Seabed disturbance by mobile bottom fishing gear has emerged as a major concern related to the conservation of essential fish habitat. Unquestionably, dredges and trawls disturb the seabed. However, the seabed is also disturbed by natural physical and biological processes. The biological communities that utilize a particular habitat have adapted to that environment through natural selection and therefore, the impact of mobile fishing gear on the habitat structure and biological community must be scaled against the magnitude and frequency of seabed disturbance due to natural causes.
Fishers operating in the mouth of Narragansett Bay, RI use trawls to harvest lobsters, squid and finfish, and dredges to harvest mussels. These mobile fishing gears impact rock, sand and mud substrates. Field observations of seabed disturbance by trawls indicate that the otter boards resuspend sediments and dig a furrow along their tow path. The trawl ground-cables and net sweep smooth micro-features on the seabed. Dredges both resuspend sediments and smooth micro-features on the seabed.
Side-scan sonar data from 1995 with 200% coverage was available from NOAA for the mouth of Narragansett Bay. Analysis of this data indicates that evidence of bottom scarring by the fishing gear is restricted to deeper waters with a seabed composition of soft cohesive sediments, despite the observation that fishing activity is ubiquitous throughout the bay mouth.
A quantitative model has been developed to compare
the magnitude and frequency of natural seabed disturbance to mobile fishing
gear disturbance. Wave and tidal currents at the seabed are coupled with
sediment characteristics to estimate the degree of seabed disturbance.
Finally, the model results are compared to the
recovery time of sediments disturbed by the interaction of the fishing
gear with the seabed. This analysis suggests that impact of mobile
The 1996 amendments to the Magnuson-Stevens Fishery Conservation and Management Act require the National Marine Fisheries Service (NMFS) and the Fishery Management Councils (FMCs) to protect and conserve the habitat of fishery resources under their jurisdiction. This habitat is referred to as "essential fish habitat" (EFH) and is defined as "those waters and substrate necessary to fish for spawning, breeding, feeding and growth to maturity." The Act ftirther requires the FMCs to amend Fishery Management Plans (FMPs) to describe and identify EFH, minimize adverse fishing effects on EFH, and finally, to identify other actions to conserve and enhance EFH.
Marine fishing activity, in general, has been identified as causing harmful environmental effects (Dayton et al. 1995; Auster and Langton 1998). Finfish and shellfish bycatch, the incidental take of manimals, turtles, and seabirds, habitat damage, the secondary effects of discards, the indirect effects of reduction of target species, and the generation of marine debris, are major concerns. Within this context, seabed disturbance by commercial mobile fishing gear has emerged as a major concern related to the conservation of EFH. However, the seabed is also disturbed by natural physical and biological processes. Bioturbation of sediments by benthic infauna mixes the sub-surface sediments with the surficial sediment layer (Rhoads et al. 1978). Bottom currents associated with surface waves and wind, tidal, and geostrophic forces also move bottom sediments creating bedforms, and causing erosion and accretion (Wright 1995). The biological communities that utilize a particular habitat have adapted to that environment through natural selection (Krebs 1994). As a result, animals adapted to a highly dynamic seabed environment due to natural causes may not be affected by seabed disturbance due to fishing. Conversely, animals adapted to a stable, quiescent seabed environment, if disturbed by fishing gear, may take a long time to recover.
Therefore, we argue that the relative significance of seabed disturbance by mobile fishing gear on habitat structure and the biological community must be scaled against the magnitude and frequency of seabed disturbance due to natural causes. We have selected the mouth of Narragansett Bay, RI as a case study where the mobile gear fisheries will be described and evidence of seabed disturbance by mobile gear will be presented. The bottom hydrodynamic environment and sediment transport processes in two environments in the lower bay will be characterized, and we will present the results of a field experiment to compare the longevity of mobile gear bottom scars as a measure of habitat recovery time. Based on the results of these studies, we will demonstrate that seabed disturbance by mobile fishing gear must be evaluated in light of natural processes, and will propose that a similar analysis for all continental shelf waters and seabed sediment environments would allow for the identification of problematic areas relative to the degradation of essential fish habitat by mobile fishing gear.
Effects of Mobile Fishing Gear on the Seabed
Understanding the extent and role of mobile fishing gear impacts is particularly important because of large increases in fishing effort over the last decade. For centuries, fishermen have used various kinds of mobile gear to capture bottom-dwelling finfish and shellfish (von Brandt 1984). Mobile fishing gear types include otter trawls, beam trawls, mussel and scallop rakes, and clam dredges. Some mobile gear can change the physical properties of surficial sediments, influence chemical exchanges between sediments and water, and alter the composition of benthic communities. Trawling and dredging can be expected to cause a number of direct and indirect changes in the ecosystem (Messieh et al. 199 1; Riemann and Hoffmann 199 1; Jones 1992). Direct, immediate effects include scraping and ploughing of the substrate, sediment resuspension, destruction of benthos, and dumping of processing waste. Indirect, delayed or long-term effects include post-fishing mortality and long-term changes to the benthic community structure.
Most experimental studies to date have been restricted to evaluating only the immediate impacts of mobile gear. However, intensive and repeated trawling in the same area may lead to long-term changes in both benthic habitat and communities. The magnitude of the effect depends on the type of gear employed, the depth of penetration of the gear into the sediment, the water depth, the nature of the substrate (mud, sand, pebbles, or boulders), the kind of benthic communities being impacted (i.e. epibenthic vs. infauna), the frequency with which the area is fished, the weight of the gear on the seabed, the towing speed, the strength of the tides and currents, and the time of year (de Groot 1984; Redant 1987; Churchill 1989; Krost et al. 1990; Jenner et al. 199 1; Mayer et al. 199 1; ICES 1995; Jones 1992; Prena et al. 1996). The parts of a trawl that leave the most distinctive marks are the otter boards. Single otter-board tracks range in width from approximately 0.2 to 2 m and their depths can vary from 3 to 30 cm deep (Caddy and Iles 1972; Krost et al. 1990). Sediment type is one of the more important factors. In sandy sediment, there is low penetration of the otter boards due to high mechanical resistance of the sediment and the seabed in sandy areas is more rapidly restored by waves and currents. Therefore, on sand bottoms, the tracks are short-lived, whereas in mud bottoms the tracks will be deeper and will last longer (Caddy 1973; Werner et al. 1976; Krost et al. 1990).
Studies indicate that dredges (Langton and Robinson 1990; Auster et al. 1996), bottom trawls (Auster et al. 1996), and beam-trawls (Kaiser and Spencer 1996) can alter the physical characteristics of the substratum, and Riemann and Hoffman (1991) note that particulate material is resuspended from the bottom into the water column from dredging and bottom trawling. Kaiser et al. (1998) investigated changes in the megafaunal benthic community in different habitats after trawling disturbance, and found that in mobile sediments effects of fishing were not even immediately detectable, and in stable sediments, after six months, seasonal changes in the benthic community masked an effect of fishing. On pebble and cobble bottoms, mobile fishing gear eliminates or severely damages the epifaunal species present prior to the gear passing the area (Eleftheriou and Robertson 1992; Auster et al. 1996; Collie et al. 1997), as well as reduces habitat complexity, species diversity, and abundance of some taxa that live in stable sediments (Auster et al. 1996; Kaiser and Spencer 1996).
Technology of Mapping the Seabed
The use of sound in the sea dates to Leonardo de Vinci in the fifteenth century who noted that the sound of approaching sailing vessels could be heard in the sea before the vessels were observed over the horizon. The modem age of sound navigation and ranging (SONAR) began with World War II and the development of electronic instruments that utilized hydroacoustics (underwater sound) to detennine water depth and the distance and bearing of objects under the sea (Urick 1983). Since 1970, hydroacoustic methods have been used for the estimation of pelagic fishery resource abundance (MacLennan and Simmonds 1992). The most advanced technology is capable of identifying, counting, and tracking individual fish as they pass around and through hydroelectric plants. The technology of using hydroacoustics to map the seabed with transducers that scanned to the sides of the survey vessel was developed in the 1960s, and has been in commercial application since the 1970s. In the 1990s, side-scan sonar is standard equipment aboard coastal survey vessels that map navigable waterways, and it is also used in nautical archeology, seabed resource mapping, and environmental surveys (Anonymous 1996). Side-scan sonar was used to map oyster reefs in the James River, VA and to identify scars in the reefs made by the propeller wash of tug boats passing over shallow reefs (DeAlteris 1988).
Side-scan sonar has also been used as a technique to demonstrate physical impacts of fishing trawls and dredges by recording and documenting tracks (Amos and King 1984; Miller 1987; Fader and Pecore 1988; Josenhans et al. 1991). Moreover, it is a fast method of data recording and is therefore well-suited for mapping large areas of the sea floor (Krost et al. 1990). Previous studies (e.g. Krost et al. 1990; Jenner et al. 199 1; Harrison et al. 199 1) clearly demonstrate that side-scan sonar can be used to determine if the seafloor has recently been disturbed by mobile fishing gear. Krost et al. (1990) identified otter trawl tracks in Kiel Bay (Western Baltic) using side-scan sonar and found that the frequency of trawl tracks was highest in mud areas.
Coastal Sediment, Transport Processes
Sediments of the coastal seabed are subject to erosion, transport, and deposition as a function of the hydrodynamic environment. Early work on sediment transport processes was restricted to fresh water environments where unidirectional currents transport sediments downstream based on the velocity of the water and the grain size of the sediment (Graf 1971). Hjulstrom. (1939) investigated the erosion, transport and deposition of uniformly sized particles in a steady current. He demonstrated that fine grain size sand with a diameter of 0.25 mm. Was eroded at the lowest average velocity (about 25 cm/sec), whereas finer and coarser sediments required substantially greater velocities. He also noted that silt and clay size sediments, because of their cohesive nature, were considerably more resistant to erosion (average velocity greater than 125 cm/sec). Hjulstrom's research was based on average velocities in channels, and therefore the critical velocities for erosion measured at I m above the bed must be adjusted downward to account for a turbulent, logarithmically decaying velocity profile (Munson et al. 1994).
Further research by Shields (193 6) and others on the mechanics of sediment transport in steady, unidirectional flows, developed the concepts of friction velocities and critical sheer stresses for erosion of cohesionless sediments. Research on the erosion, transport, and deposition of fine, cohesive sediments was lead by Partheniades (1965) and others. In the 1970s, Sternberg (1972) and Madsen and Grant (1976) investigated the coastal sediment transport processes, combining wave-generated oscillatory bottom currents with steady, unidirectional currents due to wind, tides and other forces. Sleath (1990), Cacchione and Drake (1990), and Metha and Dyer (1990) provide up-to-date research reviews of seabed boundary layer dynamics, shelf sediment dynamics and cohesive sediment transport in coastal waters, respectively.
A practical application of this research is to predict the seaward limit of significant sediment transport as related to the potential dispersal of disposed dredge material after disturbance of the sediment cap, or to compare natural disturbance to seabed disturbance by fishing gear. This estimate of the seaward limit of sediment transport is dependent on the local wave and current environment, water depth and sediment type. The Shore Protection Manual (CERC 1984) of the US Army Corps of Engineers recommends estimating the local wave climate based on hindcast analysis of the wind climate, then determining the maximum wave orbital velocity 1 m above the seabed using linear (Airy) wave theory, and finally comparing this maximum bottom current to a tabulated threshold value for sediment motion. Sherwood (1989) describes a more sophisticated analysis based on the Madsen and Grant (1976) model.
Description of the study area and mobile gear fisheries
Lower Narragansett Bay is a well-mixed (vertically.homogenous)
estuary located in southern New England, USA. It is approximately 40 km
in length, 10 km in width, and discharges into Rhode Island Sound. Sediments
in the bay range from sandy mud in the deeper portions of the bay, to pebbles
and cobbles along portions of the shore and shoal areas, to sand in other
shore and shallow areas (McMaster 1960). Tides are semi-diumal with a mean
range of 1.2 m and tidal currents at the surface reach maximum velocities
in excess of 70 cm/see (Hicks 1959). Narragansett Bay is exposed to wind
and waves of unlimited fetch from the southern quadrant (Hicks et al. 1956).
The study area is located in the lower portion of the West Passage of Narragansett
Bay and is approximately 7 km in length and 4 km in width (Figure
1). Dutch Island is located in the middle of the study area.
The State of Rhode Island issues a multi-purpose commercial fishing license and there is no trip-ticket or log reporting system. Therefore, this description of the fisheries is based on anecdotal information provided by the state regulatory agency personnel and the leaders of local fishermen's organizations. It is estimated that there have been approximately 22 mobile gear vessels annually working the lower bay for the last decade. This includes both part-time and full-time vessels, but is about equal to 15 full-time equivalent (FTE) vessels. These FTE vessels range in length from 12 to 20 m and operate for 6 months of the year, 5 days per week. Each FTE vessel makes 4 to 5 tows per day. Thus, Narragansett Bay experiences about 8000 tows annually and it is estimated that the study area, the lower portion of West Passage, experiences about 10% of the total effort or about 800 tows annually. The average tow is 1. 5 hours in duration at a speed.of about 4 km/hr. Therefore, the length of the average tow is 6 km.
Observations by divers of bottom trawls operating
on the seabed indicate that the otter boards principally impact the seabed,
smoothing an area along their path and creating a small ridge at the trawling
edge of the otter board shoe (Wardle 1993). The width of the smoothed area
depends on the angle of attack of the otter board, but is generally about
Y2the length of the otter board. The depth of the smoothed area and the
height of the ridge depend on the weight of the board and the sediment
type, but in Narragansett Bay, our observations indicate the height of
the ridge to be between 10 and 20 cm. and the depth of the smoothed area
to be 5 to 10 cm. 'Me effect of the trawl on the seabed is dependent on
the design of the sweep and the sediment type. A properly fishing trawl
net skims the seabed, and therefore only resuspends the fine surficial
sediment layer, and minimally impacts the micro-topography of the seabed.
Video surveillance shows that dredging'causes considerably more disturbance
of the seabed than the bottom trawl. The dredge rolls over gravels, pebbles,
and boulders, flattens bedforms on soft sand and mud sediments, and resuspends
fine sediments. In Narragansett Bay, dredge activity is less am 5% of the
total effort, and when it occurs, the disturbance is limited to the 2-3
m width of the single dredge towed by the local vessels.
Evidence of seabed disturbance by mobile fishing gear
Side-scan sonar data was obtained from the National Oceanic and Atmospheric Administration (NOAA). Surveys were performed from 31 August 1995 to 25 September 1995 aboard the NOAA SN Rude, a 27.4 m vessel equipped with an EdgeTech 262 Side Scan Sonar and P-code Global Positioning System. The system was adjusted to record a 50 ra wide range on either side of the trackline. Two duplicate surveys were performed within weeks of each other and each of them had I 10% coverage (10% overlap in track lines). The track lines for the two surveys were offset from each other by 50 m. The two surveys were combined into one data set.
Analysis of the side-scan sonar records was conducted by creating an interpretation scheme and applying it to all the data obtained from NOAA. The records were interpreted at approximately 50 rn spacing along each track line, thereby creating an interpretive data point area of 50m x I 00m. Trawl and dredge scars were noted and divided into 7 categories from 0 to 6, where 0 is the absence of scars and 6 is the presence of greater than 10 scars. Sediment type was interpreted based on acoustic backscatter and was divided into mud, sand, pebble/cobble, and boulder categories. Finally, structures were noted (if present), which included single boulders by sizes, shipwrecks, anchors, moorings, and lobster pots. Data was recorded in EXCEL, imported into SURFER and plotted. Post plots were created of the interpretive points, bottom type and distribution of scars. A detailed depth contour chart was constructed based on the bathymetric data (corrected to mean low water) collected concurrently with the side scan sonar data.
An estimate of total area sampled was made by multiplying the area of each interpretive data point (50m x I 00m) by the total number of interpretive points. This total was divided by 2.2 to account for the two surveys, each with 10% overlap. The total area that was scarred was estimated by assuming the width of each scar was 0.5 m (a weighted average for trawls and dredges), multiplying it by the length of each scar at each interpretive point (50 m) and by the number of scars recorded at each interpretive point, and dividing by 2.2 for duplication.
Characterization of bottom hydrodynamic environment and sediment transport processes
The hydrodynamic environment at I m above seabed was estimated for two experimental sites in the southern portion of Narragansett Bay using a methodology based on the Shore Protection Manual (CERC 1984), but using a Hjulstrom diagram to estimate the critical velocity for sediment erosion. The two sites included a shallow (7 m) sand area located at 41' 28.636'N, 71'25.027'W and a deep (14 m) mud area located at 41'28.173'N, 710 24.048'w. Both of the sites were within the study area. A surface wave climate for the mouth of Narragansett Bay was developed based on-four information sources. Hicks et al. (1956) provides daily observations of wave height and period for two locations in the mouth of Narragansett Bay (Fort Vamurn and Scarborough). Naval ship observations (SSMO) in southern New England waters (Quonset Marsden Square) for a 5-year period, 1963-1968, were summarized in the final report of a dredge material disposal study (Anonymous 1975). Hindcast wave climate data for station 83 in Rhode Island Sound for the 20-year period 1956-1975 (WIS) was taken from Hubertz et al. (1993). This data was analyzed, plotted as wave height versus cumulative probability greater than and correlated for period and height. Regression models were best fit to both data sets. Tidal currents at I m above the seabed for the two sites were derived from average tidal currents provided by a vertically integrated numerical model (Spaulding and Swanson 1984) and were adjusted to I m above the seabed using a power-law velocity profile with n=7 (Munson et al. 1994). Linear wave theory was used to estimate the maximum orbital velocity at I m above the seabed for the bay mouth wave climate at the two experimental sites' water depths. Critical velocities for erosion of sand and mud were conservatively taken from the Hjulstrom diagram.
Field verification of the longevity of bottom scars
Field studies to determine the longevity of bottom
scars were conducted between June and July of 1998. The longevity of a
scar is interpreted as a measure of both frequency of natural seabed disturbance
and the recovery time of the substrate. Experiments were conducted in two
locations, a shallow sand area located at 410 28.636' N, 710 25.027' W
with a water depth of 7 m, and a deep mud area located at 410 28.173' N,
710 24.048' W with a water depth of 14 m. These are the same locations
used in the model analysis. Sampling was conducted from a 6.4 m Boston
%aler. A stake field (DeAlteris et al. 1975) was established in both sites
using two 1.8 m long iron stakes, where the stakes were driven into the
sediment so that half was in the sediment and half was left exposed. The
stakes were spaced 1.2 m apart. Each site was marked with a buoy at the
surface attached to a concrete anchor. Divers scarred the bottom using
a hand held shovel, and the scars were approximately 15 cm deep and 1.2
m long. At both sites, the scars were made parallel and perpendicular to
the two stakes. Divers visually checked the sites routinely (daily for
the first week, and then weekly for 2 months) to monitor the longevity
of the scars.
Evidence of seabed disturbance by mobile fishing gear
The results of the analysis of the side-scan sonar
data are presented in Figures 4a to 4d ().
The survey track lines with the individual interpretative data points are
shown in Figure 4a. A total of 6163 interpretive data points were included
in the analysis, incorporating an area of 14 kM2 . The contoured bathymetric
data show a deep channel on the east side of the southern portion of West
Passage that diverges into two channels around Dutch Island, and these
converge into a single deep channel again on the east side of the northern
portion of West Passage (Figure 4b). The bottom sediment types were predominantly
mud, with boulder fields along the shallow shoreline areas, and sand with
sand waves in the shallow southwestern portion of the study area - (Figure
4c). This spatial bottom type pattern corresponds reasonably well with
sediment grain size distribution reported by McMaster (1960) based on grab
samples. The distribution of scars on the seabed attributable to the activity
of mobile fishing gear is shown in Figure 4d. The number of scars evident
in a single interpretive data point (50 x 100 m) varied from 0 to more
than 10. The spatial distribution of the scars is limited to the deeper
mud channels within the study area. A digital image of an original side-scan
sonar interpretive data point from the lower bay is shown in Figure 5 .
The scars of individual otter boards are clearly evident, and when the
original records were mosaiked, individual track pairs were observed, with
two start and end points. The total area observed to be scarred by otter
boards is estimated to be 0. 12 km , or 0.9%
Characterization of bottom hydrodynamic environment and sediment transport processes
The results of the characterizations of the hydrodynamic
environment and sediment
Field verification of the longevity of bottom scars
At the mud site, the two scars were monitored periodically and were observed to be unchanged for a period greater than 60 days. At the sand site, the first two scars lasted 4 days. Then the site was rescarred by the divers and the scars lasted 3 days. In the third and fWal experiment at the sand site, the two scars lasted only I day. Notable observations of the divers during the field work are the following: (1) large rock crabs were observed inhabiting the scars in the mud area; (2) sediments were observed in motion on all dives at the sand site; and (3) although both areas experience the activity of mobile fishing gear during certain seasons, lobster pots were found at both sites during the experimental period.
DISCUSSION AND CONCLUSIONS
With the emerging interest in essential fish habitat relative to the conservation and management of marine resources, mobile fishing gear is being scrutinized as contributing to the degradation of the marine habitat. Seabed disturbance by mobile fishing gear is recognized as an important issue, however, we argue that seabed disturbance by mobile fishing gear must be evaluated relative to seabed disturbance due to natural physical and biological causes. We selected the lower portion of Narragansett Bay, RI as a case study area to compare the seabed disturbance due to mobile fishing gear to seabed disturbance due to natural physical processes alone.
A small fleet of inshore mobile gear vessels tow bottom trawls and dredges across the seabed in Narragansett Bay: approximately 15 FTE vessels make about 8000 tows annually in the bay. The study area is the lower portion of the West Passage in Narragansett Bay. This area experiences about 10% of the total mobile gear effort, or 800 tows annually, with each tow estimated at 6 Ian in length. Mobile gear activity occurs on mud, sand and rock substrates, but we have restricted our analyses to the soft sediment bottoms (sand and mud).
The results of our analysis of side-scan sonar available for the study area indicate that bottom scarring due to trawl doors is restricted to the relatively deep sandy mud substrates, despite observations that trawling activity occurs in all the habitat types in Narragansett Bay. Our estimate of the area impacted by the bottom scars is less than 1% of the total area surveyed.
The model analysis of bottom hydrodynamic and sediment transport processes of two experimental sites in the lower portion of the study area indicates that sediment transport occurs daily in the shallow (7 m), sand substrate, but less than 5% of the time in the deep (14 m), mud substrate. This suggests that bottom scars will be short-lived in the shallow sand site and long-lived in the deep mud site.
The actual longevity of bottom scars in these two experimental sites was measured in a field study. Bottom scars dug and monitored by divers lasted only I to 4 days in the shallow sand substrate as compared to greater than 60 days in the deep mud substrate. Thus, this difference in scar longevity is also a measure of a difference in substrate recovery time.
From these analyses, we conclude that while mobile fishing gear disturbs the seabed, the significance of that disturbance must be compared to the magnitude and frequency of natural seabed disturbance. In this study, our analyses indicate that in a shallow, sand substrate, natural physical processes are disturbing the seabed regularly. Thus, the substrate's recovery from fishing gear-related disturbance is almost immediate. However, in the deep, mud substrate, the results of our analyses indicate that natural processes are rarely capable of disturbing the seabed and therefore, recovery from fishing gear-related disturbance is slow. These results correspond well with the conclusions of Kaiser et al. (1998) in their study of megafaunal benthic communities in different habitats after trawling disturbance.
Our analysis of fishing effort suggests that potentially 4.8 km2 of the seabed within the study area may be disturbed annually by mobile gear scars [6 km tow length x (0.5 + 0.5)m width disturbed x 800 tows]. However, we identified only 0. 1 kM2 of Sears in the side-scan sonar data. This indicates that habitat recovery, albeit slow, must occur even in the mud substrate.
It is recommended that an analysis of the seaward extent of active sediment transport for the continental shelf would be helpful in the evaluation of the seabed disturbance by mobile fishing gear. The Corps of Engineers has available hindcast wave climates for the shelf region, the US Geological Survey has water depth and sediment grain size data for the shelf region, and an analysis of fishing activity by gear type and location based on a National Marine Fisheries Service database could be conducted for the same region. Comparison of these analyses would allow for the identification of areas that may be problematic with respect to seabed disturbance by mobile fishing gear, i.e., areas with substantial fishing activity and minimal natural physical disturbance.
The authors thank the RI Sea Grant Program for the support of this research. Cdr. N. Perugini of the NOAA Corps graciously provided the raw side-scan sonar records and the navigational track data.
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