NOAA Technical Memorandum NMFS NE 110
Length-Length and Length-Weight
Relationships for 13 Shark Species
from the Western North Atlantic
by Nancy E. Kohler, John G. Casey, and Patricia A. Turner
National Marine Fisheries Serv., Narragansett RI 02882
Print
publication date May 1996;
web version posted April 12, 2001
Citation: Kohler NE, Casey JG, Turner PA. 1996. Length-length and length-weight
relationships for 13 shark species
from the Western North Atlantic. US Dep Commer, NOAA Tech Memo NMFS NE 110; 22 p.
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INTRODUCTION
The
rapid expansion of sport and commercial fisheries for sharks in the western
North Atlantic has created the need to manage the stocks of several species
of large sharks. In response to this need, a fishery management
plan (FMP) for sharks within the U.S. Exclusive Economic Zone of the
Atlantic Ocean (U.S. Department of Commerce 1992) was implemented in
1993. The 39 shark species included in the FMP are not managed
on an individual species basis, but are aggregated into three species
groups -- large coastal, small coastal, and pelagic. Basic biological
data needed for stock assessment are lacking for many of these Atlantic
sharks, including size values (i.e., minimum, maximum, and average)
and size relationships/conversions (i.e., length-to-weight and
fork length-to-total length). These data are essential for understanding
growth rate, age structure, and other aspects of population dynamics.
Size conversions have a practical
value in fisheries. One measure currently in practice at nearly
all shark tournaments on the Atlantic Coast is the establishment of minimum
size limits, usually a minimum weight. Since sizes must be estimated
at sea, means for converting lengths to weights are essential to anglers. Moreover,
the National Marine Fisheries Service (NMFS) conducts an extensive Atlantic
Shark Tagging Program using volunteer assistance of recreational and
commercial fishermen. Commercial fishermen generally are more confident
in estimating the weight of sharks being released, while recreational
fishermen estimate lengths. Conversions are needed to change these
estimates into common size units for analysis.
Thus, in response to the
immediate needs of tournament officials and fishermen, and for management
initiatives, we present length and weight data for 13 species of large
Atlantic sharks collected over a 29-yr period by the NMFS Apex Predator
Investigation (API) at Narragansett, RI.
MATERIALS
AND METHODS
Length and weight data were collected from sharks caught by recreational
and commercial fishermen and by biologists along the U.S. Atlantic Coast
from the Gulf of Maine to the Florida Keys during 1961-89. Sharks
were caught primarily on rod and reel at sport fishing tournaments and
on longline gear aboard research vessels and commercial fishing boats. Some
data were obtained from sharks that were harpooned or taken in gill nets. Measurements
from a white shark captured off Rhode Island in 1991 were also included
in the analysis because of the specimen’s unusually large size. Data
were obtained opportunistically throughout each year, but most (88%)
were collected during June-August off the northeastern United States
between North Carolina and Massachusetts. Only lengths and weights
measured by the authors and other members of the API or by cooperating
biologists are included in this report. Measurements of embryos
and fish known to be pregnant were excluded from the data set.
All lengths
were taken with a metal measuring tape to the nearest centimeter in a
straight line along the body axis
with the caudal fin placed in a natural position. Fork length (FL) was
measured from the tip of the snout to the fork of the tail. Total length
(TL) is defined as the distance from the snout to a point on the horizontal axis
intersecting a perpendicular line extending downward from the tip of the upper
caudal lobe to form a right angle (Figure
1).
Total weight (WT) of each shark was
measured to the nearest pound and converted to kilograms. The majority
of fish were weighed while hanging by the caudal peduncle which allowed any
water in the stomach and, in some cases, stomach contents to drain out prior
to weighing. Many
fish were examined internally; if unusually large amounts of water or contents
were found in the stomach or abdominal cavity, the weights of such were subtracted
from the overall weight to obtain a more accurate measurement.
Fork length-to-total
length relationships for 13 shark species (n = 5065) were determined by the
method of least squares
to fit a simple linear regression model. Linear regressions of fork
length-to-total length were calculated with their corresponding regression
coefficients, sample
sizes, and mean lengths. These data are combined into four family groups: Alopiidae
(thresher sharks), Lamnidae (mackerel sharks), Carcharhinidae (requiem sharks),
and Sphyrnidae (hammerhead sharks). These combined data are then graphed
for comparison.
An allometric length-weight equation
was calculated using the method of Pienaar and Thomson (1969) for fitting
a nonlinear regression model by least squares. The form of the equation
is WT = (a)FLb, where WT = total weight (kg), FL = fork length (cm), and a
and b are constants
for each species. Length-weight relationships, mean lengths and weights,
and size ranges were determined for 13 shark species (n = 9512). Literature
values for maximum fork length and fork length at maturity were also included. These
length-weight relationships were graphed with the size-at-maturity estimates
indicated on each figure. Weight (in pounds) was calculated for every
6 inches (15 cm) of length over our size range of each of the 13 shark
species to construct a chart that can be used by anglers and tournament
officials
for
setting minimum size limits on their catches.
In addition to metric units
(i.e.,
centimeters and kilograms), figure scales are also shown in English units
(i.e.,
feet and pounds) to make them more useful for U.S. tournament officials,
anglers, and commercial fishermen. Regressions of the length-weight
equations expressed logarithmically were tested for significant differences
(p<0.05)
between males and females using an analysis-of-covariance test for homogeneity
of slopes.
Fork length is used throughout this
report as the basis for all conversions and comparisons. We have
found fork length to be a more precise measurement. For comparison
purposes, all values published elsewhere as total lengths were converted
to fork lengths
using the species’ equations presented in this paper.
Minimum sizes
at maturity reported here are from published accounts with their original
sources referenced,
with the exception of the thresher shark (Alopias vulpinus) and white
shark (Carcharodon carcharias). Minimum size at maturity
for the thresher shark and the male white shark were determined by H.L.
Pratt, Jr. (pers. comm.;
Nat. Mar. Fish. Serv., Narragansett, RI, May 1993), using the following
criteria: smallest
male with calcified claspers that rotate at the base, and smallest gravid
female. When
considerable variation occurred among published accounts, traditional
sizes at maturity were chosen primarily from Atlantic populations. Maximum
sizes and maximum sizes at birth used here are summarized in Pratt and
Casey (1990).
RESULTS
AND DISCUSSION
Linear regressions of fork length-to-total length for the 13 shark species
are presented in Table
1, and linear regressions for the four shark family groups are portrayed
in Figure 2. Slopes
of the regression lines of the four families decrease with increasing
length of the upper caudal lobe (Figure 2). The
mackerel sharks (line 1) have lunate tails with the upper and lower caudal
lobes almost equal in size. The requiem (line 2), hammerhead (line
3), and thresher (line 4) sharks have heterocercal tails with the upper
lobe longer than the lower. The latter group have very long upper
caudal lobes with the fork length approximately 60% of the total length. Fork
length represents 92%, 84%, and 77% of total length for mackerel, requiem,
and hammerhead sharks, respectively.
A total of 9512 sharks representing 13 species were measured, sexed,
and weighed. There
were no significant differences in slope or intercept of the length-weight relationships
between males and females for any of the species. Therefore, one equation,
calculated with the sexes combined, was used to represent the data for each species
(Figures
3-15; Table
2).
Size at maturity for males and females is difficult to determine for
pelagic sharks, and can vary in different parts of the world (Pratt and
Casey 1990). The
discrepancy is due, in part, to the use of variable criteria in determining a
precise length at sexual maturity (Springer 1960; Clark and von Schmidt 1965;
Pratt 1979), and thus maturity is often reported as a size range rather than
a specific length. An individual author’s definition of maturity is sometimes
ambiguous or obscure. The sizes at maturity (Table
3) are from multiple reference sources, and therefore may be mixed in
definition and criteria. The original published sources should be consulted
for the basis for defining sexual maturity among different authors.
An attempt was made to obtain samples representative of the full size
range of each species. Minimum, maximum, and mean lengths and weights by species
of sharks examined in this study are reported (Table
1 and Table 3). A reliable maximum size is difficult to verify. Lengths
and/or weights for large fish are often reported inaccurately, and published
accounts usually qualify maximum lengths with “probably reach,” “possibly to,” or “may
grow up to.” Maximum lengths (FL) reported in Pratt and Casey (1990) are
included for comparison with sizes measured in this study (Table 3). With
the exception of the porbeagle (Lamna nasus) and tiger shark (Galeocerdo
cuvier), our data are within 62 cm (2.5 ft) of published maximum sizes. The
porbeagle is less common in our study area; fewer specimens were examined (< 30),
and therefore the full size range of this species is not represented. Although
the tiger shark is purported worldwide to grow to 469 cm FL (15.4 ft) (Castro
1983; Compagno 1984; Pratt and Casey 1990), Atlantic specimens may not attain
that size. Our longest tiger shark was 339 cm FL (11.1 ft) (Table 3). Maximum
reported length examined by Branstetter (1981) in a study of tiger sharks in
the north central Gulf of Mexico was 346 cm FL (11.4 ft). Maximum reported
length for the U.S. Atlantic Coast is 391 cm FL (12.8 ft) (Bigelow and Schroeder
1948). These lengths are more in agreement with individuals sampled in
this study.
Specimens from three shark species exceeded the maximum reported lengths
(Table
3): sandbar shark (Carcharhinus plumbeus), shortfin mako
(Isurus
oxyrinchus), and scalloped hammerhead (Sphyrna lewini). The
211 cm FL (6.9 ft) female sandbar shark in this study (Table
1) was measured by one of the authors (J. Casey) and is the largest
measured sandbar reported to date. This fish was caught in September 1964 by a sport
fisherman approximately 10 mi east of Asbury Park, NJ. Unfortunately, the
fish was not weighed. Two shortfin makos measured in this study were longer
than the 336 cm FL (11.0 ft) maximum size fish published in the literature. Both
of these fish were 338 cm FL (11.1 ft) females caught by sport fishermen south
of Montauk Point, NY. One was landed in July 1977 and weighed 471 kg (1039
lb). The other was caught in August 1979 and weighed 382 kg (841 lb). The
largest scalloped hammerhead [243 cm (8.0 ft) FL and 166 kg (365 lb)] was
measured at a sportfishing tournament in July 1985, and was caught 36 mi
southeast of
Highlands, NJ.
The lower ends of the length-weight curves also compare well with published
estimates of size at birth for each species of shark. Pratt and Casey (1990) give
maximum size at birth in TL for 11 of the 13 species of sharks sampled here;
all except the thresher shark are within 40 cm (15.7 inches) of those sizes. Our
smallest thresher shark is 64 cm (25.2 inches) larger than the reported
birth size.
All of the larger fish were female with the exception of the white
shark (Figure
5) and blue shark (Prionace glauca) (Figure
14). The larger size attained by females is typical of sharks
in general (Pratt and Casey 1983; Hoenig and Gruber 1990), and thus larger female
blue and white sharks very likely occur outside of our western North Atlantic
sampling area which only covers a small portion of their extensive oceanic range.
Factors Affecting Weight
Weights of individual sharks of the same length may differ depending
on several factors, including the amount of stomach contents, stage of
maturity, liver weight, and body condition. Effects of stomach
contents on the weight of the fish were minimal in this study. In
many instances, the sharks everted their stomachs prior to being weighed. For
the bigger fish, when large amounts of food were present, the contents’ weight
was subtracted to obtain the total body weight. Since not every
shark was examined internally, some pregnant fish may have been inadvertently
included in the data base.
Differences in body weight also reflect differences in body condition. Sharks
have large livers which store high-energy, fatty acids for buoyancy and use
as a food reserve (Bone and Roberts 1969; Oguri 1990). The weight of
this organ is thus a good indicator of the health or condition of a shark (Springer
1960; Cliff et al. 1989). The liver is the largest organ by weight
in the shark and can vary from 2 to 24% of body weight depending on the species
(Cliff et al. 1989; Winner 1990). This variation in liver size
accounted for the majority of the weight difference in individuals of the
same species with corresponding lengths.
Of the eight largest white sharks, six were measured for liver weight;
those liver weights ranged from 14.6 to 22.7% of body weight (hepatosomatic
index
or HSI) (Table
4). The 458 cm (15.0 ft) FL white shark in this group had the lowest
HSI value (14.6%) although it was longer than four heavier fish. The
difference in body weight between the 458 cm (15.0 ft) FL and the 463 cm (15.2
ft) FL fish is 360 kg (794 lb). When the body weights of these two fish
-- minus their liver weights -- are compared, the difference is reduced to
239 kg (526 lb). Thus, liver weight accounted for 34% of the body weight
difference between these two sharks of similar length.
The same is true for large shortfin makos. The HSI for one of the longest
makos [338 cm (11.1 ft) FL and 382 kg (841 lb)] was 5.4%, as contrasted with
17.9% for the 323 cm (10.6 ft) FL fish weighing 490 kg (1080 lb). When
the body weights of these two fish -- minus their liver weights -- are compared,
the difference is reduced from 108 kg (239 lb) to 41 kg (91 lb).
ACKNOWLEDGMENTS
The
data for this study could not have been collected without the help and
cooperation of thousands of fishermen
who allowed us to measure their shark catches over the last 29 yr. The
scientists, officers, and crew of several research vessels also assisted in
obtaining specimens during sampling cruises. We are particularly grateful
to tournament officials and participants from New York, New Jersey, Massachusetts,
and Rhode Island from whose catches a large part of the data were collected. Further,
we would like to thank the past and present members of the Apex Predator Investigation,
including Chuck Stillwell, Lisa J. Natanson, Ruth Briggs, H.L. Pratt, Jr.,
and Gregg Skomal, for their assistance and support.
REFERENCES
CITED
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Tee-Van, J.; Breder, C.M.; Hildebrand, S.F.; Parr, A.E.; Schroeder,
W.C., eds. Fishes of the western North Atlantic. Part 1.
Vol. 1. New Haven, CT: Yale University, Sears Foundation
for Marine Research.
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Mar. Biol. Assoc. U.K. 49:913-937.
Branstetter, S. 1981. Biological notes on the sharks
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Branstetter, S. 1987. Age, growth and reproductive
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Branstetter, S.; Musick, J.A.; Colvocoresses, J.A. 1987. Age
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Castro, J.I. 1983. The sharks of North American
waters. College Station, TX: Texas A&M University Press;
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Gulf Coast of Florida. Bull. Mar. Sci. 15:13-83.
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as living resources: advances in biology, ecology, systematics
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Oguri, M. 1990. A review of selected physiological
characteristics unique to elasmobranchs. In: Pratt, H.L.,
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living resources: advances in biology, ecology, systematics and
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shark, Prionace glauca. Fish. Bull. (U.S.)
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Acronyms |
| API |
= |
(NMFS Northeast Fisheries Science Center) Apex
Predator Investigation |
| FL |
= |
fork length |
| FMP |
= |
fishery management plan |
| HSI |
= |
hepatosomatic index |
| NMFS |
= |
(NOAA) National Marine Fisheries Service |
| TL |
= |
total length |
| WT |
= |
body weight |
