Measurements of adult marine fishes on the U.S. west coast are usually made using one of three methods: standard

length, fork length, or total length. Each method has advantages and disadvantages. In this paper we attempt to determine whether one method is faster and/or more reliable than the other methods.

We found that all three methods were comparable. There was no appreciable difference in the time it took to measure fish using the different methods. Fork length had the most reproducible results; however, it had the highest level of bias between researchers. We therefore suggest that

selection of measurement type be based on what other researchers have used for the species under study. The best improvement in measurement reliability probably occurs

by adequate training of personnel and not type of measurement used

Dick, Edward J.

Kahn, Reina G.

Pearson, Donald E.

English

Aquatic Commons

66(1) 31
Introduction
Reproducibility and consistency of 
measuring fish lengths depends greatly 
on the method used. The lack of a 
standard method of measuring fish is 
a problem for researchers using data 
from different sources, often requiring 
length conversion factors such as those 
given by Echeverria and Lenarz (1984) 
for rockfishes (genus Sebastes), which 
can be imprecise. Three common ways 
of measuring fish for research purposes 
are: standard length, fork length, and 
total length as described by Miller and 
Lea (1972). Standard length measures the 
distance from the tip of the longest jaw to 
Comparison of Standard Length, Fork Length, and Total Length 
for Measuring West Coast Marine Fishes
REINA G. KAHN, DONALD E. PEARSON, and EDWARD J. DICK
Reina Kahn’s current address is 2065 Royal Oaks 
Drive, Apt. 202, Sacramento, CA 95815. Donald 
Pearson and Edward Dick are with the National 
Marine Fisheries Service, NOAA, Southwest 
Fisheries Science Center, Santa Cruz Laboratory, 
110 Shaffer Road, Santa Cruz, CA 95060.
ABSTRACT—Measurements of adult ma- 
rine fishes on the U.S. west coast are usu-
ally made using one of three methods: stan-
dard length, fork length, or total length. 
Each method has advantages and disad-
vantages. In this paper we attempt to deter-
mine whether one method is faster and/or 
more reliable than the other methods. 
 We found that all three methods were 
comparable. There was no appreciable dif-
ference in the time it took to measure fish 
using the different methods. Fork length 
had the most reproducible results; how-
ever, it had the highest level of bias between 
researchers. We therefore suggest that 
selection of measurement type be based on 
what other researchers have used for the 
species under study. The best improvement 
in measurement reliability probably occurs 
by adequate training of personnel and not 
type of measurement used.
the end of the hypural bone or caudal pe-
duncle; however, there has been a debate 
regarding where the caudal peduncle or 
hypural bone ends (Howe, 2002). Fork 
length measures from the tip of the 
longest jaw to the center of the fork in 
the caudal fin. Total length measures the 
length from the tip of the longest jaw or 
the end of the snout to the longest caudal 
lobe pushed together.
Currently, on the U.S. west coast, stan-
dard length is typically used when mea-
suring juvenile and larval fish (Moser, 
1996) although some researchers use that 
measurement for adult fish as well. As the 
fish grow, total length and fork length are 
more commonly used for research and 
stock assessment purposes. 
Prior to 1990, total length was the 
conventional method for measuring 
groundfish species (i.e. rockfish, flatfish, 
sablefish, lingcod, and other species of 
roundfish) in California, while in both 
Oregon and Washington, researchers 
have used fork length. This created con-
fusion among researchers using data from 
all three states. In 1991, California began 
using fork length to end the confusion. 
While it was not clear that fork length was 
a more precise method, it did offer the 
advantage of standardization (Erwin1). 
Total length is commonly used for length 
regulations in both commercial and sport-
fishing on the west coast of the United 
States because no training is required to 
make the measurement, perhaps making 
it the most desirable measure.
We undertook this study to determine 
which measurement type was the most 
precise, and to see if one method was 
appreciably faster. To accomplish this, we 
measured a variety of fish using the three 
measurement methods and recorded the 
amount of time it took. We then examined 
the variances in length associated with 
each type of measurement.
Materials and Methods
In this study we used 50 groundfish 
of various species and lengths. We 
measured 25 sablefish, Anoplopoma 
fimbria; 7 blackgill rockfish, Sebastes 
melanstomus; 6 aurora rockfish, Sebastes 
aurora; 8 greenspotted rockfish, Sebastes 
chlorostictus; 3 greenstriped rockfish, 
Sebastes elongatus; and 1 rosethorn rock-
fish, Sebastes helvomaculatus. All fish 
were collected by longline gear as part of 
our groundfish ecology cruise program at 
the NMFS Southwest Fisheries Science 
Center’s Santa Cruz Laboratory. 
The size of the fish ranged from ap-
proximately 200 mm to 650 mm. The 
fish were placed on ice at sea and brought 
back to the laboratory for examination. 
Each fish was tagged with a unique 
number using a tagging gun to allow 
multiple length measurements to be as-
sociated with each fish.
Two researchers (hereafter called 
either reader or recorder) measured each 
fish 9 times: three times each for total 
length, standard length, and fork length. 
The reader’s measurements were timed 
by the recorder using a stopwatch and 
included the reader calling out the fish 
number and then the measurement. After 
obtaining one set of total length, standard 
length, and fork length measurements, 
the researcher stopped measuring and 
then recorded data while the other re-
searcher repeated the same process. Each 
researcher performed 450 measurements 
for a total of 900 measurements in the 
1Erwin, B. 2003. Pacific States Fisheries Man-
agement Commission, 350 Harbor Blvd., Bel-
mont, CA 94002. Personal commun. 
32 Marine Fisheries Review
Figure 1.—Comparison of mean fish lengths obtained by two readers using three 
different measurement methods: standard length (A), fork length (B), total length 
(C). A line of equality is shown on each plot. (N=50 fish, each fish was measured 3 
times, for each method, by each reader).  
study and measured 25 fish at a time 
using one method, then measured the 
same set of fish using a different method 
and repeated the process using the third 
method. 
For the purposes of this study, we con-
sidered standard length to be from the end 
of the longest jaw to the end of the caudal 
peduncle. A straight-edge device was 
used when measuring standard length 
to line up the caudal peduncle with the 
length on a measuring board.
To determine the speed of each mea-
surement method, we calculated the av-
erage amount of time it took to measure 
each fish, using each method, for each 
researcher. We then examined the results 
for any obvious trends.
For each measurement type, variabil-
ity among readers was estimated using 
the differences in lengths recorded by 
researcher 1 and researcher 2. The stan-
dard deviation of these differences is a 
direct measure of reader variability for 
each length definition (fork, standard, 
and total). The mean difference among 
readers was recorded as a measure of bias 
among readers.
To determine whether variability was 
a function of length for any of the three 
methods, the ANOVA model
lengthijk = intercept + readeri +  
fishj + error termijk 
was fitted for each measurement type, 
and plots of residuals vs. fitted values 
were visually inspected for evidence of 
variance heterogeneity.
Results
There was very little difference in the 
average amount of time it took to mea-
sure each fish using the three different 
methods; however, there was a substantial 
difference among researchers (Table 1). 
Reader 1 (the most experienced), aver-
aged about 3.6 seconds to measure each 
fish; while Reader 2 averaged about 5 
seconds per fish.
The mean size of each fish, for each 
method, for Reader 1 was plotted against 
the mean sizes obtained by Reader 2 
(Fig. 1). The results showed that mean 
total length, standard length, and fork 
length were very consistent between 
66(1) 33
Table 1.—Mean time in seconds to measure a fish using 
three measurement methods for two readers.
Measurement type Reader 1 Reader 2
Fork length 3.5 5.1
Standard length 3.7 4.8
Total length 3.7 5.1
readers with little evidence of bias. Mean 
differences in recorded length between 
Reader 1 and Reader 2 (measurement 1 
minus measurement 2), were 1.69 mm, 
–4.43 mm, and –5.13 mm for standard 
length, fork length, and total length, re-
spectively. This indicated that there was 
more bias in measurements of fork length 
and total length than for standard length.
Variability among readers was high-
est when using standard length. The 
standard deviation of the differences 
between readers was 6.60 mm. Estimates 
of among-reader variability were very 
similar for fork length (4.56 mm) and 
total length (4.70 mm).
Analysis of residuals from the three 
ANOVA models did not suggest that 
variance was a function of fish length 
when using either fork length or standard 
length. There was some evidence of 
variance heterogeneity for the model fit 
to total length data. Comparison of root 
mean squared errors among ANOVA 
models fit to log-transformed data (to 
stabilize variance) did not change the 
ranking of precision among the three 
measurement types however.
Discussion
The goals of this study were to deter-
mine whether one method of measure-
ment was faster and more accurate in 
terms of reproducibility than the others 
and to determine how much reader varia-
tion to expect with each of the measure-
ment types. We found that there was 
very little difference in the time it took to 
measure each fish using the three meth-
ods. Of more importance is the amount of 
experience the readers had with measur-
ing fish. Reader 1 was about 25% faster 
than the less experienced Reader 2. The 
time difference among readers might be 
important in studies where quick mea-
surements are desired, such as tagging 
studies where duration out of the water 
could reduce survival.
Fork length was found to be the most 
reproducible method of measuring fish; 
however, the differences were not large 
and there was evidence of bias among 
readers. Furthermore, there was very little 
difference in mean length among readers 
for the different methods (Fig. 1). 
Variations in length measurements 
can be attributed to several factors. For 
example, when using standard length, de-
termination of where the caudal peduncle 
ends greatly depends on the individual 
reader and where they feel the hypural 
bone ends. Other sources of variation 
include damage to the caudal fin, poor 
technique, a measuring board that is not 
easily readable, or inadvertently stretch-
ing the fish.
Although fork length was the most 
reproducible method of measuring fish 
in our study, it had a high level of bias 
among readers. While standard length 
had a low level of bias, it had the greatest 
variability in length measurements. When 
selecting a method, the researcher should 
consider the trade-off between precision 
and bias and evaluate the consequences 
for the intended use of the data.
This study suggests that there is no 
best way to measure fish. Proper tech-
nique and consistency is probably more 
important in producing reliable lengths 
than the method used. The decision on 
which method to use may best be based 
on what the majority of other researchers 
use, thus avoiding confusion and errors 
associated with applying length conver-
sion factors.
Acknowledgments
This work would not have been pos-
sible without the statistical advice of 
Steve Ralston (SWFSC, Santa Cruz 
Laboratory). We would also like to thank 
Brenda Erwin (Pacific States Marine 
Fisheries Commission) for her assistance 
with history of commercial data collec-
tion methods in California.
Literature Cited
Echeverria, T., and W. H. Lenarz. 1984. Conver-
sions between total, fork, and standard lengths 
in 35 species of Sebastes from California. 
Fish. Bull. 83:249–251.
Howe, H. C. 2002. Standard length: not quite so 
standard. Fish. Res. 56(1):2–7.
Miller, D. J., and R. N. Lea. 1972. Guide to the 
coastal marine fishes of California. Fish Bull. 
157:8–9. 
Moser, H. G. 1996. The early stages of fishes in 
the California Current Region. Calif. Coop. 
Fish. Invest. Atlas 33, 1,505 p. 


Comparison of Standard Length, Fork Length, and Total Length

for Measuring West Coast Marine Fishes

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Comparison of Standard Length, Fork Length, and Total Length for Measuring West Coast Marine Fishes

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