Coastal Trawl Survey
The Coastal Trawl Survey (CTS; previously known as the Shallow Water Trawl Survey or Coastal Survey) is conducted by the South Carolina Department of Natural Resources – Marine Resources Division (SCDNR-MRD). The CTS began in 1986 and standardized activities have been in place since 1990. This survey provides long-term, fishery-independent data on seasonal abundance and biomass of finfish, elasmobranchs, decapod and stomatopod crustaceans, sea turtles, horseshoe crabs, and cephalopods that are accessible by high-rise trawls in coastal nearshore waters, specifically those between Cape Hatteras, North Carolina, and Cape Canaveral, Florida (Figure 1). The CTS samples the full spatial range in multiple seasons each year.
Survey Design
Background
The pilot phase of the CTS was initiated in 1986. Using a stratified random sampling design, participating states (North Carolina, South Carolina, and Georgia) conducted daylight sampling in their respective waters in November and December using 30 ft (9.1) high-rise trawls. Sampling occurred at three to five randomly chosen sampling sites within each of 19 strata (North Carolina=7, South Carolina=7, Georgia=5). Beginning in 1987, the SCDNR took over all sampling from Cape Hatteras, North Carolina to Cumberland Island, Georgia. Using a fixed-station sampling design, four sampling locations were subdivided into three trawling sites with each site sampled once during daylight hours and once at night using 75 ft (22.9 m) mongoose-type Falcon trawls. Sampling generally occurred monthly (~7 sea days per month). In 1989, sampling moved from monthly sampling to seasonal sampling, but still varied by time of day. Sampling also expanded to central Florida during this time. Starting in Summer, 1989, the CTS took on the vast majority of standardized characteristics utilized through the present (e.g., seasonal sampling; day-time; 75 ft nets; full spatial range).
Figure 1. Strata and regions sampled by the SEAMAP-SA Coastal Trawl Survey. Inner (shallow) strata are sampled during all seasons (1990-present). Outer (deep) strata were sampled south in Spring (green) and north in Fall (yellow) from 1990-2000. In 1989, both depth zones were sampled over the entire latitudinal range in Spring and Fall. Strata are not drawn to scale.
Data Collection
Trawls were conducted in the coastal zone in Atlantic waters off the southeastern United States between Cape Hatteras, North Carolina, and Cape Canaveral, Florida (Figure 1). Historically, multi-legged cruises were conducted within three sampling seasons: Spring (April - May), Summer (July - August), and Fall (September - November). Beginning in 2023, cruises were conducted within two sampling seasons: spring/summer (April-June) and summer/fall (August-October).
Within the survey range sampling occurred within six sampling regions: Raleigh Bay, Onslow Bay, Long Bay, South Carolina (south of Long Bay), Georgia, and northeast Florida which were segmented latitudinally into 24 inner and outer strata (Figure 1). Inner strata (1989-present) were delineated by the 4.6 m depth contour inshore and the 9.1 m depth contour offshore. Outer strata (1989-2000) were delineated by the 9.1 m and 18.3 m depth contours. Outer strata were sampled to gather information on the reproductive condition of commercially important penaeid shrimp species, which, at the time, were presumed to spawn “offshore.” All 24 outer strata were sampled in 1989 in Spring, to collect spawning data for white shrimp (Litopenaeus setiferus), and Fall, to collect spawning data on brown shrimp (Farfantepenaeus aztecus). No stations in the outer strata were sampled in Summer. Beginning in 1990, 10 outer strata off Georgia and Florida were sampled in Spring and seven outer strata off North Carolina were sampled in Fall (Figure 1). Outer strata were abandoned in 2001 to intensify sampling the “inner” depth zone and because of the limited use of the data resulting from those outer samples.
Sampling conducted by SCDNR occurred on board the R/V Lady Lisa (1986-2023), a 75 ft (22.9 m) wooden-hulled, double-rigged, St. Augustine shrimp trawler owned and operated by SCDNR. The standardized gear used since 1987 is a pair of 75 ft (22.9 m) mongoose-type Falcon trawl nets (manufactured by Beaufort Marine Supply, Beaufort, SC) without Turtle Excluder Devices. The body of the trawl net was constructed of #15 net twine with 47 mm stretch mesh. Additional details of gear configuration can be found in CTS Annual Reports.
Since Summer 1989, sampling has been conducted during daylight hours (between 1 hr after sunrise and 1 hr before sunset). Trawls were towed for 20 min, excluding wire-out and haul-back time, with a target speed of 2.5 kts, relative to ground, and with limited consideration for current direction. Historically, the catch from each net was processed separately and assigned a unique collection number, with data from both nets at each station pooled for analysis to form a standard unit of effort (tow). Beginning in 2021, only the catch from one net was processed with the tail bag of the other net fished untied. Contents of the net(s) were sorted to species (limited exceptions to genus or family only), and total biomass and number of individuals were recorded for all species of finfish, elasmobranchs, decapod and stomatopod crustaceans, cephalopods, sea turtles, xiphosurans, and cannonball jellies (Stomolophus meleagris). For other miscellaneous invertebrates and algae, which were treated as two separate taxonomic groups, only total biomass was recorded. If researchers were unable to identify a specimen, data were recorded and the specimen preserved and transported to the laboratory at SCDNR - MRRI, where it was identified to the lowest taxonomic level practical. When nets contained high volume catches, all endangered species and species that posed a risk to staff during handling (e.g., elasmobranchs, catfish, scorpionfish) were removed and given priority for processing, then the remaining net contents were sub-sampled into shrimp baskets, weighed, and a randomly selected basket(s) was sorted and processed as described above. Abundance and biomass for each species was then expanded to estimate the total abundance and biomass of each species in the full catch using the ratio of the sub-sampled basket weight to the total catch weight.
When large numbers of a species occurred in the processed catch (selected basket or full net contents), all individuals of that species were weighed, but only a haphazardly selected subsample was processed for abundance, and for priority species (Table 1), lengths were measured to the nearest centimeter (cm; 1989-2017) or millimeter (mm; 2018-present) in a Length Frequency work-up (LF). The species subsample typically consisted of approximately 32 to 64 individuals. The total number of individuals in the catch was then estimated based on the ratio of the processed subsample weight to the total weight for that species, multiplied by the subsample count.
During LF, individuals of selected priority species either underwent more detailed processing or were flagged for a subsequent Life History work-up (LH; Table 2). LH selected species were designated by the SEAMAP-SA Committee at the beginning of each sampling year based on management needs. For LH finfish, fish selection was based on season, stratum, and 1 cm size bins. LH finfish specimens were measured to the nearest mm (total length, fork length, centerline length, and/or standard length), weighed to the nearest gram, and the sex of each individual was determined based on gross gonad morphology. Sagittal otoliths and, for a subset of designated species, a representative sample of gonadal tissue were preserved and transported to the laboratory at SCDNR - MRRI, where samples were processed following accepted standard procedures (Harris et al., 2004) based on the LH species list each year.
LH data, based on gross field observations only, were also collected from individual specimens of other LH selected species including penaeid shrimp, blue crabs, horseshoe crabs, and all sharks and sea turtles (Table 2). Marine turtles in good condition were released as quickly as possible according to NMFS permitting guidelines, with PIT and flipper tags generally applied if not already present, or numbers logged, if prior tags were in place. If a turtle was sick or injured, the turtle rescue coordinator for the state was contacted and consulted regarding whether the turtle should be transferred to rescue staff to receive additional care, or simply released after further observation.
Otolith sections were examined by two readers independently, a primary reader who examined all otolith sections and a secondary reader who examined one-third of otolith sections. Each reader assigned an increment count, determined by counting the number of alternating translucent and opaque bands, without knowledge of size data or capture location information. Upon completion of independent reads, increment counts (and edge type) were compared. Comparison counts were required to have at least 90% agreement, or the remaining two-thirds of the otoliths were also examined by the secondary reader. Where reader increment counts disagreed, the otolith sections were reviewed simultaneously by both readers and a consensus reached or that sample was removed from data analysis. Age was determined using increment count, month/season of capture, and edge type, in some cases using calendar age with an assumed birthdate of January 1.
All gonad samples were examined by two readers independently, without knowledge of size data or capture location information. Maturity codes were assigned based on descriptions in Brown-Peterson et al. (2011). When sex and/or maturity assignments differed, the sample was reviewed simultaneously by both readers, and if consensus could not be reached, the sample was removed from analysis.
Hydrographic data, including surface and bottom temperature and salinity measurements, were logged with a Van Dorn water sampler (1990-1992). Subsequently a surface to bottom profile of conductivity, temperature, and depth were logged with a Seabird SBE 19 SeaCAT Profiler CTD (1993-2005, 2017) or a Seabird SBE 19plus SeaCat Profiler CTD (V1 or V2; 2006-present). Additionally, sampling depth (start and end) and an estimate of wave height, as well as atmospheric data on air temperature, barometric pressure, precipitation, wind speed, and wind direction were recorded.
In 2023, the state of South Carolina purchased the R/V Lillian to replace the R/V Lady Lisa. The R/V Lillian is a 95 ft (29 m) steel-hulled trawler that will be retrofitted to meet the needs of the CTS. The R/V Lillian is expected to be operational in 2024.
Full details on survey activities and design changes are present in Table 3.
Data Analysis
The estimated area swept by a net was calculated by multiplying the average width of the net opening (13.5 m), determined by Stender and Barans (1994), by the distance in meters (m) trawled and dividing the product by 10,000 m2/ha. If area swept cannot be accurately estimated (e.g., U-turn was executed to avoid entanglement with gill net) the tow should be omitted from analyses.
Data Acknowledgment
The data user is responsible for reading information on the survey design, protocols, and data caveats (Table 1) including the Coastal Trawl Survey Methods to fully comprehend every aspect of the requested data. This information has been provided to assist the data user to be better able to query the SEAMAP-SA data from the online system in a way to receive the data desired. Additionally, this detailed information is provided to assist the data user in using the data in a responsible way. Each requester is solely accountable for any further analysis or manipulations following download. It is also the responsibility of the data user to cite and acknowledge SEAMAP and the Coastal Trawl Survey (see SEAMAP-SA Intellectual Property Protocol).
Data Availability
Field data collected by the Coastal Trawl Survey are made available to users within one year of collection through the SEAMAP-SA online database. At each station, location, depth, date, and time are recorded. Water temperature and salinity are recorded via conductivity-temperature-depth records and data are available either for the full water column or as surface or bottom values. Abundance and biomass data are available for all species encountered by the survey, except for some miscellaneous invertebrates and algae, which are treated as two separate taxonomic groups, only biomass is recorded. Additional data recorded for priority species (species of management or ecological interest designated by the SEAMAP-SA Committee; Table 2) include measurements of length or width, sex, individual weights, age, and/or reproductive information. Management agencies, scientists, and others currently have access to over 30 years of standardized trawl data from the Coastal Trawl Survey.
Future Research Goals
The Coastal Trawl Survey will continue to provide long-term, fishery-independent data on the distribution and relative abundance of species in coastal areas accessible by trawls. The CTS will continue to provide life history data and samples for species of management and ecological importance. With the change of vessels that will occur in 2024, the CTS will examine net mensuration and catch data to investigate the impact of the vessel change on comparability of time series data.