Transactions of the American Fisheries Society 117:574-580. 1988
© Copyright by the American Fisheries Society 1988
Selectivity of Gill Nets Used in the Commercial
Spanish Mackerel Fishery of Florida
NELSON M. EHRHARDT AND DAVID J. DIE
Division of Biology and Living Resources. Rosenstiel School of Marine and Atmospheric Sciences
University of Miami, 4600 Rickenbacker Causeway. Miami. Florida 33149. USA
Abstract.— Selection curves of encircling (run-around) and drifting (stab) gill nets used in the
winter fishery for Spanish mackerel Scomberomorus maculatus off southern Florida were estimated
by use of cumulative probability distributions of retention girth at length. Selection curves corresponded well with observed size frequencies obtained from each mesh size. Increasing differences
between maximum and head girth perimeters as fish grew resulted in selection curves indicating
increased selection range and efficiency. Selectivity of Spanish mackerel gill nets will change as the
condition of the fish changes with the onset of the spawning season. Apparently, selectivity also
changes with twine size.
Spanish mackerel Scomberomorus maculatus are
coastal migratory fish that concentrate in southern
Florida during the winter (Collette and Russo
1984) and disperse northerly in a spawning migration during spring and summer. Larger, thus
older, fish are usually isolated swimmers whereas
younger age-classes form dense schools. In Florida, Spanish mackerel stocks are the subject of intense exploitation by recreational and commercial
fishermen. Hook and line is predominantly used
by the recreational sector, and encircling (runaround) gill nets are used to capture schooling fish
by the commercial fishery. Larger nonschooling
fish are mostly caught incidentally in drift gill nets
(stab nets) used for Florida pompano Trachinotus
carolinus. Temporal and spatial differences in stock
availability to the fisheries, and differences in gear
selectivity, considerably affect the size composition in the landings. The fisheries are mainly managed by annual quota allocations and by a minimum size restriction of 30.5 cm fork length to
protect juveniles.
Commercial fishing activities are primarily concentrated in southwestern Florida and in the Fort
Fierce-Port Salerno area off eastern central Florida. More than 90% of the total annual commercial quota is landed in these regions from December through February. Run-around gill nets are
widely used in the fishery because of their efficiency in catching schooling fish and their ability
to select optimum market-size fish through the
control of mesh size. At present, mesh size varies
from 8.6 to 9.5 cm stretched mesh in run-around
gill nets, and from 11.7 to 12.4 cm in stab nets.
Knowledge of the selection properties of these nets
is needed to obtain unbiased estimates of growth
and mortality of the fish, and to optimize yield by
adopting a proper mesh size. The specific purpose
of our study has been to determine the selectivity
properties of Spanish mackerel gill nets used by
the commercial fishery in southern Florida.
Methods
Retention occurs when a fish penetrates a mesh
beyond its gill covers but does not pass completely
through; this implies that a fish is caught if its head
girth is smaller but its maximum girth is larger
than the mesh perimeter (Hamley 1975). Spanish
mackerel are fusiform in shape, and notoriously
lacking in protuberances and spines that might
tangle or wedge a fish in the net before it enters a
mesh beyond the gillcovers. Trent and Pristas
(1977) reported that many Spanish mackerel were
tangled or wedged by their teeth, maxillaries or
tail, but these authors did not quantify what proportion of the catches these fish represented. In
our study, visual observation of the removal by
fishermen of thousands of Spanish mackerel led
us to conclude that such proportion was negligible.
Based on this, theoretical selectivity curves derived by Sechin (1969) are particularly appropriate in estimating selection properties of Spanish
mackerel gill nets. Sechin's model estimates probabilities offish retention as a function of morphometric features of the body between the operculum (or head) and the point of maximum girth of
the fish (Figure 1). The model also incorporates
coefficients to account for body compressibility at
the retention point, elasticity of the netting material, and variability of fish size.
Selection probabilities P are estimated according to Sechin's (1969) model as
574
GILL-NET SELECTIVITY FOR SPANISH MACKEREL
575
FIGURE 1.—Region of Spanish mackerel, between the points of opercular ginh (Gc) and maximum girth ((/„,„),
where fish are retained by gill nets with mesh perimeter equal to %42m."
for the length distribution of fish small enough to
enter a mesh beyond the operculum, and
< 2**}
(2)
for that offish too large to pass through the mesh.
A combination of (1) and (2) gives
im,\t - Kc,iGCJ\
^T^J )
\\L - 4\2miWLx,
7 Km~'+iG<rfr*yi/J
for the length distribution offish that are retained
by mesh /.
In the above formulations,
S,j = probability of retention offish of sizeclassy encountering mesh size /;
GCj = mean opercular girth for fish of sizeclass j\
ere,
c.;
*•«/ = variance of G*•«/*
tf f , = factor combining the elasticity of the
twine and tissue at the point of opercular girth for mesh size /;
tfnwx.) = mean maximum girth for fish of sizeclassy;
<72m*x., = variance of GmaXty;
#max, i = factor combining the elasticity of the
twine and tissue at the point of maximum girth for mesh size /;
2m, = inside mesh perimeter of mesh size /';
ffj = variance of mesh perimeter for mesh
size /'; and
4> = cumulative distribution function of the
standard normal distribution.
The compressibility-elasticity factors at retention girth are estimated as
K = mesh perimeter (cm)Xretention girth (cm).
(4)
Data used in the analyses were collected from random samples obtained from landings in southern
Florida by run-around vessels and stab boats operating during the 1987-1988 fishing season. Samples consisted of one randomly selected 45-kg fish
basket per vessel landing, or all Spanish mackerel
landed by a stab boat. Fish were captured in runaround monofilament nylon gill nets in one of
four possible mesh sizes presently used in the fishery: 8.6, 8.9, 9.2, and 9.5 cm; stab gill nets had
12.4-cm mesh size (all mesh sizes are stretched
measure). Factory mesh specifications were cor-
576
EHRHARDT AND DIE
roborated by random measurements of several
unloaded meshes. Only one vessel operated gill
nets with 9.2-cm mesh size that was manufactured
of number 7 twine; all remaining nets were manufactured of number 9 twine. Each fish was measured (fork length, L,) to the nearest 0.5 cm and
weighed in grams. All girth measurements as required by the model were made unconstricted to
the nearest millimeter with a loop of nonstretchable synthetic measuring tape. Maximum girth was
measured on an off-perpendicular plane defined
by the anteriormost positions of the second dorsal
fin and the ventral fin (Figure 1). Retention girth,
the girth where the fish was caught, was measured
at the mark left by the mesh. We measured 109
fish caught by 8.6-cm mesh, 76 caught by 8.9-cm
mesh, 91 caught by 9.2-cm mesh, 39 caught by
9.5-cm mesh size, and 54 caught by 12.4-cm mesh.
Results
Mean head girth and mean maximum girth
were obtained by pooling the data from all mesh
sizes. Both girths were linear increasing functions
of fork length. The equations obtained for these
relations were
GCJ = 0.2\ + 0.38L,;
r = 0.84;
GmMj = -2-51 + 0.51L,; r = 0.92.
Figure 2 shows the mean head girths, maximum
girths, and the regression lines fitted. Variances of
head girth (<?£,) and maximum girth (criLxj) did not
show significant differences with respect to length
classy (Bartlett's x2 = 19.79, df = 17, P > 0.30
for G^.,; x2 = 4.46, df = 8, P > 0.80 for GCJ).
Consequently, pooled variance estimates over all
7 are
a* =0.609;
<rLx= 1.173.
The factor combining body compressibility at
the point of maximum girth and elasticity of number 9 twine for mesh size / (#„,».,•) was estimated
as the mean factor (equation 4) for fish retained
within a girth range from girths slightly larger than
Gcj to GmnJ. The compressibility factor for the
opercular region was estimated from equation 4
also for animals that were retained close to the
operculum. The less compressible bony structure
of the opercular region relative to the softer retention region in the body resulted in larger compressibility-elasticity factors (Kct) for the opercular (head) girth (^-factors are inversely related
to compressibility of the fish or elasticity of the
ffi 25
:
S 23 :
< 19 W
2S 17:
15 0
40
45
50
55
FORK LENGTH
60
65
(cm)
FIGURE 2.— Relationships of mean maximum girth
and mean head girth to fork length for Spanish mackerel
captured in gill nets with 8.6-, 8.9-, 9.2-, and 12.4-cm
stretched-mesh sizes.
mesh). Estimated AT values for different mesh sizes
were
Mesh size (cm)
8.6
0.955
0.965
8.9
0.993
0.995
9.2
0.963
0.970
12.4
0.986
0.990
Mean factors expressing compressibility and
elasticity of number 9 twine for all mesh sizes
pooled were #max = 0.975 and Kc = 0.977. Similar
factors for the number 7 twine used in nets with
9.2-cm mesh size were Kmui = 0.909 and Kc =
0.930. Compressibility-elasticity factors were
lower for 9.2-cm-mesh-size nets constructed with
number 7 twine than for nets constructed with
number 9 twine. This indicated that thinner, more
elastic twine selected larger fish than did nets with
similar mesh size constructed of thicker twine. This
characteristic was reflected in a modal fish length
for 9.2-cm meshes that deviated upwards from the
linear trend of modal lengths observed in Figure
3. Differences between mesh size measurements
and manufacturer-specified mesh sizes were negligible; consequently, variance of mesh size (of)
was assumed to be zero.
The above parameters were used to estimate the
probability of retention predicted for each mesh
size according to Sechin's (1969) model. Figure 4
shows how these selectivity curves, based on girth
measurements, fit the frequency distributions of
fork lengths observed in the fishery.
Girth-length relationships in Figure 2 indicated
that maximum girth increased faster with length
than did head girth; consequently, the location of
the cumulative distribution functions for P{GC >
2m} and P{Gmax < 2m} in equations (1) and (2)
577
GILL-NET SELECTIVITY FOR SPANISH MACKEREL
^v 70:
! 65
K gear specific
' Number 7 twine
a K gear specific
Number 9 twine
— Constant K
Upper
MESH
SIZE (cm)
8.6
—SELECTIVITY
" FREQUENCY
Optimum
ac so
Lower
1 *H
3 50:
K 45 :
O
:
*• 40:
35
0
9
10
11
12
13
14
MESH SIZE (cm)
FIGURE 3.—Observed and expected modal lengths and
expected selection ranges for Spanish mackerel caught
in gill nets used in the south Florida fishery.
moved farther apart as mesh size increased. This
resulted in an increase in selection range as mesh
size increased (Figure 5); by extension, peak efficiency (highest probability of retention) also in40
50
60
70
creased (Figure 6) as the two cumulative distriFORK
LENGTH
(cm)
butions crossed each other at a higher cumulative
FIGURE 4.—Selection curves and observed Spanish
probability. These results indicated that two funmackerel size frequencies for mesh sizes of gill nets used
damental assumptions adopted for other indirect in south Florida.
methods that could be used to estimate Spanish
mackerel gill-net selectivity were violated. These
assumptions are that selection curves are of the have different allometric growth from females. For
same shape, and that the peak efficiency is equal this reason the data collected were not separated
for all mesh sizes.
by sex. The condition of similar allometric growth
Theoretical optimum selection lengths (length between sexes is further corroborated by the relthat corresponds to the point of peak efficiency) atively high correlations (0.84,0.92) found in girthestimated with mean /f-factors for 7.6- to 12.7- length relationships from data pooled by sex.
cm mesh sizes are represented by the solid line in
Length frequencies offish captured in each mesh
Figure 3. This line follows closely the increasing size (Figure 4) fell well within the probability of
trend in optimum selection lengths obtained with capture of most mesh sizes used in this study. Two
mesh-specific K-factors and, therefore, the theo- exceptions were a shift of length frequencies toretical line can be used to estimate similar lengths ward the left of the selection curve for nets with
for other mesh sizes not included in this study. 12.4-cm mesh size, and an increased number of
Selection ranges corresponding to each modal larger fish captured with an 8.6-cm mesh size. The
length were estimated and are also shown in Fig- former case may be attributed to a decrease in
ure 3. These ranges are defined as the size range abundance of fish larger than 60 cm fork length
between 0.10 probability of retention under each as the fish approach their asymptotic sizes (males,
tail of the respective probability distributions. Ex- 79.4 cm; females, 73.9 cm) and terminal age (7amination of the selection range for each mesh 9 years) (Fable et al. 1987). Similar findings with
size indicated that significant overlapping in the large-mesh selectivity curves were reported for
probability of retention at length existed among Atlantic herring Clupea harengus harengus by
contiguous mesh sizes.
Clarke and King (1986) and for Pacific herring C
h. pallasi by Kawamura (1972). The latter case
Discussion
may be attributed to exceptionally high abunAlthough male and female Spanish mackerel dance of large fish over a short period because
show sexual dimorphism in size at age (Fable et significantly larger than normal fish were landed
al. 1987), there is no apparent indication that males in Key West on January 8, 1988, by all boats.
578
EHRHARDT AND DIE
35
40
45
50
55
60
65
70
FORK LENGTH (cm)
FIGURE 5.—Comparison of retention probabilities for Spanish mackerel encountering each mesh size of gill net
used in the south Florida fishery.
Those large fish correspond to the right tail of the
length-frequency distribution of the 8.6-cm mesh.
An important caveat related to the probabilities
of capture from our study is that the probabilities
were generated from girth measurements of fish
captured under the biological conditions prevailing during the short winter fishery (DecemberFebruary). As gonadal tissue develops in preparation for summer spawning (Klima 1959), maximum retention girth is expected to increase significantly while opercular girth will grow more
slowly for fish of the same length. The immediate
effect of that change will be reflected in an increase
1.00 -,
0.95
° 9°
< 0.85 QU
0.00
8
9
10
11
12
13
14
MESH SIZE (cm)
FIGURE 6.—Peak (modal) efficiency for gill-net meshes
used in the south Florida Spanish mackerel fishery.
in the distance between the head girth-length and
maximum girth-length relationships. Such increase will lead to an increased number of smaller
fish retained at lengths below the left-hand limb
of the "winter" selection curve of any mesh size.
This seasonal change in retention mechanism may
be responsible for differences observed in the length
frequencies of Spanish mackerel landed in 8.9cm-mesh gill nets in Alabama during the summer,
compared to length frequencies of spent fish caught
with a similar mesh size in the Florida Keys in
winter (Figure 7). This implies that Spanish mackerel gill nets with similar mesh sizes should be
expected to retain fish at a smaller optimum length
and with decreased selection range and efficiency
in the summer than in the winter.
Trent et al. (1983) reported on the selectivity of
experimental Spanish mackerel gill nets operated
in a nontraditional fishery manner in an inshore
area of northwestern Florida. They used a regression model (Holt 1957) to estimate retention
probabilities from length-frequency distributions
of fish caught by several mesh sizes, and adopted
the assumptions of normal selection, constancy of
standard deviation, and equal catch efficiency
among mesh sizes. Although differences in fisheries, methodologies, and experimental designs
preclude a strict comparison of results from that
study and those reported here, we present some
general comparative information in Table 1. Observed mean retention lengths for a range of sim-
GILL-NET SELECTIVITY FOR SPANISH MACKEREL
FLORIDA
KEYS
30 35 40 45 50 55 60 65 70
FORK LENGTH (cm)
FIGURE 7.—Length frequencies of Spanish mackerel
captured during the summer in Alabama and during the
winter in the Florida Keys with gill nets of 8.9-cm mesh
size. Sources: National Marine Fisheries Service (data
for Florida Keys) and Alabama Department of Conservation and Natural Resources (data for Alabama).
ilar mesh sizes were about 6 cm higher in our
study than in Trent et al. (1983), whereas theoretical modal selection lengths estimated by Sechin's (1969) method are 1-5 cm higher than the
mean lengths calculated by the regression model.
Also significant are the differences observed in
theoretical SDs of mean retention lengths. The
579
methodology adopted by Trent et al. (1983) requires data from two contiguous mesh sizes to
calculate theoretical SD, and the result is an SD
of selectivity curves for two contiguous mesh sizes.
In contrast, selectivity curves estimated by Sechin's (1969) model are not normal, and the computation of an SD for the modal mean selection
length is made only to characterize the range of
the selection curve.
Trent et al. (1983) also found discrepancies between pooled length frequencies from statewide
commercial gill-net fisheries and length frequencies from their experimental nettings. They concluded that "commercial fisheries data appeared
to reflect mostly the sizes of the fish that were
abundant at the time of capture rather than the
effects of selectivity." However, such differences,
as well as the reported differences between their
observed mean retention lengths and our data
(Table 1), may be explained by spatiotemporal
changes in the biological condition of the fish. Indeed, the multimodal character of pooled lengthfrequency distributions observed in Spanish
mackerel commercial landings indicate that selection is not a direct function of length but a function of gradually increasing girth. This retention
mechanism makes retention girth exclusively dependent upon fish condition and mesh size. These
selectivity features account for the lack of acceptable results when the constant normal approximation (Holt 1957) to selection is used for Spanish
TABLE 1.—Mean retention length (length of retained fish) and its SD for Spanish mackerel caught in gill nets.
Theoretical SDs calculated by Trent and Pristas (1977) refer to contiguous mesh sizes among those tested.
Mean retention length (cm)
Observed
Theoretical
Stretchedmesh size
(cm)
Trent and Pristas
(1977)
6.3
33.4±4.9
30.8
7.0
34.5±4.7
33.9
7.6
36.0±4.8
37.0
8.2
8.6
8.9
9.2
9.5
10.2
10.8
11.4
12.1
12.4
12.7
38.1 ±4.9
40.1
This study
Trent and Pristas
(1977)
This study
±5.5
±7.6
±5.4
1
Number 9 twine.
39.7±5.0
42.2±4.9
44.5 ±4.2
45.7±4.3
47.4±7.9
44.6 ±9.1
49.1 ±7.4
b
Number 7 twine.
46.8 ±5.0*
46.0±3.5a
49.7±3.3b
49.9±3.3»
54.6±2.6a
±9.7
43.2
±5.0
46.3
43.0±3.2
43.8±3.2
48.1 ±3.4
47.5±3.5
60.0±4.3
580
EHRHARDT AND DIE
mackerel and it favors the girth-probability model
(Sechin 1969).
Acknowledgments
Funds for his study were provided by the Gulf
and South Atlantic Fisheries Development Foundation, Incorporated, Tampa, Florida. We thank
William W. Fox, Jr., University of Miami, and
Jerry Samson, Organized Fishermen of Florida,
for giving support to the idea for this work. We
also thank members of the fishing industry for
allowing us to collect all the data on which this
study was based, and for their willingness to assist
in this effort. We are especially indebted to Leo
Cooper, City Fish, Incorporated, Marathon, Florida, for his help and enthusiasm that contributed
importantly to the completion of this study. Finally, we thank Captains J. A. Bass, E. Cordova,
C. Carter, W. Carter, and all the personnel from
Gulf Seafood for sharing their invaluable fishing
experience with us.
References
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Received July 18, 1988
Accepted December 19. 1988