thesis committee member and for refiningthe original work this research was based on. Partial fundingwas provided by NOAA Sea Scallop Research Set-AsideProgram NA10NMF4540474. The views expressed in thispaper are the authors’, and do not necessarily reflect theviews of NOAA.

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Editorial responsibility: Paul Snelgrove, St. John’s, Newfoundland and Labrador, Canada

Submitted: November 10, 2016; Accepted: March 12, 2018Proofs received from author(s): May 6, 2018

https://doi.org/10.3354/meps074205https://doi.org/10.1016/j.jsb.2006.10.027https://doi.org/10.3354/meps11550

estimating similarity in benthic communities over decades and in … · (asci 2016, bethoney et al....

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