70 
Fishery Bulletin 116(1) 
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Simple theoretical examples of spatially (A) persistent and (B) 
nonpersistent catches over time. (Adapted from Houghton, 1987). 
Source: ICES CM 1987/D:15, 7 p. [Available from website.1 
estimates even in a case of a complex and changeable 
spatial pattern in relative abundance. 
Our objective was to evaluate the persistence of 
relative abundance for selected species collected from 
one of the fixed-station trawl surveys conducted by the 
North Carolina Division of Marine Fisheries (NCDMF). 
Relative abundance indices derived from this survey 
have been provided to the stock assessment work 
groups at the NCDMF, the Atlantic States Marine 
Fisheries Commission, and university researchers. The 
survey data have been used to characterize nursery 
area habitat, help designate new nursery areas, and 
have also been important for the federal designation of 
Essential Fish Habitat. 
Materials and methods 
Data 
The NCDMF has operated the statewide Estuarine 
Trawl Survey, also known as Program 120, since 1971. 
The main objectives for the survey are to identify pri¬ 
mary nursery areas and produce annual recruitment 
indices for economically important species. The survey 
has also been used to monitor species distribution by 
season and area and to evaluate environmental impact 
projects. The survey targets Atlantic croaker ( Micro- 
pogonias undulatus), Atlantic menhaden (Brevoortia 
tyrannus), southern flounder ( Paralichthys lethostig- 
ma), spot ( Leiostomus xanthurus), summer flounder 
(P. dentatus), weakfish ( Cynoscion regalis), blue crab 
(Callinectes sapidus), brown shrimp ( Farfantepenaeus 
aztecus ), pink shrimp (F. duorarum), and white shrimp 
(Litopenaeus setiferus). 
The survey samples were collected in shallow-water 
areas of the Pamlico Sound system in North Carolina 
and waters south to the North Carolina-South Carolina 
border. Selection of stations was nonrandom. 
Major changes in sampling methods occurred 
in 1978 and 1989. In 1978, tow times were 
standardized to 1 min during daylight hours. 
In 1989, survey data were analyzed to deter¬ 
mine a more efficient sampling time frame 
(until then sampling occurred year-round). 
This analysis identified a fixed set of 105 core 
stations and sampling was reduced to May 
and June only, except for July when weakfish 
were sampled (dropped in 1998; sampling in 
July was reinitiated in 2004 for spotted seat- 
rout), and only a trawl with a 3.2-m (10.5-ft) 
headrope and with 6.4-mm (0.25-in) bar mesh 
would be used. Each station was sampled once 
per month. 
The current sampling gear consists of a 
2-seam otter trawl with a 3.2-m headrope, 6.4- 
mm bar mesh with body netting of 210/6-size 
twine, and a tailbag made of 3.2-mm Delta- 
style knotless nylon and with a 150-mesh cir¬ 
cumference and 450-mesh length. Three loops 
of 4.8-mm (0.19-inch) chain are hung on each wing. 
A 4.8-mm diameter tickler chain is used and 2 floats, 
each 76.2-mm (3-inch)xl01.6-mm (4-inch), are attached 
to the center of the headrope. The gear is towed for 1 
min during daylight hours during similar tidal stages 
and covers 68.6 m (75 yards). 
Environmental data are recorded during each sam¬ 
pling event and include depth, temperature, salinity, 
dissolved oxygen, wind speed, and direction. Additional 
habitat fields were added in 2008. 
Analysis 
Because not all stations were sampled in May of the 
first year (1978) we used data collected from 1979 
through 2016 and limited the analysis to those data 
collected from stations within the Pamlico Sound sys¬ 
tem. Only those stations sampled in both May and 
June over the selected period were included—a total of 
29 stations for analysis (Fig. 2). 
We first calculated the frequency of occurrence for 
each target species based on the number of tows the spe¬ 
cies was present in out of all possible tows during 1979 
through 2016 (29 stationsx2 monthsx38 years=2204 to¬ 
tal tows). Indices of relative abundance were then cal¬ 
culated for all target species to provide estimates of 
trends in abundance over time. The Mann-Kendall test 
was applied to the indices to evaluate temporal trends. 
The Mann-Kendall test is a nonparametric test used to 
evaluate a monotonic trend in time-ordered data (Gil¬ 
bert, 1987). The null hypothesis is that the time series 
is independent (observations are not serially correlated 
over time) and identically distributed (i.e., there is no 
significant trend across time, and therefore monotonic). 
The Mann-Kendall test allows for missing values and 
accounts for tied values when present. Mann-Kendall 
test statistics, including P-values, were calculated in 
R statistical software, vers. 3.4.2 (R Core Team, 2017) 
