7.0 Epibenthic Sampling in the Cape Fear Estuary
Troy Alphin and Martin Posey
7.1 Introduction
Epibenthic organisms are the taxa that live on or immediately above the
sediment surface. In southeastern North Carolina the epibenthos are dominated by mysid
shrimp, caridean shrimp (grass shrimp, white and brown shrimp), gammarid amphipods, crabs,
cumaceans, and possibly some polychaete species (Ogburn et al 1988). These animals can
form a significant portion of the diet of many juvenile fishes, making them important from
a fisheries perspective. The timing of recruitment and movement for some fish and decapods
is closely linked with the recruitment of epibenthic organisms that serve as prey (Johnson
et al. 1990). However, unlike infauna that tend to remain within the general area after
settlement, many epibenthos tend to be demersal and highly motile by nature, with
variation in abundance seasonally and spatially. The data presented here represents
samples collected at several sites within the lower, mid and part of the upper estuary of
the Cape Fear River. Although epibenthos have been sampled quarterly since November of
1996, funding through the Lower Cape Fear River Program was not available to support this
work until 1998.
In addition to epibenthic community sampling, the Benthic Ecology
Laboratory has also begun studies targeting juvenile blue crab distribution in the Cape
Fear River system (partially funded by North Carolina Sea Grant). Although the Lower Cape
Fear River Monitoring Program does not fund this investigation there has been significant
interest in the results of this project and we feel that this report provides an excellent
venue for presenting preliminary results of this research. Blue crabs are the most
valuable coastal fishery in North Carolina and roughly 20% of the overall landings in
North Carolina come from the southeastern estuaries (New River and areas south). Much of
the current management strategy for the blue crab fishery considers seagrass to be the
preferred settlement habitat. This may be true for much of the coast along the Atlantic
and Gulf of Mexico, but there is a relatively large area from southeastern North Carolina
to the Georgia coast that lacks extensive seagrass beds. However, these areas have large
populations of blue crabs. Our work has focused on the utilization of shallow water areas
as critical habitat in the Cape Fear estuary and how distribution patterns may change
throughout the year and along the estuarine gradient. We recognize the fact that older
juvenile crabs (greater than 30 mm carapace width) move up the estuarine gradient in many
areas, however this study focuses on young juvenile (<30 mm carapace width) which may
utilize low salinity areas in the Cape Fear River estuary in unexpected ways.
7.2 Methods
Sampling of epibenthos was conducted at 4 stations in the lower Cape Fear
River estuary corresponding to areas sampled during a 1995 U.S. Army Corps of Engineers
study (Fig. 7.1, site labels following Army Corps designations). These sites are all
located below the confluence with the Brunswick River, with the northernmost site located
near the Wilmington Port (SI2), SI3 located near Marker 54, SA2 located at the southern
end of Keg Island, and the southernmost site (C8) located near M31, south of Snow’s
Cut. Because epibenthos are often dominated by larval fish, shrimp and crabs,
concentration of sampling effort in the mid and lower estuary provides a better
opportunity to examine influx of these organisms. Sampling was conducted with an
epibenthic sled towed from a UNCW vessel. The sled has a rectangular frame opening (0.5m
wide x 0.3m high) with a 1 mm mesh, 2m long net attached to the frame. At the end of each
tow all material in the net is washed into a 1.4 l removable collection bucket attached to
the net. The sled frame rests on 3 metal skis that allow the net to be pulled just above
the bottom. At each location on each day, a single tow is made for 1 minute at constant
speed (~1200 rpm) against the current in a shallow area, 2-3 m depth, and in an adjacent
deep area, 8-15 m depth. A General Oceanics Model 2030 mechanical flow meter is attached
to the mouth of the sled to measure the water volume sampled, for later standardization of
analyses (note: there were no significant differences in volumes sampled between depths,
locations, or dates during the study).
As part of the Benthic Lab’s quality control procedures, tows were
only accepted if the flow meter read 4500-5000 revolutions (if reading were less than this
number another tow was conducted). This aids in standardization of field collections and
allows a field estimate of how well the trawl is fishing. Epibenthic samples are taken
twice within a one week period coinciding with infaunal grab samples during each season. A
previous study showed certain taxa (mysid shrimp as well as anomuran/brachyuran megalopae)
preferred deep habitats to shallow habitats. Although there may be some differences
between sample depth, low replication per site necessitates the combination of both deep
and shallow samples from each site. This allows us a more accurate estimation of the whole
epibenthic community at each site. All samples are preserved in 10% buffered formalin
solution and later transferred to a 70% alcohol solution for long term storage. All
organisms within the epibenthic samples were enumerated and identified to lowest possible
taxa. As part of standard QA/QC procedures representative specimens for each taxa are
maintained and verified by appropriate taxonomic experts.
Juvenile blue crab (Callinectes sapidus) distributions within
the lower Cape Fear River were monitored monthly at eight stations following the estuarine
gradient up the river. Site distribution is as follows; three sites in the Southport area
(SO, SW, and SM), a single site at Fort Fisher (FF), one site at both Carolina Beach state
park (CB) and Brunswick Town (BT) (both located in the same region of the river but on
opposite sides), a single site at Powerlines (PL) (just south of Barnard’s Creek),
and the northernmost site at Eagle Island (EI) (in the region of the Wilmington port). The
salinity range for the Southport sites averaged 18-30 ppt, while the Eagle Island site
averaged 0-2 ppt. Between these two extremes the Fort Fisher site averaged 10-15 ppt while
Carolina Beach State Park/ Brunswick Town and the power lines site averaged 0-8 ppt and
0-5 ppt respectively. These stations represent discrete areas long the estuarine gradient.
In the Cape Fear River we concentrated only on the shallow water/marsh edge habitats (i.e.
those areas without other structural habitats). In order to sample these areas we used
steel-framed push nets (Townsend 1991; Innes 1992). The opening of the net is 0.4 m wide.
Ten replicate, 10 m transects, or sweeps, were made at each area. All collections were
made in shallow water (20-50 cm depth) along the same depth contour parallel to shore.
None of these transects (sweeps) overlapped. Monthly collections were generally made
during the third week of each month. Efforts were made to avoid sampling on spring tides,
because extreme high and low tide could affect utilization of shallow water habitats. All
crabs caught at the transects were identified and enumerated. For the purposes of this
presentation we will only present data on juvenile blue crabs less than 30 mm carapace
width.
7.3 Results and Discussion
A total of 150 taxa were identified from the epibenthic samples. Because some
of these taxa represent higher taxonomic divisions (i.e. juvenile fish and amphipods too
small to identify) this number is a conservative estimate of total numbers of species.
Decapod crustaceans made up the largest single group with 35 taxa, while amphipods had 16
taxa, isopods had 14, and fish had 18 taxa represented. In addition to these groups, there
were also 17 polychaete taxa. While polychaetes are generally infaunal organisms,
individuals living near the substrate surface are susceptible to collection with this
bottom trawl. The total number of taxa present from year to year was very similar, with 89
taxa collected in 1997 and 99 taxa present in 1998. These samples were dominated by small
fish, decapod crustaceans, shrimp, amphipods, isopods, and polychaetes, similar to
findings of other studies in southeastern rivers (Allen and Baker 1990; Ogburn et al.
1988). Both years showed similar patterns in abundance among seasons with summer and fall
being the times of highest total abundance, though these results maybe driven by mysid
abundances. Other taxonomic groups show different patterns among seasons and years. Total
abundance for 1998 samples was nearly twice that of 1997. Although the pattern between
years is similar we present the data as mean abundance and standard error by season for
each site, separately for each year (Table 7.1-
7.8). Abundance of the major taxonomic
groups are also presented by site and season for each year (Table
7.9-7.10). Debris on
site in spring 1997 at SA2 caused significant damage to the equipment and prevented the
collection of samples at that site. However all other samples were collected without
incident.
The results of our epibenthic sampling vary somewhat from a previous
study at the same sites (Posey et al. 1996). In the previous study, May was the time of
high abundance in general and October was a time of lower abundance. The previous study
only focused on samples collected in May and October, unlike the current study that looks
at patterns over the course of the entire year. Our current investigation shows high
abundance in both summer (July and August) and fall (October and November) samples for all
sites. In general many epibenthic taxa tend to have the greatest recruitment into the
system in the spring of the year and are preyed upon by juvenile fish that move down the
estuary at about the same time period. Many of these organisms, including small fish, some
decapods, amphipods, and isopods also have a second, smaller recruitment period in the
fall of the year. The results from the current study may reflect poor spring recruitment
and an early fall recruitment or it could reflect a shift in the timing of recruitment for
juvenile fish possibly due to the hurricanes of 1996 and 1998 that might effect the
ability of adult fish to move up the estuary and spawn or a combination of storm recovery
in 1997 and storm perturbations in 1998. As we continue to gather data on this group of
organisms we will be able to separate inter-annual variability effects and follow effects
of large-scale perturbations. This is important because many of the fishery species within
the estuary depend on these small crustaceans and shrimp as a food resource during some
stage of their development.
Juvenile blue crab distributions show some interesting patterns, with a
portion of the early juveniles (<25 mm carapace width) utilizing lower salinity (<16
ppt) areas (Fig. 7.2). Figure 7.2 does not include data from the Southport marina site
because of possible baiting effects that might obscure natural patterns. As we follow the
estuarine gradient from the mouth region of the Cape Fear River we find the Southport
marina site has the highest overall abundance of blue crabs (Fig.
7.3). This reflects a
baiting effect seen at this site. Many of the fishing boats in this area dispose of their
catches in this basin and juvenile blue crabs have been observed on several occasions
feeding in the area. We feel that a steady food supply is the main factor responsible for
attracting and holding the crabs at this site. One area of possible future work may be to
delineate the problems faced by crab populations within this constrained area.
Consistently high abundance of juveniles in a relatively small area may have impacts on
the health of this population. This population and ones like it may experience higher than
normal rates of cannibalism, because cannibalism is a major source of mortality among blue
crabs. In addition to effects of increased cannibalism there may also be effects on this
population due to prolonged exposure within an area of low water quality, associated with
marina activities.
The Southport waterway and oysterhouse sites showed a similar abundance
pattern. The waterway site showed greatest mean number of juvenile crabs in the May 1998,
September 1998, and June 1999 but with abundances in January and February 1999 nearly as
high (Fig.
7.3). September and October 1998 and May 1999 were the times of highest mean
abundance for the Southport oysterhouse site (Fig.
7.4). It should be noted that the
oysterhouse site at Southport had the lowest mean abundance of any site in the study. The
site at Fort Fisher generally had higher mean abundance than either the Southport waterway
or oysterhouse sites. The highest abundances at this site were recorded in July 1998 and
January 1999. Further up the estuary the Carolina Beach site had highest abundances in
June 1998 but with secondary abundances in July and November 1998 comparable to highest
abundances at Fort Fisher. Brunswick Town showed highest abundances in late July 1998
(Fig. 7.5); however, abundances for the rest of the year remained consistently low
(comparable to the Southport oysterhouse site). Mean abundance at the Powerlines site
reached its highest in November 1998, while the Eagle Island site showed highest relative
abundance in December of 1998 (Fig.
7.6) and secondary increases in abundance in July 1998
and April 1999. However, the highest mean abundance for either Eagle Island or Powerlines
was only half that at Carolina Beach and Brunswick Town sites.
The crabs targeted in this study were all less than 30 mm carapace
width. The differences in timing of peak abundances among sites along the estuarine
gradient leave us with several interesting conclusions. First, recruitment into the system
may be continuous at some level but with periodic pulses of recruits possibly due to
large-scale wind or current events that drive them into the system. These types of
processes have been widely accepted as mechanisms moving larvae into the larger estuaries
of the Pamlico and Albemarle Sounds. While other studies have suggested that fall is the
time of primary recruitment into the estuary, our results indicate that larvae may be
continuously moving into the estuary throughout much of the year. Second, the presence of
early juveniles at all sites and especially in the upper sites indicates that some portion
of new recruits are by-passing the lower areas of the estuary and moving directly into the
mesohaline and oligohaline regions of the system. Although it has been shown that older
juveniles (greater than 30 mm) move up the estuary into lower salinity areas, there is
little work showing utilization by early juveniles. These areas may prove to be a valuable
refuge and/or forage habitat for this life history stage, especially considering that this
region of North Carolina lacks other structural habitats (seagrasses) that are preferred
in other systems.
7.4 Literature Cited
Allen, D.M. and D.L. Baker. 1990. Interannual variation in larval fish recruitment to estuarine epibenthic habitats. Marine Ecology Progress Series. 63:113-125.
Innes, A. 1992. Microhabitat segregation of juvenile fishes in a shallow water marsh. M.S. Thesis, UNC-Wilmington,48 pp.
Johnson, W.S., D.M. Allen, M.O. Ogburn, and S.E. Stancyk. 1990. Short term predation responses of adult bay anchovies Anchoa mitchilli to estuarine zooplankton availability. Marine Ecology Progress Series. 64:55-68.
Ogburn, M.O., D.M. Allen and W.K. Michener. 1988. Fishes, shrimps, and crabs of North Inlet estuary, S.C.: a four year seine and trawl survey. Baruch Institute Tech. Report 88-1. University of South Carolina, Columbia.
Posey, M.H., T.D. Alphin and C.M. Powell. 1996. Epibenthic fauna in shallow and channel habitats of the lower Cape Fear River - May and October 1995 sampling. Report submitted to the U.S. Army Corp. of Engineers. 30 pp.
Townsend, E. 1991. Depth distribution of the grass shrimp Palaemonetes pugio in two contrasting tidal creeks in North Carolina and Maryland. M.S. Thesis, UNC-Wilmington, 62 pp.
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