69 
National Marine 
Fisheries Service 
NOAA 
Fishery Bulletin 
o- established in 1881 ■<?> 
Spencer F. Baird 
First U S Commissioner 
of Fisheries and founder 
of Fishery Bulletin 
The forgotten need for spatial persistence in catch 
data from fixed-station surveys 
Email address for contact author: iaura.lee@ncdenr.gov 
Abstract— -Design-based estimators, 
which have well known statistical 
properties, exist for random sam¬ 
pling survey designs and provide 
unbiased estimates of the mean and 
variance of summary catch statis¬ 
tics. Although application of these 
estimators to fixed-station surveys 
is not technically valid, they can re¬ 
sult in unbiased differences between 
years if the data exhibit spatial per¬ 
sistence A persistent system is one 
in which the differences in relative 
abundance of a population that are 
recorded at fixed stations are consis¬ 
tent from year to year (i.e., there is 
no interaction between station and 
years in a 2-way classification). In a 
fully persistent system, the changes 
in relative abundance derived from 
fixed stations will be unbiased. In 
this study, the persistence in rela¬ 
tive abundance data collected from 
a fixed-station survey intended to 
monitor juveniles of economically 
important species is evaluated. The 
results can be used to infer those 
species for which changes in relative 
abundance estimated from fixed-sta¬ 
tion surveys will be more accurate 
than changes in relative abundance 
estimated from random surveys. 
Manuscript submitted 24 July 2017. 
Manuscript accepted 20 November 2017. 
Fish. Bull. 116:69-74 (2018). 
Online publication date: 12 December 2017. 
doi: 10.7755/FB. 116.1.7 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Laura M. Lee (contact author) 1 
Jason E. Rock 2 
1 North Carolina Division of Marine Fisheries 
P.O. Box 769 
Morehead City, North Carolina 28557 
2 North Carolina Division of Marine Fisheries 
Washington Regional Office 
943 Washington Square Mall 
Washington, North Carolina 27889 
Sampling is essential for studying 
fish populations. The primary objec¬ 
tive for many fisheries-independent 
surveys is to monitor the relative 
abundance of a population, which 
is an important input for stock as¬ 
sessments and management consid¬ 
erations. A wide variety of survey 
designs can be used to monitor rela¬ 
tive abundance. The advantage of 
statistically designed surveys (e.g., 
stratified random) is they are asso¬ 
ciated with design-based estimators 
for which the statistical properties 
are well known. These design-based 
estimators are designed to provide 
unbiased estimates of the mean and 
variance (when the survey design is 
implemented properly). 
What about fixed-station surveys? 
There are no design-based estimators 
associated with fixed-station sampling 
and therefore there are no estimators 
that guarantee unbiased statistics 
for these surveys. Random surveys 
are designed to provide an unbiased 
estimate of the mean, whereas fixed- 
station surveys, in general, provide a 
biased estimate; however, the magni¬ 
tude of that bias could be anywhere 
from negligible to substantial. There 
is no way of determining the extent 
of this bias from the data them¬ 
selves. Bias in any given year may 
be of little consequence if changes in 
relative abundance between years is 
accurately reflected by the changes 
in the index of relative abundance 
(Warren, 1994). This is determined 
through the property of persistence. 
Persistence, in this study, is the 
constancy of differences in the pat¬ 
terns of relative abundance at fixed 
locations during sampling years 
(Warren, 1994). The greater the per¬ 
sistence, the greater the ability to de¬ 
tect interannual changes in relative 
abundance. In a fully persistent sit¬ 
uation (Fig. 1A), changes in relative 
abundance from fixed-station surveys 
from year to year will be unbiased 
and more accurate than changes in 
relative abundance estimated from a 
random sampling design. In the case 
of a complex and changeable spatial 
pattern in relative abundance (i.e., 
where distribution pattern varies 
from year to year; Fig. IB), a fixed- 
station design will result in biased 
estimates except in exceptional and 
unlikely circumstances (ICES 1 ). A 
randomized design leads to unbiased 
1 ICES (International Council for the Ex¬ 
ploration of the Sea). 1989. Report of 
the working group on methods of fish 
stock assessments (WGMG), 10-17 No¬ 
vember 1989, Nantes, France. ICES 
CM 1990/Assess: 15, 95 p. 
