Population
Dynamics of Coral Reef Fishes
and
the Relative Abundance of their Early Life History Stage –
an
example from French Polynesia
V. Dufour[1]
Coral reef fish have a life cycle with a pelagic larval stage ending
with the colonisation of the reef and recruitment to the resident
population. Like most other marine
species, the egg production of coral reef fishes is several orders of magnitude
higher than the number of adult and juvenile fish living in the reef. The abundance of coral reef fish is thus
very high during this part of their life cycle but declines after colonisation. Therefore, if reef fish larvae could be
harvested before settlement, their abundance would probably allow sustainable
farming techniques and the juvenile and adult coral reef populations would be
preserved.
Introduction
The life
cycle of coral reef fish represents two different phases, the larval pelagic
stage and the juvenile and adult reef-dwelling stage. About 70 % of reef fish species have pelagic eggs hatching in the
open sea and the remaining percentage have demersal eggs but the larvae are
also pelagic (Leis, 1991). Compared to
the adults, conspecific reef fish larvae are considered as a different
ecospecies (Leis, 1983). This reflects
the differences between the larvae and the adult of coral reef fishes in terms
of morphology, physiology, behaviour, and environmental requirements.
At the end of the larval stage, most
species undergo a rapid and important metamorphosis to a colourful
juvenile. The metamorphosis is
conditioned by the presence of the reef.
The pelagic reef fish larvae metamorphose possibly because these cannot
survive in the open ocean at the end of their larval stage (Leis, 1991). The pelagic larval duration, back-calculated
with daily otolith increments ranges from a few weeks for small damselfish
(Wellington and Victor 1987) to several months for acanthurids (Brothers et
al., 1983).
Reef fish larvae are not abundant in coral
reef waters even in enclosed lagoons, where fish have to spawn in the reef
environment. The ones present and
abundant in lagoons are limited to a few families such as pomacentrids, eleotrids,
and gobiids (Leis, 1994; Leis et al., in press). These data show that coral reef ecosystems
are not a suitable habitat for coral reef fish larvae.
Like other marine organisms,
coral reef fishes spawn a very large number of eggs. Some species spawn more than 100,000 eggs/year (Sale, 1980). The larvae resulting from the spawn is low
but it is assumed that, on average, one fish will survive after one
reproductive cycle in order to maintain a steady abundance of reef fishes. The mortality rate of coral reef fishes is
very high during their early life history and sharply declines after the
colonisation of the reef.
It is almost impossible to measure the
mortality rate from spawning to settlement because of the dispersive nature of
the pelagic environment but an attempt can be undertaken. We have quantified the larval flux during
the colonisation of some coral reef lagoons, which allows the estimation of the
number of fish prior to settlement, and hence the mortality rate from the end
of the larval stage to the adult stage.
This information, i.e., abundance of fish larvae at colonisation
and subsequent mortality rate, permit the assessment of the dynamics of the
population of coral reef fishes and its resilience to natural harvest for fish
fry production. In this paper, I will
present a finding on the impact of harvesting coral reef fish after
colonisation and its consequences on the population dynamics. I will also present some data on larval flux
at colonisation from studies made in French Polynesia and the potential of
coral reef fish larvae for aquaculture.
French Polynesia represents an area of 5.5
x 106 km2 in the central south Pacific, comprising 120
islands from 135° to 155° W and from 8° to 28° S. The data presented here come
from Moorea Island (17°30'S and 149°50' W).
This island is surrounded by a barrier reef, enclosing a lagoon 800 to
1,300 m wide. Water in the lagoon is
exchanged with oceanic waters through channels that intersect the reef crest;
breaking waves create a unidirectional flow of water over the reef crest into
the lagoon through the channels. In
addition, the structure of the reef and the average low tides (0.2 m) at Moorea
Island result in the reef crest being only a few centimetres below the mean sea
level, allowing in-flow of water into the lagoon.
Estimates of the larval flux during
colonisation was based on 2 two sampling blocks. In 1995, we carried-out a sampling program, representing on
average 3 nets/night for two months.
The fish larvae collected in our samples were sorted into two size
classes: large fish larvae, representing individuals above 35 mm; and small
fish larvae, corresponding to individuals below 35 mm. From February to May 1996, fish larvae were
collected using nets over the reef crest.
These nets were designed as stationary devices that filter the water
flow coming from the breaking waves.
Different kinds of net were used since they were regularly damaged due
to strong wave action. The nets were
usually left overnight for sampling and fish larvae were removed every
morning. One to five nets were deployed
around the island for each sampling period.
Impact of
natural harvest of fish fry on coral reef fish population
To calculate for the number of fish after
1 year, the model used for calculating the decreasing number of a coral reef
fish population was made following the equation of mortality known as the
exponential decay model (see Sparre et al. 1989)
Nt = Ntr exp. (-Z
[t-tr])
where N is the number of fish, t: time,
tr: time at colonisation (here tr = 0) and Z being the instantaneous mortality
rate.
This model was applied using an initial
number of fish at 1,000,000 at colonisation and a variable mortality (Z)
coefficient over a period of 100 days and a fixed one after 100 days
(D100). For the first day (D1) Z was 15
and then decreasing regularly to Z = 1 at D100. This variation of Z was based on several studies showing that
mortality is at least 60 % per month after settlement for several species of
coral reef fishes (see Carr and Hixon, 1995; Doherty and Sale, 1986; Shulman
and Odgen, 1987; Victor 1986). From
D100 to D365, we chose a constant Z value of 1 because it is presumed that Z
does not present high values during the adult stage and because some species of
coral reef fishes may grow to more than 20 to 30 years old (Doherty et al.
1994). The exponential decay model was
used to fit the data previously published (see Carr and Hixon, 1995; Doherty
and Sale, 1986; Shulman and Odgen, 1987; Victor, 1986).
Larval colonisation appears to be a
constant process, occurring regularly over the reef crest at Moorea Island for
the majority of families and species of reef fishes of this island. The total number of fish larvae collected
was estimated at 2 million (1 million s. e.). in 1995.
The diversity of species collected is also
a very important feature of the colonisation process. During each of our
sampling in 1995, we have collected several species and families of fish
unknown from this area, namely, Lobotes surinamensis, Histrio histrio,
and Symphurus microrhynchus.
Most of these fishes were small fish
larvae and the number of large fish larvae was 30,500. Among small fish larvae, we estimated that
67 % of the catch represent Gobioidea (e.g. families of Gobiidae,
Callyonimidae, and Eleotridae), 10 % are Scaridae and 7% are Labridae (Dufour
and Galzin, 1993). Some of the large
fish larvae could be considered as pelagic juveniles since their body shape,
pigmentation, and behaviour were similar to small pelagic fishes such as
Scombridae or Clupeidae. However, these
large pelagic stages of coral reef fishes still metamorphose after
colonisation. These changes are
morphological (colour patterns, morphometrics, gut length) or ethological.
There were about 250 species representing
85 % of the families of coral reef fishes from the region (Figure 1) with large
larval stages. The most abundant
species was Epinephelus merra with more than 6,000 individuals collected
but it was only present in 15% of the samples.
The colonisation peaks of this serranid were very brief but the peaks
represent numerous settlers. The second
most abundant species was Acanthurus triostegus and was present in 85 %
of the samples. The peaks in abundance
of this acanthurid are protracted but the peaks represent fewer numbers of
settlers. These two species represent
the two patterns of colonisation, i. e., colonising the reef with brief
and important episodes, while other species showed a more regular colonisation
pattern with lower abundance, while other species have colonisation strategies
between these two patterns.
Densities of large reef fish larvae
settling in Moorea lagoon range from 1.9 to 3 fish/mo/m2. Since colonisation occurs regularly during 6
to 9 months per year, a very conservative estimate of the annual density at
colonisation would give 10 large fish larvae m2/y-1. As densities of small fish larvae are 50
times higher, we can assume that densities of fish larvae at colonisation
represents in one year 500 fish larvae m2/y-1. This density also represents ten times to
one hundred times the number of adult reef fish in the lagoon (usually
estimated to less than 10 fish/m2).
Impact of
natural harvest of fish fry on coral reef fish population
The data obtained from the exponential
decay model, calculated with an initial number of fish at colonisation of 1,000,000,
show that this number is lowered to 113,881 fish at D100. If the mortality rate
(Z) is then maintained at 1 from D100 to D365, the number of fish is 113,452 at
D365 (Figure 2). This shows that most
of the mortality occurs during the first days after colonisation. At D1, Z corresponds to a daily mortality of
4.16 %, while it corresponds to 3 % at D30, 2.2 % at D50, and only 0.27 % at
D100. The number of fishes remaining at
these different dates is 960,000, 350,000, 210,000 and 110,000, respectively.
The impact of harvesting 100,000 fish from
1 million at colonisation greatly depends on the mortality rate during the time
of harvest. If harvest is made at D5,
the fish population after one year represents 88 % of the population without
harvest. If the same number of fish is
harvested at D50, the remaining number of fish at D365 is only 59 % of the
population without harvest. Therefore,
by delaying collection to 45 days, the impact on the population is multiplied
by 3.5 at D365 (from 12% to 41%). It
is worth considering that the harvested fish at D5 represent 11.7% of the
population and after one year, the impact of the harvest is slightly lowered to
11 %. The harvest at D50 represents
46.8% of the population and its impact represents a decrease of 41 % after one
year.
Larval flux
during colonisation in French Polynesia
Comparison
of colonisation patterns of a variety of species of coral reef fish has shown
that some species have strong colonisation peaks while others do not have
pulses colonisation but are regularly present in the sampling devices. Epinephelinae seem to present the highest
colonisation peaks worldwide (i. e. Shenker et al., 1993,
for the Caribbean; Harmelin-Vivien, pers. comm., for Indian Ocean; Rigby and
Dufour 1996, for the Pacific Ocean).
Although unpredictable, these colonisation peaks occurs during
particular moon phases and seasons thus showing to good observers such as local
fishermen or field ecologists where they will occur. However, as these peaks are localised through time they present
certain patchiness over space. These
peaks thus represent almost all the fish coming for settlement over a
relatively large period of time (there can be only one colonisation event per
year for some species). Their abundance
needs to be estimated in order to determine the number of fish that can be
harvested.
The annual larval flux estimated from our
data represents one hundred times the density of adult and juvenile fish living
in the lagoon. This proportion greatly
varies with species, since the larger the fish at colonisation, the lower their
relative abundance. It is possible that
large reef fish larvae have a long pelagic larval duration and thus present low
abundance at colonisation and small reef fish larvae having a shorter pelagic
larval duration and a higher abundance at colonisation. This relationship needs, however, to be
balanced with the abundance of the different fish species in the lagoon. The
magnitude of the larval flux observed at Moorea Island is probably not unusual,
at least for French Polynesia (Dufour, 1994), but it strongly depends on the
sampling strategies used to collect settlement stage reef fish. In the near future, it will be necessary to
estimate this larval flux for other coral reef areas in order to compare
population of reef fish during colonisation and population of reef fish already
settled in the reef.
The culture of coral reef species is one
of the fastest growing activities in living marine resources. The ultimate
sustainable system would be to control the life cycle, especially the larval
production. However, the culture of
coral reef fishes is still largely dependent on the natural harvest of fish fry
in the wild. The collection of fish at
colonisation could be made on a sustainable way if the number of fish is much
higher during colonisation, than during the adult stage.
Impact of natural harvest of
fish fry on coral reef fish population
Local fishermen usually harvest
fish fry from wild stock of coral reef fishes.
This activity represents a very important part of seed production for
coral reef fish aquaculture in South East Asian countries. These methods seem
to be used for most species exploited for the live reef fish trade,
including groupers, snappers, and
Napoleon wrasse. The range of sizes
collected for these species are from 2 cm to more than 15 cm. This means that the harvest of fish fry is
made over a relatively long period of the life of the fish. In addition,
aquarium fishes are also collected at any size, either during juvenile or adult
stage. It is obvious that the impact of
harvesting fish fry either for cage-culture or for the aquarium will worsen as
fish get older, due to the natural decay of population through the life history.
The harvesting of larvae and juveniles for
the live-fish trade or for the aquarium trade will also depend on the mortality
rate of the population. Estimation of natural mortality for a population of
coral reef fish needs daily measurements of fish abundance at a small scale and
there are multiple possible biases from the behaviour or the migration of the
fishes. Despite these difficulties, most published studies gave relatively
close values of Z, for both the initial measurement and its evolution through
time (Carr and Hixon, 1995; Shulman and Ogden 1987; Victor, 1986).
The harvesting of larval stages has an
obvious and irreversible impact of the remaining fish population (Figure
2). However, we can estimate reasonably
that the harvest at D5 is a sustainable practice, since almost 90% of the
natural population remain undisturbed.
The age at which fish fry is collected could be easily calculated using
back-calculation from daily growth increments in fish otoliths (see Stevenson
and Campana, 1992). On the other hand,
the harvest at D50 seems close to overfishing, as the remaining population will
be lowered by more than 40 %. If any
further fishing pressure occurs on this population, the abundance would
probably rapidly drop below 50 % of the natural population. In this case,
harvesting fish fry is probably unsustainable.
These calculations show that the impact of harvesting fish fry from
natural populations of coral reef fishes is inversely proportional to the
instantaneous mortality rate. As this
mortality decreases rapidly from the time of colonisation to the juvenile
stage, the impact of harvesting fish fry will be lowest at the time of
colonisation and increases consistently to a point where overfishing is
notable.
This model also shows that traditional
theories in the management of fisheries cannot be applied to solve problems or
to help reduce threats of harvesting fish fry. Fisheries practices usually
maximise yields using total biomass caught and theories are based on equations
using fish weight or weight-derived parameters. Collection of fish fry and aquarium fishes differ markedly from
this scheme because this fishing tend to maximise the number of fish
collected. The size and the weight of
the fish are usually not taken into account by fishermen. These individuals are designated either to
aquaculture facilities or aquarium trade.
A new and more appropriate fisheries theory should therefore be
developed by fisheries scientists to provide advice for fishermen and managers
dealing with this activity in order to maintain it at a sustainable level.
Although this model is applicable in areas
such as French Polynesia, it needs to be verified in areas where the absolute
abundance of fish at settlement is usually unknown and where the harvest is
large. The sustainability of such
practices should help aquaculture technology by providing a large number of
young fish at settlement stage, when their ability to accept rearing facilities
and artificial food is good. This should also help aquaculturists to get a
better knowledge of larval stages and also to produce fish without the need to
manipulate reproductive activity.
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[1] E.P.H.E. Laboratoire d'ichtyoécologie Tropicale et Méditerranéenne, URA CNRS 1453, Université de Perpignan 66860 Perpignan Cedex France and C.R.I.O.B.E.-E.P.H.E. Research Station, BP 1013 , Moorea, French Polynesia