Download The effects of fishing on the diversity, biomass and trophic structure

Coral Reefs (1995) 14:225-235
Coral Reefs
9 Springer-Verlag1995
~
The effects of fishing on the diversity, biomass and trophic structure
of Seychelles' reef fish communities
S. Jennings 1, E. M. Grandcourt 2, N. V. C. Polunin 1
1Departmentof Marine Sciencesand Coastal Management,The University,Newcastle-upon-TyneNE 1 7RU, UK
2SeychellesFishingAuthority,PO Box 449, FishingPort, Mah6, Seychelles
Accepted: 10 January 1995
Abstract. A fishery independent underwater visual census
technique was used to assess the effects of fishing on the
diversity, biomass and trophic structure of the diurnally
active non-cryptic reef-associated fish communities of the
Seychelles. One hundred and thirty four species associated
with three significantly different types of reef habitat were
censused at one unfished ground and in six fishing grounds
subject to different fishing intensities. There was an inverse
relationship between fishing intensity and the biomass of
several species targeted by the fishery. The diversity of
families containing target species (lutjanidae, lethrinidae)
was significantly higher at unfished and lightly fished sites
as was the total biomass of the fish community and the
biomass of piscivorous, piscivorous/invertebrate feeding
and herbivorous trophic groups. However, there was no
indication that the biomass of non-target species increased
in response to the removal of their predators by fishing. The findings of this study are significant for fishery
managers because they suggest that the intensive differential cropping of top predators will not necessarily lead
to increases in the biomass and productivity of their
prey.
Introduction
Fishing is the most widespread human exploitative activity on coral reefs, and fisheries of socio-economic significance have developed in all areas where humans can
access fishing grounds (e.g. Munro and Williams 1985;
Russ 1991). Fishing activities have direct and indirect
effects on reef fish populations and their ecosystems (Russ
1991). Such effects are significant because they may lead to
changes in the size and composition to yield from the
fishery and alter subsequent fish production processes
(Jennings and Lock 1996; Jennings and Polunin 1996).
Correspondence to. S. Jennings
Given current concerns for the sustainability of reef
fisheries and a burgeoning literature suggesting approaches
for their management (Pauly 1988; Medley et al. 1993)
there are remarkably few studies of fishing effects (Munro
et al. 1987; Russ 1991; Jennings and Lock 1996). Existing
knowledge of fishing effects has been gained using two
principal approaches: temporal comparisons of fish community structure in an area subject to changing rates of
exploitation (e.g. Russ 1985; Koslow et al. 1988; Alcala
and Russ 1990) and spatial comparison between the fish
communities in two or more areas subject to different
levels of fishing intensity (e.g. Samoilys 1988; Polunin
and Roberts 1993). The former approach has typically
been used to demonstrate short-term fishing effects following the opening or closing of reserves, but these may
differ from the long term effects in established fisheries. The latter approach has generally been applied
to compare fished and unfished areas or has considered
a relatively small proportion of the fish community
(e.g. Grigg 1994). Fishing effects have rarely been
examined across quantifiable gradients of fishing intensity using fishery independent assessments of community
structure.
Decreases in the size, abundance and biomass of those
piscivorous or carnivorous species which are favoured
for consumption or sale are expected to be the most
readily detectable direct effects of fishing (Russ and Alcala
1989). These effects have been recorded in reef fisheries
as changes in catch-per-unit-effort (e.g. Ralston and
Polovina 1982; Koslow et al. 1988) or by independent
assessment techniques such as underwater visual census
(Bohnsack 1982; Samoilys 1988; Russ and Alcala 1989;
Polunin and Roberts 1993; Grigg 1994). The removal of
piscivorous fishes may have repercussions throughout the
ecosystem. Piscivorous fishes are probably the most significant consumers offish biomass (Grigg et al. 1984) and in
most reef systems will consume considerably more fish
biomass than is removed by fishing (Jennings and Lock
1996). Theorists have predicted that the removal of predatory species should result in the proliferation of their
prey (Beddington and May 1982; Beddington 1984) but
226
evidence for such effects in reef ecosystems r e m a i n s equivocal (Russ 1985, 1991; H i x o n 1991; Caley 1993).
I n the p r e s e n t study we use a fishery i n d e p e n d e n t assessm e n t t e c h n i q u e to e x a m i n e the effects Of fishing o n the
diversity, b i o m a s s a n d t r o p h i c structure o f the fish comm u n i t i e s f r o m three of the m a i n types o f fished reef h a b i t a t
in the Seychelles. W i t h i n each h a b i t a t type, fish comm u n i t i e s were investigated at o n e unfished g r o u n d a n d in
six fishing g r o u n d s subject to different fishing intensities.
W e a i m to describe the direct effects o f fishing o n target
species a n d d e t e r m i n e w h e t h e r changes i n the c o m p o s i t i o n
o f their c o m m u n i t i e s are associated with changes in the
9diversity or b i o m a s s of species a n d t r o p h i c g r o u p s which
are n o t targeted by the fishery.
55~
55~
i
lin NE
Cous I ~
U?
NI
55~
55~
Methods
Study sites
The study was conducted on reefs surrounding the granitic islands of
Mah~ and Praslin (Fig. 1). These islands rise from the SeychellesBank,
an extensive submarine platform with mean depths of 44 to 65 m.
Fringing reefs with a carbonate framework have developed around
the islands and, in deeper water, corals grow directly on the granite
substrate. Corals are scarce at depths in excess of 20 m (see review by
Stoddart 1984).
The majority of the Seychellois people live on Mah~ and Praslin
and reef-associated fishes are targeted by the artisanal fishery (Mees
1990a). Fishing is not permitted in a number of areas which have been
designated as Marine Parks or Special Reserves (Anon 1990a;
Jennings et al. 1996). Seven inshore grounds subject to different levels
of fishing intensity were selected for study (Fig. 1). Five of these
grounds (Mah~ NW, Mah6 W, Mah~ E, Praslin SW and Praslin NE)
were based on the strata used by the Seychelles Fishing Authority for
their assessment of the artisanal fishery (Anon 1993a). Sainte Anne
Marine Park (Ste. Anne) was established by the Government of
Seychelles in 1973 (Anon 1973) and Cousin Island Special Reserve
(Cousin) was established by the International Council for Bird
Preservation (now Birdlife International) in 1968 (Diamond 1975).
Poaching takes place at Ste. Anne and a few local families have been
granted fishing rights (Anon 1993b). Cousin is considered to be an
unfished site (Jennings et al. 1996).
Fig. 1. Location of the seven grounds subject to different fishing
intensities. Continuous lines bordering Ste. Anne and Cousin indicate
boundaries of the reserves. Broken lines indicate fringing reefs.
Habitat types: 9 = 1, well-developed fringing reef with a carbonate
framework. O -- 2, Coral growth on a granitic substrate in exposed
locations. 9 = 3, small rock and coral patch reefs on a predominantly
sandy substrate
Assessment offish community structure
Quantitative estimates of the size and abundance of fish in the seven
grounds were made using an underwater visual census (uvc) technique. This was an appropriate technique for censusing the fish communities because speartishing is banned in Seychelles and the fish were
not expected to avoid divers. Moreover, recreational diving and the
feeding of fish, which can result in fish gathering around divers, do
not take place at the Cousin reserve. One hundred and thirty four
non-cryptic diurnally active reef associated species which could be
positively identified and censused effectively were selected for study
(Table 1).
Fish were counted and their lengths were estimated at three sites
within each fishing ground. The sites were selected to include each of
the three main types of fished reef habitat, i.e. (1) well-developed
fringing reef with a carbonate framework, (2) coral growth on a
granitic substrate in exposed locations and (3) small (typically 3-7 m
circumference) rock and coral patch reefs on a predominantly sandy
substrate. Counts were conducted by the same observer (SJ) from
26 April to 12 June 1994. Sixteen replicate counts were completed at
each site during daylight hours at depths of 3.0 to 13.0 m below chart
datum. Replicates were placed randomly with the proviso that the
boundaries of adjacent counts were separated by a minimum of 15 m
(to avoid bias due to push-pull effects in preceding counts; Samoilys
1992).
The uvc point count technique was based on that developed by
Samoilys and Carlos (1992). The sizes and abundance of target species
within a census area of 7 m radius were determined by counting each
individualand making an estimate of its length (to + 1 cm). Counts of
the species in each area were conducted sequentially, the most active
species being recorded first. When a count for one species was complete, all further movements of that species in or out of the census area
were disregarded because counts of incoming fish would bias biomass
estimates (Samoilys 1992). When the active, non-territorial species
had been counted, a 7 m line was laid to confirm estimates of the
position of the census boundary. Smaller territorial species such as
Stegastes and Plectroglyphidodonwere then recorded. Only those fish
estimated to be >__8 cm were included. The accuracy of length estimation was maintained by practising length estimation with objects of
known size (60 lengths of 2 cm diameter plastic pipe cut to lengths of
5-65 cm and threaded onto a line in random order) during the census
227
Table 1. Species for which size and abundance estimates were obtained, the trophic groups to which they were assigned (co corallivore,
hb herbivore, ip invertebrate feeder and piscivore, iv invertebrate
feeder, om omnivore, pi piscivore,pk planktivore, dt detritivore), their
maximum lengths (cm; from Smith and Heemstra 1986; Randall
1992 and personal observation) and their role in the artisanal fishery
Trophic
group
Aeanthuridae
Acanthurus leucosternon Bennett, 1832
A. lineatus (Linnaeus, 1758)
A. nigrofuscus (Forsskgd, 1775)
A. triostegus (Linnaeus, 1758)
A. tenneti Gtinther, 1861
Ctenochaetus binotatus Randall, 1955
C. striatus (Quoy and Gaimard, 1825)
C. strigosus (Bennett, 1828)
Naso lituratus (Forster, 1801)
Paracanthurus hepatus (Linnaeus, 1766)
Zebrasoma scopas (Cuvier, 1829)
Z. desjardini (Bennett, 1835)
Balistidae
Sufflamen chrysopterus
(Bloch and Schneider, 1801)
Chaetodontidae
Chaetodon auriga Forsskfil, 1775
C guttatisimus Bennett, 1823
C. kleinii Bloch, 1790
C lineolatus Cuvier, 1831
C. lunula Lacep6de, 1803
C. madagaskariensis Ahl, 1923
C. melannotus Bloch and Schneider, 1831
C. meyeri Bloch and Schneider, 1831
C. trifaseialis Quoy and Gaimard, t824
C trifasciatus Park, 1797
C. xanthocephalus Bennett, 1832
C zanzibariensis Playfair, 1867
Haemulidae
Diagramma pictum (Thunberg, 1792)
Plectorhinchus gibbosus (Lacep6de, 1802)
P. orientaIis (Bloch, 1793)
P. schotaf (Forsskfil, 1775)
Labridae
Anampses meleagrides Valenciennes, 1840
Bodianus axillaris (Bennett, 1831)
Cheilinus digrammus (Lacep6de, 1801)
C. fasciatus (Bloch, 1791)
C. trilobatus (Lacep~de, 1801)
Corisformosa (Bennett, 1834)
Epibulis insidiator (Pallas, 1770)
Gomphosus caeruleus Lacep6de, 1801
Halichoeres cosmetus Randall
and Smith, 1982
H. hortulanus (Lacep6de, 1801)
H. marginatus Riippell, 1839
H. scapularis (Bennett, 1831)
Hemigymnusfasciatus (Bloch, 1792)
H. melapterus (Bloch, 1791)
Labrichthys unilineatus (Guichenot, 1847)
Labroides biocoIor (Fowler and Bean, 1828)
L. dimidiatus Valenciennes, 1839
Macropharyngodon bipartus Smith, 1957
No vaculichthys taeniourus (Lacep6de, 1801)
hb
hb
hb
hb
hb
dt
dt
dt
hb
pk
om
om
Length in
cm and
role
23 d
38 ~
21 d
27 ~
31 ~
22 d
26 ~
19 d
45 d
31 ~
19 ~
40 d
iv
20 d
iv
20 ~
11 d
12 d
30 ~
20 ~
14 d
15 ~
20 d
17 d
14d
ROd
15c
iv
co
CO
iv
iv
CO
CO
co
co
iv
iv
iv
iv
iv
iv
iv
iv
pi
iv
iv
iv
iP
iv
iv
1V
1V
1V
ap
xp
1v
1V
1V
1V
1V
90 b
70"
55 a
40"
21 d
20 d
30 ~
36 c
40 b
60 ~
35 d
28 d
12 d
27 d
17 d
20 d
45 r
50 ~
17 d
14d
12a
12d
25 d
Table 1. (Continued)
Stethojulis albovittata (Bonnaterre, 1788)
Thalassoma hardwicke (Bennett, 1828)
T. herbracium (Lacep6de, 1801)
T. lunare (Linnaeus, 1758)
Lethrinidae
Lethrinus concyliatus (Smith, 1959)
L. enigmatus undescribed
L. harak (Forsskgd, 1775)
L. lentjan (Lacep+de, 1802)
L. mahsena (Forssk~d, 1775)
L. mahsenoides Valenciennes, 1830
L. nebulosus (Forsskgd, 1775)
L. obsoletus (ForsskN, 1775)
L. olivaceus Valenciennes, 1830
L. rubrioperculatus Sato, 1978
Monotaxis grandoculis (Forsskgd, 1775)
Lutjanidae
Aprion virescens Valenciennes, 1830
Lutjanus argentimaculatus (Forsskfd, 1775)
L. bohar (ForsskN, 1775)
L. fulviflamma (Forsskgd, 1775)
L. gibbus (ForsskN, 1775)
L. kasmira (ForsskS,1, 1775)
L. monostigma (Cuvier, 1828)
L. rivulatus (Cuvier, 1828)
L. russelli (Bleeker, 1849)
Macolor niger (Forsskfil, 1775)
Monacanthidae
Cantherines pardalis (Rfippell, 1837)
Oxymonocanthus longirostris
(Bloch and Schneider, 1801)
Mullidae
Mulloidesftavolineatus (Lacepbde, 1801)
P. barberinus (Lacep~de, 1801)
P. bifasciatus (Lacep6de, 1801)
P. ciliatus (LacepSde, 1801)
P. cyclostomus (Lacep6de, 1801)
P. rubescens (Lacep6de, 1801)
P. macronema (Lacep~de, 1801)
Trophic
group
Length in
cm and
role
iv
12 d
18 d
23 ~
27 c
iv
iv
iv
lV
76 a
40 a
60 a
40 a
38 a
25 a
87 a
40 a
100 a
50 a
56"
iol
px
pl
11o
11o
11o
lp
lp
lp
pk
100 a
100 a
95 ~
30 a
50 a
30 b
45 a
60 b
40 b
70 b
iv
21 c
10d
lp
lp
lp
lp
lp
lp
lp
lp
lp
lp
CO
ip
ip
ip
ip
iv
40 c
50 ~
35 c
40 ~
50 ~
42 c
32 c
iv
28 c
Pomaeanthidae
Apolemichthys trimaculatus (Lacep~de, 1831) i v
Centropyge multispinis (Playfair, 1867)
iv
Pomacanthus imperator (Bloch, 1787)
iv
P. semicireulatus (Cuvier, 1831)
iv
21 d
12d
40 c
38 d
Nemipteridae
Scolopsisfrenatus (Cuvier, 1830)
Pomacentridae
Ambyglyphidodon leucogaster
(Bleeker, 1847)
Chromis atripectoralis
Welander and Schultz, 1951
C. ternatensis (Bleeker, 1856)
C. weberiFowler and Bean, 1928
Dascyllus trimaculatus (Riippell, 1828)
Neoglyphidodon melas (Cuvier, 1830)
Plectroglyphidodon dickii (Lienard, 1839)
P. johnstionus Fowler and Ball, 1924
P. lacrymatus Quoy and Gaimard, 1824
iv
ip
pk
14a
pk
14d
pk
pk
pk
iv
11 d
14d
14d
186
11 d
10 d
12 d
om
om
hb
228
Table 1. (Continued)
Description o f habitat characteristics
Trophic
group
Length in
cm and
role
hb
hb
12d
12 d
hb
hb
hb
hb
54c
80 b
75 b
35 c
hb
hb
hb
hb
hb
hb
hb
hb
hb
hb
hb
hb
hb
hb
hb
70 b
476
476
75 b
70 b
27 ~
356
306
75 b
37 b
356
386
40 b
33 ~
ip
pi
61 ~
50 b
Cephalopholis argus
pi
50 ~
(Bloch and Schneider, 1801)
C. leopardus (Lacep6de, 1801)
C. miniata ((Forssk~d, 1775)
C. urodeta (Bloch and Schneider, 1801)
Epinephelus caeruleopunctatus (Bloch, 1790)
E. fasciatus (Forsskgd, 1775)
E. hexagonatus (Bloch and Schneider, 1801)
E. rnerra Bloch, 1793
E. spilotoceps Schultz, 1953
E. tukula Morgans, 1959
ip
pi
ip
pi
ip
ip
ip
ip
ip
206
41 ~
28 b
60 a
35 a
25 a
28 b
356
200 a
Siganus argenteus
hb
37 a
(Quoy and Gaimard, 1825)
S. puelloides Woodland and Randall, 1979
S. stellatus Forsskfd, 1775
S. sutor (Valenciennes, 1835)
hb
hb
om
356
30 b
45"
iv
22 d
Pomacentrus trilineatus Cuvier, 1830
Stegastesjasciolatus (Ogilby, 1889)
Searidae
Calotomus carolinus (Valenciennes, 1840)
Cetoscarus bicolor (Rtippell, 1829)
Hipposcarus harid ((Forsskgd, 1775)
Leptoscarus vaigiensis
(Quoy and Gaimard, 1824)
Scarus atrilunula Randall and Bruce, 1923
S. caudofasciatus (Gunther, 1862)
S. falcipinnis Playfair, 1867
S. frenatus Lacep6de, 1802
S. ghobban Forsskgd, 1775
S. gibbus Riippell, 1829
S. globiceps Valenciennes, 1840
S. niger Forsskgd, 1775
S. psittacus Forssk~l, 1775
S. rubroviolaceus (Bleeker, 1849)
S. scaber Valenciennes, 1840
S. sordidus Forssk~l, 1775
Scarus sp
S. tricolor Bleeker, 1847
S. viridifucatus (Smith, 1956)
30 b
Serranidae
Aethaloperca rogaa (ForsskN, 1775)
Anyperodon leucogramma
(Valenciennes, 1828)
Siganidae
Habitat was described both within the perimeter of each replicate
count area and within the overall boundaries of the study site. When a
count was complete the percentage cover (based on plan view) of sand,
rubble, rock, massive coral, soft and branching coral was estimated
and topographic complexity of the substrate was described using the
6 point scale of Polunin and Roberts (1993). An exposure index was
calculated for each site by determining the 15~ sectors (first sector
beginning at true north) from which the site was fully exposed to an
unobstructed wave fetch of 3 km or more, and summing the product
of the mean annual windspeed on Mah6 (km h ~) and duration (hours)
in each of these sectors (Seychelles Meteorological Office, unpublished
data). The general seabed habitat at each site was described in terms of
composition and profile. The number of counts in which the dominant
underlying substrate consisted of carbonate framework, sand or granite was expressed as a proportion of the total number of replicate
counts within each site, and the minimum and maximum depth within
each count area was recorded to provide a crude index of site rugosity.
D a t a analysis
Estimates of fish length were converted to mass using published
length-weight relationships (from Kulbicki et al. 1993 and unpublished sources). When the weight-length relationship for a given
species was not available, then the relationship for a species from the
same genus and with similar morphology was used. For 19 species this
was not possible and the relationships were selected for fish with
similar morphology. Fish were assigned to trophic groups (Table 1) on
the basis of information provided by Hiatt and Strasburg (1960),
Vivien (1973), Sano et al. (1984), Randall (,1983, 1992), Blaber et al.
(1990), Randall et al. (1990) and observations during the census
period.
An examination of the relationships between habitats at the sites
selected for study was made by subjecting the means of all replicatespecific habitat indices and the values of site specific habitat indices to
an agglomerative hierachical clustering procedure, using the average
linkage method (Sokal and Michener 1958). All data were standardized to mean = 0 and SD = 1 before clustering. The significance of
differences between clusters was assessed using a test of dissimilarity
(Carr 1991).
The significance of differences in the biomass of species or trophic
groups between habitats and grounds was assessed using two-way
crossed A N O V A with loge +1 transformed data. When interactions
between habitats and grounds were not significant (P>0.05) and
when differences between habitats were not significant (P > 0.05)
biomass data were presented as pooled means. Otherwise, the data
were re-analyzed within-habitats using one-way ANOVA. Tukey's
method (P = 0.05; Day and Quinn 1989) was used to determine the
significance of differences in mean biomass between specific habitats
or grounds. One-way and two-way crossed ANOVA were also used to
compare the number of species recorded per count in the different
grounds or habitats.
Zanelidae
Zanclus cornutus (Linnaeus, 1758)
a Primary target species caught by most of the fishing techniques in use
b Secondary target species caught using a limited number of techniques
~ Bycatch sometimes retained if caught
d Species which are not retained if caught; from Seychelles Fishing
Authority (unpublished data)
period and assessed using methods of Bell et al. (1985) and Polunin
and Roberts (1993). The time required to complete a census was not
standardized since this is dependent on the number and diversity of
fish in the census area.
Estimation o f fishing intensity
The Seychellois fish from small outboard, powered open boats and a
small proportion ofunpowered pirogues (Anon 1993a). Larger boats,
such as 'whalers' and 'schooners', rarely fish around inshore reefs
although they do land catches on Mah6 and Praslin (Mees 1990b).
The Seychelles Fishing Authority conducts a regular census of the
number of small boats in operation and their landing sites (Anon
1991, 1992, 1993a). Data from the 1992 censuses (Anon 1993a) were
used to estimate the monthly mean number of boats fishing with lines
or traps (the methods used on inshore reefs) on a sea-area specific basis
within each fishing ground. This provides a reasonable index of fishing
intensity because the majority of small boats fish in areas relatively
229
close to their landing sites and a similar proportion of the fleet use
specific gear types in the different grounds (Anon 1993a).
There are no records of the number of boats fishing or poaching in
Ste. Anne, so the effective number of boats fishing was calculated from
knowledge of the number of traps and lines licensed to local families
(Seychelles Division of Environment, unpublished) and records of
illegally positioned traps confiscated by wardens of the National
Parks Service (Anon 1993b). It was assumed (1) that 50% of the traps
set by poachers were confiscated, (2) that the ratio of trap to line
fishing effort was 1:1 and (3) that a single boat with fishing power
equivalent to those in other grounds would deploy four traps day-~.
Areas of reef habitat (of the three types where fish counts were
conducted) within each fishing ground were determined on the basis
of sea-surface area using digitized navigational charts (UK hydrographic office charts 722 and 742) or maps (UK ordnance survey
series Y851) and records of substrate type collected during diving
o
o
o
f-
qS:
-c,"
o
o
o
o
o
I
I
I
I
I
I
I
I
7
6
5
4
3
2
1
0
dissimilarity
Fig. 2. A dendrogram which shows the grouping of habitat types
formed by hierachical classification analysis. For symbols see Fig. 1
surveys. Areas were measured from the coast to the 20 m isobath for
grounds based on Seychelles Fishing Authority strata and to the
reserve boundary at Ste. Anne and Cousin.
Results
C l u s t e r i n g o f sites o n the basis o f their h a b i t a t c h a r a c t e r istics (Fig. 2) p r o d u c e d t h r e e significantly different g r o u p s
( P < 0 . 0 5 ) . T h e g r o u p s c o r r e s p o n d e d With the c a r b o n a t e
b a s e d c o r a l r e e f (code 1), r o c k b a s e d c o r a l reef (2) a n d
p a t c h reef (3) h a b i t a t t y p e s selected for study, w i t h the
e x c e p t i o n o f the c a r b o n a t e b a s e d r e e f site in M a h 6 W
(Fig. 1) w h i c h c l u s t e r e d w i t h the p a t c h reefs o n s a n d y
s u b s t r a t e . C h a r a c t e r i s t i c s o f the different h a b i t a t t y p e s are
s u m m a r i z e d in T a b l e 2. T h e d i s t r i b u t i o n o f fishing intensity indices ( T a b l e 3) in r e l a t i o n to the g e o g r a p h i c l o c a t i o n
o f the g r o u n d s (Fig. 1) i n d i c a t e s t h a t g r o u n d s w i t h similar
fishing i n t e n s i t y were n o t g r o u p e d o n the s a m e islands o r
c o a s t s o f islands.
A l l l u t j a n i d , lethrinid, c h a e t o d o n t i d , m u l l i d a n d scarid
species e n c o u n t e r e d d u r i n g the survey were i n c l u d e d in the
census a n d their w i t h i n - f a m i l y species richness is s h o w n in
Fig. 3. T h e n u m b e r o f l e t h r i n i d species r e c o r d e d p e r c o u n t
was significantly h i g h e r at the unfished g r o u n d in h a b i t a t s
1 ( P < 0.005) a n d 2 ( P < 0.001) a n d the least intensively
fished g r o u n d in h a b i t a t 3 ( P < 0.005). C o r r e l a t i o n s bet w e e n the n u m b e r o f l e t h r i n i d species p e r c o u n t a n d fishing
i n t e n s i t y were significant in h a b i t a t 1 (r = - 0 . 7 7 , P < 0.05)
b u t n o t in h a b i t a t s 2 o r 3 ( P > 0.05). T h e n u m b e r o f
l u t j a n i d species r e c o r d e d p e r c o u n t was h i g h e r in the
u n f i s h e d o r least intensively fished g r o u n d s o f all h a b i t a t
t y p e s ( P < 0 . 0 5 ) a n d c o r r e l a t i o n s b e t w e e n species
n u m b e r a n d fishing i n t e n s i t y were significant at h a b i t a t s 2
(r = - 0 . 8 8 , P < 0.01) a n d 3 (r = - 0 . 8 8 , P < 0.05) b u t n o t 1
(P>0.05).
Differences in the m e a n n u m b e r
of
c h a e t o d o n t i d , m u l l i d a n d s c a r i d species were significantly
different in all h a b i t a t s ( P < 0.05), w i t h the e x c e p t i o n o f
m u l l i d s in h a b i t a t 3 ( P > 0.05). H o w e v e r , t h e r e were n o
significant c o r r e l a t i o n s b e t w e e n fishing i n t e n s i t y a n d the
Table2. Characteristics ofthe habitat types selected for study. Vahies are means + SD(n = 112) unless indicated otherwise. Habitat codes follow
Fig. 1
Coral cover (%)
Sand cover (%)
Rubble cover (%)
Rock cover (%)
Mean depth(m)
Mean (max-min depth) (m)
Habitat complexitya
Exposurea,c
Calcareous substrateb,c
Sand substrateb'~
Rock substrateb.~
Habitat 1
Habitat 2
Habitat 3
34.1
7.1
32.7
13.4
5.8
2.2
0.62
0.18
0.86
0.05
0.09
24.6
4.1
25.0
40.3
7.1
4.1
0.64
0.55
0.09
0.13
0.79
19.22
13.4
22.1
30.5
6.8
2.8
0.65
0.38
0.44
0.48
0.08
_+ 1t.ll
+ 9.70
+ 20.40
+ 12.50
+_ 0.61
+ 0.35
+ 0.039
+ 0.236
+ 0.152
+ 0.076
+ 0.134
a Expressed in relative terms where maximum possible value = 1
u Expressed as the proportion of replicate counts in which a given substrate was dominant
~
_+ 10.91
+ 6.28
+ 25.25
+_ 28.73
_+ 1.55
+ 0.70
+ 0.069
+ 0.156
+ 0.087
+ 0.114
+ 0.157
_+ 7.10
+ 12.44
+ 13.15
+ 10.36
+_ 1.23
+ 0.57
+ 0.053
+ 0.137
_+ 0.051
+ 0.059
+ 0.059
230
Table 3. Fishing intensity indices for seven Seychelles' fishing grounds
Ground
Cousin
Ste. Anne
Mah6 N W
Mah6 W
Mah6 E
Praslin SW
Praslin NE
Boats (mean
number fishing
month ~)
Reef area
0
3.7
25.9
27.0
45.2
31.2
20.2
0.4
2.1
2.7
4.6
9.0
4.7
5.7
(km 2)
- Intensity
index
(boats km-2)
0
3.5
19.9
11.7
10.0
13.3
7.0
Lutjanidae
4
Lethrinidae
2
o
0
I
I
Chaetodontidae
o
ID
1
o
9
OoO
I
E
I
I
Mullidae
E
9
9
0
A9
I
lk
O
I
I
I
(P< 0.05).
Scaridae
-*o!
0 e
0
;
o
t
10
between grounds (P< 0.05). The biomass of A. virescens
and L. obsoletus was significantly higher in the two least
heavily fished sites and that ofL. bohar significantly higher
in the unfished site (P < 0.05). Similarly, in two of three
habitat types the biomass of C. argus and S. argenteus was
significantly higher in the unfished site (P < 0.05). For the
ten,species considered to be Of secondary importance as
target species (Fig. 4b) the mean biomass at different
grounds differed significantly only for L. harak in habitat 2
(P < 0.05) L. fulviflamma in habitat 3 (P < 0.05), S.
ghobban (pooled data, P < 0.05) and S. frenatus (pooled
data, P < 0.01). In addition, there was a significant negative correlation between the biomass of S. frenatus and
fishing intensity (r = -0.86, P < 0.01). Differences between
the biomass of the ten non-target and by-catch species in
different grounds (Fig. 4c) were frequently significant.
However, there were no significant correlations between
fishing intensity and mean biomass of these ten species
(P<0.05) and no trends indicating consistently higher
biomass at specific sites. Hence the significance of decreases in the biomass of individual target species was generally
attributable to differences between the unfished and/or
most lightly fished site and all others.
The total biomass of all fish censused (Fig. 5) was
significantly higher in the unfished and least heavily fished
site than at all others in all habitats (P<0.05). Total
biomass in habitats 1 and 3 was significantly correlated
with fishing intensity (r=-0.84, P<0.05; r=-0.75,
P < 0.05) but not in habitat 2 (P < 0.05). Differences
between mean biomass in different grounds were significant (P < 0.05) for herbivores and invertebrate (Fig. 5;
feeders in all habitats (with the exception of invertebrate
feeders in habitat 2). However, the only significant correlations between biomass and fishing intensity were
for herbivores and invertebrate feeders in habitat 3
(r =-0.81, P < 0.05; r =-0.74, P< 0.05). Piscivore and invertebrate feeder/piscivore biomass (Fig. 6) was significantly correlated with fishing intensity (r = -0.76, P < 0.05;
r=-0.85, P<0.01) and the biomass was significantly
higher at the unfished and least intensively fished sites
2'0
fishing intensity (boats km-2)
Fig. 3. The relationship between the numbers of Species recorded and
fishing intensity within five families of reef fishes. Symbols indicate
habitat types and follow Fig. 1
mean number of chaetodontid, mullid or scarid species
recorded per count (except for chaetodontids in habitat 3
(r---0.82, P<0.05) and no indication of consistently
higher numbers of these species at specific sites.
Twenty-five species each contributed more than 1%
of the total recorded biomass, and their biomass at the
different grounds is shown in Fig. 4. The mean biomass
of primary target species (Fig. 4a) differed significantly
The total biomass of corallivorous, detritivorous,
omnivorous and invertebrate feeding fish which grow to a
maximum size of less than 20 cm (Fig. 7), and which are
unlikely to be directly affected by fishing due to their small
size and feeding strategy, differed significantly between
grounds in all habitat types (P<0.05), but in no case
was biomass significantly correlated with fishing intensity
(P< 0.05).
Discussion
Evidence for fishing effects
Clearly, differences in the diversity, biomass or trophic
structure of reef fish communities may be attributed to a
number of factors and may not be persistent in space or
time. However, there is evidence to suggest that fishing
effects were responsible for some of the significant trends
we observed. In particular, trends were examined across a
231
A. virescens
.
eel
0
9
C. argus
'~
L. obsoletus
2
~0
0
H. harid
~0
~176
L, harak
L, bohar
2
e
L. fulviflamma
S. argenteus
Coo
Scarus frenatus
S. ghobban
1t~
9
E
;
o
1
.I=~
"i~
"o
S. gibbus
~,o ; ~
S. niger
9
t
0
9
350
~o
S. scaber
9
l9
9
Ill
0
~O
o9
9
0
"
9
9
S. puelloides
04 I 9
9 9 9o I
0
9
9
9
S. sordidus
0
9
OO O
9
~0
I0
9 ~176176
e~
A. leucostemum
C, trilobatus
9
0
L , e~ e ' 9
C. atripectoralis
.
~ ~" ' ~
q 0
H. fasciatus
~0
,"
P.barberinus
0
20
o-
-'o
0
9
io
20
i o ;~
~'
all groups
8oI
0
I
I
40
,r
~o
0
20 0
fishing intensity (boats km-2)
I
~40m
~'
0
I
0
$
9 ,,t
herbivores
I
!20
I
i
I
{
0
I
~ 0
I
20
i ]t ~
",
0
~ IA~
"o~
20
grounds and habitats or differences between habitats were not
significant (two-way crossed ANOVA, P> 0.05). In other cases symbols indicate habitat types and follow Fig. 1. Values of zero are not
plotted
invertebratefeeders
2of
I
9 :
Scolopsis frenatus
~'o
20 r
{
i
oo
P. lacrymatus
Fig. 4a-e. The relationship between biomass and fishing intensity
for a primary target species; b secondary target species and e species
which are retained as bycatch or not fished. Mean values for replicates pooled across habitats are presented when interactions between
80
C.strigosus
51 o0o.
P. ciliatus
'I:
o
C. striatus
4i:
0
I
I
I
I
I
I
10
I
i
0
i
4oI t
00
20
p
5
r
10
lt5
i 0
20
0
10
t
5
i
10
i
15
~i 0
20
0
1tO
115
20
fishing intensity (boats km-2)
Fig. 5. Relationships between mean biomass (+95% CL, n = 16) of
trophic groups and fishing intensity (habitats: 1. upper, 2. centre,
3. lower). Interactions between grounds and habitats, and differences
between habitats, were significant (two-way crossed ANOVA,
P < 0.05). Symbols indicate habitat types and follow Fig. 1
range o f seven g r o u n d s subject to different fishing
intensities rather than using the paired (in space or time)
c o m p a r i s o n s which typified m a n y previous studies. Whilst
h a b i t a t characteristics such as coral cover, reef size, reef
height, t o p o g r a p h i c c o m p l e x i t y o f the substrate, current
n o w and exposure h a v e been s h o w n to influence fish
a b u n d a n c e (review by Williams 1991), the analysis o f
h a b i t a t d a t a f r o m the different g r o u n d s suggested that
similarities within h a b i t a t types between fishing g r o u n d s
are considerably greater t h a n those between h a b i t a t types
232
piscivores
20I
lO
,{
E 0
}{},
v
invertebrate feeders/piscivores
0
o
I
I
I
I
5
10
15
20
fishing intensity (boats kin-2)
Fig. 6. Relationships betweenmean biomass (+95% CL, n = 48) of
piscivores and invertebrate feeders/piscivoresand fishing intensity.
Mean values for replicates pooled across habitats are presented
because interactions between grounds and habitats, and differences
between habitats, were not significant (two-way crossed ANOVA,
P>0.05)
0
2
~
oE
~5
0
I
I
I
I
r
i
i
,
1;
1'5
20
}
I
5
I
fishing intensity (boats km -2)
Fig. 7. Relationships between mean biornass (+95% CL, n = 16) of
small (maximum length < 20 cm) corallivorous, detritivorous, omnivorous and invertebrate feeding fishes and fishing intensity in three
habitats (habitats: 1. upper, 2. centre, 3. lower). Interactions between
grounds and habitats, and differences between habitats, were significant (two-way crossed ANOVA, P<0.05). Symbols indicate habitat
types and follow Fig. 1
and within grounds. Furthermore, the distribution offishing intensity indices in relation to geographic location
of grounds indicated that grounds with similar fishing
intensity were not grouped on the same islands or coasts of
islands and, therefore, geographic trends in fish distri-
bution were unlikely to lead to differences between fish
populations which would be falsely attributed to fishing.
Existing descriptions of the movements and migrations of
many reef fish species (review by Parrish 1989; Jennings
and Lock 1996) suggest that only a relatively small
proportion of fish would move between relatively large
fishing grounds and thereby affect biomass estimates.
However, given the proximity of individual grounds, the
current flows on the Seychelles Bank (Seychelles Division
of Environment, unpublished) and the duration of pelagic
egg and larval phases in many reef fish (review Victor
1991) it is reasonable to assume that fish populations within
individual grounds are not predominantly self-recruiting. As
a result the observed fishing effects are expected to indicate
the responses of the resident post-recruitment fish
community &fishing.
Diversity and biomass
The findings of the present study corroborate existing
evidence that fishing causes detectable decreases in the
diversity and biomass of target species (review by Russ
1991). For the species which showed the strongest responses to fishing (predominantly scarids, lethrinids and
lutjanids) differences in the response to fishing in different
habitats were rare and inconsistent. This suggests that the
responses to sustainable fishing intensities were no more
marked on true carbonate reefs than in sandy areas with
low coral cover or on reefs where corals form a veneer on
the rock substrate. There was no indication that target
species were replaced by non-target species with similar
functional roles in the censused community. If such effects
were typical responses to fishing, it is likely that they would
have occurred because the relative differences in fishing
intensities between grounds have been sustained for at
least seven years (Anon 1991, 1992, 1993a).
The results suggest that increases in effort within
sustainably fished areas would have a smaller impact
on biomass than starting to fish in an unfished system.
Jennings and Polunin (1995a) noted similar effects when
using catch-per-unit-effort as an index of fishing effects in
six Fijian reef fisheries subject to different levels of exploitation. The two factors which determine yield at a given
biomass will be the rate of recruitment to the fishery and
the growth of these recruits. On the basis of information
available it is impossible to determine their relative significance. Biomass may not change across a range of fishing
intensities because the fishers are consistently cropping the
majority of those fish which recruit to the fishery. In
addition, an increased rate of biomass regeneration (fish
production) may compensate, in part, for the removal of
fish by the fishery. This possibility is compatible with the
widely reported decreases in size that have been attributed
to fishing (reviews Russ 1991; Jennings and Lock 1996)
since the form of fish growth trajectories is such that,
within a population of given biomass, decreases in mean
size are expected to lead to increases in mean production.
An additional consequence of there being a greater proportion of the biomass in smaller fish of fewer dominant
age classes may be that fluctuations in recruitment will
233
have greater relative effects on the total stock biomass and
thus lead to greater instability (variance) in population
size. This may make fishing effects harder to detect in
heavily fished areas.
Intensity of exploitation
The results of the present study suggest that, despite
decreases in total biomass, processes of recruitment and
fish production are sufficient to allow yields to be maintained at existing fishing levels. The total catch of reef
associated species landed by the small boat fishery off
Mah6, Praslin and La Digue was 759 t in 1992 (Anon
1993a) and the area (based on sea surface area) to 20 m
depth around these islands is approximately 170 km 2. The
seabed in this area is often sand, rock, seagrass or rubble
bottom and may not be fished as intensively as reef areas,
but it nevertheless forms a habitat crudely comparable
with those measured in other studies of area specific reef
fish yields (Russ 1991; Jennings and Polunin 1995b). The
mean yield from this area would be 4.5 t km 2 U x, which
suggests that the fishing grounds we studied were fished
with a range of fishing intensities from unfished to close
to the limits of sustainability (Russ 1991). It should be
emphasized, however, that the yields from heavily fished
areas may only be maintained because larvae can recruit
from adjacent and less heavily fished areas. Thus the
collective effects of fishing must be considered and the
study does not necessarily indicate that it is safe to increase
effort to a theoretical maximum throughout the islands.
Predator-preyrelationships
Whilst piscivory is one of the major processes of energy
transfer within the reef ecosystem (Grigg et al. 1984;
Parrish et al. 1985, 1986; Norris and Parrish 1988; Hixon
1991) the present study provides no evidence for the proliferation of prey species following a significant reduction
in the biomass of some of their predators. Thus small
(<20 cm maximum size) corallivores, detritivores, omnivores, and invertebrate feeders are neither target nor
retained bycatch species and yet there were no signs of
increased biomass in grounds where predator populations
had been reduced. Furthermore, if any responses were not
detected due to the variability in the data, they would have
been too small to compensate for the reduction in the
biomass of target species, and thus total community
biomass decreased significantly in response to fishing.
Other studies of predation effects have also shown no
marked responses in prey populations but have been
hampered by the immigration of new piscivores to the
study area and experimental artifacts or other extraneous
factors which affect prey abundance (e.g. Lassig 1982;
Stimson et al. 1982; Thresher 1983; Doherty and Sale
1985; Sphigel and Fishelson 1991). However, in the
present study there were significant differences between the
biomass of the censused piscivorous and invertebrate
feeding/piscivorous species at the different grounds,
although it is not clear how fishing has affected the density
of cryptic piscivores such as muraenids or roving
piscivores such as carangids and some elasmobranchs.
These species may play an important functional role in the
ecosystem. Sudekum et al. (1990) calculated the fish
consumption of two carangid species on Hawaiian reef
and they consumed more fish per unit area than are
characteristically removed by man from the most heavily
fished reef areas (Russ 1991). The lack of significant prey
release may be a function of the complexity of trophic
interactions within the reef ecosystem. Hixon (1991)
suggested that diffuse predation may mask predator-prey
relationships because the overall effect of all piscivores on
the prey population is substantial and yet the impact of
each species is minor. Bohnsack (1982) demonstrated
significant decreases in the biomass of a few piscivorous
species in a fished area and also found no convincing
evidence for widespread changes in prey populations.
Caley (1993) on the other hand removed all reef species
likely to consume fish (with the exception of roving
species) and found evidence for increases in the abundance
of some prey species.
The findings of the present study are important for
fishery managers because they suggest that the intensive
differential cropping of top predators may not necessarily
lead to increases in the biomass or productivity of their
prey. In enclosed lake fisheries there are examples of prey
release following piscivore removal (Marten 1979 a,b).
However, these systems are not open and lack the complexity of trophic linkages between fish species which
characterize reef ecosystems and the diversity of piscivorous species is considerably lower (Jones 1982). Munro
and Williams (1985) and Grigg et al. (1984) suggested that
actively fishing piscivorous species may lead to increases in
catches of prey species. It is unlikely that such a strategy
would provide sufficient additional yield to be effective
unless a very wide range of predators could be cropped,
using a method akin to the complex age, size and situation
related feeding strategies of the piscivorous fishes. A fishing strategy of this type is likely to be impractical. It may
be more beneficial to crop prey species on the basis that
they form a major proportion of total community biomass
and may be faster growing and therefore more productive
than species at higher trophic levels.
Acknowledgements. We wish to thank: Philippe Michaud, David
Boull6, Joel Nageon de Lestang and other staff of Seychelles Fishing
Authority for considerable technical and logistical assistance: JeanClaude Michel, Suzanne Marshall and staffof the Seychelles Division
of Environment for permission to work in Ste. Anne Marine Park,
logistical assistance and the provision of unpublished data; Robbie
Bresson and Jim Stevenson of BirdLife International for their
assistance with the work at Cousin; Chris Mees of the Renewable
Resources Assessment Group for useful advice; and the referees for
some valid comments which have been incorporated in the text. The
study was supported by the British Overseas Development Administration through its Fishery Management Science Programme.
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