Lee and Rock: The forgotten need for spatial persistence in catch data from fixed-station surveys 
73 
Table 2 
Results of Mann-Kendail trend analyses of the rela¬ 
tive abundance indices of 10 target species the data of 
which were collected from trawl surveys conducted dur¬ 
ing 1979-2016 in the Pamlico Sound system of North 
Carolina. P-values are the 2-tailed probabilities for the 
trend test. The arrows in the column marked “trend” in¬ 
dicate the direction of the trend (decrease and increase 
in relative abundance over time) if a statistically sig¬ 
nificant temporal trend was detected (a=0.05). NS=not 
significant. 
Species 
P-value 
Trend 
Atlantic croaker 
0.0010 
4 
Atlantic menhaden 
<0.0001 
4 
Southern flounder 
0.0165 
T 
Spot 
0.0104 
4 
Summer flounder 
0.1183 
NS 
Weakfish 
0.0197 
4 
Blue crab 
0.9636 
NS 
Brown shrimp 
<0.0001 
T 
Pink shrimp 
0.4002 
NS 
White shrimp 
<0.0001 
T 
Table 3 
Results of the 2-way analysis of variance tests for the 
year-station interaction (1036 degrees of freedom) for 
the period of 1979-2016 for target species collected dur¬ 
ing the North Carolina Division of Marine Fisheries Es¬ 
tuarine Trawl Survey conducted in the Pamlico Sound 
system in North Carolina. The F value measures the 
ratio of the variance that is caused by the interaction 
effect to the unexplained variance. A significant interac¬ 
tion (a=0.05) indicates a lack of persistence. 
Species 
Mean 
square error 
F-value 
P(>F) 
Atlantic croaker 
5611 
1.46 
<0.0001 
Atlantic menhaden 
6418 
1.01 
0.4210 
Southern flounder 
37.90 
1.43 
<0.0001 
Spot 
38,524 
0.79 
1.0000 
Summer flounder 
0.15 
1.02 
0.3930 
Weakfish 
194 
0.97 
0.6840 
Blue crab 
33.79 
1.55 
<0.0001 
Brown shrimp 
4859 
1.26 
0.0001 
Pink shrimp 
0.70 
1.02 
0.3950 
White shrimp 
6.47 
0.93 
0.8930 
The results of the 2-way ANOVA tests for main in¬ 
teraction effects indicate a lack of persistence (i.e., of 
statistically significant interaction) for Atlantic croak¬ 
er, southern flounder, blue crab, and brown shrimp 
(Table 3). The data for all other target species showed 
the presence of persistence over the time period and 
stations analyzed. 
Discussion 
The idea of persistence made its first appearance in 
the primary literature in Warren’s article (1994) on 
sampling with partial replacement. Despite the ubiq¬ 
uity of fixed-station surveys, the practice of testing 
for persistence has largely been ignored. A cited ref¬ 
erence search (via Web of Science Core Collection) 
identified 7 papers that referenced Warren (1994). 
Of these articles, only one evaluated persistence in a 
fixed-station sampling survey (Li et ah, 2015). Anoth¬ 
er briefly described the idea of persistence (Kimura 
and Somerton, 2006). Given that bias can be intro¬ 
duced into a relative abundance index by a lack of 
persistence, it seems reasonable to recommend that 
persistence be routinely evaluated and documented 
for fixed-station surveys. 
The results of this study suggest that the presence 
of persistence is species-specific within the NCDMF Es¬ 
tuarine Trawl Survey. A lack of persistence was shown 
for Atlantic croaker, southern flounder, blue crab, and 
brown shrimp and indicated that this survey may be 
ineffective in tracking abundance for some of its tar¬ 
get species, at least for the time series and stations 
selected for our analysis. Investigation into those life 
history characteristics that lead to more or less per¬ 
sistence may be warranted. Further examination into 
the spatial distribution of the catches and the changes 
in environmental conditions may lead to an improved 
understanding of persistence. The selection of stations 
may be species specific. For example, removal of sta¬ 
tions where species of interest are not consistently 
caught may lead to improved or reduced persistence. 
Additionally, preliminary investigations conducted in 
preparation for this article suggest that choice of time 
series and stations can have a substantial influence on 
whether persistence is present. 
In summary, fixed-station sampling is acceptable 
when random sampling is impractical, so long as per¬ 
sistence is evaluated and is present for the species of 
interest. Fixed-station surveys can be easier to imple¬ 
ment, especially if a substantial part of the target sam¬ 
pling area is inaccessible to the gear. Van der Meer 
(1997) found that fixed-station sampling could have 
a higher power of detecting change over time than 
random sampling. In another comparison of fixed and 
random designs, Quist et al. (2006) found that a fixed- 
station design can provide a more sensitive measure of 
temporal variation. 
How the data collected from a fixed-station survey 
will be used should be carefully considered. If bias in 
annual changes is a concern, persistence should be 
evaluated so that the data can be examined in con¬ 
text. This approach is especially important if the sur¬ 
vey is going to be used to develop a relative index of 
abundance for use in a stock assessment because it is 
currently unknown how that bias may affect stock as- 
