Gongora, M.

English

Aquatic Commons

824
  PRELIMINARY ANALYSIS OF GROWTH OF QUEEN
CONCH, STROMBUS GIGAS, IN SAN PEDRO, BELIZE.
 
                MAURO GONGORA
           Fisheries Department, Ministry of Agriculture and Fisheries
                     Belize City, Belize. 
 Current Address: Centro de Investigaciones de Quintana Roo
              Zona Ind. #2 Carretera Chetumal-Bacalar A.P. 424
                C.P. 77000 Chetumal, Quintana Roo, Mexico
ABSTRACT
Belize?s fishing industry is the fourth largest foreign exchange earner from
exported products. The queen conch fishery is third in economic importance. Queen
conch fisheries has been studied by Blakesly, 1977 and Gibson et al., 1982. During
the months of November and December 1992 a survey was carried to evaluate the
conditions of conch populations at Hol Chan Marine Reserve and Tres Cocos near
San Pedro Town, Belize.
In this study 2065 organisms were collected and their siphonal lenght were
measured. Siphonal lenght measurements fluctuated between 7 and 27cm, with a
mean of 14.23 cm at Tres Cocos and 14.59 cm at Hol Chan Marine Reserve.
Juvenile queen conch were found to be predominant at both sites and sparce
seagrass beds of primarily Thallasia testudinum, Siringodium filiforme and
Laurencia Sp. was the habitat type where juvenile queen conch were most
abundant. Lenght-frequency data were used to estimate growth parameters using
ELEFAN (K 0.42, 0.40; Loo 31.5, 30.5; to - 0.78, - 0.68 at Tres Cocos and Hol
Chan Marine Reserve respectively).
Key words: Belize, Fisheries, Growth, Populations, Queen Conch. 
  
INTRODUCTION
The queen conch, Strombus gigas, is the most important gastropod in the caribbean
region. Belize started commercial exploitation of the resource in the early 1960's.
Production reached its highest level in 1972 when over 562.5 tons of conch meat
was exported to the U.S markets. The high demand for marine products attracted
many people to start fishing not only for conch but also for the spiny lobster,
Panulirus argus, and scale fish. Belize?s queen conch population was being heavily
exploited without any regulations to control the fishery. Therefore, based on the
limited biological data available, the Government of Belize introduced its first
fisheries laws in 1977. The regulations prohibit scuba gear for conch fishing and set
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
825
a minimum size limit of 17.78 cm shell lenght, a minimum clean meat weight of
85.05 grams and a closed season which extends from July 1 to September 30
inclusive in any year (Fisheries Laws). 
To date queen conch investigations which have been carried out in Belize include
that of Howard L. Blakesly (1977) A contribution of the fisheries and biology of the
queen conch, strombus gigas L., in Belize and Janet Gibson (1982) The status of the
Conch Industry in Belize and Janet Gibson (1983) A summary of conch fisheries
in Belize. The average lenght of conch with a fully formed lip is 23.4 cm with a
market clean weight of 110 g. (Gibson et al, 1982). A conch size of 22cm was
found to be most economical in terms of maximum meat total weight (Blakesly,
1977). However, at this size conch may not be sexually mature. A large percentage
of immature conch can be legally harvested at the current size limit if lipless conch
are considered sexually immature (Gibson et al. 1982). Even though these two
papers showed that conch in Belize were being harvested at an immature stage, the
fisheries regulations of 1977 are still in effect today. This may be the major reason
why conch population drastically decreased from 1972 to 1985. The objective of
this study is to evaluate the condition of aggregations and to determine if there is
any similarities in population parameters in two areas which support both adult and
juvenile conch. Hol Chan Marine Reserve was formed in 1987 and is patrolled on
a daily basis. However, conch fishing is allowed in certain areas within the reserve.
Tres Cocos is a harvestable area and has been known to support very large
populations of juvenile conch for many years. Both of these areas are located
immediately at the north side of channels on the Belize Barrier Reef.
METHOD
Queen conch were collected from two sites near San Pedro Town. Tres
Cocos is located 3.2 km. north of San Pedro and Hol Chan Marine Reserve with a
total area of 8.0 square km.is located approximately 6.4 km. southeast of San Pedro
Town, Ambergris Caye (Figure 1). These sites were selected to conduct the study
because of their close proximity to the island as well as their relatively large
populations of juvenile and adult conch. Ten transects were conducted in each site
during the month of November and December 1992. Each transect consisted of a
150 m fiberglass tape baseline and 2 - 50 m lines perpendicular to the baseline. One
50 m line was placed on each side of the baseline and were on the 50 and 100
meters points. All conch found within a 2m swath on each side of the lines were
counted and measured. Shell lenght was calculated as the distance from the apex
to the end of the siphonal canal. In addition to measuring shell lenght, divers also
recorded bottom coverage of the transect which provided valuable data on conch
distribution and bottom habitat. Aluminum calipers were used to measure the conch
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826
to the nearest millimeter.   
Figure 1.
Study
areas where sampling was carried out.
Having shell lenght data grouped into lenght-frequency, size structure of
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
827
the populations was determined. Data obtained was processed to estimate growth
rate by ELEFAN method (Gayanilo et al, 1988). Once age structured was estimated,
cohort structure was determined. Natural mortality rates were estimated from the
catch curve (Fig. 2 and Fig. 3).
Figu
re 2. Catch Curve for Tres Cocos, Belize.
RESULTS
Table 1 shows abundance of organims in the two areas sampled. Because
conch data did not fulfill ANOVA basic requirements, especially on normal
distribution of residues and therefore original data are presented. A total of 2065
queen conch, Strombus gigas, were counted and measured. Out of this total only 24
or 2.4% and 94 or 9.91 % in Tres Cocos and Hol Chan Marine Reserve
respectively, were considered adults. Adult conch were classified as having a fully
developed flared lip which is > 4mm in thickness. The mean lengths of year class
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
828
conch is shown in Table 2 and Table 3. An average of 5.1% of total conch
measured were considered adults.
Figure 3. Catch curve for Hol Chan Marine Reserve
Sparse grasses composed primarily of turtle grass, Thallassia  testudinum,
Syringodium filiforme and Laurencia Sp. was the most abundant habitat type
encountered and constituted 68 % in Tres Cocos and 40.0 % in the Hol Chan
Marine Reserve with an average of 53.75 % of total area covered (Table 5). The
highest density 0.85 conch/m2 was found in sparse grass at a depth of 3 - 5 m. The
lowest density was found associated to areas with a high percentage of coral rubble,
usually over sand or rock and dense turtle grass. The survey showed that conch
were mostly found in the back reef and lagoon area. Transects were not carried out
on the shallow forereef because no conch were observed while snorkeling on this
area. The conch population surveyed was found on the north side of channels or
breaks on the barrier reef. High juvenile conch recruitment on north sides of
channels has also been observed on other areas in Belize. Juvenile conch were most
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829
abundant at approximately 450-600 m north of the channels and 250-300 m
westward from the reefcrest which suggests a high inflow of pre-metamorphosed
larvae through these channels.
Table 1. Strombus gigas abundance (D =org/m2) in Tres cocos and Hol  Chan
marine reserve.
Tres Cocos, Belize: Average density 0.2059 conch/m²
T1 T2 T3 T4 T5 T6 T7 T8 T9 T10
# 339 218 291 29 36 3 150 13 6 21
area 400 1000 400 1000 1000 1000 1000 1000 1000 1000
Den. 0.85 0.22 0.73 0.03 0.04 0.003 0.15 0.01 0.006 0.02
Hol Chan Marine Reserve, Belize: Average density 0.101 conch/m²
T1 T2 T3 T4 T5 T6 T7 T8 T9 T10
# 84 6 36 23 58 175 127 136 139 175
area 800 1000 1000 1000 1000 1000 1000 1000 1000 1000
Den. 0.11 0.01 0.04 0.02 0.06 0.18 0.13 0.14 0.14 0.18
Lenght-frequency analysis (Figure 4) showed that the juvenile conch
population in the Hol Chan Marine Reserve is composed of two distinct age classes
and the adult class was faintly represented (Table 3). The population at Tres Cocos
is composed of only two cohorts (Table 4). Juvenile conch in the first year class had
a mean lenght of 16.63 cm and 15.05 cm in Tres Cocos and Hol Chan Marine
Reserve, respectively. Using age structure data obtained (Fig.3), number of
organisms in each age class was analyzed and mortality rates were estimated and
are therefore used to provide a preliminary description of condition of conch
resource in San Pedro, Belize. Using ELEFAN program values for von Bertalanffy
prameters (Table 5) shows no significant difference between the two sites. The
natural mortality in Tres Cocos was slightly higher than in Hol Chan Marine
Reserve. The fishing mortality was almost double than in the reserve. The
exploitation rates however, show that both sites are being overexploited.
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
830
DISCUSSION
From results obtained it is clear that queen conch densities in Tres Cocos
(0.2059 conch/m2) and Hol Chan marine Reserve (0.101 conch/m2) are similar to
those reported for the region. In Venezuela, conch density in protected area was 0.4
conch/m2 (Weil and Laughlin, 1984) and in Florida was 0.005 conch/m2 (Stoner
and Waite, 1990) and in southern Quintana Roo, Mexico density was 0.015
conch/m2 (de Jesus et al, 1992). Siphonal lenght fluctuated between 7 and 27 cm.
The survey 
Table 2: Set of lenght-at-(relative)age data, pertaining to queen conch in Tres
Cocos, Belize.
Age group 
(relative age, in years)
Mean length (cm) Number
I 16.63 1100
II 21.75 4
III 25.10 0
IV 27.30 0
Table 3: Set of lenght-at-(relative)age data, pertaining to queen conch in Hol Chan
Marine Reserve, Belize.
Age group 
(relative age, in years)
Mean length (cm) Number
I 15.05 781
II 20.19 170
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
831
III 23.62 7
IV 25.91 1
showed that the majority of the organisms measured (94.9%) were in their juvenile
stage. This is no surprise, in Belize several areas such as Half Moon Caye in
Lighthouse Atoll and in Turneffe Atoll have been identified with large populations
of juvenile conch and may be an important nursery area which is subject to
exploitation by Belizean and illegal fishermen from our neighbouring countries of
Guatemala and Honduras. Tres Cocos and Hol Chan Marine Reserve support large
populations of juvenile conch. Due to their close proximity to San Pedro and
shallow depths much of first and second year class organisms are caught before
they reach sexual maturity. This may a reason for exploitation as shown in the
results obtained from ELEFAN. 
Growth parameters obtained by ELEFAN (Table 5) does not show any
significant differences between the two areas and are within the range reported by
other authors (Dominguez-Vivieros et al,1992). Total mortality was greater in Tres
Cocos (2.430) than in Hol Chan Marine Reserve (1.380) as was expected beacuse
there was a greater number of juvenile conch in that area and therefore more
predation. Fishing mortality was also greater in Tres Cocos. Both areas show slight
over exploitation. The adult class in the Hol Chan Marine Reserve is faintly
represented. It was expected that a larger portion of the population would be in their
adult stage. Adult conch or those that are becoming sexually mature in the
non-fishing area (1.6 Km2) can easily migrate to the fishing area for food, mating
or to lay eggmasses because of its relatively small area. Conch fishermen will catch
conch on the margin which delineate the fishing from the non-fishing area. The high
natural mortality in the Tres Cocos area is as a result of the large number of juvenile
conch in this area. Queen conch predators such hermit crab, Petrochirus diogenes
and tulip shell, Faciolaria tulipa were frequently observed while conducting the
transects. The high exploitation rates estimated by ELEFAN is as a result of very
little adults and a high abundance of juveniles present in both areas.
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
832
Fig
ure
4.
Le
ngt
h-Frequency of queen conch, Strombus gigas, in Tres Cocos and Hol Chan 
Marine Reserve, Belize                     
RECOMMENDATIONS
Since the majority of queen conch in Tres Cocos and Hol Chan Marine
Reserve are juveniles and the adult population is very small it is important to
protect the organisms until they become sexually mature. To do this it will be
necessary to implement changes in the management of these areas. Hol Chan
Marine Reserve should increase size of non-fishing area. No new fishermen should
be issued licenses to conduct conch fishing activities in the fishing area of the
reserve. Implement a quota system where fishermen are allowed to harvest a limited
number of conch per season. Conduct annual evaluation of the resource and if
exploitation rates continue to rise, conch fishing should be halted for a period not
less than 2 to 3 years. The Tres Cocos area should be reserved and protected as a
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
833
nursery area.
LITERATURE CITED
Blakesley, H. L. 1977. A Contribution to the Fisheries and Biology of the of the
queen conch,strombus gigas, in Belize. (Abst.) Am. Fish. Soc. 107 ann.
mtg. Vancouver, Sept.15-17.
Dominguez-Viveros, M., E. Sosa-Cordero y A. Medina-Quej. 1992. Abundancia
y ParametrosPoblacionales del caracol, Strombus gigas, en Banco
Chinchorro, Q. Roo, Mexico. Proc.GCFI 45Th. Meeting.
Gayanilo, F.C. Jr., M. Soriano and D. Pauly. 1988. A draft guide to the compleat
ELEFAN.ICLARM Contribution 435:65pp
Gibson, J., S. Strasdine and K. Gonzales, 1982. The status of the conch industry of
Belize.Proc. GCFI 35Th. Meeting. 
Stoner, A.W. and J. M. 1990. Distribution and behaviour of queen conch, Strombus
gigas,relative to seagrass standing crop. Fish Bull. U.S. 88:573-585.
Weil, L. and R. Laughlin. 1984. Biology, Population Dynamics and Reproduction
of the queenconch Strombus gigas L. in the Archipiélago de Los Roques
National Park. J. of shellfishRes. 4(1):45-62 
 
Table 4.  Length - Frequency Distribution of queen conch, Strombus gigas, in Tres
Cocos and Hol  Chan Marine Reserve.
Proceedings of the 47th Gulf and Caribbean Fisheries Institute
834
Shell length (cm) Tres Cococ Frequency Hol Chan Marine
Reserve Frequency
7 0 1
9 111 58
11 355 377
13 99 86
15 179 124
17 292 226
19 63 99
21 3 1
23 0 6
25 0 0
27 0 1
Table5. Population parameters for Queen conch in Hol Cham Marine Reserve and
Tres Cocos, Belize.     SL= starting length sample    
Area L4 K to C W.P. SL RN
Tres
Cocos
31.5 0.42 0.78 0.00 0.00 16.0 0.446
Hol
Chan
30.5 0.40 0.68 0.00 0.00 11.0 0.285
                               


A Contribution to the Fisheries and Biology of the of the queen conch,strombus gigas,

A draft guide to the compleat ELEFAN.ICLARM Contribution 435:65pp

Biology, Population Dynamics and Reproduction of the queenconch Strombus gigas L. in the Archipiélago de Los Roques National Park.

Distribution and behaviour of queen conch, Strombus gigas,relative to seagrass standing crop.

Preliminary analysis of growth of queen conch, Strombus gigas, in San Pedro, Belize

http://aquaticcommons.org/14300/1/gcfi_47-51.pdf

Preliminary analysis of growth of queen conch, Strombus gigas, in San Pedro, Belize

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