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The goal of this study was to assess the impact of recreational harvesting on the populations of two sea urchin species, Paracentrotus lividus and Arbacia lixula, via their popu- lation densities and mean individual size at three sites in shallow water in the Gulf of Lion (NW Mediterranean Sea) characterised by different anthropogenic pressures. We observed a positive relationship between mean size and population density for P. lividus at one of the three sites. The increase in anthropogenic pressure was linked to a decrease of both population density and individual size. Differences in mean size and densities were detected for P. lividus between a site with high anthropogenic pressure and sites with moderate to low anthropogenic pressure. These results suggest the existence of an impact of recreational sea urchin harvesting on popula- tions of P. lividus and A. lixula, and stress the necessity of a regulation of such practices, as their effects are likely to affect local ecosystems
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Several studies have reported that sampling activites
(fishing, harvesting) have direct consequences on the
benthic macrofauna, potentially decreasing density, indi-
vidual size, and leading to sex ratio imbalance (Uphoff
1998, Brazeiro & Defeo 1999, Guidetti et al. 2004, Diele
et al. 2005). Some indirect effects on the benthic commu-
nity assemblages of rocky coasts have also been reported
(Sala et al. 1998, Guidetti et al. 2003, Guidetti 2004).
Sea urchins are perhaps the most studied group of ben-
thic macrofauna in the Mediterranean Sea (Le Direach et
al. 1987, Lecchini et al. 2002, Guidetti et al. 2004, Pais et
al. 2007). Their nutritional value and the delicacy of their
gonads, along with the growing demand of the Japanese
market (Cook & Kelly 2007), put some urchin species
under intense harvesting pressure, potentially resulting in
a decrease of population size in some inshore areas. In the
Mediterranean Sea, Paracentrotus lividus (Lmk.) is some-
times intensively harvested, mostly along French, Span-
ish and Italian coasts (Le Direach et al. 1987, Palacín et
al. 1988, Lawrence 2001, Pais et al. 2007). Furthermore,
because they are frequently found in shallow waters,
edible sea urchins are also subject to recreational fishing
(Gianguzza et al. 2006, Pais et al. 2007). Recently, Duran
et al. (2004) have suggested from population genetics that
P. lividus population stocks are healthy on the Mediterra-
nean and Atlantic coasts, but the increasing exploitation of
edible sea urchins suggests stock monitoring along these
coasts should be intensified to avoid any negative conse-
quences for stocks, such as decreases in density, biomass
and individual size (Lecchini et al. 2002, Guidetti et al.
2004, Pais et al. 2007).
Fishing of sea urchins have indirect effects on the
benthic community assemblages of rocky coasts, as they
constitute keystone species of the Mediterranean infralit-
toral communities through their important role in the food
web structuration (Kempf 1962, Lawrence 1975, Ver-
laque 1987). Many sea urchins species are herbivorous
and graze on brown algae, calcareous algae, and seagrass
species in the Mediterranean Sea (Verlaque 1987). Each
urchin species has its specific diet. This herbivorous diet
leads sea urchins to account for an important part of the
dynamics of seaweeds (Frantzis et al. 1988, Palacín et
al. 1988, Lecchini et al. 2002). In areas where P. lividus
are abundant, their grazing on brown algae may trigger
a shift in algal community composition, allowing the
establishment of encrusting algae (Kempf 1962, Vukovic
1982, Verlaque & Nédélec 1983, Verlaque 1987, Spirlet
et al. 1998). Some studies have shown that the presence
of P. lividus exerts a role on the distribution of benthic
assemblages (Sala & Zabala 1996, Guidetti 2004, Guidet-
ti et al. 2004).
Several studies investigated the impact of commercial
harvesting on sea urchin populations in the Mediterra-
nean Sea (Le Direach et al. 1987, Gras 1987, Pfister &
Bradbury 1996, Guidetti et al. 2004, Pais et al. 2007),
but few considered the practice of recreational harvesting
(Gianguzza et al. 2006, Pais et al. 2007). The objective of
the present study was to assess the impact of recreational
VIE ET MILIEU - LIFE AND ENVIRONMENT, 2010, 60 (4): 299-305
UPMC Univ Paris 06, FRE 3355, Biologie Intégrative des Organismes Marins, Observatoire Océanologique,
66650 Banyuls/Mer, France
CNRS, UMR 7232, Biologie Intégrative des Organismes Marins, Observatoire Océanologique, 66650 Banyuls/Mer, France
Université Montpellier II, Institut des Sciences de l’Évolution, UMR 5554, Place Eugène Bataillon,
34095 Montpellier Cedex 05, France (present address)
* Corresponding author:
ABSTRACT. – The goal of this study was to assess the impact of recreational harvesting on the
populations of two sea urchin species, Paracentrotus lividus and Arbacia lixula, via their popu-
lation densities and mean individual size at three sites in shallow water in the Gulf of Lion (NW
Mediterranean Sea) characterised by different anthropogenic pressures. We observed a positive
relationship between mean size and population density for P. lividus at one of the three sites.
The increase in anthropogenic pressure was linked to a decrease of both population density and
individual size. Differences in mean size and densities were detected for P. lividus between a
site with high anthropogenic pressure and sites with moderate to low anthropogenic pressure.
These results suggest the existence of an impact of recreational sea urchin harvesting on popula-
tions of P. lividus and A. lixula, and stress the necessity of a regulation of such practices, as their
effects are likely to affect local ecosystems.
Vie Milieu, 2010, 60 (4)
harvesting on P. lividus and A. lixula by comparing popu-
lation density and individual size in protected (Cerbère-
Banyuls Marine Reserve) and unprotected areas along a
portion of the French Gulf of Lion coast.
Study area: Samples were collected in May 2008 along the
French Catalan coast (North-Western Mediterranean Sea) both
inside and outside the Cerbère-Banyuls Marine Reserve (Fig. 1)
over a three weeks period. The study was conducted at three sites
(Le Racou, Paulilles (Le Fourat) and Tancade) with a decreasing
impact of recreational harvesting of sea urchins from Le Racou
to Tancade (Table I). Le Racou is subjected to commercial and
recreational fishing, Paulilles is subjected to recreational fish-
ing only, while all fishing is prohibited in Tancade (personal
communication of the managers of the Cerbère-Banyuls Marine
Reserve), which is located within a protected area.
Sampling: Mean size and abundance for both species were
recorded in situ. The current practice of estimating size of sea
urchins is to measure the diameter of the test in its widest part
(ambitus) without spines (Barnes & Crook 2001, Lecchini et al.
2002, Barillé-Boyer et al. 2004, Pais et al. 2007). However, this
technique damages the urchins, because the use of vernier calli-
pers breaks their spines. We chose to use a non-destructive tech-
nique because part of our work was conducted in a protected
area. A sample of urchins was measured with spines, using a cal-
ibration to the millimeter level. Harvesting Paracentrotus livi-
dus individuals, on which the two measurement techniques were
performed in the laboratory, allowed to validate this method.
These urchins, used for validation, were collected at a location
different from the three study sites. Pearson’s correlation coef-
ficient shows a strong linear relationship between the size with
and without spines (N = 122, R = 0.950; p = 0.001, Fig. 2).
Table I. – Characteristics of the sampling sites.
Topology Principal algal coverage
Le Racou Outside reserve High
Bottom rough: low-depth areas (0.5 to 1 m) with
sand and rock and deepest areas (1-2 m) with
medium-sized pebbles
Grass algae and
encrusting algae
Paulilles Outside reserve Moderate
Depth increasing gradually (0-2 m) with
medium-sized rocks
Grass algae (0-1 m depth)
and encrusting algae
(1-2 m depth)
Within reserve
(harvest of sea-
urchin prohibited)
Depth increasing gradually (0-2 m), large-sized
Raised algae
Fig. 1. Location of the three
study sites (Le Racou, Paulilles
and Tancade) along the Fench
Catalan coast; Le Racou: high
anthropogenic pressure, Pau-
lilles: moderate anthropogenic
pressure and Tancade: low
anthropogenic pressure
Vie Milieu, 2010, 60 (4)
The three sites were sampled in the 0-2 m depth range by
snorkelling (because recreational fishing is usually done on foot
or by snorkelling). We chose to use a random sampling design
to estimate individual density and body size in the study area.
In randomly chosen zones (“sample units) of 1 meter radius
circles delineated using a rope and a lead, the number and body
size of all individuals of both urchin species were determined.
At each site, 10 surveys of one hour each were conducted. This
was repeated on four days at each site (for a total 40 replicates)
and one person conducted all the samplings. Each area was ran-
domly selected using a specially designed table listing random
directions (indicated by randomly generated angles uniformly
distributed in the range of 1-360 °) and distances (indicated by
randomly generated fin kicks, uniformly distributed between 1
and 15).
Data analysis: The effects of species, sampling site, and
interaction between these factors on density (expressed in indi-
viduals per square meter) were tested using a two-way ANOVA
(random effects) with significance assessed via 999 permuta-
tions (Anderson & Legendre 1999). Because this test requires a
balanced design (an equal number of observations in each con-
dition), six datasets (corresponding to the six conditions implied
by the combination of three sites and two species) containing
28 measures (size of the smallest sample) were reconstructed
by random sampling within the original datasets. To ensure that
sampling does not induce a bias in further analyses, means and
variances of reconstructed and original datasets were compared.
Following the observation that data were not normally dis-
tributed (Shapiro-Wilk’s test), Bartlett’s test using 999 permuta-
tions was used to assess homogeneity of variances (Anderson &
Legendre 1999). When two-way ANOVA resulted in a signifi-
cant association between site or species and density, a posteriori
tests were used to identify the modalities of the factor associated
with the variation in the response variable. Whenever variances
were found to be equal among samples, t-tests with 999 permu-
tations were used. Otherwise, non-parametric tests (Kruskal-
Wallis and Mann-Whitney tests) were used.
The difference in mean size among sites for Arbacia lixula
(at Paulilles and Tancade) was assessed using a Student’s t-test
with 999 permutations, as variances were homogeneous (Flign-
ers test) but data were not normally distributed. The differences
in mean size among sites for Paracentrotus lividus (sites Le
Racou, Paulilles, and Tancade) were measured by a one-way
ANOVA, as data were normally distributed and variances were
homogeneous (according to Fligner’s test). To characterize pair-
wise differences between sites, Tukeys Honestly Significant
Difference a posteriori tests were used.
For each species at each site, correlations between popula-
tion density and individual size were assessed using Pearsons
correlation tests (using 999 permutations, as density data were
not normally distributed).
Analyses were conducted using the R 2.9.0 software (R
development core team 2008). All tests were declared signifi-
cant for p < 0.05.
Density and size
When testing for the effects of sampling site and urchin
species on population density, we found that mean popula-
tion density is different both between species (p = 0.001)
and sampling site (p = 0.005; Table II), but no interac-
tion was found between these two factors (p = 0.132).
The mean population density for Paracentrotus lividus
(2.55 ind.m
) was greater than for Arbacia lixula (0.14
, p < 0.0001).
Population density varied amongst sampling sites both
when considering A. lixula and P. lividus simultaneously
(p = 0.005) and separately (A. lixula p = 0.003; P. lividus
p = 0.01). Densities were similar between Paulilles and
Tancade (both species, p = 0.211; A. lixula, p = 0.601;
P. lividus, p = 0.096). Densities observed in Le Racou
were significantly smaller than in the two other sites, for
both species (0.80 vs. 1.35 for Paulilles, p = 0.03, and 1.82
for Tancade, p = 0.01), as well as when considering only
A. lixula (no individuals observed, vs. 0.27 for Paulilles,
p = 0.001, and 0.22 for Tancade, p = 0.012) (Fig. 3) or
only P. lividus (1.62, vs. 2.44 for Paulilles, p = 0.038, and
3.43 for Tancade, p = 0.001) only (Table II; Fig. 4).
We found no significant difference in mean individual
size for A. lixula between Tancade (mean size = 6.91 cm)
and Paulilles (mean size = 7.30 cm; p = 0.743; Table II).
Individual sizes were different between sites for P. livi-
dus (all sites, p < 0.0001) (Table II). Sizes were also sig-
nificantly different between each pair of sites (Tancade
- Paulilles, p < 0.0001; Le Racou - Paulilles, p < 0.0001;
Tancade - Le Racou, p < 0.0001, Fig. 5). Individuals were
larger in Tancade (n = 26, mean size = 6.60 cm) than in
Fig. 2. – Scatterplot depicting the link between the diameter in
cm of 122 Paracentrotus lividus individuals measured with and
without spines (R = 0.950, p < 0.001)
Vie Milieu, 2010, 60 (4)
Paulilles (n = 38, mean size = 5.48 cm) and Le Racou
(n = 28, mean size = 4.29 cm).
Relation between density and size
Except for a positive correlation for Paracentrotus
lividus in Paulilles (p = 0.004), we found no correlation
between density and mean individual size in any configu-
ration (sites-species).
While slightly less precise, size with spines was found
to be a good estimator of the test diameter, allowing in
situ measurements of live animals. Previous studies have
shown that echinoderm spines regenerate (Heatfield
1971, Dubois & Ameye 2001), for example after an envi-
ronmental perturbation triggering their loss. Regeneration
could induce some variability in the ratio of the diameter
at the ambitus with and without spines (in that mature
individuals may nonetheless display shorter spines than
expected). Future research should characterize this vari-
ability within and between sampling sites and seasons to
better estimate the effect of this process on the spine size
We observed a significant difference in the density of
individuals of Paracentrotus lividus and Arbacia lixula,
between a site with a high anthropogenic pressure and
two sites with moderate and low anthropogenic pressure.
This observation supports the hypothesis that anthropo-
genic pressure has an impact on sea urchin populations in
the study area. The impact of recreational fishing, as well
as commercial fishing, on the density of the sea urchins
Table II. – Mean and standard deviation for density and size for Paracentrotus lividus and Arbacia lixula at the three sampling sites,
and percentage of P. lividus at each site. N is the number of individuals and X the number of replicates. “Significance” refers to the sta-
tistical test (ANOVA) of the comparison of means.
Variable Le Racou (N, X) Paulilles (N, X) Tancade (N, X) Signicance
Total density of both species
0.81 ± 1.52
1.36 ± 1.74
1.82 ± 2.42
P = 0.005
Density of P. lividus (ind.m
) 1.62 ± 1.83 (194,38)
2.44 ± 1.85
3.43 ± 2.51
P = 0.01
Density of A. lixula (ind.m
0.27 ± 0.50
0.22 ± 0.50
P = 0.003
Size of P. lividus (cm)
4.29 ± 1.33
5.48 ± 1.34
6.60 ± 1
P < 0.0001
Size of A. lixula (cm)
7.30 ± 1.26
6.91± 1.64
P = 0.743
Percentage of P. lividus
91.33 ± 16.30
95.59 ± 9.88
P = 0.607
Fig. 3. – Mean density (mean number of individuals.m
) of
Arbacia lixula at the sites of Tancade and Paulilles. No signifi-
cant difference was found between the sites (p = 0.71)
Fig. 4. – Mean density (in number of individuals. m
) of Para-
centrotus lividus at the sites of Tancade, Paulilles and Le Racou.
No significant difference was found between Tancade and Pau-
lilles (p = 0.25), but mean density at Le Racou was significantly
lower (p = 0.04)
Vie Milieu, 2010, 60 (4)
has been shown globally (Carter & Blaricom 2002). The
low density of sea urchins at Le Racou could, if recre-
ational fishing intensifies, lead to a total stock loss at this
site, provoked by either the loss of reproductive individu-
als or a greater impact of stochastic population size varia-
tion. However, to assess the reality of such a risk, further
work is required to understand population dynamics at
this site. It must be assessed whether or not the individu-
als found at this site are reproducing, as fishermen tend to
pick the larger, sexually mature, individuals, thus poten-
tially decreasing the fraction of breeders in the popula-
tion. If no reproducers are present in Le Racou, an extinc-
tion/colonization balance might nonetheless maintain the
population of P. lividus. Difference between population
density between the sites may also be explained by their
topography, which although being rocky, is mainly hori-
zontal. This should affect more A. lixula, known to live
mostly on vertical rock walls (Kempf 1962, Bulleri et al.
1999, Guidetti & Mori 2005), than P. lividus, which pos-
sesses fewer adhesive tube feet and a larger test diam-
eter and colonizes rather horizontal rocky areas (Bulleri
et al. 1999). The difference in density observed between
Le Racou and the two other sites contrasts with previ-
ous studies that found the opposite pattern: a higher den-
sity in an unprotected area compared to a protected area
(Sala & Zabala 1996, Guidetti et al. 2004). In these stud-
ies, the difference may be explained by a cascade effect:
the establishment of a protected status in an area leads to
an increase in the density, biomass and diversity of fish
due to a decrease in fishing pressure (Polunin & Rob-
erts 1993), leading in turn to an increase in predators of
sea urchins such as Coris julis and Diplodus spp. (Sala
1997, Barrett et al. 2009), therefore resulting in a lower
density of sea urchins in protected areas. We report the
opposite, most likely because our study was carried out
at low depths where such sea-urchin predators are rare,
suggesting that their effect is low or absent. Such higher
urchin densities in protected vs. unprotected areas has
been previously observed in the same location (Lecchini
et al. 2002), as well as in other regions (Gianguzza et al.
2006, Pais et al. 2007). This suggests that the dynamics
of urchin population in protected (vs. unprotected) area is
controlled by a balance between the effects of biodiver-
sity conservation on the food chain [the so-called “reserve
effect”, see e.g. Harborne et al. (2008)] and the release of
direct anthropogenic pressure.
Only P. lividus presents a difference in mean size, at
each site. The individual size of P. lividus individuals
is 1.5 times lower at Le Racou than at Tancade and 1.2
times lower at Paulilles compared to Tancade, suggesting
that individual size varies along a gradient of anthropo-
genic pressure. Such an effect is predictable, as fishermen
tend to pick the largest individuals. In shallow Mediter-
ranean rocky reefs, intense (and unregulated) exploitation
probably leads to a reduction in the mean size of P. lividus
(Gras 1987, Guidetti et al. 2004, Pais et al. 2007). Har-
vesting mainly takes place during the spawning period,
between May and August. Previous studies indicate that
the main spawning period of P. lividus is in spring and
summer (Lozano et al. 1995, Lopez et al. 1998, Spirlet et
al. 1998). This harvesting of the largest individuals may
lead to a lower rate of reproduction, strengthened by the
fact that urchins are harvested during the spawning sea-
son. The reduced reproduction rate may cause an irrevers-
ible decline of the population, which will in turn impact
the local food web. The lack of significant differences of
mean size of A. lixula between Tancade and Paulilles is
to be taken with caution, as the numbers of replicates for
this condition is low (n = 4). We could have expected an
inverse relationship between population density and mean
individual size, as shown for Diadema antillarum, con-
sistent with the hypothesis that urchin body size is regu-
lated by food availability (Levitan 1988). However, we
observed that mean size increases with density for P. livi-
dus at Paulilles. This could be linked to the shallow depth
at which our study was carried out: larger (older) indi-
viduals are indeed found deeper than the smaller (young)
specimens (Lecchini et al. 2002). This pattern is also
expected if the difference in individual size and popula-
tion density observed between sites is due to harvesting,
because fishermen sample the largest individuals, then
impacting both density and size.
In conclusion, the present study suggests that anthro-
pogenic pressure such as recreational harvesting affects
negatively the density and mean size of the urchin popu-
lations, especially P. lividus, at the sampling sites with a
high or moderate anthropogenic pressure. This study is
preliminary and it would be necessary to characterize pre-
cisely the nature of this pressure along the French Cata-
Fig. 5. – Mean sizes of Paracentrotus lividus individuals at the
sites of Tancade, Paulilles and Le Racou. Mean sizes at all sites
were significatively different and all pairwise tests indicate sig-
nificant differences; individuals at Tancade were bigger
(p = 0.0001), and individuals at Le Racou were smaller
(p = 0.0003).
Vie Milieu, 2010, 60 (4)
lan coast (relevant measures include number of harvest-
ers, time spent harvesting, and mean number, size and sex
ratio of harvested individuals). Some replicates of poten-
tially harvested vs. non harvested sites should be included,
and more environmental variables at each sites should be
taken into account in the analysis and interpretation of the
results. The effect of predators should also be more thor-
oughly investigated. In this study, we cannot rule out that
this variation between sites is due to other factors such
as population dynamics or seasonal processes, but previ-
ous data for comparison are lacking. However, our results
suggest that, given the key role of these sea urchins in
the coastal ecosystems, recreational harvesting should be
more closely monitored in this area.
Ac k n o w l e d g m e n t s .- D Borcard kindly provided the R script
for the Bartlett’s test with permutations, and P Legendre helped
with the permutational two-way ANOVA. We would like to
thank three anonymous reviewers for their constructive com-
ments on this manuscript. This study is part of the Cybelle
Méditerranée biodiversity monitoring program, led by C
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Received February 25, 2010
Accepted August 3, 2010
Associate Editor: J Orignac
... Le sex-ratio des adultes peut être influencé (i.e. déviation à partir d'un rapport de 1 : 1 de sexe) par : la période d'échantillonnage (Ozvarol et Turna, 2009) ; la disponibilité des conditions alimentaires (des habitats sociaux plus typiques) qui pourraient attirer un nombre plus élevé des femelles (McPherson, 1965(McPherson, , 1968Lessios, 1979) ; la différence dans la croissance et la mortalité pourraient être à l'origine de la dominance d'un sexe par rapport à l'autre (McPherson, 1965) ; la distribution spatiotemporelle des deux sexes, leur potentiel reproducteur (nombre de sperme et d'oeufs déposés) et leur comportement reproductif avec différents moment de l'émission de gamètes (Glutton-Brock et Vincent, 1991;Levitan, 2004) ; la densité des individus adultes (Gianguzza et al., 2007) ; prédation sélective naturel selon le sexe (Fenberg et Roy, 2008;Gianguzza et al., 2009) ; la saison de migration (Levitan, 2004) ; les activités anthropogéniques (la pêche et la récolte) (Uphoff Jr, 1998;Brazeiro et Defeo, 1999;Guidetti, 2004;Diele et al., 2005;Tessier et al., 2010). ...
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This study is to assess the quality of Algerian coastal waters, according to an ecological approach, the level of contaminant (as the traces metals and radionuclides) bioaccumulation was measured, in the mussels Mytilus galloprovincialis (Lmk, 1816) and the sea urchin Paracentrotus lividus (Lmk, 1816). Sampling of M. galloprovincialis performed over six months at three (03) regions of Algerian coastal: West coast "Kristel, Oran" (S1), Center coast "Sercouf, Algiers" (S2) and Est coast "Collo, Skikda" (S3). In a first step, trace metals (Pb, Cd, Cr, Ni, Co, Cu, Zn, Mn et Fe) were determined in the mussel ecosystem of M. galloprovincialis (sediments, suspended particulate matter, mussels), to assess metal biogeoavailability and to assess the human health risk. Secondly, transplantation of M. galloprovincialis individuals from different regions (East, Central and West) has been carried at the joint site of Sercouf, Algiers (S2), to discount the trophic effect and physicochemical effect on the bioaccumulation of trace metals and radionuclides in the mussels. The transplantation of individuals began in May and ended towards the end of August. Physicochemical descriptors of seawater (temperature, salinity, pH, dissolved oxygen, suspended particulate matter, and nutrient) were analyzed during the implantation period and during the retrieving mussel samples. At the same time, in the context of quality control (QC), morphometric and physiological measurements of individuals were carried out. Regarding the study of bioaccumulation of chemical contaminants (trace metals), in the sea urchin Paracentrotus lividus, seasonal sampling was carried out throughout the year. Bioaccumulation of trace metals (Pb, Cd, Cu and Zn) in gonads of sea urchin measured in three sites: (S4) Sidi Mejdoub, (S5) Willis (Mostaganem) and (S6) Bateau cassé (Algiers). Seasonal monitoring of the morphometric and physiological parameters of the sea urchin was carried out at all sites (S4, S5 and S6). Our results show that the M. galloprovincialis having different bioaccumulation capabilities depending the metal biogeoavailability. However, the Cr represents the metal limiting consumption (0.5 kg/d) of this species, and the treated individuals are not contaminated with Cs (137). As for the analysis of the contamination in P. lividus, the maximum bioaccumulation of Pb, Cu and Zn recorded in the site S6 "Bateau Cassé", probably related to the proximity of this site to the spill of the two oueds: "Oued El Harrach" and "Oued El Hamiz". The sea urchins in the site S5 "Willis" are the most contaminated by Cd. Keywords: Biomonitoring; Hydrological descriptors; Metal availability, Gonadic index; Risk assessment.
... This result agrees with the one obtained by Guettaf (1997). The sex ratio could be influenced by the following factors: the sampling period, collection and fishing (Guidetti, 2004;Ozvarol and Turna, 2009;Tessier et al., 2010); the food conditions which could attract a higher number of females (McPherson, 1965(McPherson, , 1968Lessios, 1979); the growth and the mortality of the species (McPherson, 1965); the reproductive behaviour (Brock-Clutton and Vincent, 1991;Levitan, 2004); the density of the adult individuals (Gianguzza et al., 2007); the selective predation according to the sex (Gianguzza et al., 2009); the migration season (Levitan, 2004). The seasonal variability of the gonad index (GI) presents the same evolution in the all stations (p > 0.05). ...
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This paper presents a goods and services approach to determine the economic value of Paracentrotus lividus with the aim of clarifying the role of valuation in the management and conservation of marine biodiversity. More specifically, it uses the Contingent Valuation Method to estimate the existence value of the sea urchin population that is predicted to be lost in the future if we do not take any measure today to protect them. According to the results a great number of respondents were willing to pay for the conservation of P. lividus population. Moreover, individual characteristics have distinctively different effects for explaining responders behaviour against their conservation. The empirical results from the proposed models link individuals opinion about eco-nomic value of biodiversity with people total willingness to pay (WTP). The results would help the choice of management strategies for P. lividus conservation and can help design incentive schemes to make conservation policy both effective and efficient.
The purple sea urchin Paracentrotus lividus is a Mediterranean and north eastern Atlantic species. It is particularly common in the subtidal, down to depths of 10-20 m and in tidal pools, mainly on solid rocks, boulders and in seagrass meadows. Densities usually range from a few to a dozen individuals per square meter, but very high densities (over 50 to 100 ind·m-2) may occur. In the field, when food resources are not limited, P. lividus is basically herbivorous, with a number of species of MPOs (Multicellular Photosynthetic Organisms) clearly 'preferred' or 'avoided'. Under conditions of limited food resources, it appears to be a very opportunistic generalist, able to exploit any kind of food resource, including drift material, sponges, hydrozoans and even small conspecifics (cannibalism) and particulate organic matter (POM). Competition between P. lividus and other herbivores (e.g., the sea urchin Arbacia lixula and the teleost Sarpa salpa) and predation by teleosts, crustaceans and mollusks may affect its abundance and behavior, and hence its effect on benthic communities. The density of P. lividus shows a general and conspicuous negative correlation with erect MPO coverage and biomass. It is a key species and structuring force, not only in overgrazed habitats with high sea urchin densities (barren grounds), but also in habitats with low sea urchin densities and a dense cover of MPOs (forests). In addition, it may control the shift between these two alternative 'stable' states.
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Commercial harvest of red sea urchins began in Washington state in 1971. Harvests peaked in the late 1980s and have since declined substantially in Washington and other areas of the U.S. west coast. We studied effects of experimental harvest on red sea urchins in San Juan Channel (SJC), a marine reserve in northern Washington. We recorded changes in density and size distribution of sea urchin populations resulting from three levels of experimental harvest: 1) annual size-selective harvest (simulating current commercial urchin harvest regulations), 2) monthly complete (non-sizeselective) harvest, and 3) no harvest (control) sites. We also examined re-colonization rates of harvested sites. The red sea urchin population in SJC is composed of an accumulation of large, old individuals. Juvenile urchins represent less than 1% of the population. Lower and upper size limits for commercial harvest protect 5% and 45% of the population, respectively. Complete harvest reduced sea urchin densities by 95%. Annual size-selective harvest significantly decreased sea urchin densities by 67% in the first year and by 47% in the second year. Two years of size-selective harvest significantly altered the size distribution of urchins, decreasing the density of legal-size urchins. Recolonization of harvested sites varied seasonally and occurred primarily through immigration of adults. Selective harvest sites were recolonized to 51% and 38% of original densities, respectively, six months after the first and second annual harvests. Yields declined substantially in the second year of size-selective harvest because of the fishing down of the population and because of low re-colonization rates of harvested sites. We recommend that managers consider the potential efficacy of marine harvest refuges and reevaluate the existing upper and lower size limits for commercial harvest to improve long-term management of the sea urchin fishery in Washington.
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We surveyed the population structure of the sea-urchin Paracentrotus lividus, considering the impact of depth, habitat and protection on its abundance and size distribution. No difference was found between habitats (walls vs. boulders) whereas a depth gradient was highlighted for the abundance and the size distribution of the sea-urchin. Most of the population (about 80%) is located in shallow areas (less than 10 m depth) whatever the location. Shallow water populations were made of small and medium size individuals (< 50 mm in diameter) while deep water populations were made of large individuals (> 50 mm in diameter). These large individuals accounted for 57% of the population in deep areas while they only represented 11% in shallow habitats. Since the recruitment in the deep waters cannot explain the abundance of large individuals, we suggest that larger individuals originate from shallow water populations, migrating to deep habitats while growing. In addition to differences linked to depth, we also observed significant differences between localities, higher abundances of sea-urchin being observed in the marine protected area than outside (193.6 vs. 82.5 ind. per 10 m2). However, rather than a protection effect, such result seems to be the consequence of a lower recruitment outside the protected area as the lower abundance of juveniles was observed out of the protected area. This last observation demonstrates the existence of a micro-geographic variability in the population structure of Paracentrotus lividus and much attention should be paid on this aspect prior to test the protection effect.
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The red sea urchin, Strongylocentrotus franciscanus, is a conspicuous member of subtidal communities in the north Pacific. Within the last decade, this ecologically important species has been exposed to intense harvesting for the first time ever. Analysis of population census data suggest that harvestable size urchins have rapidly declined in shallow regions while catch-per-effort and landings data suggest that divers have maintained high landings by exploiting more distant and difficult fishing areas, including deeper areas. We present a size-structured model for the red sea urchin both to estimate what previous levels of harvesting mortality were and to explore what effect future harvesting strategies might have on population trajectories. Using population census data, we explore three models: one that would result in an equilibrial population size in the absence of fishing, one that includes positive density dependence (an "Allee effect"), and one that incorporates realistic variability in recruitment. Our principal findings are that annual fishing mortality levels that best fit the observed census data in the past (1984-1993) were 0.38-0.49 and represent a 70-90% decrease in the survivorship of harvestable urchins in the years of fishing. Under a variety of fishing strategies, 100-yr projections indicated that the inclusion of an Allee effect or variability in recruitment could drive the harvestable population to <50% of present estimated population size at relatively low annual fishing mortality values. Our simulations also indicated that, although a yearly fishery would have higher yields, a rotational fishery would maintain populations at a level less likely to cause irreversible decline. The wealth of empirical evidence that sea urchins are an important component of nearshore communities should encourage management strategies that emphasize long-term population viability.
Conference Paper
I combined stock-recruitment, environment-recruitment, and spawner biomass per recruit analyses to analyze risk of blue crab (Callinectes sapidus) stock deflation using fishery-independent trawl survey data collected from Maryland's portion of Chesapeake Bay. Spawning stock biomass indices varied from 0.06-0.46 kg per trawl and recruit indices above the 1977 to 1995 median did not occur once the spawner biomass index decreased below 0.10 kg per trawl. Stock-recruitment would be stable for 4 to 5 years and then would undergo a rapid change in status. Ricker stock-recruitment models that included a term for either winter water or air temperature were significant. Analyses of resistance to overfishing and risk of deflation concentrated on best- and worst-case stock-recruitment models; 1976 to 1986 year-class spawner biomass per recruit produced mean spawner biomass near where maximum mean recruitment was expected. Spawner biomass per recruit was near the deflation threshold for 1987 to 1993 year-classes; mean stock size was near 0.10 kg per trawl in both cases, and mean recruitment was reduced 24% for the worst-case stock-recruitment model. Risk of stock deflation under the 1987 to 1993 year-class harvest pattern was 14-281 and doubled that of the 1976 to 1986 year-class harvest pattern. An increase in exploitation, a consequence of decreasing oyster (Crassostrea virginica) value, and rising blue crab value, during the mid-1980s and 1990s was inferred from decreasing spawner biomass per recruit and decreasing legal size male mean carapace width. Female blue crabs increasingly may be the focus of a directed fishery that could lead to even lower spawner biomass per recruit and increased risk of stock deflation.