Download Steeper biomass spectra of demersal fish

195
Steeper biomass spectra of demersal fish communities
after trawler exclusion in Sicily
Christopher J. Sweeting, Fabio Badalamenti, Giovanni D’Anna, Carlo Pipitone,
and N. V. C. Polunin
Sweeting, C. J., Badalamenti, F., D’Anna, G., Pipitone, C., and Polunin, N. V. C. 2009. Steeper biomass spectra of demersal fish communities after
trawler exclusion in Sicily. – ICES Journal of Marine Science, 66: 195 – 202.
The effects of trawling on Mediterranean demersal fish communities were assessed by comparing them with the normalized biomass
spectra among three gulfs on the north Sicilian coast, one of which had been subject to 15 years of trawler exclusion. Comparisons
were conducted across seasons and among depth strata. Biomass-spectra slopes were significantly steeper in the gulf that was closed to
trawling than in the unprotected gulfs. This was attributed to the exclusion of trawlers, which lacked catch-size selectivity, and to the
continued fishing by artisanal gears that are more size selective. The biomass of all size classes was higher in the protected gulf than in
unprotected areas, and increases were greatest in smaller size classes. Community mean individual fish mass was similar among all
areas with a wider range of body masses present in the trawl exclusion area, compensating for the greater abundance of small
fish. Size-spectra slopes were generally shallower and midpoint heights lower with increasing depth and were greater in autumn
than spring; the effect of these seasonal and depth factors was as great as that of protection. Depth patterns are explainable by bathymetric trends in within- and among-species fish size. Seasonal differences were attributed to variation in spawning behaviour and
fishery recruitment, with seasonal differences being greater in unprotected locations as a result of recruitment overfishing.
Keywords: community structure, ecosystem, fisheries, indirect effects, management, marine reserve, size spectra.
Received 26 October 2007; accepted 13 September 2008.
C. J. Sweeting, F. Badalamenti, G. D’Anna, and C. Pipitone: CNR-IAMC, Laboratorio di Ecologia Marina, via G. Da Verrazzano 17 91014,
Castellammare del Golfo, (TP), Italy. C. J. Sweeting and N. V. C. Polunin: School of Marine Science and Technology, Ridley Building, Newcastle
University, Newcastle upon Tyne NE1 7RU, UK. Correspondence to C. J. Sweeting: tel: þ39 44 191 222 5868; fax: þ39 44 191 222 7891; E-mail:
[email protected]
Introduction
In marine ecosystems, the exploitation of fishery resources often
has consequences beyond the reduction of fish biomass
(Jennings and Blanchard, 2004), giving rise to community
changes in fish body size and diversity (Bianchi et al., 2000;
Greenstreet and Hall, 1996), and damage to the physical habitat
(Jennings and Kaiser, 1998). Fishing can change the mean fish
community trophic level (Pauly et al., 1998) and the foodweb
source of production (Kaiser and Spencer, 1994). The ecosystem
approach to fishery management can account for fishing impacts
on different ecosystem components and recognizes that species
are not exploited in isolation from each other (Jennings, 2004).
The impacts of fishing in the Mediterranean are wide ranging,
the result of a very high fishing intensity employing a great variety
of fishing gears and practices, and exploiting a great diversity of
species and habitats (Tudela, 2004). Catches and earnings are
dominated by bottom-trawl fleets, which utilize small mesh sizes
(40 mm in Italy; Machias et al., 2004). This results in a multispecies fishery characterized by a large proportion of juvenile fish in
catches, high bycatch and discard rates, and frequent violation of
minimum landing size (Machias et al., 2004; Tudela, 2004).
Finding robust ecosystem metrics to test ecosystem effects of
fishing has proved difficult (Rice, 2000; Rochet and Trenkel,
# 2009
2003). Of the measures available, size-based indicators such as
size spectra seem promising. The abundance or biomass of individuals decreases log-linearly with size, the result of energy use
and energy transfer efficiency among organisms of different size.
Body size is also a significant determinant of catchability
(Bianchi et al., 2000) and influences a species’ ability to recover
from fishing through links to life history and population dynamics
(Charnov, 1993).
Fishing can directly alter the size composition of exploited
communities by triggering disproportionate mortality on larger
individuals or species. Indirectly, fishing may increase abundance
of small prey species through predation release (Shin et al., 2005).
Therefore, as fishing mortality increases and the relative abundance of large individuals decreases, the mean size of individuals
in the community may decrease, resulting in a steeper decrease
in biomass– size spectra and changes in species composition
(Figure 1).
Fishing closures may mitigate many of the direct and indirect
effects of exploitation (Collie et al., 2000; Pipitone et al., 2000),
and closed areas have proved useful in elucidating ecosystem
effects of, and recovery from, fishing (Murawski et al., 2000;
Badalamenti et al., 2002). However, most protected areas are too
small to assess the impacts of commercial trawl fisheries on the
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196
Figure 1. How the size spectra from (a) an unexploited community,
(b) alters as a result of size-selective fishing, and (c) potential prey
release. The dashed lines in (b) and (c) represent the unexploited size
spectra, and the circles define the midpoint height relative to the
unexploited size spectra.
size composition of associated communities at appropriate scales.
Such work is essential to understanding both what impact trawling
has on the ecosystem and whether or not areas closed to trawling
represent a viable management tool for ecosystem restoration or
fishery management.
This study examines the effects of trawling and subsequent
closure on the biomass –size spectra of demersal marine species
by comparing the Gulfs of Castellammare (GCAST), Termini
Imerese (GTERM), and Sant’Agata (GSANT; North Sicily;
Figure 2). The Gulf of Castellammare has been closed to trawling
since 1990, and the total fish biomass there has increased eightfold
(Pipitone et al., 2000). The assumptions that trawler exclusion
resulted in an increased size-spectrum midpoint height (a
measure of community-wide abundance) and a shallower size
spectrum were tested, and effects examined across depths and
seasons for consistency of response.
Material and methods
GTERM and GSANT are next to each other in the middle of the
north coast of Sicily. GCAST, which is located 200 km farther
west (Figure 2), has been subject to a 200 km2 ban on commercial
trawling since 1990 (Pipitone et al., 2000). Fishing within the trawl
exclusion area is restricted to artisanal vessels employing mainly
size-selective static gears such as trammelnets and set gillnets.
Artisanal fishing extends over the whole continental shelf of
GCAST. GTERM and GSANT have large trawling fleets
(Cavaliere et al., 1988; Greco et al., 1993), as well as artisanal fisheries. The only regulation imposed on GTERM and GSANT is a
ban on trawling in water shallower than 50 m, which applies to
all Italian coastlines. Both GTERM and GSANT are overfished
(Greco, 1994).
C. J. Sweeting et al.
All three areas are characterized by large, central, sandy shores
with cliffs at both outer edges. The seabed is largely soft sediment,
except immediately under the surrounding cliffs where there are
boulder slope habitats. Sampling was done only over soft sediment
habitats.
The demersal community was sampled using a benthic otter
trawl, with headline and groundrope lengths of 31.3 and 41 m,
respectively, and a stretched mesh codend of 35 mm deployed
from the commercial fishing vessel FV “Giaguaro”. Trawls were
undertaken at 2.5 knots and lasted 30 min in GCAST and
60 min elsewhere.
Each gulf (location) was divided into three depth strata (10 –
50 m (A), 51 –100 m (B), 101–200 m (C)), each of which was
divided into boxes of approximately 2.25 square nautical miles,
a number of which were randomly sampled during each survey
in each stratum. Surveys were carried out over four consecutive
seasons in GCAST starting in summer 2004; surveys were carried
out in autumn 2004 and spring 2005 only in GTERM and GSANT.
Fish catch was sorted on board. Where the catch of one or more
species was excessively large, a subsample was taken after sorting.
Samples were returned to the laboratory daily for subsequent
measurement. Total length and weight of all fish were measured
to the nearest 5 mm and 1 g. Catch was standardized to a
60 min trawl, then all individuals were allocated to log2 body
mass classes (grammes) from 22 (,0.5 g) to 11 (.2048 g),
regardless of taxonomy.
Size-spectra analyses are only applicable to data in the size
range over which the gear effectively samples the community
(Jennings and Dulvy, 2005). Selectivity was determined based on
theoretical expectations that normalized fish biomass and body
weight class are negatively log-linearly related in marine ecosystems. This expectation was not met below size class 4 (8 –16 g)
and was direct evidence of inefficient gear sampling for fish
,8 g, which were excluded from the analysis.
The slopes of size spectra were calculated from the linear
regression of log2 normalized biomass [total biomass (g) per
weight class divided by weight span (g) of the weight class]
against the midpoint of each log2 weight class. Normalized
biomass spectra were centred on the predominant median
weight class (weight class 6; 16 –32 g) to calculate midpoint
height, rather than use the regression intercept. This was to
remove the influence of correlation between slope and intercept
(Daan et al., 2003). These spectra were compared among gulfs
and within GCAST among all four seasons.
Abundance size spectra were also constructed using log10 fish
abundance instead of normalized biomass on the y-axis.
Abundance size spectra exhibited patterns very similar to those
based on normalized biomass with the same interpretation and,
therefore, are not presented here.
A permutational analysis of variance (PERANOVA; Anderson,
2001) was used to test the differences in slopes and midpoint
heights of the size spectra. The PERANOVA approach was selected
to overcome difficulties in testing differences in the first design,
where the number of protected and unprotected gulfs was different
(one protected, GCAST, vs. two unprotected, GSANT and
GTERM). Heteroscedascity was checked using Cochran’s test
and, when possible, removed using the appropriate transformations (see tables for details). Two analytical models were used
to assess (i) the effects of the trawl exclusion in different seasons
and strata and (ii) the seasonal variability within GCAST. The
first design included three factors, treated as fixed and
197
Biomass spectra of demersal fish communities after trawler exclusion
Figure 2. Charts for each of the three gulfs, showing depth strata (10– 50, 51– 100, and 101 –200 m), the associated elemental statistical units
(ESUs) for subsampling, and the no-trawl area in the Gulf of Castellammare. Areas in which commercial fishing is allowed, but that are not
conducive to scientific trawling, are shown in grey.
orthogonal: gulf (Lo—three levels), season (Se—two levels), and
stratum (St—three levels), with experimental trawls treated as
replicates (n ¼ 6). Variance was partitioned for gulf (Lo) by contrasting protected vs. unprotected gulfs and among unprotected
gulfs because the a priori hypothesis was that there were differences
between the three gulfs mediated by the trawling ban. The second
design included two fixed and orthogonal factors: season (Se—
four levels) and stratum (St—three levels), with experimental
trawls treated as replicates (n ¼ 6).
Similarity matrices were created using PERMANOVAþ for
PRIMER v. 6 (Anderson et al., 2008), choosing Euclidean distances
as a distance measure. For all tests, 9999 permutations were
allowed under the reduced model. For both designs, significance
was set at a ¼ 0.05. A posteriori pairwise tests were also
performed within the PERANOVA test (Anderson, 2001;
Anderson et al., 2008).
Results
The slopes and midpoints of the normalized biomass spectra displayed significant season stratum interactions and differences
among the gulfs (Table 1). The slopes and midpoints were
similar between the two unprotected sites, GSANT and GTERM,
with the slope significantly shallower, the midpoint height
198
C. J. Sweeting et al.
Table 1. Analysis of the effects of trawl exclusion on size spectra variables: (top panel) PERANOVA for analysis of protected (P) vs.
unprotected (UP) locations; (bottom panel) pairwise comparisons of main effects and interaction terms.
Source
d.f.
Slope
Midpoint
MS
Pseudo-F
p (perm)
MS
Pseudo-F
p (perm)
Season
(Se)
1
0.9477
14.403
0.0003
0.0023
0.1269
0.7219
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Stratum
(St)
2
1.5900
24.164
0.0001
0.0813
4.5608
0.0123
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Location
(Lo)
2
1.1323
17.208
0.0001
2.4064
134.9500
0.0001
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
P vs. UP
1
2.2419
33.809
0.0001
4.8114
281.4600
0.0001
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
UP
1
0.0227
0.27836
0.5881
0.0014
0.0776
0.7842
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Se
St
2
0.2060
3.1303
0.0456
0.1762
9.8826
0.0002
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Se Lo
2
0.0361
0.54862
0.5777
0.0082
0.4620
0.6384
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Se
P
vs.
UP
1
0.0151
0.22835
0.6364
0.0002
0.0105
0.9153
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Se UP
1
0.0571
0.70118
0.4091
0.0163
0.8799
0.3608
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
St
Lo
4
0.0494
0.75068
0.5569
0.0088
0.4961
0.7316
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
St P vs. UP
2
0.0069
0.10371
0.9013
0.0087
0.5100
0.5981
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
St
UP
2
0.0919
1.1295
0.3327
0.0090
0.4844
0.6196
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Se
St Lo
4
0.0517
0.78531
0.5309
0.0045
0.2545
0.9031
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Se
St
P
vs.
UP
2
0.0131
0.19712
0.8208
0.0088
0.5146
0.5993
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Se
St
UP
2
0.0903
1.1094
0.3384
0.0003
0.0151
0.9836
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Residuals
90
0.0658
0.0178
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Total
107
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Cochran’s
test
C ¼ 0.2022 (n.s.)
C ¼ 0.2022 (n.s.)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Transformation
None
Ln(X þ Constant), Constant ¼ 3
Pairwise
comparisons:
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Location
(Lo)
GCAST , GSANT ¼ GTERM
GSANT ¼ GTERM , GCAST
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Se
St
interactions:
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Season (Se)
A
B
C
A
B
C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
n.s.
Au
,
Sp
n.s.
Au
,
Sp
n.s.
Au . Sp
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Stratum (St)
Au
Sp
Au
Sp
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
A¼B.C
A¼B,C
A,B¼C
A,B¼C
Significant values of p are emboldened. GCAST, Gulf of Castellammare; GSANT, Gulf of Sant’Agata; GTERM, Gulf of Termini Imerese; A, 10–50 m;
B, 51–100 m; C, 101–200 m; Au, autumn; Sp, spring; n.s., not significant.
significantly lower than the slope, and midpoint height found in
the gulf closed to trawling (Tables 1 and 2).
Seasonal differences in slope were observed in depth stratum B
(50– 100 m depth) only, with slopes being steeper in autumn than
in spring (Tables 1 and 2). For both strata A and C, no seasonal
effects on slope were found. In autumn, strata A and B had
similar slopes, and both were steeper than stratum C; in contrast,
in spring, strata B and C were comparable, and both had shallower
slopes than stratum A (Tables 1 and 2).
Midpoint height did not vary among seasons in stratum B, but
in the shallower stratum A, the midpoint heights were lower in
autumn than in spring, whereas they were lower in spring than
in autumn in the deeper stratum C. In autumn, the midpoint
heights were lowest in stratum A, whereas strata B and C were
comparable. In contrast, in spring, the midpoint heights were
comparable between strata A and B and significantly lower in
stratum C (Tables 1 and 2).
Despite differences in the slope of size spectra, mean mass of
the fish assemblage was comparable among locations (Table 3),
except for significantly greater mean fish mass in GCAST in
spring. The largest individuals collected in both seasons were
obtained from GCAST (Table 3), and more than 70% of individuals .500 g were caught in GCAST.
Within GCAST, slope was independent of season but differed
among strata (Table 4). Statistically, slopes in strata A and B
were comparable and both steeper than observed in stratum C,
so that slope became shallower with depth (Tables 4 and 5).
Table 2. Slope and midpoint height of normalized biomass– size
spectra for the fish communities sampled.
Slope
Midpoint height
Term
Mean
1 s.d.
Term
Mean
1 s.d.
GCAST
21.383
0.260
GCAST
6.225
1.091
. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .
GSANT
21.059
0.400
GSANT
2.874
0.757
. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .
GTERM
21.095
0.291
GTERM
2.951
0.924
. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .
A*Au
21.423
0.318
A*Au
3.404
2.132
. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .
A*Sp
21.348
0.349
A* Sp
4.291
1.828
. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .
B*Au
21.366
0.252
B*Au
4.342
1.757
. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .
B*Sp
21.006
0.296
B*Sp
4.340
1.558
. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .
C*Au
21.029
0.302
C*Au
4.272
1.769
. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .
C*Sp
20.902
0.211
C*Sp
3.452
1.803
GCAST ¼ Gulf of Castellammare, Gulf of Sant’Agata ¼ GSANT, and
GTERM ¼ Gulf of Termini Imerese; A ¼ 10–50 m, B ¼ 51–100 m, and
C ¼ 101–200 m; Au ¼ autumn, Sp ¼ spring.
199
Biomass spectra of demersal fish communities after trawler exclusion
Table 3. Differences in fish mass among locations represented by
mean weight and the weight at the 95%-ile of the biomass spectra
of all fish .8 g at each location.
Season
Parameter
Fish mass (g)
GCAST
GSANT
GTERM
95%-ile
211
215
225
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .
Mean
61
66
62
. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .
Spring
95%-ile
363
227
332
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .
Mean
89
63
70
Autumn
For midpoint height, the season stratum interaction was significant (Table 4), but was driven by a significantly greater midpoint height in stratum B than the other two strata in summer
only (Table 4). In all other seasons, midpoint height was similar
among strata within season and among seasons within stratum,
suggesting only minor effects of season and depth on midpoint
height (Table 4).
Discussion
Size spectra are believed to be robust ecosystem metrics for examining the impact of fishing, where fishing causes mortality on
larger individuals or species (Rice, 2000; Rochet and Trenkel,
2003). The resultant disproportionate removal of large individuals
and theoretical increase in small individuals through prey release
(Shin et al., 2005) commonly results in steeper size spectra and
lower midpoint height with increased fishing pressure (Rice and
Gislason, 1996; Dulvy et al., 2004; Graham et al., 2005). It was
hypothesized that establishment of the trawl exclusion area in
GCAST would reverse these trends, resulting in shallower sizespectra slopes and higher midpoint heights.
Response of the size spectra
Observations of midpoint height were consistent with previous
size-spectra studies, where increasing fishing pressure resulted in
lower midpoint heights (Bianchi et al., 2000; Dulvy et al., 2004),
and protection from fishing reverses the decline as recovery
occurs (McClanahan and Graham, 2005). Given that the area
under the size spectrum (a function of slope and midpoint
height) is a measure of total community abundance, the results
also agree with an eightfold increase in total cpue in GCAST
(Pipitone et al., 2000). Increased fish abundance and biomass
have also been observed for other Mediterranean MPAs
(Guidetti and Sala, 2007), although increases varied among
species and were based largely on rocky reef or seagrass habitats
rather than open, soft sediments.
The response of the size-spectra slopes was not consistent with
expectations from previous studies on fishing effects (Bianchi
et al., 2000; Dulvy et al., 2004). The unprotected gulfs exhibited
shallower slopes than the slopes calculated for GCAST, where
trawling was excluded. This effect was unexpected. For size
spectra to be a suitable indicator of the impacts of fishing, sizespectra responses should be predictable in relation to fishing
(Macpherson and Gordoa, 1996; Bianchi et al., 2000).
Expectations that increasing fishing pressure results in steeper
size spectra stem from fisheries that are more size selective, e.g.
120 mm mesh applied in North Sea trawl fisheries (Rice and
Gislason, 1996). In contrast, the codend mesh size employed by
most commercial fisheries in Italy is 40 mm (Machias et al.,
2004), which exploits species over the entire size spectrum that
was studied (.8 g). Under such conditions, three mechanisms
link patterns of fishing to size-spectra response. Perhaps,
(i) trawler exclusion did not uniformly reduce fishing mortality
across all size classes. Legitimate artisanal gears were allowed in
the protected area, which are more size selective (Pipitone et al.,
2000). Therefore, although total fishing mortality was reduced
by trawler exclusion, this effect was greater for small size classes.
Table 4. Analysis of the effects of season on size-spectra variables within GCAST: (top panel) PERANOVA table of the seasonal analysis,
and (bottom panel) pairwise comparisons of main effects and interaction terms.
Source
d.f.
Slope
Midpoint
MS
Pseudo-F
p (perm)
MS
Pseudo-F
p (perm)
Season
(Se)
3
0.0494
1.3866
0.2435
0.3158
0.3513
0.7984
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Stratum
(St)
2
0.9346
26.2190
0.0001
3.8684
4.3041
0.0156
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Se St
6
0.0130
0.3654
0.9007
2.4319
2.7059
0.0212
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Residuals
60
0.0356
0.8988
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Total
71
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Cochran’s
test
C ¼ 0.148 (n.s.)
C ¼ 0.419 (p , 0.01)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Transformation
None
Unsuitable
Pairwise
comparisons:
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Season
(Se)
n.s.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Stratum
(St)
A¼B,C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Se
St
interactions:
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Season
A, B, C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
n.s.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Stratum
Su
Au, Wi, Sp
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
A¼C,B
n.s.
Significant values of p are emboldened. A, 10–50 m; B, 51– 100 m; and C, 101– 200 m; Su, summer; Au, autumn; Wi, winter; Sp, spring; n.s., not significant.
200
C. J. Sweeting et al.
Table 5. Slope and midpoint height of normalized biomass –size
spectra among seasons within the Gulf of Castellammare.
Slope
Midpoint height
Term
Mean
1s.d.
Term
Mean
1s.d.
A
21.523
0.231
A*Su
5.618
0.865
. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .
B. . . . . . . . . . . . . . . . . . . . . . ..21.457
0.154
A*Au
5.498
2.126
. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .
C
21.153
0.160
A*Wi
7.222
1.519
. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .
A*Sp
6.491
1.219
. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .
B*Su
7.236
0.373
. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .
B*Au
6.695
0.422
. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .
B*Wi
6.596
0.654
. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .
B*Sp
6.747
0.269
. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .
C*Su
5.992
0.379
. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .
C*Au
6.587
0.557
. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .
C*Wi
5.836
0.444
. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .
C*Sp
5.832
0.506
A, 10–50 m; B, 51–100 m; C, 101–200 m; Su, summer; Au, autumn; Wi,
winter; Sp, spring.
Alternatively, similar effects may be induced where fishing
with small-mesh nets in unprotected areas results in either (ii)
recruitment-overfishing (limited production of small fish) or
(iii) heavy fishery mortality on small size classes. All three mechanisms link fishing directly to the size-spectra response and
account for the observed patterns in response to protection from
trawling. The similarity among unprotected locations suggests
that differences were a direct effect of fishing.
Although biomass spectra slopes were steeper in GCAST than
elsewhere, the size range of fish caught was commonly greater
and, consequently, the mean across-community fish size was
similar or slightly greater in protected than in unprotected
locations. Positive MPA effects on fish size have been noted in
other Mediterranean MPAs, e.g. Bell (1983), although these
effects are by no means ubiquitous and have been rarely tested
at community levels.
Alternative drivers of slope
Although the above mechanisms are believed to be the most probable causes of observed differences in slope with protection, other
possibilities should be considered. These include the correlation
of body size with other biological characteristics. Smaller species
generally have faster life histories (Jennings et al., 1999) and
shorter population doubling times, allowing smaller size classes
to increase faster. Qualitatively, the species that increased the
most (e.g. Lepidotrigla cavillone; Pipitone et al., 2000) did have a
short population doubling time. Such an interpretation supports
the assertions that MPAs may take many years to develop
mature community structures (McClanahan and Graham, 2005),
despite rapid initial change (Halpern and Warner, 2002).
Body size also correlates positively with mobility
(Schmidt-Nielsen, 1984). Individuals that are more mobile have
greater potential for movement outside the trawl exclusion area
and may therefore experience greater fishing mortality. It is
unlikely that this explains the pattern here, because the narrow
continental shelf of GCAST presents a physiological and physical
boundary, reducing movement potential and therefore the magnitude of this effect.
Finally, greater survival of large, fecund individuals in the initial
years following MPA establishment can result in high recruitment
and disproportionately large numbers of small individuals
(Agardy, 1994). Within the GCAST trawl exclusion area, a high
recruitment of some species after protection has been observed,
e.g. Mullus barbatus, a species that accounts for 20 –30% of total
biomass. However, after .15 years of protection, the size spectrum should have settled to a new equilibrium (Bianchi et al.,
2000). Based on size spectra alone, it is not possible to determine
which of the above mechanisms drive patterns in slope, although
processes operating through fishing mortality are most
parsimonious.
Background variation
The effect of fishing on size spectra should be considered against
other sources of variation. The slope of size spectra decreased
with depth, and these changes were of a magnitude similar to
that of protection. Few studies have specifically examined depth
effects on size spectra, despite expectations that the relative proportions of small to large fish will vary with depth. Larger individuals within species are frequently observed in deeper water,
reflecting a migratory ontogenetic movement to cooler water
(Macpherson and Duarte, 1991), and many species utilize
complex shallow inshore habitats as nursery areas (Franco et al.,
2006; Planes et al., 2000). Some studies control for this depth
effect (Bianchi et al., 2000; Stobberup et al., 2005), whereas
others have been conducted over relatively narrow depth ranges
(,50 m) and are unlikely to be confounded by depth. However,
given that the main or interaction effects of depth and season on
slope are of a magnitude similar to those of protection in our
results, the importance of accounting for depth in size-spectra
analyses is evident.
The patterns in size-spectra slope with respect to depth varied
seasonally. Fish populations are rarely static, but move inshore or
offshore for feeding, spawning, or as a result of seasonal environmental conditions. Such movements should be reflected in the size
spectra. However, when strata were pooled, seasonal effects were
still evident, suggesting that seasonal effects cannot be explained
by within-gulf movement alone. Seasonal effects appear greater
in the unprotected locations, being largely absent from analyses
of GCAST across four seasons. Size-spectra analyses are increasingly being promoted as useful metrics to assess the impacts of
fishing at the community level (Rice, 2000; Rochet and Trenkel,
2003; Jennings and Dulvy, 2005), and they demonstrate predictable changes in response to fishing for temperate demersal fisheries
(Gislason and Rice, 1998). This study’s observations that slope
does not always decrease with declines in fishing mortality highlights the connection between fishing practices and the response
of size spectra, but cautions against generalized assumptions that
increasing slope represents greater fishing pressure. This study
also suggests a need to control size spectra for depth and season.
However, more research into seasonal distribution of species by
size class would be needed to examine this issue fully.
Acknowledgements
The authors sincerely thank Simon Jennings for initial discussion
in establishing the project, M. Coppola and G. Di Stefano for
assistance with fish sampling, and N. A. J. Graham for comments
Biomass spectra of demersal fish communities after trawler exclusion
on the manuscript. This study was co-funded by the Italian
Ministry of University and Research.
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