Student no.: 610026241
The use of nursery habitats by Haemulon sciurus, Chromis cyanea and Thalassoma
bifasciatum in the Bahamas
Summary
The use of so-called ‘nursery habitats’ such as mangroves and seagrass meadows is an
important part of early development for many coral-reef species, including in the Bahamas.
The degree of use of these back-reef systems, as well as by which species, can vary and
understanding the differences is vital to informing policy on specially designated Marine
Protected Areas (MPAs). Here we show that two species, Haemulon sciurus and Thalassoma
bifasciatum, use the mangroves of San Salvador, but not the seagrass meadows, as nursery
habitats. A third species, Chromis cyanea, was not found to use either as nursery habitats.
Summary word count: 98
Introduction
Coral-reef inhabiting fish have long been known to utilise ‘nursery habitats’ as safe
environments for juvenile development, considered important to many Caribbean species1.
Specifically, mangroves and seagrasses serve as intermediary habitats2, providing maximum
food availability and minimum predation risk in order to allow a variety of species to grow
and develop with minimal stressors3. This nursery role of back-reef systems has only recently
begun to be properly understood4. Indeed, it has been found that proximity of coral reef fish
communities to nursery habitats greatly increases the biomass of these species, even
outweighing the benefits of no-take Marine Protected Areas (MPAs) 5, in addition to strongly
impacting community structure on the reef2. The assemblages of species inhabiting seagrass
beds and mangroves vary, with different species inhabiting one or both of these habitatswith mangroves in particular exclusively containing several juvenile species3. With
anthropogenic activity threatening the stability and preservation of nursery habitats, the
importance of understanding which species utilise these environments, and to what extent, is
greater than ever. Mangroves, for example, are among the most threatened coastal biotypes6
whilst also one of the most important.
We studied three coral-reef fish species - Haemulon scirus (Blue-striped Grunt), Chromis
cyanea (Blue Chromis) and Thalassoma bifasciatum (Blue-headed Wrasse) – on the Bahamian
island of San Salvador. Their sizes were measured over three different habitats of coral reef,
seagrass and mangroves, to determine whether these species utilised the back-reef systems
as nurseries. It was expected that all three would be found to use at least one back-reef
environment as a nursery for the smaller, juvenile stages, in agreement with previous
observations of some of these species in the area of San Salvador7 and the Bahamas8.
Methods
Line transects of 30m x 2m, the exact position of which chosen randomly, were run at each
of the three different habitats. Several different sites were used for each habitat and the
results pooled together to give a more complete representation of each of the three habitat
types on the island. Numbers of each of the three studied fish species and their estimated
Student no.: 610026241
length were recorded by visual census along each line. Length was estimated as 0-5cm, 510cm, 10-15cm, etc. and then assigned a value of 3cm, 8cm, 13cm etc. respectively. For the
mangrove habitat, transects were run along the tree line and the visual census completed by
looking into the root systems. Surveys were carried out during the same week in June 2013,
between 8am and 5pm.
Average size and standard deviation were then calculated for each fish species at each site,
and a single factor ANOVA test was carried out to determine whether or not the differences
in average fish size, for each species, between the habitats was significant.
Results
The average size of T.bifasciatum (fig.1) was found to be smallest in the mangrove habitat
(5.07cm) than the reef habitat (6.59cm), with the differences in sizes between the three
areas found to be significant (p=0). The size variation of H.sciurus (fig.2) was found to follow
a similar pattern, with the mangrove population appearing significantly smaller (6.26cm) than
the reef populations (9.16cm); size differences between all three habitats were again
significant (p=0). For both T. bifasciatum and H. sciurus, seagrass populations were found to
be, on average, the largest. Data for C. cyanea was only available for reef populations, as
none of this particular species were seen in either seagrass or mangrove habitats during the
visual surveys. Standard deviation is used for all data sets, as it refers to individual and
separate data sets.
Fig. 1
12
10
Mean Size (cm)
8
6
4
2
0
Reef
Seagrass
Mangrove
Habitat Type
Mean Size (cm)
Fig. 2
16
14
12
10
8
6
4
2
0
Reef
Seagrass
Habitat Type
Mangrove
Student no.: 610026241
Fig. 3
Habitat Type
Reef
Seagrass
Mangroves
T. bifasciatum
3518
48
544
Number of Individuals (n)
H. sciurus
2482
5
614
C. cyanea
194
0
0
Figure Legends
Figure 1 – Mean size of T. bifasciatum in Reef and Back-reef habitats - A bar graph
showing the mean size of T. bifasciatum (Blue-headed Wrasse), in cm, over the three
different habitats surveyed. P=0 (<0.05). Error bars represent standard deviation (SD=3.88,
2.07 & 0.8 respectively).
Figure 2 – Mean size of H. sciurus in Reef and Back-reef habitats - A bar graph showing
the mean size of H. sciurus (Blue-striped Grunt), in cm, over the three different habitats
surveyed. P=0 (<0.05). Error bars represent standard deviation (SD=6, 2.74 & 2.91
respectively).
Figure 3 – Individuals of species surveyed – A table to show the total number of individuals
(n) recorded in each of three habitat types surveyed.
Discussion
Both T. bifasciatum and H. sciurus can be determined to use the mangroves of San Salvador
as nursery habitats, with the average size of both species significantly smaller at the
mangrove site than on the reef sites (fig. 1 & fig. 2). This conforms to expectations, as well
as previous findings both on San Salvador in regards to H. sciurus7 and to reefs more
generally for most fish species. Perhaps unexpectedly, these two species were found overall
to be larger in the seagrass habitat. There are, however, several potential reasons for this.
Most importantly, the sample sizes of species seen in the seagrass habitat were very small
(fig. 3). This could be due to experimental error – the seagrass beds were much deeper than
the other habitats when surveyed, with identification of fish therefore much more difficult if
they were hiding amongst the seagrass. However, it may be that these fish do not use this
Student no.: 610026241
environment as a nursery habitat at all but instead solely as feeding grounds. Indeed, the
importance of seagrass meadows does appear to vary with a number of factors – whilst
juvenile survival in seagrass habitats tends to be greater when compared to unstructured
habitats9, structured mangrove habitats are much more preferred10.
Our visual surveys only recorded the presence of C. cyanea in reef environments. The
overwhelming lack of C. cyanea at the back-reef systems would seem to strongly indicate
that this species does not use the seagrass or mangrove habitats as nurseries. As there is
little literature on this species’ juvenile stage, it is hard to compare to any previous findings
- though our initial hypothesis presumed that this species would have showed similar
patterns to the other two species studied. Planktivores such as C. cyanea form an important
trophic link between reef and open-water communities11, and as such further investigation of
C. cyanea’s juvenile habitats is important for better understanding of much wider interactions
and trophic interactions.
Whilst the data presented here should be expanded upon by further study to potentially
improve the accuracy, it provides a firm basis from which decisions can begin to be made
regarding local habitat protection. The use of nursery habitats by H. sciurus and T.
bifasciatum on San Salvador may be indicative of much wider use by other species, including
those considered more economically or culturally important to the local area. Therefore,
these results should be used primarily as a starting point in the on-going discussion regarding
the future of Marine Protected Areas on the island.
Word count:1,000
References
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(2001).
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