1. Introduction
With the
increasingly threat from anthropogenic disturbances to marine environments,
biodiversity is eroding at alarming rates, even faster than new species are
being discovered [1,2]. Reduction of species may
negatively affect human communities economically and socially by causing
irreversible knock-on effects toward degradation of habitats harboring
economically and ecologically important species. Unbalances caused by erosion
of biodiversity may come about from breaking interactions among species. Such
effects have mostly unknown consequences, but are generally agreed to be
negative [3-7]. Efforts to protect one species,
although less preferred over protecting assemblages, may be justified more
strongly when this species is key in regulating an ecosystem, such as with top
predators. Dusky groupers (Micteroperca marginata) may be fall under such
group, as its presence contributes to the maintenance and viability of
ecosystems [8,9].
Dusky groupers in Brazil are widely distributed throughout its southeastern
and southern coast [10,11]. This species is
sedentary, have a high site fidelity, and a low growth rate [12-15]. Dusky groupers are listed as threatened by
the IUCN, especially due to overfishing and habitat degradation [16]. In Brazil, even though listed as over-exploited
by regulating authorities, there is little more than length limits attempting to
manage its fishery [17,18]. Lack of effective
regulation, associated with its slow growth, late maturation, and behavioral
characteristics, is contributing to the disappearance of this species along the
coast of Brazil. Measures toward its protection and recovery are of immediate
need.
Using
captive-reared fish for enhancing stocks and repopulating degraded habitats has
met with varying degrees of success over the years. Much skepticism existed in
the past as to its efficiency in stock enhancement [19-22],
but recent advances in mariculture and monitoring techniques have revived its
adoption in recovery programs [23,24]. Such advances
have contributed to recovery of invertebrates [25-28]
and fish [29,30], especially sedentary fish with
high habitat fidelity, as for dusky groupers.
The objective of this study was to investigate the viability of using
captive-reared dusky groupers in population recovery of reef habitats.
Mariculture of dusky groupers may still be cost prohibitive for commerical
applications, but can be a viable alternative for conservation and management
of this species. In a world where overfishing has gone unabated, tools to
mitigate its effect need to be as diverse as the environments we are seeking to
preserve. The opportunity to revert overfishing trends cannot rest solely on
regulations, especially for slow growing species with a late maturation, as for
dusky groupers.
2.
Materials
and Methods
Released fish from this study were produced in captivity from eggs to
juveniles. Eggs were produced from wild dusky groupers induced to maturity with
cholesterol and GnRHa [31] at 30 mg/kg. After
fertilization, eggs were put in incubators at a 1g/l density until hatching. Larvae
were fed rotifers for approximately 15 days post-hatching, followed by artemia
for another 45 days. After the 60 days, 60 juvenile fish were selected and placed
in net pens for growout. Growout fish were fed a fish diet daily to satiation. The
growout phase lasted for 10 months, when juveniles reached 20 cm total length
on average.
Twenty-five fish
survived through the growout phase and used for the release phase. All
twenty-five fish were implanted with an acoustic tag of 9 mm diameter (Vemco V9).
Implants were conducted by an approximately 1 cm long abdominal incision. Fish
were observed for mortality for two days after implants. After observation,
fish were transported in oxygenated tanks to the release site (Figure 1). The release site
was characterized by a rocky substrate of depths less than 10 m. The release
site was chosen based on prefered habitat for dusky groupers at the juvenile
phase [9,12]. All released fish were tracked by acoustic receivers (Vemco VR2W) for
7 month post-release.
Stations 1, 2 and 3 were defined for receiver installation. Station 1
was at the southern most, Station 2 in the middle, and Station 3 at the
northern most location within the study site (Figure
1). One acoustic receiver was placed at each station, separated from each
other by 300 m, the reach of receiver detection, at depths varying between 6
and 9 m. Five tagged groupers were released at Station 1, 15 at Station 2, and
five at Station 3. Fish acoustic signals were downloaded from receivers at the
end of the experiment and stored in a database for analyses. Telemetry analyses
consisted of signal counts per day to detect signal appearance/disappearance
patterns and frequency of signal disappearance for tagged fish. Signal
disappearance from the detection dataset was interpreted as mortality.
As an alternative release experiment, larval fish were released to
observe their behavior and estimate viability for stock recovery programs.
Larval dusky grouper were placed in oxygenated plastic bags and taken to the
release site. Experiment were conducted over two days. The release site
comprised of reef habitats approximately 6 m deep. Larval release was conducted
at 3 m depth to afford an opportunity for fish to move either to the surface or
the substrate. Three bags with approximately 100 larvae each and 3 with
approximately 500 larvae each were opened at the release site. This same
operation was repeated the next day at a similar site in a different location.
After release, larvae were followed by divers for one minute and observed as to
their average movement direction.
3.
Results
and Discussion
Technological improvements in telemetry allow greater numbers of fish to
be tracked at a reduced cost. Information as to home-range, habitat preference,
inter- intra-specific interactions, migratory patterns, and distribution are
but a few examples where telemetry excels at providing knowledge on fish and fisheries
ecology. This knowledge, in turn, may prove essential for conservation of
ecosystems and management of commercially important species [32-35].
Telemetry data
suggested that groupers produced in captivity acclimated to reef habitats to varied
degrees. There was higher mortality, as indicated by disappearance of acoustic
signal, at Station 2, where 15 individuals were released. Five individuals
survived to the end of the 7-month experimental period at Station 2. At Station
1, only one individual remained through the experimental period (Figure 2).
At Station 1,
however, there were only 2 individuals detected from the start of the
experiment. This may be due to either failure of transmitters in the other 3
individuals released, or because those same 3 individuals immediately left the
receiver’s area of detection. At Station 3, there was no loss of signal from
any of the individuals released, indicating complete acclimation.
Stations 1-3,
although of similar environments, were located at least 300 m apart from one
another. Moreover, the orientation of the shoreline from the island where
releases were performed were different among the stations. Station 3 faced
north, while Station 1 and 2 faced east. The difference in orientation could
partly explain the different degrees in acclimation, due possibly to slight
microhabitat differences [9,36,37]. Most
probably, the differences were due to the numbers released. Groupers are known
territorial species [12], which could explain
the higher mortalities at Station 2.
Data in this study
also indicated the reappearance of acoustic signals after a period of
disappearance of the signal. This may be due to the typical behavior of
groupers when seeking shelter [9,38]. Groupers
may spend the hours of daytime hiding in holes and crevices of their habitat.
When in shelters, the acoustic signal may be weak or non-existent, due to
interference of the rocky hard substrate. Active seeking of shelter is one more
indication of the high degree of adaptability and acclimation of groupers
raised in captivity to their natural environment. This calls for telemetry
studies, such as this one, to extend in duration for at least a few days, to be
able determination of degree of fish acclimation to the new habitat.
For habitats
similar to the experimental study from this work, an optimal number to release
groupers for population recovery centers around 5 juvenile individuals per 600
m shoreline. Certainly, the release of 15 individuals was excessive and
counter-productive. This result should be considered in programs aiming at
recovery and enhancement of grouper stocks in habitats where overfishing or
degradation has taken place. The result of this study does not only have
managerial applications, but also economic implications. The cost of producing
groupers in captivity is still prohibitive for commercial undertakings, but may
be justified in marine conservation. If, however, an excessive number of
juvenile groupers is released, conservation programs may be deemed ineffective,
their success never being realized. Results from studies such as this may avoid
premature conclusions being drawn against potentially effective conservation tools.
Data on release of
captive-reared groupers in their larval stages showed the expected behavior for
fish during this life-stage. All larvae were as active during release as they
were when collected, indicating little or no stress during the trip from the
aquaculture facility to the release site. Without exception, larvae were
positively phototactic, without much dispersion during release, indicating a
strong pattern to surface migration during this life-stage. Due to the high
probability of predation, using larval groupers in repopulation programs may
only be effective when large numbers of larvae are employed. Using larvae in fish release programs has a cost
advantage over releasing juveniles. Employing larvae also allows for a much
higher number of released individuals. The transition from larvae to juvenile
in captivity is associated with a high mortality, sometimes close to 100
percent. This could be due to the techniques used in laboratory. If this is the
case, releasing larvae has yet the additional advantage of keeping
metamorphosis-related mortality to a minimum. The downside of releasing larvae,
however, is the as of yet rudimentary means to track released fish, which make
estimating this alternative weak at best for stock recovery. It is, however,
encouraging that all observed larvae did behave as expected, demonstrating
immediate acclimation to the natural environment.
In conclusion,
grouper aquaculture may still fall short of economic viability for commercial
ventures. Groupers raised for market need to reach a size that may not come
about before two or three years from larvae. Also, during metamorphosis from
larvae to juvenile, high mortality, sometimes, total, may be observed [39,40]. Alternatively, grouper raised for stock
replenishment may be viable, if the right life-stage and numbers are employed
during release. This study demonstrated that groupers raised in captivity are
highly adaptable and readily acclimate to reef and rocky environments. The
foundation has been laid. The next step to build on that foundation is to
undertake studies to determine the quantities of fish to be released and at
what life-stage. The results of such studies will depend on the environment
being studied and on the budget for conservation projects. According to the
findings herein, however, the optimal numbers and most adequate life-stage seem
to approximate 5 juveniles (20 cm) groupers per 600 m shoreline of rocky reefs.
research article 
