Johnson, A.G.

Koenig, C.C.

English

Aquatic Commons

906
AGE AND SIZE STRUCTURE OF THE FISHERY AND
JUVENILE ABUNDANCE OF GAG (MYCTEROPERCA
MICROLEPIS), 
FROM THE NORTHEASTERN GULF OF MEXICO.
ALLYN G. JOHNSON AND CHRISTOPHER C. KOENIG
National Marine Fisheries Service
Southeast Fisheries Science Center
Panama City Laboratory
3500 Delwood Beach Road
Panama City, FL  32408, U.S.A.
ABSTRACT
The age and size structure, year class abundance in the fishery and juvenile
abundance in the seagrass habitat of gag, Mycteroperca microlepis, in the
northeastern Gulf of Mexico were examined from 1989 to 1994.  We examined
growth marks on otoliths from 1,539 gag (55% commercial and 45% recreational
fishery); age ranged from 1 to 26 years for fish 332 to 1,290 mm total length (mm
TL).  Von Bertalanffy growth equation parameters for 1991-1994 were Lì
(maximum attainable length) = 1261.0 mm TL, K (growth coefficient) = 0.139, and
to (hypothetical age at zero length) = -1.3143. Juvenile gag absolute abundance was
estimated in seagrass habitat of St. George Sound, Florida by using a 5 meter otter
trawl with known sampling efficiency.  Abundance estimates (number per hectare)
for the years 1989-1994 were 388 in 1989, 3.9 in 1990, 460 in 1991, 180 in 1992,
1,035 in 1993,and 11 in 1994.  The alternating year pattern of high-low abundance
was also evident in the age-structure of the fishery (1989, 1987, 1985 were high and
1988, 1986, 1984 were low), and suggests that juvenile abundance estimates may
be useful to forecast future fishery production.
Key Words:  Abundance, Age, Fishery, Gag, Juvenile
INTRODUCTION
The gag (Mycteroperca microlepis: Serranidae) is a reef-dwelling grouper
that inhabits temperate to tropical waters of the western Atlantic Ocean and Gulf
of Mexico.  It is a protogynous hermaphrodite (McErlean and Smith, 1964) that
occurs from Massachusetts, U.S.A., to Rio de Janeiro, Brazil (Briggs, 1958).
McErlean (1963) indicated that the center of abundance in the Gulf of Mexico was
the reef environment of the west coast of Florida. Gag are common along shallow
reef tracts as well as shelf break areas in depths of only a few meters to at least 150
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
907
m (Bullock and Smith, 1991).  Total fishery landings of gag in the United States are
between 1.5 and 3 million kg per year (Schirripa and Goodyear, 1994; J. Bennett,
NMFS, Miami, FL, pers. comm.). Peak spawning of gag occurs in February and
March (Collins et al., 1987; Gilmore and Jones, 1992; Hood and Schlieder, 1992),
and pelagic juveniles settle in high-salinity estuaries during April and May
(McErlean, 1963; Keener et al., 1988; Mullaney, 1991).  After 5 to 6 months of
rapid growth, juvenile gag leave the estuarine habitat in early fall and migrate to
shallow offshore reef habitats (Keener et al., 1988; Mullaney, 1991).  Mean juvenile
settlement dates in St. George Sound, Florida, were May 7, April 27, and May 8,
for 1989, 1991, and 1992, respectively.  The mean  size at estuarine egress in 1991
was 145 mm standard length (SL) and ranged from 130 to 160 mm SL (Koenig,
unpublished data). Shallow estuarine seagrass beds appear to be an important
habitat of early juvenile gag along the gulf coast of Florida (McErlean, 1963).
Juvenile settlement is synchronized with the spring increase in primary and
secondary productivity in the northwest Florida seagrass habitat (Koenig,
unpublished data).  On the gulf coast of Florida there is extensive potential seagrass
nursery habitat; 3,000 km2 occur in the Big Bend area alone (Zieman and Zieman,
1989). Age and growth information on Gulf of Mexico gag collected from the
fishery indicate that gag are a long-lived (26 years) and grow rapidly during their
first 5 years of life (McErlean 1963, Hood and Schlieder 1992, and Johnson et al.
1993). We present here the results of two independent studies on the life history of
gag off northwest Florida.  These studies are: (1) age- size structure and relative
abundance of year classes in the fishery of gag in the commercial and recreational
fisheries, and (2) juvenile abundance from a seagrass nursery habitat in St. George
Sound, FL.  These studies were conducted independently over a five year period
(1989-1994).
MATERIALS AND METHODS
Two independent investigations on gag are integrated in this report, thus
the materials and methods are present in two sections.
 Age and size structure and abundance in the fishery The gag fisheries
(commercial and recreational) at Panama City, Florida, were randomly sampled
(sampled without regard to month, fish size, etc.) from February 1991 to August
1994.  These fisheries are hook and line fisheries that operate in the panhandle area
of Florida.  The commercial fishery may exceed the geographic range of the
recreational fishery, but it is confined to the northeast sector of the gulf. Of 1,608
fish that were sampled 1,539 were used for the determination of age structure in the
fishery.  These 1,539 (95.7% of the total) were used because two independent
annuli counts of sectioned and unsectioned otoliths were in agreement.  Forty-five
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
908
percent of these specimens were collected from the recreational fishery and 55%
from the commercial.  Processing of otoliths (whole and sections), back-calculation
of total length (TL) at mark deposition, and statistical analyses followed the
procedures of Johnson et al. (1993).  We back-calculated TL at age using the final
annulus present at time of capture (following the recommendations of Vaughan and
Burton 1994) using the direct proportional method of Lea (1910).  The year class
membership was determined using the following procedure: if the month of
collection was January to July, then age is the number of completed opaque marks
plus 1, but if the month of collection was July to December then age is the number
of completed opaque marks.
Juvenile Abundance in s Seagrass Habitat
Estimates of absolute abundance of juvenile gag in seagrass habitat of
eastern St. George Sound, Florida, were made annually from 1989 to 1994.
Methods of determining juvenile gag abundance were given in Koenig and Colin
(in press).  In general, the methods involved applying the Jolly-Seber mark-
recapture method within each of three square sampling stations (each 150 meters
on a side) located in seagrass beds totalling 15.4 km2 in area.  Samples were taken
by standard tows (150 meter, 1.8 km/hr) tows with a 5-meter otter trawl.  Koenig
and Colin (in press) provided estimates of absolute abundance (numbers per unit
area) which allowed the estimation of capture efficiency and the effects of habitat
condition and fish size on capture efficiency.  The appropriate capture efficiency
estimates were then applied to spatially random standard tows made annually over
the seagrass reference area of St. George Sound.  We assume that this seagrass
reference area is representative of juvenile gag abundance over a broad area of
coastal north Florida from St. Marks to St. Andrew Bay and that juvenile gag
recruit to the fishery located offshore in the same geographical area.
RESULTS AND DISCUSSION
The Fishery
Analysis of the timing of otolith margin deposition indicated that the
opaque margin was deposited between January and August; the greatest proportion
of fish with an opaque margin occurred in May (Fig. 1).  This information is in
agreement with the analysis of gag otolith bands by Hood and Schlieder (1992).
This in combination with the tetracycline age-validation work of Collins et al.
(1987) indicates that the opaque mark is annually deposited during spring and
summer in the northeastern Gulf of Mexico. The deposition of the marks on the
otolith with regard to distance of distal edge of the mark from the primordium was
similar to the mark distribution reported by Hood and Schlieder (1992).  Our age
1 to age 6 mark distance ranges and means in mm were: mark 1 range = 1.1-2.4 and
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
909
mean = 1.7, mark 2 range = 1.6-3.9 and mean = 2.3, mark 3 range = 2.0-4.5 and
mean = 2.8, mark 4 range = 2.3-4.8 and mean = 3.2, mark 5 range = 2.7-4.6 and
mean = 3.5, and mark 6 range = 2.9-4.8 and mean = 3.9. Von Bertalanffy growth
equations were developed using the last back-calculated TL at age data pooled for
1991-1994 (Table 1).  The equation indicates that TL at age of gag is larger (20-60
mm) than the equations of Hood and Schlieder (1992).  Both equations indicate that
gag are larger at age than the equation for 1979-1980 northwest Florida gag
(Johnson et al. 1993).
Fi
g
ur
e
1. Gag otolith margin appearance by month from the northeastern Gulf of
Mexico, 1991-1994. 
These differences indicate that age, size and growth relationships should
be determined on a regional basis and periodically revised to obtain currently
accurate growth parameters. The age composition of the gag fishery during 1991-
1994 shows that the year classes present contributed differently to the fishery
(Table 2), and these differing contributions can be followed from year to year.  For
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
910
example, 1989 produced a strong year class that contributed 2.43% of the 1991
sample, 23.67% of the 1992 sample, 52.46% of 1993 sample, and 49.81% of 1994
sample, while the 1988  year class was poor and contributed 0.81% of 1991 sample,
7.73% of 1992 sample, 7.60% of 1993 sample, and 11.62% of 1994 sample.  This
alternating year pattern of strong and weak year classes is evident in all four years
of sampling the fishery.  The high productionyears were (odd numbered) 1989,
1987, and 1985 and the lowproduction years were (even numbered) 1988, 1986, and
1984. The minimum legal size for gag in the fishery is 20 inches TL (508mm TL)
(Gulf of Mexico Fishery Management Council 1994).  Usingback-calculations of
TL at age for the last annulus we estimatethat 31.5% of age 2, 79.1% of age 3,
97.2% of age 4, and 98 to 100%of the fish age 5 and older have obtained the legal
size of 20inches TL.  Based on our four years of data, the mean cumulative
exploitation of age 5 gag is 59%, age 6 is 81% and age 7 is 90%.   Thus, by the age
of seven only about 10% of the fish in a cohort remain.
Juvenile Abundance
The annual abundance of juvenile gag in seagrass beds in St. George
Sound was high in odd years and low in even years (Table 3). This same pattern
appears to persist in the fishery age structure back to 1985 (Table 2).  The causes
of this pattern of alternating high years of juvenile abundance are unknown. In this
paper we assume that annual abundance determinations made in St. George Sound
are indicative of juvenile gag abundances throughout the panhandle area of Florida
from St. Marks to St. Andrew Bay.  There is some evidence to support this
assumption.  In 1993, when juvenile gag abundance in St. George Sound was high,
there were reports of high juvenile abundance in St. Joseph Bay (Michael Beck,
Department of Biological Sciences, Florida State University, pers. comm.) and in
St. Andrew Bay (William Fable, NMFS, Panama City, pers. comm.).  We also
assume that high levels of juvenile abundance in seagrass result in high regional
catch levels several years later.  That is, we assume that adult gag do not, for the
most part, migrate great distances and become part of fisheries distant from the
nursery areas.  Evidence to support this assumption, at least for late juveniles,
comes from fishermen who report large numbers of late juveniles offshore
following egress of a large year class. The two patterns of alternating years of high-
low abundance (fishery and juvenile) are summarized in Table 4 where the year
class contributions to the fishery are pooled over the four years of sampling.
Because the various cohorts have had different histories of exposure to the fishery,
the fishery abundance values are only a relative estimate of abundance.  The
comparison of the alternating year high-low fishery and juvenile abundance patterns
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
911
s u g g e s t s
that the
f i s h e r y
w i l l
experience
s t r o n g
recruitmen
t from the
1991 year
c l a s s
beginning
in 1995,
and again
from the
1993 year
c l a s s
beginning
in 1997.
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
912
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
913


Age and size structure of the fishery and juvenile abundance of gag (Mycteroperca microlepis), from the northeastern Gulf of Mexico

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Age and size structure of the fishery and juvenile abundance of gag (Mycteroperca microlepis), from the northeastern Gulf of Mexico

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