NOAA Technical Memorandum NMFS NE 171
Length-Weight Relationships for 74 Fish Species
Collected
during NEFSC Research Vessel
Bottom Trawl Surveys, 1992-99
by Susan
E. Wigley,
Holly M. McBride, and Nancy J. McHugh
National Marine Fisheries Serv., 166 Water St., Woods Hole, MA 02543
Print
publication date March 2003;
web version posted June 23, 2003
Citation: Wigley SE, McBride HM, McHugh NJ. 2003. Length-weight relationships for 74 fish species collected during NEFSC research vessel bottom trawl surveys, 1992-9. NOAA Tech Memo NMFS NE 171; 26 p.
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Abstract
This study is the first comprehensive examination of spatially and temporally
synoptic length-weight observations collected along the Northeast coast
of the United States during the Northeast Fisheries Science Center's
(NEFSC's) research vessel winter, spring, and autumn bottom trawl surveys
from 1992 to 1999. Linear regression using natural logarithmic transformation
data was performed to calculate a and b coefficients. Analysis
of covariance was used to test for seasonal and gender differences. Length-weight
parameters were calculated for 74 fish species: 39 species showed seasonal
differences, and 28 species showed gender differences. Minimum and maximum
length observations for the first 37 years of the time series (i.e.,
1963-99) of NEFSC research vessel bottom trawl surveys are also presented.
Results from this study can be used within the "real-time" auditing of
length-weight data collected by the Fisheries Scientific Computer System,
the NEFSC's at-sea electronic data acquisition system.
Keywords: length-weight relationship, groundfish, demersal fish
INTRODUCTION
At the Northeast Fisheries Science Center (NEFSC), length-weight parameters
are routinely used in the estimation of numbers of fish landed in order
to estimate fishery removals for stock assessments (e.g., see
Wigley and Serchuk 1992). Additionally, length-weight relationships have
been used in the auditing of NEFSC research vessel survey catch and biological
data. However, many of the length-weight parameter values currently being
used in the audit applications were derived from studies with limited
sample sizes, combined genders, incomplete length ranges, restricted
seasonal and/or geographical coverage, or, in many cases, from studies
of unknown origin. Since the advent of recording individual fish weight
during NEFSC bottom trawl surveys in 1992, numerous spatially and temporally
synoptic length-weight observations have been obtained. This study is
the first comprehensive examination of these data to update the length-weight
parameters.
A consideration for updating these length-weight parameters is their
use in the NEFSC's survey auditing procedures. Those procedures use length-weight
equations at two levels: the species catch level and the individual fish
level. At the species catch level, the audit calculates a derived weight
for the catch based upon the length frequency of the catch, and compares
that derived weight with the observed weight of the catch. At the individual
fish level, the audit compares the derived weight of the individual fish
with the observed weight of that individual. Updated parameters, especially
those sensitive to gender and seasonal differences, could improve the
effectiveness of the auditing procedures.
Another consideration for updating the length-weight parameters is
the recent implementation of the Fisheries Scientific Computer System
(FSCS), an at-sea electronic data acquisition system on board NEFSC research
vessels. Within this system, data (i.e., lengths and weights)
are digitally recorded, thus enabling real-time auditing of these data
using a length-weight equation during the data collection phase. When
observed weights deviate from predicted weights, the FSCS produces an
error message requiring a manual override which, in turn, slows down
data collection. Updated parameters could minimize error messages within
the FSCS for those species for which current length-weight relationships
are problematic.
METHODS
DATA SOURCES
The NEFSC has conducted research vessel bottom trawl
surveys to assess the distribution and relative abundance of groundfish
along the east coast of the United States during the past three decades
(Grosslein 1969; Azarovitz 1981). The survey employs a stratified random
sampling design with tows at depths ranging from 5 to 366 meters. Geographic
coverage of the spring and autumn surveys is from Cape Hatteras to Nova
Scotia, and the winter survey from Cape Hatteras to the southern flank
of Georges Bank (Figure 1).
Beginning in 1992, biological sampling procedures were expanded to include
recording individual fish weight, in addition to recording the fish's length,
gender, and maturity stage. Aboard ship, the fish are measured live or
freshly killed to the nearest centimeter (total length or fork length,
depending on the species, except for rays, where disk width is measured
from wing tip to wing tip), and weighed (whole fish) to the nearest 0.001
kg. Gender and maturity stage of the fish are examined macroscopically
and recorded into the following categories: 1) unsexed, male, or female;
and 2) immature, developing, ripe, ripe & running, spent, or resting
(Burnett et al. 1989).
Although many species exhibit dimorphoric growth by sex, the survey procedures
for measuring and enumerating fish is, for the most part, conducted at
an unsexed species level. However, a few exceptions occur for those species
for which gender can be determined from external physical characteristics.
Length data by species and gender are collected for spiny dogfish, smooth
dogfish, American lobsters, and various crab species. Common and scientific
names used throughout this study are in accordance with those endorsed
by the American Fisheries Society (Williams et al. 1989; Robins et
al. 1991; Turgeon et al. 1998), with the exception of some flounders
(Cooper and Chapleau 1998) and rays (McEachran and Dunn 1998) which have
undergone subsequent systematic revision.
Species for which five or more length-weight observations existed were
analyzed in this study. For most species, data from two or three seasons
were available; however, for a few species, a limited number of observations
were available for a single season. Due to the limited geographic coverage
of the winter survey, data from the winter surveys were excluded from analyses
for several species, such as Acadian redfish, whose primary distribution
occurs within the Gulf of Maine.
LENGTH-WEIGHT PARAMETERS
Length and weight observations were transformed using natural logarithms,
and were plotted for visual inspection of outliers. Only extreme outliers
attributed to data error were omitted from the analyses.
Where sufficient data were available, analysis of covariance (i.e.,
test for homogeneity of slopes) was performed using PROC GLM (SAS Institute1985)
to detect significant differences (P <0.05) between season and
gender. Since the NEFSC species audit compares observed catch weight with
predicted catch weight based on the length frequency of observed fish,
the first task was to test for seasonal differences in length-weight parameters
for each species (with genders combined). Further exploration of the data
was then conducted to determine if gender differences existed within seasonal
group. Length-weight parameters were estimated by gender within a seasonal
group as appropriate, according to the following linear regression using
PROC REG (SAS Institute 1985):
ln W = ln a + b ln L
where W = weight (kg), L = length (cm), a = y-intercept, and b =
slope.
Residuals from the linear regressions were plotted and visually inspected
for trends. Since raw data for existing length-weight relationships were
not available for more rigorous statistical analysis, comparisons with
length-weight relationships derived in this study were performed as follows:
1) 95% confidence intervals were derived for each length-weight relationship;
2) significant differences occurred if the predicted weights from the existing
relationship fell outside the 95% confidence interval of the new length-weight
relationship.
LENGTH RANGES
To evaluate whether the 1992-99 data used in this study to derive length-weight
relationships were representative of the length ranges available to the
survey for each species, historical observations of minimum and maximum
lengths collected during NEFSC spring, autumn, and winter research vessel
bottom trawl surveys from 1963 to 1999 were updated and compared to minimum
and maximum lengths in this study. Additionally, range ratios were calculated
by dividing the study length range by the historical length range.
The update of these length ranges was expanded beyond the 74 species
in this study to include all species sampled during the surveys. This updated
information can be utilized as part of the real-time audit of length measurements
within the FSCS.
RESULTS
LENGTH-WEIGHT PARAMETERS
Between 1992 and 1999, a total of 24 NEFSC research
vessel bottom trawl surveys were conducted, during which 242,693 individual
fish length and weight observations were recorded for the 74 fish species
(comprising 2 classes, 9 orders, and 35 families) analyzed in this study.
Sample sizes ranged from six for greater amberjack, northern kingfish,
and smooth butterfly ray, to 26,590 for spiny dogfish (Table 1). In Table 1 and all subsequent tables, species are ordered according
to the National Oceanographic Data Center (NODC) taxonomic code.
For 19 species, data were not available for multiple seasons, precluding
seasonal analyses for those species. Of the remaining 55 species, 16 did
not exhibit significantly different (P <0.05) length-weight relationships
by season; 39 did. For four species (i.e., round herring, Atlantic
thread herring, Atlantic spadefish and buckler dory), data were not available
on gender, precluding gender analyses for these species. Of the remaining
70 species, 28 had significantly different length-weight relationships
by gender; 42 did not. Sample sizes, length ranges, length-weight parameter
estimates and the standard deviations (standard errors) of those estimates,
standard errors of the weight estimates, and regression correlation coefficients
are presented in Table 1 by species for appropriate season or seasonal
groups and for gender.
Although residual patterns generally showed no trend, a few exceptions
should be noted. For some species, residual patterns (either positive or
negative) existed for smaller fish, and may relate to the sensitivity of
the Marel weighing scales in open-deck environments and/or recorder bias
in determining a true weight during scale fluctuations in heavy seas. Small
sea ravens exhibited negative residuals, which might be attributed to some
size specificity in the characteristic behavior of this species to "gulp" water
when captured. The analysis of bluefish in the spring revealed a pattern
in which residuals were negative at smaller sizes and became positive at
larger sizes; although this pattern would normally result in rejection
of the regression, the analysis was retained due to the possibility that
fish greater than 40 cm (the length around which the residuals pivoted)
collected in the southern portion of the survey might be reproductively
active and therefore include the weight of the maturing gonad. Scup exhibited
a funnel-shaped residual pattern, with decreasing deviation as fish length
increased; this pattern may be related to the log-log transformation model
used in the study (Pienaar and Thomson 1969).
Comparisons of length-weight relationships established by this study
with those currently used in the NEFSC auditing process indicated no significant
differences at the catch level (genders combined) for 42 of 74 species.
However, there were nine instances in which the weights predicted by the
current length-weight relationships fell entirely outside of the 95% confidence
intervals of the weights predicted by this study. These species were: chain
dogfish, rosette skate, southern stingray, bluntnose stingray, cownose
ray, Atlantic thread herring, fawn cusk-eel, Atlantic spadefish, and spot.
For Atlantic angel shark, predicted weights from the current relationship
for intermediate-sized fish occurred within the confidence interval, but
those for smaller- and larger-sized fish did not. There were five species
for which predicted weights from the current relationship were significantly
different for larger fish (i.e., Atlantic sturgeon, round herring,
Atlantic herring, greater amberjack, and cunner), and 17 species for which
smaller fish were problematic (i.e., spiny butterfly ray, Atlantic
sharpnose shark, smooth dogfish, spiny dogfish, clearnose skate, Spanish
sardine, red hake, alewife, cusk, silver hake, Acadian redfish, bluefish,
black sea bass, scup, southern kingfish, fourspot flounder, and witch flounder).
There were seven species for which gender-specific predicted weights
from the current length-weight relationship were significantly different
from gender-specific predicted weights in this study (i.e., rosette
skate, red hake, white hake, Acadian redfish, striped bass, weakfish, and
yellowtail flounder). No comparisons were possible for the seven species
for which no current gender-specific relationship existed (i.e.,
clearnose skate, little skate, winter skate, spiny butterfly ray, longhorn
sculpin, ocean pout, and sea raven).
LENGTH RANGES
Generally, the length ranges used to derive length-weight
relationships in this study represented a significant proportion of the
ranges which have been historically observed, as evidenced by an all-species
average range ratio of 72% (Table 2). For 14
species, range ratios were below 0.50, suggesting that the length ranges
utilized in this study may not have represented the historically observed
length range. However, for 10 of those 14 species, sample sizes were quite
small (i.e., Atlantic torpedo ray, smooth butterfly ray, Atlantic
sturgeon, alewife, northern searobin, greater amberjack, vermilion snapper,
northern kingfish, Atlantic spadefish, and tautog). There appeared to be
adequate sample sizes for the remaining four species with range ratios
below 0.50, but larger-sized specimens of those species were noticeably
absent from the study data set (i.e., Atlantic sharpnose shark,
Spanish sardine, fawn cusk-eel, and Spanish mackerel; Table 2).
The observed minimum and maximum lengths (cm) for all
species measured during NEFSC bottom trawl surveys since 1963 are summarized
in Table 3. Approximately 2.83 million lengths
have been obtained from species comprising 9 phyla, 25 classes, 89 orders,
and 171 families. For species which are sorted by gender during the survey
(e.g., spiny dogfish, American lobster), length values for males,
females, and unknown genders are reported in Table 3.
DISCUSSION
Zar (1968), Glass (1969), and more recently Hayes et al. (1995)
presented information supporting the use of nonlinear least-squares regression
techniques for allometric modeling; however, Xiao and Ramm (1994) concluded
that the use of log-transformed data was appropriate for describing length-weight
relationships in fishes. In this study, the small sample sizes associated
with several species were potentially problematic with respect to asymptotic
variance properties of nonlinear regression. Our choice of an allometric
model was practical; linear regression using log-log transformed data facilitated
statistical comparisons of gender and seasonal relationships, and allowed
a single method to be applied to all species within the study, regardless
of sample size.
Length-weight relationships derived in this study generally compare favorably
with those of other published studies. For 35 of 78 species analyzed by
Wilk et al. (1978) which were also examined in this study, only
the relationship for fawn cusk-eel was significantly different. While this
might be attributed to the larger sample size and greater size range available
to Wilk et al. (1978), the use of the Wilk et al. (1978)
relationship within the FSCS during the NEFSC spring 2001 bottom trawl
survey resulted in numerous real-time audit messages indicating an erroneous
weight for a given length. When the parameters derived in this study were
substituted into FSCS, these error conditions were eliminated for subsequent
fawn cusk-eel samples.
There were also no apparent differences between length-weight relationships
for four of the six flatfish species derived by Lux (1969) and the relationships
derived by this study; differences for witch and fourspot flounders may
be related to the restricted geographical range of Lux's (1969) samples.
This similarity in relationships is somewhat remarkable given the difficulties
in obtaining accurate fish weights at sea prior to the development of modern
electronic motion-compensated scales. Wilk et al. (1978) froze fish
at sea and obtained thawed weights back at the laboratory, while Lux (1969)
weighed his samples at sea with hand-held spring scales. This similarity
in findings suggests both the diligence of these investigators as well
as the underlying robustness of fish length-weight relationships to measurement
error.
In summary, this study updates length-weight parameters for many species
routinely encountered during NEFSC bottom trawl surveys, utilizing uniform
methods and modern scale technology. The availability of whole live body
weight from sexed fish collected across seasonal surveys takes into account
annual cycles of fish feeding and reproduction, allowing derivation of
length-weight relationships at the gender and/or season level. Analysis
of these data provided insights into areas, such as length range or sample
size for some species, in which additional sampling can be targeted in
future surveys. The length-weight relationships derived in this study also
support the improved processing of survey data within the recently-implemented
FSCS environment, providing critical fishery independent data in a more
timely fashion.
ACKNOWLEDGMENTS
We wish to express our appreciation to the sea-going scientific staff
of the NEFSC who diligently collected the biological observations used
in this study. We thank the anonymous reviewers for their helpful comments
and review of this manuscript.
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Acronyms
FSCS = Fisheries Scientific Computer System
NEFSC = Northeast Fisheries Science Center
NODC = National Oceanographic Data Center
