ICES CM 2000/Mini:13 (Poster)
Effects of commercial trawl fishing in the Strait of Sicily on the diversity of
demersal resources
M. Gristina, G. Garofalo, G. Bono and D. Levi
IRMA-CNR, Via L. Vaccara, 61 – 91026 Mazara del Vallo, Italy (tel.: ++39 0923 948966,
e-mail: surname@irma.pa.cnr.it
Abstract
The effects produced by commercial trawl fishing on the diversity of demersal resources in two
zones of the Straits of Sicily subject to different fishing effort were analysed. Data were available
from a total of 79 hauls, carried out annually in autumn 1997 and 1998 between 200 and 400 meter
depth and pertained to the bathyal muddy bottoms biocenosis (Perès and Picard, 1964) on the
whole.
Data on fishing effort, recorded since 1995, come from the Harbour Office and have been verified
from interviews to crews and captains of Mazara del Vallo trawl fisheries.
Excluding pelagic and rare (appearing in <= 5% of hauls) species from the analysis, diversity
indices, cluster analysis, K-dominance and size spectra were investigated.
The cluster analysis (employing the Bray-Curtis similarity matrix among species) shows the
presence of two distinct groups of hauls corresponding to the areas with different fishing effort.
High values of the main diversity indexes seem to characterize the hauls subject to a more intensive
fishing effort. K-dominance curves for the two areas show that, in “Zone 1” the main part of the
catch is concentrated on commercial species (Merluccius merluccius, Parapenaeus longirostris),
while in “Zone 2” the catch is mainly characterized by massive species, Elasmobranchs and
Crustaceans with low or null commercial value.
Introduction
In the last two decades there has been great concern that some ecosystems were being impacted by
fishing activities (Dayton et al., 1995) and that the pattern of fishing effort characterises diversity
and structure of the demersal community (Jennings and Cotter 1999).
The aim of this papers it to value, on spatial scale, the possible effects of the trawl commercial
activities on the demersal communities of two zones subject to different fishing effort.
The Strait of Sicily is a very intensively fished area. The Mazara del Vallo trawl fleet (about 180
trawl vessels; mean GRT = 23.421), one of the most big in the Mediterranean Sea, swept from
about 30 years the trawlable grounds of the Strait of Sicily looking for unexploited fishing grounds
moving farther from the Sicilian coasts.
Actually the eastern border of the Mazara del Vallo trawl fleet is represented by the ideal line that
links Malta with the Libyan coast; easternward the fishing grounds are nearly totally unexploited.
Material and Methods
Effort data, available since 1995, come from the Harbour Office and have been
interviews to crews and captains of Mazara del Vallo trawl fisheries. Two “Zones”
identified which are characterised by different intensity of fishing effort; “Zone 1”
exploited by commercial fishery of Mazara del Vallo (Fig.2), while “Zone 2” is
untrawled area.
verified from
(Fig. 1) were
is intensively
a completely
Data on demersal communities in the two zones were available from two trawl surveys (Autumn 97,
Autumn 98) carried out in the Strait of Sicily by the commercial vessels Santa Anna with an “Italian
Otter Trawl” (28 mm mesh size at the codend). In order to compare the two zones only the hauls
within the depth range 200 – 400 meter and belonging to the bathyal muddy bottoms biocenosis
were selected and analysed. Table 1 indicates the number of hauls in each zone and year.
Abundance and biomass standardised to 1 Km2 for each haul and for each species (excluding
pelagic, undetermined and rare species) were utilised to calculate the Diversity indices (Shannon
H’, alpha, Simpson) and perform a Bray-Curtis Cluster Analysis (Single Link). Moreover, Kdominance curves were calculated for the two zones, based on mean species biomass. Finally, data
were pooled across fourteen commercial species to analyse the size spectra. Specimens lengths were
divided into 10-cm size classes and the natural logarithm of the mean abundance was plotted
against the ln of the midlength of the corresponding size class.
Results and Discussion
No temporal trend in fishing effort was observed over the time-period considered, although there
were small seasonal fluctuations in Zone 1 (Fig. 2).
A total of 83 species were analysed in the two surveys examined. Although the number of species
collected is quite similar in the two “Zones” investigated, abundance/km2 and biomass/km2 in the
trawled area were significantly lower than in the untrawled one (Mann-Withney U test, p<0.001)
(Tab. 1).
In both zones, species abundance and biomass were higher in Autumn 1998 than in Autumn 1997 ,
even though the differences were not significant ( Mann-Withney U test, p>0.05 for all).
Because of the high concordance of the results in all the analysis carried out for the two years we
discuss the data and present figures relatively of Autumn 1998.
The Bray-Curtis analysis shows two different clusters representing the hauls realized in the Zones
with different fishing impact (Fig. 3) .
The quite similar number of species in the two zones linked with high value of abundance in “Zone
2” explain the low diversity indices recorded in the unexploited area (Fig. 4). This result, in contrast
with other experiences carried out on the benthic community (Tuck et al. 1998), could be explained
with the moderate impact of the fishing activity on the trawled area (Zone 1); the low level of
trawling disturbance probably creates an “acute impact” (in time and space) on the demersal
community that increases the diversity values (Connell, 1974).
Elasmobranches seem to be particularly affected by fishing activities; K- dominance shows that
they represent a very important fraction of the top 20 ranked species in “Zone 2” while they become
quite negligible in “Zone 1” (Fig. 5). On the contrary the species with a high commercial value
assume an important role in the demersal assemblage on the exploited zone.
Recent studies (Gislason & Rice, 1998, Rice & Gislason, 1996) investigating the relationship
between size-spectra and fishing mortality, suggested that abundance-size spectrum seems to be a
metric sensitive to changes in fishing activity. Accordingly to theory, the slope of the size spectrum
in the exploited community is steeper and the intercept is higher with respect to same parameters of
the unexploited one.
The size structure of the demersal population in “Zone 1” is affected by selective removal of larger
fish. The main part of the specimens (more than 60 %) results comprises in the first two classes
while in "Zone 2" the catches appear distributed in a more wide range of length classes. (Fig. 6).
Patterns in diversity and demersal community structure seem strictly related to the fishing effort.
The analysis of the diversity indices does not seem the best way to investigate the impact of the
fishing activity on the demersal communities; in fact, significant lower diversity values are not, at
all, clearly linked to a trawl disturbance. On the contrary, the structure of the demersal assemblages
and the analysis of the size spectra result more sensitive to detect changes in the demersal
communities.
References
Connel, J.H., 1978. Diversity in tropical rain forests and coral reefs. Sience, 199: 1302 – 1310.
Dayton, P.K., Trush, S.F., Agardy, T., Hofman, R.J., 1995. Environmental effects of marine fishing.
Acquat. Conserv., 5, 205 - 232.
Gislason, H. and Rice J., 1998. Modelling and response of size and diversity spectra of fish
assemblages to changes in exploitation.
Jennings, S. and Cotter A.J.R., 1999. Fishing effects in northeast Atlantic shelf seas: patterns in
fishing effort, diversity and community structure. I. Introduction. Fish. Res., 40: 103 - 106.
Perés, J.M. and Picard 1964. Nuveau manuel de bionomie bentique de la Mer Mediterranèe. Recueil
Trav. Stat. Mar. Endoume, 31 (47).
Rice, J. And Gislason, H., 1996. Patterns of change in the size spectra of numbers and diversity of
the North Sea fish assemblage, as reflected in surveys and models.
Tuck, I.D., Hall, S.J., Robertson, M.R., Armstrong, E. and Bradsford, D.J., 1998. Effects of
physical trawling disturbance in a previously unfished sheltered Scottish sea loch. Mar. Ecol.
Prog. Ser., 162: 227 - 242.
"Zone 1" - Fishing effort
25
1500
Fishing days/season
Vessels per season
Fig. 1 – Map of the Strait of Sicily showing the location of the hauls in the two zones investigated.
20
1000
15
10
500
5
0
0
Win. 98
Aut. 98
Sum. 98
Win. 97
Aut. 97
Sum. 97
Win. 96
Aut. 96
Sum. 96
Win. 95
Aut. 95
Sum. 95
N° Vessels
Fishing-days
Fig. 2 – Fishing effort in the Strait of Sicily from ’95 to ’98.
ZONE 1
Autumn 97'
Autumn 98'
Autumn 97'
N° Hauls
22
Total Species
67
N° Hauls
24
Total Species
70
Abundance/km
Biomass/km
Autumn 98'
2
Abundance/km
Biomass/km
2
2
-
69
-
16
-
61
-
Mean
St. dev. Mean
St. dev.
19.235
16.455 46.389
23.522
292
2
ZONE 2
18
29.363
407
251
1.143
552
21.104 61.390
24.855
348
1.381
583
Table 1 – N° hauls, total species, Abundance/km2 , Biomas/km2 per zone per season.
Mann-Whitney U test p<0.001 for between zone comparisons; p>0.05
for between time period comparisons.
Fig. 3 – Bray-Curtis cluster analysis (Single link).
SHANNON H'
ALPHA
6
No of observations
No of observations
10
8
6
4
5
4
3
2
2
1
0
0.4
0.6
0.8
ZONE:
1
1
1.2
0.4
0.6
0.8
ZONE:
2
1
0
2.4
1.2
2.8
3.2
3.6
4
ZONE:
1
4.4
4.8
5.2
2.4
2.8
3.2
3.6
4
ZONE:
2
4.4
4.8
5.2
SIMPSONS
No of observations
7
6
5
4
3
2
1
0
0
2
4
6
8
ZONE:
1
10
12
14
0
2
4
6
8
ZONE:
2
10
12
14
Fig. 4 – Histograms of diversity indices in each zone. Mann-Whitney U test, all p<0.001 for between
zone comparisons.
"Zone 1"
"Zone 2"
Macrorhamphosus scolopax
Terebratula vitrea
84,7
Phycis blennoides
74,8
Helicolenus dactylopterus
67,1
Parapenaeus longirostris
Lepidopus caudatus
54,8
Scyliorhinus canicula
35,6
63,0
Chlorophthalmus agassizi
55,8
47,1
Argentina sphyraena
19,5
Illex coindetii
Peristedion cataphractum
9,9
Chlorophthalmus agassizi
69,0
Raja clavata
28,4
Capros aper
72,3
Squalus blainvillei
41,4
Gadiculus argenteus
75,0
Raja oxyrhynchus
46,4
Todaropsis eblanae
80,3
77,8
Merluccius merluccius
51,3
Merluccius merluccius
82,6
Trigla lyra
61,5
Raja clavata
84,2
Coelorhynchus coelorhynchus
58,1
Argentina sphyraena
85,7
Gadiculus argenteus
64,4
Scyliorhinus canicula
87,2
Glossanodon leioglossus
69,8
Coelorhynchus coelorhynchus
88,6
Macrorhamphosus scolopax
72,4
Raja oxyrhynchus
Helicolenus dactylopterus
89,9
Todaropsis eblanae
77,2
Nephrops norvegicus
92,3
91,1
Mullus surmuletus
79,4
Lepidotrigla dieuzeidei
93,4
Raja melitensis
81,2
Boops boops
94,4
Mullus barbatus
83,0
Munida intermedia
36,4
Capros aper
Target Commercial Sp.
Massive species
19,2
Other species
Elasmobranchs
Fig. 5 – K-dominance of biomass for the top 20 ranked species.
a) Cumulative length-frequency
distribution
b) Number-size spectrum
8
7
100
Ln(numbers)
Cumulative%
120
80
60
40
20
Zone 1
y = -2.5338x + 14.297
2
R = 0.9379
p=0.001
6
5
4
3
y = -3.314x + 15.865
R2 = 0.9138
p=0.003
2
1
Zone 2
0
Zone 1
Zone 2
0
0
10
20
30
40
Length
50
60
70
80
2,5
3
3,5
4
4,5
Ln(length)
Fig. 6 – Size structures obtained pooling fourteen commercial species and elasmobranches. (a) Cumulative
Lenght-frequency distribution. (b) Number-size spectrum with linear model fit.