Biogeographical considerations of fish diversity in Bioko

Biodiversity and Conservation 3, 808-827 ( 1994)

Biogeographical considerations of fish diversity in Bioko

RAMON CASTELO Asociacidn Amigos de Doriana. Cl Panamri No. 6, 41080, Sevilla, Spain

Received 4 July 1994; accepted 12 August 1994

The present study on the fish fauna of Bioko reports a total of 43 species of 21 families; 18 of which are new records for the island. Bioko’s icthyofauna is made up of 17 freshwater and 26 saltwater and brackish water species. Freshwater fishes are primarily restricted to a narrow belt along the north and west of the island, similar to the adjacent volcanic region around Mount Cameroon. The rest of the island, however, is inhabited by salt tolerant fishes, as found in SLo Tome, Principe and Annobon. Fish communities also vary according to elevation since rivers on Bioko are characterized by steep gradients from source to sea. Pressures on Bioko’s river systems are not alarmingly high but may increase in the future because of the use of toxic products (lime and insecticides) for fishing. There is no doubt that this is having a significant effect on northern and western rivers which are precisely the most species-rich. Aphyosemion oeseri, which is endemic to Bioko, and which has a distribution restricted to the small northern rivers is in great danger of extinction. P. monkei is in a similarly precarious situation, given that it is extremely rare and will probably disappear altogether in a few years from the island’s rivers. Before the situation becomes irreversible urgent measures must be taken to improve matters through educational campaigns as well as through cleaning up heavily-polluted rivers and re-stocking them with fish species from neighbouring river basins.

Keywords: fishes, Bioko, endemisms, biogeography, distribution

Introduction

According to Trewavas (1974) and Roberts (1975), Bioko and Mount Cameroon form part of a subregion within the Lower-Guinea icthyological province. This province encompasses Equatorial Guinea as well as coastal Gabon, Cameroon and SE Nigeria. Roberts’ (1975) empirical analysis of freshwater fish diversity coincides with the recent study by Hugueny (1990). However, Roberts includes the river basins in the Ivory Coast and Gambia in the ‘Upper Guinea’ province, whereas Hugueny considers them part of a Sahelo-Sudanese province. Likewise, rivers between the Ogun and Cross Rivers, which are classified by Roberts within the ‘Lower Guinea’ province, are regarded by Hugueny as also belonging to the Sahelo-Sudanese province. In this evaluation, the Bioko-Mount Cameroon subregion is considered to be between the Sahelo-Sudanese and the East Guinea regions.

Since the short and rather incomplete Bioko expeditions by Thys van den Aude- naerde (1967), Scheel (1968a,b) and Eisentraut (1973), little work has been done on the island’s rivers or on their fish fauna. Thys van den Audenaerde (1967, 1968), in fact, re-

0960-3115 ~CI 1994 Chapman & Hall

Fish diversity in Bioko go9

cognized the importance of additional investigations on Bioko’s icthyofauna and the need to study further some newly described species.

A research project, initiated in 1985, set out to investigate the epicontinental icthyofauna of Equatorial Guinea, including Bioko island. A seven-month study of the distribution and ecology of fishes in Bioko was undertaken between November 1988 and June 1989. This paper presents some results on the species richness and biogeography of the fishes encountered. All specimens are deposited in the scientific collection at the Asociacion Centro de Estudios Tropicales (Asociacion Amigos de Doiiana), Sevilla, Spain.

Survey methods

The present study is based on approximately 1800 specimens collected from 34 sampling stations in most of the main Bioko rivers (Fig. 1). Most fish were caught with the use of gill nets (1 and 1.5 cm) or with long-pole nets. In some instances, hook and line and traditional fishing methods were also employed. Fish specimens were fixed in 10% form01 and preserved in 70% alcohol.

Water samples were taken from 41 different localities. Water analyses consisted of pH determination using an ORION (Model SA 250) with a 91-06 electrode. Conductivity was measured with an INSTRAN (Model 10) and dissolved oxygen concentrations and temperature with a SYLAND (Model Simplair-S).

The aquatic environments

Bioko’s icthyofauna is restricted to rivers since there are not fishes present in the crater lagoons (Lago Biao, Lago Loreto etc.). Aquatic biotopes are similar to those found in the adjacent Mount Cameroon area and may be subdivided into three main types, as shown in Table 1. Not of all Bioko’s rivers are composed of these basic physical features; some have only the first part (Type I) because their slopes are extremely steep and they cascade directly into the sea; others have no middle section but have an estuary (Type II); and Type III are rivers that possess all three features.

Because of Bioko’s abrupt relief and relatively small size, the island has numerous, fast-flowing rivers (or, more aptly called, mountain torrents) radiating from the two main systems around Pica Basil& and the Southern Highlands (Fig. 1). There are abundant gullies, rapids and falls. Some rivers flow only a few kilometres to the sea, often descending from heights of over 1000 m. Heaviest flows are typical of the southern parts of the island where rainfall can be as high as 11 m per year (Fa, 1991). Monthly flow data from three different rivers in southern Bioko (Fig. 2), illustrate how maxima are typical between September and October and minima between April and May; a sharp drop occurs in August when precipitation decreases. During the dry season, the smallest rivers may dry up completely and, in many cases, (especially in the south), estuaries can remain separated from the sea by sand barriers that form on the beaches.

River beds are lined with rough gravel and large basaltic boulders derived from volcanic lava; most rivers are between I-10 m wide and 0.5 m deep. Water properties (analysed at the end of the dry season between 20 February 1990 and April 1990) differ significantly from continental waters in Rio Muni (personal observations) as well as

Caste10

Figure 1. Distribution of the main river systems in Bioko.

from those in the Cross River, Nigeria-Cameroon (Teugels et al., 1992) due to contrasts in the underlying geology. Oxygen levels in Bioko rivers fluctuate between 4.7 (25.3”C) mg I-’ and 8.6 mg I -- ’ (21,8”(Z) and pH values vary from slightly acid (6.60) to alkaline (8.33) (average pH 7.5). Characteristically, electrolyte content is very high; conductivity differing with river length from between 36.5 to 260 microsiemens cm-‘. Water temperature averages around 24°C but may range from 17.5”C at high altitudes to 25.8”C in the lower regions.

Fish diversity in Bioko

Table 1. Classification of the main biotopes found in rivers in Bioko

811

Upper reaches: Found at high elevations. Usually long stretches of very steep slopes and correspondingly fast-flowing waters. These parts are not suitable for coloni- zation by any fish species except those of the genus Sicydium. There is prac- tically no submerged vegetation except for some colonies of Anubias heterophylla. Plants, such as Impatiens spp. may be found along the banks.

Middle reaches: These are narrow stretches of water, usually found 100 m above sea level. Currents are often slow and banks provide sandy sheltered areas with rotting vegetation. The most common aquatic plant is Crinum natans and Apomogetum sp. occurs to a lesser extent. The most common vegetation is Commelina sp. and, particularly, Anubias heterophylla; along sheltered river- banks there may be some floating Pistia stratoides with Nimphaes lotus in some stagnant areas or muddy riverbeds. These are the richest parts of the rivers in terms of epicontinental icthyofauna and are inhabited by all freshwater fishes.

Lower reaches: Some of Bioko’s rivers form estuaries. As with most rivermouths in the adjacent continental areas, Bioko’s northeastern river vegetation is primarily mangroves with species such as Avicenna nitida, Laguncularia racemosa, Dal- bergia ecastophyllum, Drepanocarpus lunatus and Rhizophora mangle and, in places (e.g. the mouth of the Bosao river), associated halophytic Cyperaceae. Crinum natans occurs in some faster-flowing parts of the estuaries whereas the dominant riverbank vegetation consists of Cyperus spp., Kyllinga c$ odorata, Echinocloa spp., Cyrtosperma senegalensis and Raphia vintfera. Lep- toderris brachyptera is also very common in river estuaries and there are small colonies of Pandalus candelabrum in some scattered areas (e.g. Riaba, Ureca). This is the favoured stretch of river of Group II fishes (see Compo- sition of fish fauna).

Composition of the fish fauna

A total of 43 species of 21 families within nine fish groups was collected during the present study. Most of the families are represented by l-3 species, the largest family, the Gobiidae, being composed of seven species (Fig. 3). Although some nomenclature changes have altered Thys van den Audenaerde’s species list (1967, 1968), the present inventory adds another 18 new taxa to the island’s icthyofauna. All species collected by Thys van den Audenaerde (1967, 1968), except Arius latiscutatus, were recovered. Although the present study has added to the number of species found by Thys van den Audenaerde, his observation still holds that fish species richness in Bioko is much lower than in continental waters due to the ‘island factor’. Thys van den Audenaerde (1968) argued that it is because of the relatively small size of Bioko (many species may have become extinct after isolation from the continent) and the restricted range of biotopes, that fish fauna on and around the island is considerably poorer relative to the continent.

Thys van den Audenaerde (1967) divided the fish species in Bioko into two main groups: fishes of little or no saltwater tolerance (Group I) and marine and coastal fishes (Group II). Following this classification, the actual species list is composed of 17 Group I taxa and 26 Group II species.

Caste10

JAN FEE MAR APR MAY JUN JUL AUG SEF’ GCT NOV DEC

Figure 2. Flow fluctuations in three rivers in southern Bioko. (Source: Electricit& de France Inter- national

Group I: freshwater jishes

In Bioko, these fishes are primarily restricted to the lower parts of the permanent freshwater river basins in the NW and W of the island, along a narrow margin to the Bay of Luba. According to Hugueny (1990) six main subgroups can be recognized (Table 2):

Subgroup 11: Species endemic to Bioko and species common to the volcanic subregion of Bioko and Mount Cameroon (three species).

Subgroup 12: Species restricted to the Lower-Guinea region (six species). Subgroup 13: Species restricted to the coastal regions of northern Lower-Guinea and

southern Nigeria (Sahelo-Sudanese icthyological region) (one species). Subgroup 14: Species inhabiting the Lower and Upper Guinea coastal strip and

southern Nigeria (two species). Subgroup 15: Species found in the Lower Guinea, Southern Nigeria (Sahelo-Sudanese

region) and Middle Zaire areas (one species). Subgroup 16: Species of a wide geographical distribution (four species).

Fish diversity in Bioko 813

Gobildae

I ARIIDAE

BAGRIDAE

CARANGIDAE

CLARIIDAE

CHARACIDAE

DASYATIDAE

GERREIDAE

HAEMULIDAE

KYPHOSIDAE

MALAPTERLJRIDAE

MASTACEMBELIDAE

MONODACTILIDAE

Families with more than one species

Figure 3. Distribution of species according to fish families in Bioko.

Group II: saltwater and brackish water fishes

Species in this group (Table 3) are found in saltwater or brackish water areas adjacent to freshwater river mouths around most of the island. From Senegal to Angola the composition of this fish fauna is relatively homogeneous, thus Thys van den Aude- naerde (1967) argues that this group of fishes is of little local zoogeographical importance. This statement is probably valid for all species except A. m. scheeli, which is an endemic subspecies in the Lower Guinea coast. Subdivisions of this group, following the categories and terminology used by Thys van den Audenaerde (1967), are as follows:

Subgroup 111 Shore fishes. Only P. barbarus is included. This species inhabits the intertidal areas.

Subgroup 112 Lagoon fishes. These are fishes which spend a large part of their lives in coastal saltwater lagoons and estuaries (14 species).

Subgroup 113 Sporadic freshwater fishes. These are marine fishes tolerant of fresh or brackish water (eight species). It is certain that the number of species included in this group will probably increase with future explorations, since Osorio (1898) and Gunther (1859- 1870) have reported Cephalopholis nigri (Serranidae), Selene dorsalis, Trachinotus goreensis (Carangidae), Lutjanus fulgens, L. griseus (Lutjanidae). Surveys by the author in intertidal pools have revealed Abubdefduf analogus (Pomacentridae) and Hypleur- ochilus langi (Blennidae), all of which have been cited by other authors (Quero et al., 1990; Schneider, 1990; Leveque et al., 1992) in brackish water estuaries in different parts of West Africa.

Subgroup 114 Complementary fishes. These are species which live in freshwater except during their larval stages which develop at sea.

Tabl

e 2.

Gro

up

I fis

hes

in

Biok

o

Geog

raph

ical

distri

butio

n Fa

mily

Sp

ecies

Isl

and

distri

butio

n Co

mm

ents

Biok

o Cy

prino

dont

idae

Aphy

osem

ion

oese

ri (S

chm

idt.

1928

) Al

In

Bi

oko,

A.

oe

seri

is sy

mpatr

ic wi

th

A.

volca

num

, bo

th

of

which

ar

e pr

esen

t in

riv

ers

in

the

NW

of

the

islan

d.

Biok

o/M

ount

Ca

mer

oon

Cypr

inida

e

Cypr

inodo

ntida

e

Barb

us

thysi

(Tre

wava

s, 19

74)

Aphy

osem

ion

volca

num

(R

adda

an

d W

ildek

amp,

19

77)

A Al

Thys

va

n de

n Au

dena

erde

(1

967.

19

68)

iden

tifie

d a

three

- sp

otte

d ba

rbus

as

B.

tri

spilo

s bu

t po

inte

d to

th

e hig

her

posit

ion

of

the

seco

nd

spot

. La

ter

Trew

avas

(1

974)

su

gges

ted

that

th

is sp

ecies

, to

geth

er

with

th

ose

in

Mun

go

and

Mem

e riv

ers

be

nam

ed

Barb

us

thysi.

In

Bi

oko,

it

is pr

esen

t in

th

e low

er

parts

of

th

e M

irupu

ruru

, Ol

sina,

M

opob

ill an

d Ba

supu

riv

ers

in

the

north

we

st as

we

ll as

in

th

e so

uthwe

st (T

iburo

nes).

Radd

a an

d Fu

rzl(l9

87)

conc

luded

th

at

the

Chro

map

hyos

emion

sp

ecies

in

Bi

oko

and

Mou

nt

Cam

eroo

n we

re

Aphy

osem

ion

volca

num

. Th

ey

claim

it

to

be

ende

mic

to

the

Kake

riv

er

basin

an

d ot

her

sprin

gs

in

the

Mou

nt

Cam

eroo

n ar

ea,

altho

ugh

they

did

not

spec

ify

Biok

o.

They

be

lieve

it

to

be

symp

atric

with

A.

oe

seri

and

its

habit

at

‘smal

l str

eams

an

d sw

ampy

pa

rts

of

broo

ks

and

river

s in

th

e ra

infor

est

on

volca

nic

soils

’. In

Bi

oko,

A.

vo

lcanu

m

is pr

esen

t wi

th

A.

oerse

ri in

riv

ers

in

the

NW

of

the

islan

d.

Lowe

r Gu

inea

regio

n Cy

prini

dae

Cypr

inida

e

Bagr

idae

Barb

us

camp

taca

nthu

s (B

leeke

r. Al

18

63)

Barb

us

bate

sii

(Bou

lenge

r. 19

03)

A2

Para

uche

nogla

nis

mon

kei

(Keil

hack

, 19

10)

Al

Foun

d in

Bi

oko

in

some

NW

riv

ers

(Tim

babe

, Os

ina,

Cons

ul)

Foun

d in

fo

reste

d ar

eas

from

S. C

amer

oon/

N.

Gabo

n to

th

e Ch

ad

syste

m an

d Ti

besti

(D

aget

er

al.

. 19

84);

in

the

latte

r pla

ced

with

B.

by

nni

occid

enta

lis

(Bou

lenge

r, 19

11)

by

Leve

que

and

Gueg

an

(199

0).

In

Biok

o re

strict

ed

to

Rio

Tibu

rone

s.

Foun

d in

S.

Cam

eroo

n.

In

Biok

o it

is ve

ry ra

re

in

the

Osina

an

d Ba

sapu

riv

ers

in

the

NW.

Cypr

inod

ontid

ae

Cypr

inod

ontid

ae

Proc

atop

us

similis

(A

hl,

1927

)

Cich

lidae

Lowe

r-Gui

nea/

Sout

h Ni

geria

Ci

chlid

ae

Lowe

r-Gui

nea/

Sout

h Ni

geria

/Mid

dle

Zaire

M

asta

cem

belid

ae

Epipl

aiys

sexfa

rciatu

s ra

rhke

i (R

adda

, 19

71)

Chro

mid

otila

pia

finley

i (T

rewa

vas,

19

74)

Pelvi

cach

romi

s ta

eniat

us

(Bou

leng

er,

1901

)

Caec

omas

tace

mbe

luF

crypla

canr

hus

(Gun

ther

, 18

67)

A Le

vequ

e et

al

. (1

992)

rec

ogni

zes

two

subs

pecie

s of

E.

sexf

asci

atus

Gill

(186

3) f

or W

. Af

rica.

E.

s.

rath

kei

occu

rs

from

th

e Ku

mba

re

gion

to

Rio

M

uni

and

E.

s. to

golen

sis

from

S.

Tog

o to

Ben

in

and

Nige

ria

(Cro

ss R

iver).

The

two

diffe

r in

the

ir co

lour

pa

ttern

s.

E.

s. ra

thke

i ha

s a

blac

k ba

nd

in i

ts u

neve

n fin

s, a

s fo

und

in B

ioko

sp

ecim

ens.

In

Biok

o,

the

spec

ies

is fo

und

in t

he l

ower

re

ache

s of

som

e riv

ers

in t

he N

W

and

W (

Tibu

rone

s-

Luba

).

Al

Base

d on

the

pos

ition

of t

he p

elvic

fin

s,

shap

e of

fe

mal

es a

nd

no y

ello

w on

the

bac

k, T

rewa

vas

(197

4)

conc

lude

d th

at t

his

spec

ies

was

close

to

P.

glauc

icaud

us.

Radd

a (1

972)

an

d W

ildek

amp

et a

l., (

in

Dage

t et

al.,

19

86)

and

Rom

and

(in

Leve

que

et

al.,

1992

) pl

ace

it wi

th

P.

similis

.

Al

Acco

rdin

g to

Dag

et e

t al.

, (1

991)

, C

. finl

eyi

is r

estri

cted

to

the

Mun

go

and

Mem

e riv

ers

in M

ount

Ca

mer

oon,

Bi

oko

and

the

Krib

i riv

er b

asin

. In

Bio

ko,

it is

foun

d in

th

e lo

wer

area

s of

som

e N

W

river

s.

Al

Lim

ited

to t

he c

oast

al a

reas

of

S. N

iger

ia

and

S.

Cam

eroo

n.

In B

ioko

, it

has

been

cau

ght

in t

he O

sina

and

Basu

pu

river

s in

the

NW

of

the

isla

nd.

Al

Larg

er

spec

imen

s ha

ve a

rou

nded

ca

udal

fin

com

pare

d to

a p

oint

ed

one

in s

mal

ler

ones

(< 2

02 m

m).

Prea

nal

leng

th

diffe

rs r

elat

ive

to t

he p

osta

nal

betw

een

0.65

in

the

larg

er

spec

imen

s to

up

to 0

.85

in t

he s

mal

ler

ones

. Th

is

varia

tion

may

poi

nt

to t

he l

arge

r sp

ecim

ens

bein

g C

. lon

gicau

da

as s

ugge

sted

by

Trav

ers

(198

4a).

Foun

d in

the

M

iddl

e Za

ire

river

a

cent

ral

west

Afri

can

subl

ittor

al

rain

fore

st

zone

fro

m

the

Ntem

(R

io

Mun

i) riv

er

basin

to

the

Oue

me

(Ben

in)

river

but

exc

ludi

ng

the

Nige

r ba

sin.

In B

ioko

, it

has

only

been

fou

nd

in t

he u

nder

wate

r ve

geta

tion

of s

helte

red

river

bank

s in

NW

riv

ers

(Bas

upu,

O

sina

and

Miru

puru

ru).

Tabl

e 3.

Gro

up I

I fis

hes

in

Biok

o

Type

Fa

mily

Sp

ecies

Isl

and

distri

butio

n Co

mm

ents

Shor

e fis

hes

Gobii

dae

Perio

phth

almus

ba

rbar

us

(Linn

aeus

. 17

66)

A-B1

Th

is ge

nus

was

first

cons

idere

d in

th

e fa

mily

Pe

rioph

thal

mid

ae

but

Murd

y (1

989)

pla

ces

it wi

thin

th

e fa

mily

Go

biida

e.

Base

d on

os

teolo

gy,

Bird

song

et

al.

, (1

988)

de

scrib

es

this

grou

p as

un

ique

wi

thin

th

e fa

mily

, sin

ce

P.

barb

arus

is

a re

lict

spec

ies

foun

d in

W

est

Afric

a,

of

an

esse

ntial

ly we

stern

In

do-P

acific

dis

tribu

tion

This

spec

ies

is fo

und

in

brac

kish

water

s an

d so

met

imes

fre

shwa

ters

betw

een

Sene

gal

and

Ango

la.

In

Biok

o,

it wa

s fo

und

arou

nd

the

river

mouth

s al

ong

the

north

-wes

t.

Estu

arine

an

d la

goon

fis

hes

Cypr

inodo

ntiad

ae

Aploc

heilic

hthy

s sp

ilauc

hen

(Dum

eril.

1861

) A-

B1

Aploc

heilit

chhy

s m

acro

phth

almus

A2

sc

heeli

(R

oman

. 19

71)

Thys

va

n de

n Au

dena

erde

(1

967)

hi

ghlig

hted

th

e lar

ger

size

(max

imum

le

ngth

92

.5

mm

) of

th

is sp

ecies

in

Bi

oko

comp

ared

to

co

ntine

ntal

forms

. In

th

e pr

esen

t stu

dy

a m

ale

mea

surin

g 10

3 m

m

was

colle

cted.

Rom

and

(Lev

eque

et

al.

, 19

92)

cons

idere

d th

is a

subs

pecie

s be

caus

e of

co

loura

tion

diffe

renc

es

and

geog

raph

ical

distri

butio

n.

Wild

ekam

p et

al.

, (in

Da

get

ef

al.,

1986

) lis

ted

it as

a

full

spec

ies.

Ther

e ar

e no

sig

nifica

nt

diffe

renc

es

in

the

mer

istico

-mor

pholo

gical

relat

ions

given

by

Ro

man

(1

971)

for

th

e su

bspe

cies

and

thos

e of

Da

get

(197

9)

and

Radd

a an

d Pu

rzl(l9

87).

Howe

ver,

ther

e ar

e re

giona

l va

riatio

ns

that

dis

tingu

ish

this

subs

pecie

s in

Bi

oko:

it

is mu

ch

large

r (to

tal

leng

th

50

mm

) an

d gr

eate

r nu

mbe

r of

ra

dii

in

the

anal

fin

(1

3m

15).

It wa

s fo

und

in

fresh

an

d sa

ltwat

er

river

m

outh

s in

co

asta

l Ca

mer

oon

and

Rio

Mun

i. In

Bi

oko

it inh

abits

so

me

river

s ar

ound

th

e Lu

ba

area

,

Syng

nath

idae

Enne

ucam

pus

anso

rgii

(Bou

lenge

r, 19

10)

A-B1

Le

vequ

e et

al.

, (1

992)

re

ports

th

e sp

ecies

as

fo

und

alon

g co

asta

l W

est

Afric

a fro

m Se

nega

l to

An

gola

. Bi

oko

spec

imen

s wa

s co

llecte

d at

Ur

eca

and

in

Rio

Tibu

rone

s.

Tabl

e 2.

Con

tinue

d

Geo

grap

hica

l di

strib

utio

n Fa

mily

Sp

ecie

s Is

land

di

strib

utio

n Co

mm

ents

Lowe

r-Upp

er

Gui

nea/

So

uth

Nige

ria

Mor

myid

ae

Char

acid

ae

Isich

thys

henr

yi (G

ill,

1863

)

Bryc

inus

long

ipin

nis

(Gun

ther

, 18

64)

Al

Al

In B

ioko

, fo

und

in t

he B

asup

u an

d M

irupu

ruru

riv

ers.

Pres

ent

in t

he l

ower

re

ache

s of

som

e N

W

river

s in

Bi

oko.

Wid

e di

strib

utio

n Cy

prin

idae

Bu

rbus

ca

llipte

rus

(Bou

leng

er,

1907

) Al

Clar

ias

came

rune

nsis

(Lon

nber

g,

A 18

95)

This

spe

cies

is fo

und

in p

ract

ically

th

e en

tire

Suda

nese

pr

ovin

ce,

inclu

ding

th

e Ch

ad

river

bas

in,

sout

hern

Up

per

Gui

nea

and

north

ern

Lowe

r G

uine

a.

Repo

rted

in B

ioko

in

the

low

er M

irupu

ruru

riv

er

(Thy

s va

n de

n Au

dena

erde

, 19

67,

1968

) an

d in

the

Olsi

na

and

Basu

pu

river

bas

ins

in t

his

stud

y.

Clar

iidae

Th

ys v

an d

en A

uden

aerd

e (1

967)

ide

ntifie

d fiv

e sp

ecim

ens

from

Bi

oko

as C

. wa

lkeri.

G

unth

er

(189

6)

plac

ed

this

spe

cies

with

C

. poe

nsis

(de&

bed

as a

Bio

ko

ende

mic)

. Te

ugel

(19

86),

on t

he o

ther

ha

nd,

inclu

des

C.

came

rune

nsis

with

C.

wa

lkeri.

Th

is s

pecie

s is

foun

d th

roug

hout

th

e co

asta

l re

gion

fro

m

Togo

to

the

mou

th

of t

he Z

aire

riv

er.

In B

ioko

, it

was

foun

d in

the

low

er

north

ern

river

s fro

m

Cons

ul

west

to T

ibur

ones

.

Mal

apte

rurid

ae

Mala

pter

urus

ele

ctricu

s (G

mel

in,

1789

)

Hem

ichro

mis

bimac

ulatu

s (G

ill,

1862

)

Al

Cich

lidae

A2

This

spe

cies i

s di

strib

uted

th

roug

hout

Af

rica.

Re

porte

d on

ly in

the

NW

riv

ers

in B

ioko

.

Biok

o sp

ecim

ens

are

larg

er

(37-

43%

) as

opp

osed

to

3 l-

37%

fou

nd

in T

euge

ls (1

986)

and

Thy

s va

n de

n Au

dena

erde

(L

eveq

ue

et

al.,

1992

). Bi

oko

figur

es

are

simila

r to

tho

se r

epor

ted

by D

aget

(1

954)

for

the

Upp

er

Nige

r. Tb

e sp

ecie

s is

wid

ely

dist

ribut

ed

alon

g th

e co

asts

fro

m

the

Zaire

riv

er

to S

eneg

al a

nd t

he N

ile b

ut

not

in

Rio

Mum

. In

Bio

ko,

it is

onl

y fo

und

in t

he T

ibur

ones

riv

er

basin

in

the

sout

hwes

t.

Tabl

e 3.

Con

tinue

d

Type

Fa

mily

Sp

ecies

Isl

and

distri

butio

n Co

mm

ents

Syng

nath

idae

Micr

ophis

br

achy

urus

ac

uleaiu

s (K

aup,

18

56)

A-B1

Th

is sp

ecies

is

cons

idere

d an

adro

mou

s by

M

cDow

all

(198

8).

Thys

va

n de

n Au

dena

erde

(1

967)

fo

und

the

spec

ies

in

Biok

o.

Spec

imen

s we

re

colle

cted

from

weste

rn

and

sout

hern

riv

ers

durin

g th

e pr

esen

t stu

dy

beca

use

of

its

pref

eren

ce

for

vege

tatio

n ar

ound

br

ackis

h wa

ter

estu

aries

. Pr

obab

ly pr

esen

t th

roug

hout

th

e co

ast

arou

nd

Biok

o.

Cich

lidae

Ti

lapi

a gu

ineen

sis

(Blee

ker,

1862

) Al

Mon

odac

tylida

e Mo

noda

crylu

s se

bae

(Cuv

ier,

1829

) A-

B1

Mug

ilidae

Mug

ilidae

Mug

ilidae

Liza

falci

pinnis

(V

alenc

ienne

s, 18

36)

A-B1

Liza

gran

disqu

amis

(Vale

ncien

nes,

1836

)

Mug

il cu

rema

(V

alenc

ienne

s, 18

36)

A-B1

A-B1

This

spec

ies

occu

rs al

ong

the

W.

Afric

an

coas

t fro

m Se

nega

l to

An

gola

. In

Bi

oko,

it

has

been

ca

ught

in

th

e NW

of

th

e isl

and

in

fresh

an

d br

ackis

h wa

ter

area

s. It

prob

ably

occu

rs th

roug

hout

riv

er

estua

ries

in

the

north

west.

This

is th

e on

ly re

pres

enta

tive

of

this

typica

lly

Indo

- Pa

cific

fam

ily

in

Afric

a.

It ha

s a

wide

dis

tribu

tion

alon

g th

e ea

stern

At

lantic

co

ast

from

Cape

Ve

rde

to

Ango

la.

In

Biok

o,

it is

prob

ably

foun

d in

mo

st es

tuar

ies.

This

spec

ies

was

foun

d in

th

e m

outh

of

th

e Ba

supu

riv

er.

Cata

drom

ous

spec

ies

(McD

owall

, 19

88)

which

inh

ibits

both

fre

sh

and

salt

wate

r al

ong

all

river

es

tuarie

s an

d ad

jacen

t ar

eas

at

sea

level.

Ve

ry co

mm

on

betw

een

Sene

gal

and

the

Zaire

riv

er.

Acco

rding

to

Qu

ero

et

al.

(199

0)

the

rang

e of

th

is sp

ecies

is

from

Sene

gal

to

Cam

eroo

n.

Leve

que

et

al.

(199

2).

in

agre

emen

t wi

th

Sere

t an

d Op

ic (1

981)

ex

tends

its

dis

tribu

tion

to

the

Zaire

riv

er.

Restr

icted

to

co

asta

l fri

nge

from

Sene

gal

to

Zaire

. In

Bi

oko,

on

ly on

e sp

ecim

en

has

been

ca

ught

in

th

e no

rth

of

the

islan

d alo

ngsid

e L.

falc

ipinn

is.

The

spec

ies

is pr

obab

ly fo

und

in

an

area

sim

ilar

to

L. f

alcipi

nnis.

Gob

iidae

Gob

iidae

Gob

iidae

Eleo

trida

e Bo

stryc

hus

afric

amu

(Ste

inda

chue

r, 18

80)

Eleo

trida

e El

eotri

s vit

tata

(D

umer

il,

1858

)

Spor

adic

fresh

wate

r fis

hes

Dasy

atid

ae

Barh

ygob

ius

sopo

rator

(V

alen

cienn

es,

1837

)

Nem

atog

obius

br

achy

nemu

s (P

faff,

19

33)

Nem

atog

obius

m

aindr

oni

(Sau

vage

, 18

80)

A-B1

A-B1

A-B1

A-B1

A-B1

Dasy

atis

mar

garit

a (G

unth

er,

1870

) A-

B1

Mille

r an

d Sm

ith (

1989

) co

nsid

er

this

spe

cies

to b

e di

stin

ct

from

ot

her

cong

ener

ics

foun

d in

the

reg

ion.

Th

is

is a

n ea

st A

tlant

ic sp

ecie

s wh

ich

has

colo

nize

d th

e Af

rican

co

asts

from

M

aurit

ania

to

Ang

ola.

Spec

imen

s co

llect

ed

far

exce

ed th

e m

axim

um

size

re

porte

d fo

r th

e sp

ecim

ens

by M

iller

(199

0).

Biok

o sp

ecim

ens

mea

sure

d 11

9-15

3 m

m a

s op

pose

d to

57

mm

. Un

til

now,

N.

brac

hyne

mus

had

neve

r be

en f

ound

ou

tsid

e m

arin

e ha

bita

ts.

In B

ioko

an

d SH

o To

mb,

sp

ecim

ens

have

bee

n ca

ught

in

tidal

wa

ters

as

well

as in

pe

rman

ent

fresh

wate

r se

ctio

ns o

f st

rong

cur

rent

s.

Biok

o sp

ecim

ens

were

fou

nd

in m

outh

of

the

016

rive

r in

S.

Biok

o.

Harri

son

(199

0) c

onsid

ers

this

spe

cies

as s

ynon

ymou

s to

N.

an

sorg

ii; th

e on

ly tw

o sp

ecie

s re

stric

ted

to f

resh

wate

rs

in t

he c

entra

l an

d we

ster

n tro

pica

l Af

rica.

Q

uero

et

al.

(199

0) r

ecor

ds t

he s

pecie

s as

a b

enth

ic,

insh

ore

mar

ine,

es

tura

ine

and

fresh

-wat

er

fish

foun

d fro

m

Sene

gal

to

Cong

o an

d th

e G

ulf

of G

uine

a isl

ands

.

Durin

g th

is s

tudy

, tw

o sp

ecim

ens

were

col

lect

ed

burie

d in

the

ben

thic

mud

on

the

edg

e of

the

Tib

uron

es

river

.

This

fis

h is

foun

d al

ong

the

lowe

r re

ache

s of

mos

t riv

ers

from

Se

nega

l to

Zai

re

as w

ell

as in

all

Gul

f of

Gui

nea

islan

ds.

Acco

rdin

g to

Ser

et (

in l

eveq

ue e

t al.,

19

92)

this

spe

cies i

s co

mm

on

in t

he c

oast

of W

. Af

rica

but

less

abu

ndan

t th

an

D.

mar

garit

ella.

It

rang

es f

rom

Cab

o Bl

anc0

to

Ang

ola.

O

ne s

pecim

en w

as c

olle

cted

in

the

Elo

belo

riv

er

(Ure

ca),

200

m fr

om

the

river

’s m

outh

, at

a d

epth

of

1 m

.

Tabl

e 3.

Con

tinue

d

Type

Fa

mily

Sp

ecies

Isl

and

Distr

ibutio

n Co

mm

ents

Cara

ngida

e Ca

ranx

hip

pos

(Linn

aeus

, 17

66)

A-B1

Th

is sp

ecies

is

wide

ra

nging

in

bo

th

sides

of

th

e At

lantic

an

d in

th

e M

edite

rrane

an.

In

W.

Afric

a,

it oc

curs

as

far

sout

h as

S.

Ang

ola.

Bi

oko

samp

les

were

co

llecte

d at

Ur

eca

but

are

prob

ably

this

spec

ies

foun

d in

al

l es

tuarie

s an

d riv

er

mou

ths

on

estu

aries

.

Lutja

nida

e

Gerre

idae

Haem

ulid

ae

Ariid

ae

Lutja

nus

ende

caca

nthu

s (B

leeke

r, A-

B1

1863

)

Lutja

nus

gore

ensis

(V

alenc

ienne

s, 18

30)

A-Bt

l

Gerre

s m

elano

pter

us

(Blee

ker,

1863

) A-

B1

Pom

adaq

s ro

gerii

(Cuv

ier,

1830

) A-

B1

Arius

lat

iscut

atus

(G

unthe

r. 18

64)

A-B1

Very

com

mon

al

ong

the

coas

tal

strip

of

the

Gulf

and

in

Biok

o.

It is

foun

d in

es

tuarie

s an

d ad

jacen

t fre

shwa

ter

area

s. In

S.

Bi

oko,

th

is fis

h ca

n of

ten

be

seen

tra

pped

in

po

ols

in

small

riv

ers

which

ar

e se

para

ted

from

the

sea

by

sand

ba

rrier

s du

ring

the

dry

seas

on.

They

als

o co

me

in

from

the

sea

at

high

tid

e.

In

Leve

que

et

al.

‘Bau

chot

(in

19

92)

cons

iders

this

fish

to

be

distri

bute

d al

ong

the

west

Afric

an

coas

tal

area

s in

th

e Gu

lf of

Gu

inea

as

well

as

arou

nd

the

Cape

Ve

rde

islan

ds.

Alth

ough

th

is fis

h is

a m

arine

litt

oral

spec

ies

it of

ten

pene

trate

s es

tuarie

s es

pecia

lly

durin

g ris

es

in

river

lev

els.

It is

wide

ly dis

tribu

ted

from

Sene

gal

to

Ango

la

as

well

as

alon

g th

e W

ester

n At

lantic

.

This

is a

mar

ine

fish

which

inh

abits

th

e co

asta

l ar

eas

from

Sene

gal

to

Ango

la.

Spec

imen

s we

re

colle

cted

in

saltw

ater

ar

eas

in

S.

Biok

o.

This

spec

ies

(A.

gamb

ensis

) wa

s no

t fo

und

durin

g th

e pr

esen

t stu

dy.

Com

plem

enta

ry

fishe

s Go

biida

e

Kyph

osida

e

Gobii

dae

Gobii

dae

Kyph

osus

se

ctator

(L

innae

us,

1766

) A-

B1

Chon

opho

rus

busta

man

tei

(Gre

eIT,

1882

) A-

B1

Sicy

dizzm

br

evzji

le (O

gilvie

- Gr

ant,

1884

) A-

B1

Sicy

dium

bu

rtam

ante

i (G

reeff

, 18

82)

A-B

This

littor

al sp

ecies

, ha

s hit

herto

be

en

reco

rded

on

ly in

m

arine

en

viron

ment

s bu

t ha

s a

wide

dis

tribu

tion

alon

g th

e W

. Af

rican

co

ast,

in

the

Gulf

of

Guine

a no

rthwa

rds,

inclu

ding

th

e M

edite

rrane

an

(Ser

et

and

Opic,

19

81).

A sp

ecim

en

was

foun

d ab

out

200

m

near

th

e El

obel

o riv

er

mou

th

poss

ibly

brou

ght

in

by

the

tide.

Mau

ge

(198

6)

desc

ribes

th

is sp

ecies

as

fo

und

in

fresh

water

an

d re

strict

ed

to

Biok

o,

SBo

Tom

e an

d Ro

las.

Acco

rding

to

hi

m,

this

spec

ies

and

S.

bzzs

tama

ntei

are

ende

mic

to

the

Gulf

of

Guine

a isl

ands

bu

t th

is as

certi

on

may

be

incor

rect

since

th

eir

life

cycle

is

partl

y de

pend

ent

on

a m

arine

sta

ge,

will

enab

le

the

spec

ies

to

be

mor

e wi

dely

distri

bute

d.

Obse

rvatio

ns

from

the

pres

ent

study

, ind

icate

th

at

it ma

y liv

e in

fre

sh

and

saltw

ater

ar

eas

alon

g riv

er

mou

ths,

altho

ugh

its

first

stage

s de

velop

at

se

a. Sa

mple

s we

re

colle

cted

in

the

inter

tidal

zone

sw

imm

ing

upcu

rrent

from

the

sea

towar

ds

Eola

-Bai

ta.

Juve

nile

stage

s ar

e tra

nspa

rent

an

d on

ly de

velop

co

lour

when

the

y en

ter

fresh

water

, as

in

th

e ge

nus

Sicy

dium

.

Alth

ough

Ri

sch

and

Thys

va

n de

n Au

dena

erde

(1

979)

lim

it th

is sp

ecies

’ ra

nge

to

the

coas

tal

river

ba

sins

arou

nd

the

Gulf

of

Guine

a,

it is

foun

d in

th

e low

er

and

mid

dle

parts

of

al

l riv

ers

in

the

Gulf

islan

ds.

Its

life

cycle

is

prob

ably

simila

r to

S.

bu

stama

rzte

i.

Very

little

is

know

n ab

out

this

genu

s in

Af

rica.

It

has

an

amph

idrom

us

life

cycle

, sim

ilar

to

Pacif

ic an

d W

. In

dian

Oc

ean

cong

ener

ics.

Adult

s sp

end

most

of

their

liv

es

in

river

s, ev

en

durin

g sp

awnin

g.

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822 Castelo

Geographical zonation of Bioko’s fish fauna

The distribution of fresh and saltwater fish in Bioko can be classified into two main icthyological zones (Zones A and B) (Fig. 4).

ZONE A

Within this zone, most rivers are Type III and resemble those of the continental mainland. The zone encompasses the river mouths west of the island from below 100 m above sea level (depending on orogeny), north to Rebola and as far south as Luba. All first and second group fishes are present. The zone extends onto a littoral platform (at 50-100 m in depth), which has been the only connecting bridge between the continent and the island during the various geological stages. Zone A is therefore an area which has acquired its fish fauna as a result of migration of forms when Bioko was connected to the mainland. An important distinguishing geomorphological feature of this zone is the gentler sloping ground present when compared to the rest of the island. Because of this feature, vegetation is more abundant and this favours the development of life cycles of the different fish species.

This zone can be subdivided at the narrowing of contours around Basacato de1 Oeste at (3”35’35”N, 8”37’25”E) into a northern (Subzone Al) and a southern sector (Subzone A2).

Subzone AI This zone contains the majority of Group I species (12 species exclusive to it and another three - C. walkeri, E.s. rathkei and B. thysi. - shared with subzone A2).

Subzone A2 or centre- West Only two species are exclusive to it (H. bimaculatus and B. batesii).

ZONE B

Rivers in this zone are typically Types I and II and are identical to those found in the other Gulf of Guinea islands (Sao Tome, Principe and Annobon). It can be further subdivided altitudinally into an upper region (Subzone Bl) and a lower part (Subzone B2).

Subzone Bl This is a narrow coastal strip stretching from the sea to 100 m above sea level. It covers the northeastern, eastern, southern and southwestern perimeter of the island from Rebola to Luba. Only Group II fishes are present and in its steeper parts are only colo- nized by the genus Sycidium. In this subzone, the river basins have heavily undulating profiles and some cascade directly into the sea. There is no littoral platform here, so fishes are restricted from expansion into the interior by orographic barriers whilst freshwater fishes must contend with the sea water as another barrier (Myers, 1949).

Subzone B2 This area covers the highest parts of Bioko where only S. bustamantei is able to colonise after overcoming the steep slopes of the river basins, impassable to the other species.


41’30’

4%2

3030

7”: i 1000

1 Q , 2Km

I I I

8030’ 9000 9030’

Figure 4. Map of Bioko and adjacent regions showing the different icthyological zones, considered in the text.

824 Castelo

Status and conservation of species

In Bioko there are two factors which affect important icthyological areas making them extremely vulnerable to the extinction of species. These are: (i) the small size of the island and (ii) the fact that the two most densely populated towns, Malabo and Luba, are within icthyological subzones Al and A2, respectively. Human activity in the lower reaches of Bioko’s most important rivers (zone A) has, for decades, been direct and very intense since the surrounding areas are used almost exclusively for cocoa growing. However, it does not appear to have damaged the fish communities excessively despite the almost complete replacement of primary forest in the area and the periodic use of potentially toxic agents in the plantations e.g. copper sulphate and pesticides. Similarly, neither the hydroelectric plant in Semu (Musola river) nor the recently constructed one in Riaba (Ruma river) have affected fish communities, thanks to their particular design which does not stop water-flow and to their location, which in the case of the Semu plant is in the higher reaches of the river. The Riaba plant is outside any icthyological area of relevance.

Pollution of Bioko’s river systems is not alarmingly high but fishing pressures - ex- pressly forbidden by Equatorial Guinea’s laws -- using toxic products like lime and insecticides are all too common (according to our observations) in the dry season. There is no doubt that this is having a significant effect on northern and western rivers which are precisely the most species-rich. Indeed, the endemic Cyprinodont A. oeseri restricted to the small northern rivers is in great danger of extinction. P. monk& is in a similarly precarious situation, given its rarity. It will probably disappear altogether in a few years from the island’s rivers if no action is taken.

Before the situation becomes irreversible urgent measures must be taken to improve matters: (i) public education campaigns to make local people aware of the dangers of indiscriminate use of toxins and the abuse of the environment in general; (ii) the conservation of species in danger of extinction as well as (iii) the cleansing of heavily-polluted rivers and their re-stocking with fish species from neighbouring river basins.

Acknowledgements

The present study was undertaken as part of a project directed by Drs Benign0 Roman Gonzalez and Javier Castroviejo Bolivar within the Programa de Investigation y Con- servacion de la Naturaleza en Guinea Ecuatorial (Estacibn Biologica de Doiiana- Agencia Espafiola de Cooperation International). This study would not have been possible without the encouragement and help of the Ministerio de Cultura, Turismo y Promotion Artesanal and its Minister, Exmo Sr D. Leandro Mbomio Nsue.

I would like to acknowledge the assistance received from colleagues at the Asociacion Amigos de Dofiana at Huevar (Sevilla) in organizing the collection. My thanks also go to Dr Raymond Romand for assistance in identifying fish species and to Mr Manuel Fidalgo de Carvalho for help with plants. To Dr John E. Fa, I am grateful for his comments and assistance in reviewing first drafts of the manuscript and to MS Monique Williamson for translation of the paper from Spanish to English.


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Fish diversity in Bioko

Appendix

Vernacular names of$sh species in Bioko

Abat (F): Acung (F): Aferec (F): Afve-ngo (F): Aiieng (F): Dodo (Bu):

Ebabo (Bu): Ebome (C): Econo (F): Ec6-mb6 (F): Edaaduma (F): Efutbele (Bu): Esuale (Bu): Lobi (Bu):

Meloco (Bu):

Modoco (Bu): MO&h (Bu): Motond6 (C): Musongo (C):

Ndora (F): Ndoo (FN): NfvPa (F):

Nfvoga (F): Nfvon (FO): Nfvong (F):

Ngoo (F): Nguala (F):

Noco (Bu):

Ntotom (F): Nwong (F): Nyobo (F): Nyoc (F): fiuma (Bu): Sibith-sadola-dola (Bu): Sibith-sihilo (Bu): Siquellh (Bu):

(F): Fang (FO): Fang Okak (FN): Fang Ntumu (C): Combe (Bu): Bubi

Brycinus longipinnis Tilapia guineensis Chromidotilapiajinleyi Monodactylus sebae Malapterurus electricus Nematogobius brachynemus Nematogobius maindroni Aplocheilichthys spilauchen Caranx hippos (Juv.) Tilapia guineensis Gerres melanopterus Aplocheilichthys spilauchen Kyphosus sectator Eleotris vittata Liza falcipinnis Liza grandisquamis Mugil curema Sicydium brevtjile Sicydium bustamantei Periophthalmus barbarus Chonophorus bustamantei Caranx hippos Enneacampus ansorgii Microphis b. aculeatus Epiplatys s. rathkei Parauchenoglanis monkei Barbus camptacanthus Barbus thysi Barbus batesii Parauchenoglanis monkei Aphyosemion oeseri Aphyosemion volcanum Aplocheilichthys m. sheeli Procatopus similis Clarias camerunensis Hemichromis bimaculatus Pelvicachromis taeniatus Bostrychus africanus Eleotris vittata Isichthys henryi Caecomastacembelus cryptacanthus Barbus callipterus Bathygobius soporator Dasyatis margarita Lutjanus goreensis Lutjanus endecacanthus Pomadasys rogerii

827