Dredging activity and associated sound have negligible effects on adult Atlantic sturgeon migration to spawning habitat in a large coastal river.

Dredging is considered a major threat/impedance to anadromous fish migrating to spawning habitat. Due to the perceived threat caused by dredging, environmental windows that restrict dredge operations are enforced within many rivers along the east coast. However, it is generally unknown how anadromous fish react to encountering an active dredge during spawning migrations. Atlantic sturgeon (ATS) are an endangered, anadromous species along the Atlantic slope of North America. To determine if and how an active dredge may affect ATS spawning migration, a Vemco Positioning System array was deployed around an active hydraulic-cutterhead dredge that adult ATS must traverse to reach spawning habitat in the James River, VA. Telemetry data showed that all ATS that entered the study area survived. ATS that migrated upstream during dredge operations (N = 103) traversed the dredge area and continued upstream to spawning habitat. Many ATS made multiple trips through the study area during dredge operations. There was no noticeable difference in swim behavior regardless of whether the dredge was absent or working within the study area. We suggest that dredging in the lower James River does not create a barrier for adult ATS migrating to spawning habitat or cause adults to significantly modify swim behavior. This is the first study to utilize fine-scale telemetry data to describe how an organism moves in relation to an active dredge. This methodology could be used to describe dredge-sturgeon interactions on different life stages and in other locations and could be expanded to other aquatic organisms of concern.

Introduction

Through Coastal Zone Management Acts, many states have implemented time-of-year restriction windows, or environmental windows, on dredging projects to minimize possible negative impacts they may have on natural resources. In 2012, the National Marine Fisheries Service declared that distinct population segments of Atlantic sturgeon Acipenser oxyrinchus oxyrinchus (ATS) were either threatened or endangered throughout their entire range along the Atlantic coast [1-2]. ATS are both anadromous and philopatric species meaning they return to upstream freshwater reaches in their natal streams to spawn [3]. Currently, areas along the U.S. east coast have established riverine environmental windows for the spring anadromous fish spawning run. When anadromous fish windows were implemented it was thought that ATS spawning was limited to the spring season. However, studies have shown that in some river systems, ATS spawning occurs during both the spring and fall seasons [4-9]. There are no riverine environmental windows currently in place to protect fall spawning ATS migrating to spawning habitat. Although the supporting data are sparse, dredging is listed as a serious threat to ATS recovery [3] and resource managers are concerned about potential impacts an active dredging operation may have on fall run adult ATS migrating to spawning habitat.

Hydraulic-cutterhead dredging has occurred in the James River during the summer-fall every year since 1998 except for 2000, 2006 and 2013. Dredging occurs annually between river kilometer (rkm) 60 and 80 of the James River, Virginia, due to shoaling (i.e., buildup of bottom sediments in river channels) in the federal navigation channel. Dredging typically begins soon after the spring anadromous fish environmental window ends in July and continues through November to create safe passage for shipping. Fall spawning ATS usually stage in the lower part of the James River near rkm 30 until water temperatures decrease below 28°C during August-September and then move upstream above rkm 130 to spawn [7,10]. This creates a situation where adult ATS must traverse annual dredging operations to reach spawning habitat. Since 2016, collections of fall spawn young-of-year and age-1 ATS were verified using genetics and show successful spawning during the past few years when dredging occurred during the fall spawning migration (Matthew Balazik, Unpublished Data). Even though successful reproduction by fall ATS exposed to dredging operations has been proven, information is needed to elucidate potential impacts of dredging in a riverine environment on migrating ATS. The goal of this study was to determine how migrating adult ATS move in relation to an active hydraulic-cutterhead dredge by utilizing Vemco Positioning System (VPS) technology. Implications of this work will help determine if environmental windows are an effective management policy.

Methods

This work was carried out in guidelines set by Virginia Commonwealth University’s Institutional Animal Care and Use Committee (#AD20127) and the National Marine Fisheries Service endangered species permit (#16547).

Study area

The study area is a tidal, 12 km reach of the James River federal navigation channel (Figs 1 and 2). Federal regulations state that a 91.4 m wide, 7.6 m deep channel must be maintained for navigation. Due to severe shoaling in the federal navigation channel, dredging occurs annually from July through November to maintain legally required channel depths. In 2017 two portions of the 12 km stretch were dredged (Fig 2). The average river width of the 12 km stretch is 4.3 km. Because the study area was located in a public area, no special permits were required to access the field site.

Fig 1

Map showing study location.

Basemap reprinted from Ocean Basemap under a CC BY license, with permission from ESRI, original copyright 2012.

Fig 2

Map of study design.

The white polygon is the navigation channel and the colored dots within the channel are where the hydraulic-cutterhead dredge was working during the study period. The colored dots within the channel show where the dredge was working. The different colors show where the dredge was working each day. The black dots are collocated receiver locations of the Vemco Positioning System array. The red dot is a reference tag to assist with determining fine-scale positions for the telemetered Atlantic sturgeon. The gray dots are receiver locations for the supplemental array upstream of the dredge area. The black grid in the southern area of the array is where the dredged material from the lower dredge area was placed. Basemap reprinted from Ocean Basemap under a CC BY license, with permission from ESRI, original copyright 2012.

Vemco Positioning System array

A VPS utilizes trilateralization technology to estimate down to 1 m spatial resolution of telemetered objects within a study area [11]. This technique requires acoustic receivers to be strategically placed to allow a single ping from a telemetry tag to be detected by at least three receivers. The position of the tag can be estimated using difference in arrival time to multiple receivers. The tagged organisms in this study were verified to be sexually mature fall spawning ATS when the telemetry tag was placed. During previous years, Virginia Commonwealth University has surgically implanted Vemco telemetry tags into various sized ATS in the James River [5,10]. There were about 140 adult James River population ATS at large of which about 100 enter the James River annually to spawn at the time of this project.

There were not enough receivers to cover the entire dredge area (Fig 2). The VPS was deployed around the lower dredge area where the river was narrower and would provide coverage for almost the entire width of the river. A 37 bottom mounted VPS receiver array with collocated synchronization tags was deployed around rkm 65 to monitor movements of telemetered ATS in the lower dredge area (Fig 2). One reference tag was placed in the middle of the array to improve positioning data of the ATS (Fig 2). The lower dredge area was chosen because the width of the river is narrower; therefore, the VPS would cover more of the overall width of the river. It is very important to minimize receiver movements when conducting VPS studies [11]. VPS Receivers were placed inside 0.7–1 m long, 8 cm diameter PVC pipes with the receiver hydrophone exposed on one end with the other end of the pipe embedded in a 61X61X12 cm concrete slab weighing about 45 kg (Fig 3). Rebar cross members that extend about 50 cm from each direction of the concrete slab were added to strengthen the concrete. The reinforced concrete slab and extended rebar helped prevent the receiver from moving or tipping over in the water current. Rebar bent in a U-shape was placed in the concrete to provide handles to make moving the receiver stands easier and create a grappling point. A 30 m piece of positively buoyant rope was secured to the receiver stand and the other end of the rope was secured to a 2 kg anchor or cinder block (Fig 3). The gear was retrieved by grappling the rope between the two anchors. The VPS array deployment was initiated on July 20, completed on August 6 and was removed October 29, 2017.

Fig 3

Setup to reduce movements of receives in the Vemco Positioning System array.

A supplemental receiver gate comprising 4 receivers was placed on July 20 about 2 km upstream of the upper dredge area to provide additional receiver coverage at the upper dredge area. (Fig 2). Each supplemental receiver was affixed to the bottom with an 11 kg anchor. The supplemental gate was used to determine when ATS passed through the upper dredge area but did not provide fine-scale positions.

Position analysis

Other telemetry studies have been used to describe sturgeon swim behavior [12,13] but none have focused on movements around something that could be perceived as a threat such as a dredge. VPS positions used for data analysis were vetted by only using positions estimated to be within the array grid (Fig 4). Positions were vetted further following methods by Roy et al. [11] using horizontal positioning error scores. The Optimized Hot Spot Analysis tool (ESRI ArcMap 10.5.1 Pro) was used to determine areas that ATS more frequently used while moving through the study area. The Optimized Hot Spot Analysis tool removes the subjective aspect of determining point clusters by assigning confidence values to each cell as either a significant hot spot, a non-significant spot, or a significant cold spot. This technique utilizes inferential statistics that applies z and p scores to hexagon polygons using Getis-Ord Gi* Cluster Analysis [14]. The hotspot analysis was confined to the rectangular bounding grid used for vetting the VPS positions (Fig 4). Position data was divided into three timeframes: 1) dredge operational in lower area, 2) dredge operational in upper area, and 3) no dredge operations occurring.

Fig 4

Points showing all adult Atlantic sturgeon positions generated by the Vemco Position System array.

The black box is the extent of the Getis Gi* Hotspot analysis. In total 99 adults were represented and 5839 positions were recorded, of which 5,011 were used after the points were vetted for accuracy. Basemap reprinted from Ocean Basemap under a CC BY license, with permission from ESRI, original copyright 2012.

Gaps in a fish’s swim patch occur because its position is only determined when the tag sends a ping and pings are not always detected on enough receivers in a VPS for a position to be determined. The longer the time between positions will likely lead to more error when describing swim behavior because we are assuming a linear swim path between two positions. To reduce the effects of describing swim behavior only fish tracks extending at least 1 km with no consecutive points greater than 15 minutes apart were used for the swim behavior analyses. Each path was classified according to whether the direction of movement was upstream or downstream, as well as whether the dredge was at the downstream site, the upstream site or absent. Swim speeds were determined by measuring the distance between all points of an individual track and dividing by elapsed time. Permutation tests with 25,000 iterations using the ‘independence_test’ function in R Statistical Software, version 3.5.2, package “coin 1.3–0” were run comparing ATS swim speed versus movement direction through the study area and whether the dredge was working in the lower area, upper area, or if the dredge was absent [15]. Another aspect of swim behavior analyzed was how much an ATS deviated from a straight swam path, i.e. meandered, while moving through the study area. The difference in the actual distance swam and the straight-line distance between the first and last point of the track was calculated in meters. Using the same methods as the speed permutation test, separate permutation tests were run on how much ATS meandered while traversing through the study area.

Water quality

Water temperature is a critical factor influencing the timing of ATS migration to spawning habitat [16]. Hourly water temperature measurements were obtained 2 km downstream of the study site by The National Oceanic and Atmospheric Administration. [17]. Average hourly water temperature measurements were estimated from July 20 to November 3, 2017. In previous years, male fall spawning ATS tended to start migrating upstream when the daily mean water temperatures fell below 29°C (Matthew Balazik, Virginia Commonwealth University, Unpublished Data).

Dredge

The hydraulic-cutterhead dredge, Lexington, conducted maintenance dredging during the study period. The Lexington is 61 m long, 125 m wide, 3 m draft barge with a 2000 HP pump connected to a 51 cm diameter intake pipe located just shipward of the cutterhead. The cutterhead tip had a 1.4 m diameter. Dredge operations started at 1500 on July 27 and where continuous until the project ended at 1700 on September 16, 2017. Dredging occurred in the lower area from July 27 to August 23 and September 12 to 14. Dredging occurred in the upper area from August 23 to September 12 and September 15 and 16 (Fig 5). The dredge removed 278,056 m3 of sediment from the lower dredge area and 227,311 m3 from the upper area (Fig 5). The dredge’s location was determined at 30 s intervals using the United States Army Core of Engineers Automatic Identification System Analysis Package (AISAP) tool. All dredged material was placed via a partially floating and sinking pipeline to a pre-determined site located south of the navigation channel (Fig 2).

Fig 5

The primary Y-axis shows how much material was removed from the lower (solid gray line) and upper (dashed gray line) dredge area. The secondary Y-axis shows temperature (red line) and number of adult Atlantic sturgeon moving passed the dredge or through the area (black line) each day. Serendipitously dredging started when water temperatures dropped to levels that trigger upstream spawning migrations.

Results and discussion

During the study period, 106 adult ATS (7 female and 99 male) were detected in the study area, 103 of which swam past the active dredge. Accurate positions could not be determined for 7 males, but the dates of movement are known. A total of 5,839 fine-scale positions were determined by the VPS of which 5,011 from 7 females and 92 males were used for analysis (Fig 4). All ATS were detected by the supplemental array. The 7 males that had no fine-scale positions estimated moved upstream in late August during the peak of upstream migration (Fig 5). In the lower area, 48 different adult ATS made a total of 88 passes by the active dredge (Fig 5). Seventy-four adults moved past the dredge a total of 129 times while the dredge was working in the upper area (Fig 5). After dredging finished on September 16, 93 adults traversed the study area 125 times (Fig 5).

Three male ATS moved through and staged upstream of the study area prior to dredge operations. As in previous years, most adults started moving upstream when daily mean water temperatures at the water quality buoy fell below 29°C (Fig 5). Mean daily water temperatures fell below 29°C in the study area on July 30. ATS staging downstream started migrating upstream to spawning habitat on July 29 when the dredge was working in the lower area. Adult activity gradually increased in the study area but then decreased as water temperatures began increasing around August 16 (Fig 5). As water temperatures dropped relatively quickly starting August 28, upstream migration activity peaked while the dredge was working in the upper area (Fig 5). Based on real-time receivers around hypothesized spawning habitat, the peak of the 2017 spawning occurred from September 5 to 14. When dredging operations were finishing in the upper area there was a spike in migrating adults on September 15 as the post-spawn ATS began to return to the ocean. Most telemetered male ATS stayed around spawning habitat for weeks after the peak spawning season but started migrating downstream in early October (Fig 5).

The dredge did not limit ATS movement or cause mortalities within the James River. All ATS that reached the study area while the dredge was working moved upstream to spawning habitat. Telemetry data showed that ATS swam past dredge operations 217 times. Over half (n = 59, 56%) of the spawning ATS made one pass by the dredge. Forty-four adults (41%) passed the active dredge multiple times with one ATS passing the dredge 13 times (Fig 6). If the active dredge and associated noise were a deterrent it is unlikely adult ATS would make unnecessary, repeat trips past the dredge. Three males moved upstream in June before dredge operations and downstream in October, completely missing the dredge activity altogether (Fig 6).

Fig 6

The number of times adult Atlantic sturgeon moved past the hydraulic-cutterhead dredge.

The Y-axis is the total number of fish that passed the dredge the number of times listed on the X-axis.

Hot Spot Analysis showed adult positions were more densely packed in the immediate channel area during upstream and downstream migration (Fig 7). When the dredge was working in the lower and upper areas, most of the adults were making upstream migrations to spawning habitat. Adult positions were concentrated in the channel during upstream migration regardless of the presence or absence of an operating dredge in the immediate area and avoided areas away from the channel (Fig 7). During the main downstream migration after dredging was complete, adult ATS still had high densities in the channel but were more scattered throughout the study area compared to when ATS were mostly moving upstream to spawning habitat (Fig 7).

Fig 7

Results of the Getis Gi* Hotspot Analysis for when the dredge was working in the lower area (Panel A), working in the upper area (Panel B) and when no dredging was occurring (Panel C). The black dots in the middle of the hot spot in Panel A shows where the dredge was working during the lower area. The white polygon marks the federal navigation channel. All three plots show Atlantic sturgeon’s preference for the navigation channel either with or without a hydraulic-cutterhead dredge conducting dredge operations. Basemap reprinted from Ocean Basemap under a CC BY license, with permission from ESRI, original copyright 2012.

It is possible that dredge activity attracts migrating adult ATS to the channel. However, it’s more likely the adult fish naturally utilize the channel during upstream migration due to depth, current flow or some other factors. Catch and telemetry data show migrating adults have a high affinity for deep channels [18] in the James River and results from this study suggest the preference continues even if dredging is occurring. Based on passive telemetry data during the previous 5 years, telemetered adult ATS have traversed active dredge operations hundreds of times in the James River and data indicate that no mortalities of telemetered ATS resulted. The VPS telemetry data from this study indicate that no telemetered adults were killed during dredging operations and none stopped upstream progress due to the dredge.

ATS traversed the VPS study area 298 times, 170 of which satisfied our criteria for individual movement analysis (Fig 8). ATS swim speed or meandering did not vary significantly when factoring for dredge location and ATS swim direction (Table 1, Fig 9). These data suggest that there was no difference in swim speed or meandering in the VPS area whether the dredge was operational within the lower or upper areas or not operational.

Fig 8

Individual tracks (colored lines) of adult Atlantic sturgeon swimming in an upstream to spawning habitat during dredge operations.

The colored dots represent the dredge location for when the sturgeon of the same color line moved through the area. Basemap reprinted from Ocean Basemap under a CC BY license, with permission from ESRI, original copyright 2012.

Table 1

Results of permutation tests comparing swim speed and meandering behavior factoring in swim direction and dredge location.

Fish movement direction	Number of tracks			Average swim speed (m/s)				Average meandering (m)
	Absent	Lower	Upper	Absent	Lower	Upper	p-value	Absent	Lower	Upper	p-value
Upstream	8	34	36	0.71	0.83	0.78	0.43	458	454	297	0.44
Downstream	57	15	20	0.57	0.59	0.67	0.76	472	511	447	0.96

Fig 9

Left boxplots show swim speed (meters per second) and right boxplots show meandering (difference in actual swim path from a linear swim path) when the dredge was not in the area (Absent), when the dredge was working in the lower area (Lower) and when the dredge was working in the upper area (Upper). The top boxplots are when Atlantic sturgeon where swimming upstream through the study area while the bottom boxplots are when Atlantic sturgeon were swimming downstream through the study area. Permutation tests showed no significant difference between swim speed and dredge location for upstream movements (p = 0.429) and dredge location for downstream movements (p = 0.756). Permutation tests also showed no significant difference between meandering and dredge location for upstream movements (p = 0.44) and for dredge location and downstream movements (p = 0.96).

Conclusions

The results of this study indicate that hydraulic-cutterhead dredging in the study area does not deter adult ATS from migrating to spawning habitat and had no observable effect on swim behavior. This study along with Reine et al. [18] found no evidence of any subadult or adult ATS being killed by a hydraulic-cutterhead dredge in the James River. The combined evidence of this study and past data suggests that the physical presence of a hydraulic-cutterhead dredge and associated underwater sounds during the spawning season does not hinder ATS spawning migration.

Data from this study suggest that dredge restrictions in this area where eggs and larvae are likely not to be present may not be an effective management policy. This study was conducted in a relatively broad, >4 km wide, section of the James River; however, ATS spawning occurs further upstream where the river is typically <200 m wide. While in this study many adults passed within 100 m of the dredge, there may be a different reaction in a much narrower channel. Mimicking this type study in a narrower channel or closer to spawning habitat would provide useful information for resource managers. More work is also needed to look at potential impacts of various dredge types on juvenile ATS in different environments. It would also be beneficial to conduct fish movement-dredge VPS studies to examine other species of concern, such as striped bass (Morone saxatilis) and Alosines, which are protected by the anadromous fish environmental dredge window.

(ZIP)

Click here for additional data file.

6 Jan 2020

PONE-D-19-29177

Dredging activity and associated sound have negligible effects on adult Atlantic sturgeon migration to spawning habitat in a large coastal river

PLOS ONE

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The study sample size is extremely limited regarding females. Suggesting dredging does not have an impact on adult ATS migration is a rather sweeping claim and a bit presumptuous given the study’s samples and geographic size of the study area. I strongly encourage the authors limit this study to male subjects or address the limited number of females in the study within the discussion.

In figure 1, it is unclear what the colors along the dredge lines indicate (“the colored dots within the channel are where the hydraulic-cutterhead dredge was working during the study period”). Do warmer colors signify the amount of time a dredge working in an area or the time of year the dredge was working? Please clarify.

To this point, why did the authors set up the VPS at the downstream site over the upstream site? Please incorporate the reasoning into the manuscript.

The authors report the number of fish in the study area used in the hot spot analysis but was there a minimum number of positions per fish required? Did these positions need to be captured within a certain window of time, similar to the swimming speed requirements? Incorporating a fish's positions in July versus September might be very different. Was a seasonal component or individual effect included? The authors elude to the differences in behaviors across seasons so this should be accounted for in the analyses.

Figure 7 is very interesting and I believe the authors should expand text in the discussion around this figure. Its very interesting that in plot C the fish are in a very narrow cluster along the channel (presumed preferred habitat) but in plots A and B the cluster is much broader and fish are avoiding banks. To me, this suggests dredging does influence migratory behavior. From your analyses it does not appear to impact the upstream or downstream movement or meandering in the lower dredging area but there appears to be an impact that should be discussed.

Remove or expand figure 8. Either include all fish's paths or none.

In general, the manuscript's discussion could be expanded and incorporate related studies.

Reviewer #2: The study assesses the effects of dredging on migration of sturgeon for purportedly spawning purposes. It is laudable in the sense that the study uses existing fish with transmitters and essentially, employs an array of Vemco receivers. However, my main two concerns are the writing styles and the interpretations or inferences from their study.

I found this manuscript to be very difficult to read and follow given the number of inverted, double negative and redundant sentences. I have made annotations on a pdf of your manuscript to highlight some problem areas and suggested some potential edits. I think you would be able to relay your message much more effectively if you conducted a very hard edit on your manuscript.

Your study was about the affects of dredging on migrating sturgeon, I recommend you adhere to that in your conclusions. You cannot make any statement about spawning success or efficacy of policy on recruitment because you didn't test it in your study. I have provided comments about the structure of your manuscript. For example, your conclusion is not a conclusion. You shouldn't be presenting new material in the conclusion. I think your abstract captures your study nicely, your conclusion doesn't.

Minor comments

Can you conduct statistical analyses to support your observations on initiation of movement (e.g., logistic regression or GLM using temperature and upstream movement)? It could strengthen your study.

This study was conducted predominately on males. I understand that you are relying on previously transmittered fish and therefore have to rely on movement, but could you address potential limitations in your discussion (e.g., sex or size effect). Additionally, could you provide metrics of the fish in the study with the caveat that they were of that size upon sample.

Several figures are, in my opinion, not necessary (e.g., Figures 2 and 4); Figure 6 would be better if it was a histogram.

Do not start sentences with acronyms.

**********

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Reviewer #2: No

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Submitted filename: PONE-D-19-29177_reviewer.pdf

Click here for additional data file.

25 Jan 2020

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A figure (Figure 1) was added to provide additional location information. The lat/lon points for the fish are also in the supplemental information.

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Added a line saying this was public area so no special permits were required to access the study site.

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Added Ethics Statement at the beginning of the methods section.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

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Reviewer #1: Yes

Reviewer #2: No

________________________________________

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: No

________________________________________

3. Have the authors made all data underlying the findings in their manuscript fully available?

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Reviewer #1: Yes

Reviewer #2: Yes

________________________________________

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Reviewer #2: No

________________________________________

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Overall this is a very well written manuscript and the authors are commended for their efforts.

The study sample size is extremely limited regarding females. Suggesting dredging does not have an impact on adult ATS migration is a rather sweeping claim and a bit presumptuous given the study’s samples and geographic size of the study area. I strongly encourage the authors limit this study to male subjects or address the limited number of females in the study within the discussion.

We understand that the sample size is highly skewed towards males over females. However, this does not change the fact that all the females in the study moved passed the dredge and reached spawning habitat. Even with the small female sample size, the claim is still valid that the dredge did not have any noticeable effects on adult sturgeon moving up to spawning habitat. We added text in the discussion to point out there was a relatively small amount of females but the point is still valid.

In figure 1, it is unclear what the colors along the dredge lines indicate (“the colored dots within the channel are where the hydraulic-cutterhead dredge was working during the study period”). Do warmer colors signify the amount of time a dredge working in an area or the time of year the dredge was working? Please clarify.

The different colors show where the dredge was working each day. We added text to the figure description. The figure would have been too crowded to show what color each day represents; however, the description of when the dredge was working on each day is described later in the methods section.

To this point, why did the authors set up the VPS at the downstream site over the upstream site? Please incorporate the reasoning into the manuscript.

We did not have enough receivers to cover both areas. We chose the narrower river width so our receivers would cover most of the overall width of the river. We added text to the methods section.

The authors report the number of fish in the study area used in the hot spot analysis but was there a minimum number of positions per fish required? Did these positions need to be captured within a certain window of time, similar to the swimming speed requirements?

No, all the points that fit the vetting criteria described in the position analysis were used to the hot spot analysis. This is stated in the methods section. If the only points used for the hot spot analysis had to pass the same conditions as the swim speed analysis there would not have been enough data to the analysis to show hot and cold spots.

Incorporating a fish's positions in July versus September might be very different. Was a seasonal component or individual effect included? The authors elude to the differences in behaviors across seasons so this should be accounted for in the analyses.

We agree that there might be a seasonal component in regards to pre/post spawn fish. The fish are moving back and forth during from July-November and it is possible that pre and post spawn fish move differently. During the July/early September most the fish were making their initial run to spawning grounds while in late September/November the fish were leaving for the year. Since dredging ended in the middle of September, comparing figures 7a and 7b to figure 7c is like taking into account pre and post spawning movements.

Figure 7 is very interesting and I believe the authors should expand text in the discussion around this figure. Its very interesting that in plot C the fish are in a very narrow cluster along the channel (presumed preferred habitat) but in plots A and B the cluster is much broader and fish are avoiding banks. To me, this suggests dredging does influence migratory behavior. From your analyses it does not appear to impact the upstream or downstream movement or meandering in the lower dredging area but there appears to be an impact that should be discussed.

This is a common misinterpretation of the figure, we thought it as well but had ERSI (maker of the tool) explain things to us. So according to the analysis in general the fish were closer to the channel, and therefore the dredge, and avoided the fringes (7a and b) compared to when the dredge had stopped working (7c). The fish were more dispersed when the dredge was not working which means the dredge had some sort of attraction effect. This is highly unlikely and is already noted in the discussion section. The only way to prove this would be to have the dredge work during October/November and not during August/September while still having the same water quality parameters. The dredge can not wait so late in the season to work because shipping would have to stop.

Remove or expand figure 8. Either include all fish's paths or none.

If all paths are plotted (170) it would be just on big blur because all the lines would be overlapping and wouldn’t make sense. We think it helps provide a scale to how close the fish swam passed the active dredge.

In general, the manuscript's discussion could be expanded and incorporate related studies.

The discussion was expanded; however, the only truly related study that tracks telemetered fish around an active dredge is already cited in this paper.

Reviewer #2: The study assesses the effects of dredging on migration of sturgeon for purportedly spawning purposes. It is laudable in the sense that the study uses existing fish with transmitters and essentially, employs an array of Vemco receivers. However, my main two concerns are the writing styles and the interpretations or inferences from their study.

I found this manuscript to be very difficult to read and follow given the number of inverted, double negative and redundant sentences. I have made annotations on a pdf of your manuscript to highlight some problem areas and suggested some potential edits. I think you would be able to relay your message much more effectively if you conducted a very hard edit on your manuscript.

We conducted a very hard edit to the manuscript. We might have a different writing style from reviewer 2. We would like to point out the reviewer 1 said the manuscript was very well written.

Your study was about the affects of dredging on migrating sturgeon, I recommend you adhere to that in your conclusions. You cannot make any statement about spawning success or efficacy of policy on recruitment because you didn't test it in your study. I have provided comments about the structure of your manuscript. For example, your conclusion is not a conclusion. You shouldn't be presenting new material in the conclusion. I think your abstract captures your study nicely, your conclusion doesn't.

The text was modified to stating that adults reached spawning habitat. We addressed the comments in the PDF.

Minor comments

Can you conduct statistical analyses to support your observations on initiation of movement (e.g., logistic regression or GLM using temperature and upstream movement)? It could strengthen your study.

We would have to look at multiple years of data and beyond the scope of this study. Physical cues to sturgeon movements are going to be a major point for another paper we plan to write.

This study was conducted predominately on males. I understand that you are relying on previously transmittered fish and therefore have to rely on movement, but could you address potential limitations in your discussion (e.g., sex or size effect). Additionally, could you provide metrics of the fish in the study with the caveat that they were of that size upon sample.

This was addressed in comments by reviewer 1. These fish were proven to be adults during initial capture. Most we tagged over 4 years prior to this study and growth rate is highly variable due to the fact that some years adults skip spawning and likely grow more than fish that do spawn and whether the fish spend their ocean time north or south of VA. We think there are too many variables to account for a size effect during this study when there is so much variability of growth from fish tagged 4+ years ago.

Several figures are, in my opinion, not necessary (e.g., Figures 2 and 4); Figure 6 would be better if it was a histogram.

Figure 4 will be removed. However, the setup shown in figure 2 is something that researchers are always asking for us to explain. We feel keeping figure 2 will help other conduct their own VPS studies.

Do not start sentences with acronyms.

20 Feb 2020

Dredging activity and associated sound have negligible effects on adult Atlantic sturgeon migration to spawning habitat in a large coastal river

PONE-D-19-29177R1

Dear Dr. Balazik,

We are pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it complies with all outstanding technical requirements.

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Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: N/A

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: The authors did an excellent job addressing most of my concerns and edits. Personally, I think the paper is stronger. I still feel the manuscript is qualitative vs quantitative and some of their conclusions could be strengthened by statistical analyses.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #2: Yes: Tim Haxton

24 Feb 2020

PONE-D-19-29177R1

Dredging activity and associated sound have negligible effects on adult Atlantic sturgeon migration to spawning habitat in a large coastal river

Dear Dr. Balazik:

I am pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

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