ICES Journal of Marine Science, 57: 15–23. 2000
doi:10.1006/jmsc.1999.0572, available online at http://www.idealibrary.com on
The origin of the Saharan Bank cephalopod fishery
E. Balguerı́as, M. E. Quintero, and
C. L. Hernández-González
Balguerı́as, E., Quintero, M. E., and Hernández-González, C. L. 2000. The origin of
the Saharan Bank cephalopod fishery. – ICES Journal of Marine Science, 57: 15–23.
The Saharan Bank (West Africa, between 21N and 26N latitude) has been fished
since the fifteenth century. Bottom trawls were introduced during the Second World
War. Catches of cephalopods were very limited until the 1960s, when a spectacular
increase was observed in their landings. The apparent replacement of finfish by
cephalopods has been attributed to a change in the ecosystem due to overexploitation
of Sparidae, but the hypothesis has never been quantitatively confirmed. The evolution
of the profile of Spanish catches from the region from 1933 to 1996 shows a decreasing
trend in the numbers of Sparidae, which virtually disappeared from landings at the
beginning of the 1970s, with a simultaneous sharp increase in cephalopod catches.
However, results from surveys carried out on the bank in 1942, 1962, 1974, and 1990
are not entirely consistent with the replacement hypothesis. The data suggest that there
may have been some adjustment in the faunistic communities in response to fishing,
but that the change has by no means been of the magnitude suggested by fishery
statistics. We suggest that the changes observed are caused by a combination of
factors, including economic incentives as well as oceanographic variations and
competition for food, which have ultimately favoured benthic cephalopod populations
at the cost of most finfish populations.
2000 International Council for the Exploration of the Sea
Key words: assemblages changes, cephalopods fishery, Saharan Bank, West Africa.
Received 29 January 1999; accepted 15 October 1999.
E. Balguerı́as, M. E. Quintero, and C. L. Hernández-González: Centro Oceanográfico de Canarias, Instituto Español de Oceanografı́a, Carretera San Andrés s/n,
38120 Santa Cruz de Tenerife, Spain. Correspondence to E. Balguerı́as: tel: +34 922
549439; fax: +34 922 549554; e-mail: [email protected], [email protected],
[email protected]
Introduction
The Saharan Bank, situated along the northwest African
coast approximately between Cape Blanc (21N) and
Cape Boojador (26N) (Fig. 1) is considered one of the
richest fishing grounds in the world, attributed to the
broad continental shelf and the large and permanent
upwelling. Fishing in the region has been a continuous
and well documented activity since the fifteenth century,
almost exclusively by fishermen based in the Canary
Islands (Garcı́a Cabrera, 1970, in Balguerı́as, 1995;
Rumeu de Armas, 1977; Macı́as, 1982). Typically,
seasonal fisheries followed the biological cycles of target
species, mainly composed of Sparidae, Serranidae and
Sciaenidae, using handlines from dories transported to
the fishing grounds by sailing motherships. During the
two World Wars (especially the Second), fleets operating
in the North Atlantic moved to the Saharan Bank and
introduced bottom trawling as a new fishing method.
1054–3139/00/010015+09 $30.00/0
This was the beginning of a profitable fishery for demersal finfish, which for years was dominated by boats from
outside the region, particularly Portugal, France, Italy,
and the Spanish mainland.
Catches of cephalopods, mainly octopus (Octopus
vulgaris Cuvier, 1797), cuttlefish (Sepia officinalis
hierredda Linnaeus, 1758) and squid (Loligo vulgaris
Lamarck, 1798), were very small until the 1960s when a
spectacular increase was observed in their landings,
inducing boats already operating in the area to shift
their fishing strategy (Balguerı́as, 1995). The years from
1962 to 1980 were characterized by the massive presence
of trawlers from many different countries, which were
attracted by the new cephalopod fishery. In 1976, Spain
abandoned the Western Sahara and Morocco took over
the administration of this former Spanish province.
Most foreign fleets left the region at that time, and only
Moroccan and Spanish vessels have been exploiting the
demersal resources of the Saharan Bank since. The
2000 International Council for the Exploration of the Sea
16
E. Balguerı́as et al.
30°N
Canary Islands
Morocco
28°
Cape Juby
26°
Cape Bojador
Western
Sahara
Ba
nk
Sa
ha
ra
n
24°
Dakhla
22°
Cape Blanc
19°
17°
15°
Mauritania
13°
11°W
Figure 1. Geographical location of the Saharan Bank.
access of Spanish trawlers has been regulated by the
technical conditions imposed in successive fishing
agreements signed with Morocco, and later with the
European Commission.
The reasons for the change in dominant species in the
landings are unclear. Several authors have attributed it
to a change in the ecosystem due to an overexploitation
of Sparidae, possibly overlapping with temporal
anomalies in prevailing oceanographic features (Garcı́a
Cabrera, 1968, 1969, 1970 and Bas et al., 1970, in Bravo
de Laguna, 1982; Caddy, 1981, 1983; Bravo de Laguna,
1982). However, this hypothesis has never been
quantitatively confirmed.
We describe the apparent replacement of finfish by
cephalopods on the Saharan Bank over the period when
the process took place and compare changes in the
fishery with apparent changes in faunistic assemblages
occurring in the area. It was hoped that this would
provide some class, which could help eventually to
identify factors governing such a substitution. To
achieve this, historical Spanish catches on the bank have
been reconstructed for the period 1933 to 1996 using
different sources of information, and analyzed to determine the major episodes in the fishery from the beginning of bottom trawling up to the present day. This
information is compared with qualitative and quantitative information on the composition of faunistic assemblages obtained during four scientific surveys conducted
at different stages of development of the fishery.
Material and methods
Official records of Spanish landings in Canarian ports
between 1933 and 1972 (excluding 1935–1939 when no
statistics were collected) were used to estimate the catch
of demersal species from the Saharan Bank. All pelagic
species, as well as those known not to occur off the
African coast, were eliminated from the dataset. This
simple procedure is likely to provide a reasonably good
approximation to the real catches made by Spanish
vessels fishing on the Saharan Bank, because then most
of the vessels participating in the fishery operated from
the Canary Islands, while catches from littoral fisheries
around the archipelago were relatively minor and mainly
composed of small pelagics and other species easily
identifiable in the statistical record.
In 1974, a research programme was established to
assess the state of the resources exploited in the Eastern–
Central Atlantic. A statistical network was set up in
most ports along the southern Spanish mainland and in
the Canaries and information was collected on a trip-bytrip basis to produce fisheries statistics for the different
Spanish fleets operating along the African coast.
Spanish boats fishing the Saharan Bank can be grouped
in several categories, but only trawlers targeting
cephalopods (octopus, cuttlefish and squid) and a very
small artisanal fleet that concentrates on rocky bottoms
exploit demersal species on the continental shelf in
waters of less than 200 m depth. Information from the
former group has been utilized to complete the landings
series of demersal species between 1975 and 1996. No
catch data are available for 1973 and 1974 in the data
sets described. However, information on cephalopods
has been derived from FAO (1982). Unfortunately, no
data have been found on the by-catch in these two years.
The reconstructed series of Spanish statistics by major
taxonomic groups is obviously far below the actual
production, especially during the period between 1964
and 1980 when fleets from many different countries were
operating in the region. However, this dataset is the
longest and the only one providing information on
catches of both cephalopods and other species (mainly
finfish), thereby allowing comparative analyses.
Data from four surveys carried out in years representative for major episodes of bottom trawling were
re-analyzed to explore possible changes in faunistic
assemblages. The surveys selected represent the situation
at the beginning of the fishery (1942), the phase of
supposed expansion of cephalopod populations (1962),
the period when the cephalopod targeted fishery
attained its maximum activity (1974), and the current
situation of a more or less regulated fishery (1990).
Scientific names of fish species recorded were standardized according to Eschmeyer’s nomenclature (e.g.
Froese and Pauly, 1997). For other taxonomic groups,
FAO species identification sheets for the Eastern Central
Atlantic (Fischer et al., 1981) were used as reference.
To obtain comparable results from the surveys, only
hauls conducted between 21N and 26N and at depths
of less than 200 m were selected. Survey information
The origin of the Saharan Bank cephalopod fishery
17
Table 1. General survey information on and relative abundance indices from selected research cruises
conducted on the Saharan Bank.
1942
General survey information
Vessel
Cierzo
Country
Spain
Date
24 July–4 Sept
Hauls analysed
151
Hours fishing
341.3
Depth (m)
32–180
Latitude (N)
2001–2433
Longitude (W)
1623–1735
1962
1974
1990
Thalassa
France
10 Nov–8 Dec
18
20.9
30–102
2010–2555
1442–1738
Atlor V
Spain
7–28 April
20
19.3
15–146
2040–2538
1452–1726
Congel
Morocco
29 Sept–27 Oct
93
54.2
17–105
2034–2531
1425–1713
Relative abundance indices (kg h 1)
Fish
2003
Crustaceans
12
Molluscs
149
Echinoderms
2
Other invertebrates
9
Total
2175
Relative abundance indices (%)
Fish
Crustaceans
Molluscs
Echinoderms
Other invertebrates
92.12
0.54
6.83
0.09
0.42
(after haul selection) is provided in Table 1. Relative
abundance indices were calculated for all taxa occurring
in each survey using two different approaches. During
the cruise conducted in 1942, information on abundance
was recorded according to a qualitative six-point scale:
1, very rare; 2, rare; 3, not very abundant; 4, abundant;
5, very abundant; and 6, extraordinarily abundant. The
total of points attributed to each taxon divided by the
total number of hauls has been used as a mean qualitative index of abundance. The report of this survey
(Navarro, 1943) also provides indications of the total
catch of commercial species, by-catch and discards,
which has permitted an estimate of relative abundance
expressed in kg/h trawling. This total abundance index
was distributed among taxonomic groups in proportion
to their mean qualitative index of abundance. Information from the three other cruises includes numbers
and weights of the different taxa caught in each haul,
allowing indices of relative abundance to be calculated.
Percentage representation of each taxon in the total
relative abundance index (expressed as a qualitative
index for the 1942 cruise and in number of individuals
caught per hour trawling for the others) within each
cruise were also calculated. Thus, only relative indices
are available for comparative analyses among cruises
due to the absence of real abundance or biomass
numbers by taxon in the dataset from 1942, which is
used as a baseline. The problem is that these do not
allow us to follow the evolution of species abundance in
absolute numbers. While decreases in the percentage
457
0
96
16
569
80.32
0.02
16.78
2.88
152
40
20
211
71.66
18.70
9.64
252
3
31
0
36
322
78.20
0.86
9.61
0.05
11.28
representation between cruises of a particular taxon may
reflect decreases in abundance, the opposite is not
necessarily true.
Results
The evolution of Spanish catches by taxonomic groups
(Fig. 2) shows that the total catch ranged from a
minimum of around 500 t in 1933 to a maximum of
more than 100 000 t in 1976. The gradual decline in
Spanish fishing rights since 1981 is responsible for the
subsequent decline in catches.
The catch composition reveals three welldifferentiated periods. Until the beginning of the 1960s,
catches were small and mainly composed of finfish that
were salted and sold in local markets. Other minor
groups observed were crustaceans (lobsters and crabs)
and molluscs, including flying squids and several
bivalves species. Fluctuations in the production from
that period were probably related to a combination of
fishing effort and abundance of target species. Between
1961 and 1970, the fishery entered a transition phase still
characterized by the dominance of finfish but increasingly consisting of cephalopods. Crustaceans and other
molluscs dissappeared completely during this phase. The
third period extends from 1971 until the present and
reflects a highly productive cephalopod-based fishery
where only a small percentage of the landings is
represented by finfish.
18
E. Balguerı́as et al.
(a)
120 000
Octopus
Squid
Other taxa
100 000
Cuttlefish
Flying squid
80 000
60 000
40 000
Catch (t)
20 000
0
(b)
120 000
100 000
80 000
Haemulidae
Other taxa
Sciaenidae
Sparidae
Serranidae
Other fishes
Pleuronectiforms
60 000
40 000
20 000
0
33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 78 81 84 87 90 93 96
Year
Figure 2. Composition of Spanish landings (in tons) from
the Saharan Bank, 1933–1990. (a) By major cephalopod taxa,
(b) by major fish taxa.
The species composition of the cephalopod landings
(Fig. 2(a)) also reveals three major shifts in the fishery.
At first, the interest was focused on squid (Loligo
vulgaris), a valuable species on Spanish markets. During
the transition phase starting in the 1960s, squid landings
were supplemented by cuttlefish, but within a few years,
octopus dominated the catches. During this period,
Japanese vessels entered the fishery and an international
market for octopus opened up. The third shift coincides
with the final stage of the fishery when octopus accounts
on average for more than 90% to the total landings of
cephalopods and where squid is generally subordinate to
cuttlefish.
Similar trends are observed in finfish (Fig. 2(b)).
The bulk of the finfish landings until 1960 consisted
of species belonging to the Sparidae and Sciaenidae.
During the transition phase, production of these two
families decreased gradually until 1972, whereas the
Haemulidae increased significantly. The catch composition between 1975 and 1982 is uncertain, but
finfish were only a by-catch. After 1982, Sparidae and
Sciaenidae, major families in the past, have been almost
completely substituted by Pleuronectiforms.
According to the survey data, the combined
abundance of all faunistic groups in the Saharan Bank
appears to have decreased substantially from 2000 kg/h
in 1942 to less than 600 kg/h in 1962, and even smaller
values in the two most recent years (Table 1). This
decline apparently involves mainly finfish and to a
lesser extent molluscs. Interpretation of changes in
groups other than finfish is difficult since the survey
objectives concentrated on commercial species and
therefore less attention was paid to the correct identification of secondary species, which were grouped into
different categories depending on criteria set by the
responsible scientists. For instance, during the ATLOR
V cruise, all invertebrates other than cephalopods were
recorded as ‘‘other invertebrates’’. Nevertheless, the
data clearly show that finfish have progressively diminished from 92% in 1942 to 78% in 1990, while crustaceans and molluscs have remained fairly stable and
other invertebrates have increased. These changes in
abundance do not seem to have had a marked effect
on species richness of finfish occurring on the Saharan
Bank (39–45 families and 70–109 different species
identified by survey; Table 2). However, the number of
taxa caught is, among others, a function of the number
of hauls (habitats visited) and fishing time, which
varied considerably during the different surveys
(Table 1).
When considering individual families (Table 2),
several changes are observed. A number of families have
clearly dropped in numbers of individuals from 1942
until now. Within the elasmobranchs (Chondrichthyes),
all families but one (Scyliorhinidae) experienced severe
decline in relative abundance. Also the total number of
families and species represented in the surveys decreased
from 1942 to 1990.
Among the bony fishes (Osteichthyes), Psettodidae,
Anthiidae, Chaetodontidae, Pomatomidae, Tetraodontidae, Trichiuridae, Uranoscopidae, Sciaenidae, and
Serranidae have diminished in abundance. Within the
Serranidae Epinephelus aeneus (Geoffroy Saint Hilaire,
1817) accounted for 5.3% of the mean total abundance
of fish in 1942 and declined to 0.001% in 1974. The
species was not recorded during the cruise conducted
in 1990. Other bony fish such as flatfish (except
Psettodidae), and families Haemulidae, Mullidae,
Sparidae, and Triglidae, exhibit increasing trends in
percentage representation. The Sparidae accounted for
the bulk of the fish within each survey, both in terms of
number of species and average number of individuals caught. Within this family, Pagellus erythrinus
(Linnaeus, 1758), Pagrus caeruleostictus (Valenciennes,
1830), Pagrus pagrus (Linnaeus, 1758), Diplodus
puntazzo (Cetti, 1777), and Sparus aurata Linnaeus,
1758 show decreasing trends in relative abundance.
These losses are compensated by increase in Diplodus
bellotti (Steindachner, 1882), Spondyliosoma cantharus
(Linnaeus, 1758) and Dentex macrophthalmus (Bloch,
1791), which occasionally represent a considerable
percentage of the total mean abundance of the finfish
caught in different surveys.
The origin of the Saharan Bank cephalopod fishery
19
Table 2. Percentage contribution to the total fish catch and number of species (n) by family during the
research cruises on the Saharan Bank by year. The overall number of species observed during all
cruises is also given, as well as sub-totals for elasmobranchs and bony fish separately.
Family
Chondroichthyes
Carcharhinidae
Dasyatidae
Gymnuridae
Leptochariidae
Mobulidae
Myliobatidae
Rajidae
Rhinobatidae
Scyliorhinidae
Squalidae
Squatinidae
Torpedinidae
Triakidae
N families
n species
% contribution
Ostheichthyes
Ammodytidae
Antennariidae
Anthiidae
Ariidae
Balistidae
Batrachoididae
Bothidae
Callionymidae
Caproidae
Carangidae
Carapidae
Centracanthidae
Centrolophidae
Cepolidae
Chaetodontidae
Citharidae
Clupeidae
Congridae
Cynoglossidae
Diodontidae
Echeneidae
Engraulidae
Ephippidae
Fistulariidae
Gobiidae
Haemulidae
Labridae
Lophiidae
Macroramphosidae
Macrouridae
Melanonidae
Merlucciidae
Monacanthidae
Mugilidae
Mullidae
Muraenidae
Myctophidae
Odontaspididae
Ophichthidae
Ophidiidae
All
(n)
(2)
(2)
(1)
(1)
(1)
(1)
(10)
(1)
(1)
(2)
(2)
(2)
(3)
(13)
(29)
(1)
(1)
(1)
(1)
(1)
(1)
(8)
(2)
(2)
(9)
(1)
(1)
(1)
(1)
(1)
(1)
(3)
(3)
(1)
(2)
(2)
(1)
(1)
(1)
(2)
(2)
(1)
(1)
(2)
(1)
(2)
(1)
(3)
(2)
(2)
(1)
(5)
(3)
1942
(n)
0.50
0.16
0.11
0.32
0.18
0.02
2.93
0.98
(2)
(1)
(1)
(1)
(1)
(1)
(2)
(1)
0.70
0.32
6.76
(2)
(1)
(3)
(11)
(16)
13.0
0.02
(1)
2.18
0.07
1.29
0.02
(1)
(1)
(1)
(1)
9.34
(6)
0.05
(1)
0.18
(1)
0.14
1.11
0.11
(1)
(2)
(2)
0.27
0.52
(1)
(1)
2.61
(2)
0.09
0.39
0.09
(1)
(1)
0.09
(1)
0.02
(1)
1962
(n)
0.03
<0.01
(1)
(1)
<0.01
1.16
0.55
1.22
0.02
0.03
0.04
0.14
(1)
(6)
(1)
(1)
(2)
(1)
(2)
(2)
(10)
(18)
3.2
1974
(n)
1990
(n)
<0.01
<0.01
0.19
(1)
(7)
1.76
0.04
<0.01
<0.01
<0.01
(1)
(1)
(1)
(1)
(1)
(7)
(13)
2.0
0.14
<0.01
0.60
<0.01
0.25
1.0
(3)
(1)
(1)
(1)
(6)
(6)
0.01
(1)
0.07
(1)
<0.01
(1)
<0.01
(1)
0.06
2.09
(1)
(6)
0.04
20.94
0.02
0.78
0.03
(2)
(5)
(1)
(1)
(1)
0.04
0.84
0.04
1.81
3.09
(1)
(4)
(1)
(1)
(2)
1.02
1.08
1.17
0.02
14.48
(1)
(1)
(1)
(1)
(2)
0.03
2.15
0.05
0.06
(1)
(1)
(1)
(2)
<0.01
(1)
1.24
1.04
<0.01
(1)
(1)
(2)
0.69
2.28
0.02
(1)
(3)
(2)
0.25
(1)
1.29
(1)
0.45
0.13
<0.01
<0.01
9.28
(1)
(1)
(1)
(1)
(2)
0.43
0.10
<0.01
(1)
(1)
0.15
(1)
<0.01
<0.01
<0.01
4.52
0.55
0.38
(1)
(1)
0.03
(1)
0.13
0.01
0.03
(1)
(1)
(1)
0.25
<0.01
0.06
(1)
(1)
(1)
0.81
0.36
(3)
(1)
0.02
0.28
(2)
(2)
(2)
(1)
(3)
0.06
Continued over
20
E. Balguerı́as et al.
Table 2. Continued.
Family
Ostheichthyes continued
Peristediidae
Pomadasyidae
Pomatomidae
Psettodidae
Sciaenidae
Scombridae
Scorpaenidae
Serranidae
Soleidae
Sparidae
Sphyraenidae
Sphyrnidae
Syngnathidae
Synodontidae
Tetraodontidae
Trachichthyidae
Trachinidae
Trichiuridae
Triglidae
Uranoscopidae
Zeidae
Unidentified
N families
n species
% contribution
Total
N families
n species
All
(n)
(1)
(1)
(1)
(2)
(7)
(5)
(7)
(9)
(11)
(23)
(1)
(1)
(1)
(3)
(4)
(1)
(4)
(2)
(10)
(2)
(1)
61
172
74
201
1942
1962
(n)
0.66
4.54
5.60
3.72
0.50
(1)
(1)
(1)
(5)
(4)
7.07
4.72
33.83
0.59
0.05
(4)
(3)
(11)
(1)
(1)
<0.01
0.41
0.04
0.14
1.37
0.14
0.63
0.74
2.98
48.47
(1)
(1)
(1)
(1)
(2)
(1)
(6)
(5)
(7)
(15)
(3)
(1)
(3)
(2)
(6)
(1)
(1)
0.05
(2)
0.03
0.01
0.86
1.38
0.20
4.58
0.09
(1)
(1)
(2)
(1)
(1)
1.66
0.14
7.96
2.64
0.80
87.0
35
66
(46)
(82)
The evolution of the species composition of molluscs
shows an increase in the percentage of cephalopods, and
consequently a decrease in ‘‘other molluscs’’ (Table 3).
Irrespectively of trends in actual abundance, cephalopods make up almost the entire mollusc fauna in trawl
catches on the Saharan Bank (98% in 1990). The
Octopodidae and especially Loliginidae experienced
increases in relative abundance, Loligo vulgaris (22% in
1942 and 83% in 1990) being responsible for most of
the increase. Ommastrephidae diminished in relative
abundance, and Sepiidae did not show a clear trend.
Discussion
Observed changes in the catch composition of multispecies fisheries have often been of concern in different
areas because of management implications. Most
changes have been simply described qualitatively,
although sometimes indications have also been provided
to explain the ecological causes and consequences.
Recently more work has been devoted to try to quantify
the magnitude of such changes and to identify major
factors driving the underlying processes (e.g., Coutin
and Payne, 1989; Harris and Poiner, 1991; Gabriel,
(n)
96.8
38
91
(48)
(109)
1974
(n)
0.03
0.17
<0.01
(1)
(1)
(1)
<0.01
(1)
0.21
0.97
10.59
58.90
(3)
(2)
(8)
(16)
0.34
(1)
<0.01
<0.01
1.44
0.34
5.86
0.04
0.07
(1)
(1)
(3)
(1)
(6)
(1)
(1)
98.0
38
78
(45)
(91)
1990
(n)
9.65
(1)
<0.01
0.12
<0.01
0.04
0.08
1.14
52.39
<0.01
(1)
(2)
(1)
(1)
(3)
(20)
(1)
0.03
(1)
0.62
<0.01
7.34
0.04
0.36
<0.01
(3)
(1)
(5)
(1)
(1)
99.0
33
64
(39)
(70)
1992). Regarding cephalopods, extensive reviews
directly or indirectly related to the subjects have been
prepared by Caddy (1981, 1983) and Caddy and
Rodhouse (1998). Caddy and Rodhouse (1998) in particular deal with the transition from finfish-targeted
fisheries to cephalopod-targeted fisheries at different
geographical scales and from the perspective of oceanic,
neritic, and benthic cephalopods. The case of the
Saharan Bank fishery represents an example in these
studies where the main cause alluded to the species
substitution is overfishing of the Sparidae stocks in the
area, as previously proposed by other authors (see Bravo
de Laguna, 1982).
The evolution of the Spanish catches on the Saharan
Bank is, in principle, consistent with this hypothesis. The
rapid decline of Sparidae landings and the parallel
increase in those of cephalopods during the 1960s suggest a direct replacement in the ecosystem of one group
by the other. However, facts are not so simple. The
traditional fisheries on the Saharan Bank were conducted by fishermen from the Canaries. They used
hand-lines on hard bottoms to catch big specimens of
Sparidae, Sciaenidae, Serranidae and Haemulidae which
were preserved dry and salted, because this was the only
The origin of the Saharan Bank cephalopod fishery
21
Table 3. Percentage composition of cephalopod species in the total catch of molluscs during the
research cruises on the Saharan Bank by year.
Family
Loliginidae
Octopodidae
Ommastrephidae
Sepiidae
Species
Alloteuthis africana
Alloteuthis media
Alloteuthis subulata
Loligo vulgaris
Total
Octopus macropus
Octopus vulgaris
Scaergus unicirrus
Total
1942
bertheloti
elegans
officinalis hierredda
orbignyana
sp.
conservation method available at that time. Fishing
strategy, preservation method, and market demands
determined target species and sizes. Only a marginal
amount of relatively small-sized fish, mainly composed
of species belonging to the Sparidae, was retained to
be sold as a low-quality product at a low price. When
bottom trawls were introduced, target species and
preservation methods remained the same in the beginning but some unwanted species, for which there was no
market in the islands, appeared in larger quantities than
before due partly to gear selectivity and partly to the
necessity of moving to fishing grounds suitable for
trawling. Some of these species were transformed
into fishmeal in factories appearing in the archipelago.
Others, such as octopus, did not have any useful destination. Old fishermen and factory managers confirm
that in the 1950s octopus was the predominant species in
individual hauls. An initiative to establish a trade
market for dried octopus with companies from the
Spanish mainland, where consumption of this species
was popular, even failed (Balguerı́as, 1995). When the
Japanese fleet arrived at the Saharan Bank with new
preservation methods (i.e. freezing) that guaranteed the
durability and high quality of the product, and offered a
high market value, most trawlers rapidly modified their
practices and shifted their target to cephalopods.
1990
0.73
0.51
0.06
25.01
7.31
34.93
67.31
4.98
25.65
26.89
1.53
1.31
1.53
1.31
1.02
0.04
0.07
1.13
37.00
0.04
58.65
2.44
37.00
61.13
Undetermined
Total Cephalopods
Total other Mulluscs
1974
22.32
22.32
Illex coindetii
Ommastrephes sagittatus
Todarodes sp.
Todaropsis eblanae
Total
Sepia
Sepia
Sepia
Sepia
Sepia
Total
1962
10.99
0.06
11.06
0.31
82.97
87.94
0.01
7.46
7.47
0.19
0.09
0.31
0.28
6.39
0.06
6.55
6.74
19.69
1.37
0.94
0.01
2.37
98.36
1.64
98.06
1.94
0.15
60.86
39.14
90.60
9.40
This event, partly reported by Caddy (1981), had a
great influence on the fishery. Pair trawlers constituted
the larger part of the fleet at that time, but were
gradually substituted by modern freezing trawlers exclusively targeting cephalopods. Finfish species that in the
past had constituted their target became a by-catch or
were discarded and no longer appeared in the statistics.
In fact, the criteria for keeping or discarding a particular
species are well established in the fishery after it had
reached the developed phase in the 1970s. In general,
only individuals of valuable species are retained for
marketing, such as Solea vulgaris Quensel, 1806,
Dicologlossa cuneata Moreau, 1881, Merluccius
merluccius (L., 1758), Lophius piscatorius L., 1758, as
well as some large Sparidae, Sciaenidae, Haemulidae,
and Serranidae accidentally caught in sufficient
quantities in single hauls (Balguerı́as et al., 1990). All
other species are discarded. This practice has been
assessed several times (Bravo de Laguna et al., 1976,
1977a, 1977b; CECAF, 1982; Balguerı́as et al., 1993;
Balguerı́as, 1996), showing that discards of all taxa
combined constitute around the 50% of the total catch in
weight. Thus, changes in landings composition do not
necessarily reflect changes in faunistic assemblages.
The survey results indicate that during the studied
period, ecological changes have occurred over a period
22
E. Balguerı́as et al.
of 50 years, but these are much less than would be
inferred from landings statistics. The most outstanding
change is the severe reduction in the total catch rate
from 1942 to 1990 (by a factor of seven), which might
be exclusively attributable to intensified fishing. This
reduction does not seem to have affected all taxa evenly.
Finfish have been reduced in relative abundance,
whereas other groups, such as crustaceans and molluscs,
have increased. This may reflect an adjustment of the
ecosystem. However, it remains difficult to evaluate
these changes in detail because grouping criteria differed
between cruises. Moreover, survey gears were not standardized and number of hauls and haul duration also
varied considerably.
Although the methodology used in the surveys does
not permit us to follow the evolution of the faunistic
assemblages quantitatively, some interesting conclusions
can be drawn. Crustaceans have not been well analyzed,
but commercially important species such as big crabs
and lobsters have become very rare. This is corroborated
by the landing statistics in which they have not appeared
since 1972. Other crustaceans seem to have increased
quite significantly in numbers (Balguerı́as, 1996).
Molluscs represent another interesting case. Most
bivalve species that were abundant during the 1942
cruise seem to have disappeared almost completely. This
might be related to the continuous disturbance of their
habitat by bottom trawls. In contrast, marked increases
in relative abundance have been observed among the
cephalopods, particularly Loligo vulgaris. Strangely, this
species has only been targeted at the beginning of the
cephalopod fishery. This may be partly explained by its
patch distribution and partly by the high market price of
octopus. L. vulgaris tends to aggregate in unispecific
schools of the shelf break, which renders exploitation
difficult and less profitable. Another group that has
apparently become more abundant is the genus
Cymbium belonging to the gastropods (Balguerı́as,
1996).
With reference to finfish species, the cruise results
indicate that only the most sensitive species have been
reduced in relative abundance and probably even more
so in absolute numbers. These include territorial
predators such as Serranidae and some Sparidae, low
fecundity species such as Uranoscopidae, and most
Chondrichthyes, as well as species living at the limits of
their distributional range. Decreasing trends observed in
other fish families are not so easily attributable to
intensive fishing and other factors such as catchability or
seasonal availability (typical of some Sciaenidae) might
bias the results to some degree. Cruise results show no
marked changes among the Sparidae. Although some
larger species were less frequently caught in recent years
than before, the family as a whole still constitutes the
major component of the fish community, both in
number of species and abundance.
FISHERY
BENTHIC CEPHALOPODS
(Octopus and Cuttlefish)
FISH
OTHER INVERTEBRATES
??
DISCARDS
CRUSTACEANS
Figure 3. Proposed trophic relationship between benthic
assemblages on the Saharan Bank.
The major conclusion to be drawn from the observation is that faunistic communities have been adjusted
in favour of the cephalopod populations but the changes
are much smaller than fisheries statistics would suggest.
This makes it easier to accommodate the different
hypotheses that have been put forward to explain the
supposed replacement process (see Caddy, 1981, 1983;
Caddy and Rodhous, 1998).
In our view, there is no prevalent event responsible for
the observed changes but, more likely, a combination of
factors is behind driving these processes. First of all,
fisheries statistics present an exaggerated picture of
the ‘‘replacement’’ of finfish by cephalopods, and this
picture is severely biased by changes in the economic
value of the different components over time. Heavy
trawling has originally contributed to more favourable
conditions for cephalopods by removing main predators
(Chondrichthyes, Serranidae, big Sparidae) and competitors and possibly by preparing substratum for
colonization. Propitious oceanographic conditions leading to subsequent good recruitment and the opportunistic characteristics of cephalopods may have contributed
to their gradual expansion. Nevertheless, dense concentrations were reported by fishermen long ago and had
also been detected already during the survey in 1942.
The introduction of trawling led to discarding of
species of low economic value. Initially, cephalopods
were discarded but the shift in fishing strategy had
consequences for other species. Discarding practices
may have played an important role in the expansion and
maintenance of benthic cephalopod populations. The
possible ecological effects of discards have been widely
discussed by different authors (Saila, 1983; Alverson
et al., 1994; FAO, 1996). In the case of the Saharan
Bank, the amount of organic matter discarded has been
estimated at around 114 000 t per year (Balguerı́as,
1996), which may have affected trophic relationships.
Figure 3 summarizes conceptually the major pattern
in the present food web. Fishing and discarding of
The origin of the Saharan Bank cephalopod fishery
non-marketable species may have had a double effect on
the system by keeping predator populations and competitors of the benthic cephalopods at a low level and by
providing scavengers with the necessary food to keep
their populations at a high level (Balguerı́as et al., 1993;
Balguerı́as, 1996). Most of these scavengers, particularly
crustaceans, many of which are important prey items in
the diet of cephalopods, are tough, and survive after
being discarded themselves. This quality could also have
contributed to their increases. The idea of implications
of discards for the food web has also been raised in
relation to cephalopod fisheries in other areas (Guerra,
1978) as well as to other fisheries (Harris and Poiner,
1991; Caverivière and Rabarison Andriamirado, 1997).
However, the alternative hypothesis that cephalopods
may also feed directly on discards has not yet been
explored.
Acknowledgements
We are grateful to all colleagues participating in the
different cruises. Special thanks are due to Dr J.-C.
Quero (IFREMER – France) for providing us with basic
information from the THALASSA cruise to West
Africa. We also greatly appreciate constructive comments on the original manuscript from Dr N. Daan and
two unknown referees.
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