Depth and Seasonal Effects on the Settlement Density of Two Mussel Species (Perna Perna and Mytilus Galloprovincialis) in Offshore, Agadir (Morocco)
European Scientific Journal April 2018 edition Vol.14
Depth and Seasonal Effects on the Settlement Density of Two Mussel Species (Perna Perna and Mytilus Galloprovincialis) in Offshore, Agadir (Morocco)
Mohamed Id Halla 0
Jawad Kassila 0
El Mustafa Ait Chattou 0
Yassine Ouaggajou 0
Fatima El Aamri 0
Abdelbasset Benbani 0
Student Hassan Nhhala 0
0 National Institute of Fisheries Rechearch (INRH) , Morocco
A study of spat settlement of two mussel species was carried out in Agadir area on the North Atlantic Coast, between Jan 2002 and Jan 2003. The preferred depth of settlement and settlement period of both species were monitored on collectors suspended in offshore at three different depths (1, 5, 10 m). For Perna perna, the effect of season on settlement was consistent, with relatively higher settlement both in spring (286-462 spats.m-1) and in summer (406-594 spats.m-1). Similar abundances of settlers were found at 1 m and 5 m depth whatever the season, which suggests a homogeneous distribution of settlers of P. perna in the first 5 m of the sea water. For Mytilus galloprovincialis, the settlement was less patchy in time in regard to P. perna (ρ<0.05). Thus, the settlement was continuous from spring to autumn until 10 m depth. The filamentous structures (laces in polypropylene) used in this study are often designed to enhance the amount of settlers, however, the settlement densities of both species were very low. Consequently, the results suggest that larval supply has been the limiting factor in the settlement success, but not the lack of suitable substrates. Moreover, the study area has poor spat falls and seems to be not suitable for collection of mussel spat.
Mussels spat; offshore; depth; season
the generally abundant supply. The seasonal nature of spat collection also
allows recovery periods of up to a year, which should be ample time for the
recovery of the ecosystem. However, the supply of mussel seed is often critical
for the development of industrial mussel cultivation
(Fuentes & Molares,
. Thus, supplies of seed mussels from natural sources have been scarce
over the last few years severely affecting the profitability of mussel fisheries.
While wild beds are still in use for juvenile supply in several countries,
reliability was obtained through the development of spat collecting techniques
(ropes, shell). Spat recruitment using rope collectors has been used about 65
years ago in Galician estuaries as an alternative to scraping
Molares, 1994; Cáceres-Martínez & Figueras, 1998)
. Interestingly, the use of
mussel juveniles from collector ropes has been recommended because of their
significantly larger size and weight at harvest than juveniles from intertidal
beds (Fuentes et al., 1998).
In Morocco, the development and scale-up of the mussel aquaculture
sector are limited. The mussel seed used in cultivation is still obtained by
scraping directly from intertidal exposed rocky shores where mussel seed is
attached. The gathering of mussel seed from artificial collector ropes was
carried out only on an experimental basis
(Aghzar et al., 2012)
. Due to
overfishing, the mussel beds tend to disappear on the exposed rocky shores
thus making the manual collection of spat less efficient. The feasibility of
using techniques such as rope cultures has been investigated in offshore, in
(Id Halla et al., 2017)
. The results indicated that the submerged
longline system tested in the study area could be commercially convenient.
Thus, about 90% of mussels in both species reached the commercial size (≥
60 mm) after 10 months of culture. However, it is necessary to move towards
a culture technique which combines seed collection and growth in suspended
culture to allow shorter production cycle.
The offshore mussel culture system has been often suggested as a
solution to meet the growing demand
(Langan & Horton, 2003; Karayücel et
. Well-protected and bays are preferred than open unprotected areas
for mussel culture. However, these ecosystems are often subject to
environmental constraints associated with pollution from industrial and
domestic sources. Major difficulties in establishing the rope cultures in
offshore are the high tidal currents, which require special mooring systems and
the reliance on natural larval supply. A mussel cultivation business is
dependent on a regular annual supply of seeds for growing on to market size.
Such supplies may not be sufficiently regular to sustain restocking on an
annual basis. The capture of wild stocks of mussel spat for use in aquaculture
was more reliable through accurate forecasting of peak settlement period and
knowledge of the preferred depth of settlement of different species. Thus, the
present study was designed to test the influence of season and depth on spat
settlement of two coexisting mussel species (Mytilus galloprovincialis and
Perna perna) in offshore, in Agadir area. This is the first investigation of
mussel settlement using an offshore submerged system in Morocco. The study
also aims at investigating the seed availability of both species for commercial
Material and Methods
The study was conducted 25 km north of Agadir at 20-22 m depth of
the offshore site in the North Atlantic Ocean (Fig. 1). This marine area,
commonly called “PK25” (30°34’N, 9°46’W), was chosen due to its harsh
weather conditions (currents and high waves), far from any pollution sources
(domestic and industrial). The designated area located at 1 mile from the coast
was exposed to relatively energetic waves from the North Atlantic. More than
70% of waves in winter have significant heights ranging 1.5-3.5 m
et al., 2016)
. In summer, the directional range also covers the North. More than
90% of incoming waves during this season have significant heights ranging
12 m. This clear seasonal wave pattern highlights the predominance of a
swelldominated regime in winter and more fair-weather mixed swell and
shorterfetch wave conditions in summer. Tides are semi-diurnal and mesotidal with a
mean spring range of 2.9 m and a mean neap range of 1.3 m.
Fig 1. Map showing the study area
A submerged longline system was deployed at 1 m below the water
surface. This system was designed according to site conditions. It was
anchored at both sides with heavy cement blocks (350 kg), and buoys (30 L)
attached along the longline were used to keep it afloat. Four marker buoys
were used on the edges of the experimental area to mark and protect the site
for navigation purposes. Experimental collector ropes were suspended on the
longline system from February 2002 to January 2003. During this spat
collecting trial, one type of collector rope was installed. Four 15 m (0.8 cm in
diameter) nylon ropes with filamentous lacing sections (40 cm of length) were
used. These polypropylene sections were attached to the ropes at three
different depths (1, 5, 10 m). Five kg concrete weights were tied to the ends of
the ropes to guard against the wave action and the tangling of ropes. The
collectors were hung on the mainline.
Collectors deployed in February 2002 and data collection started in
March 2002. Monthly sampling was conducted to determine the density of
spat on ropes at three different depths (1, 5, 10 m) throughout the study period.
Four replicates were sampled by divers at each depth. Individual samples were
grazed from a 40 cm length of the filamentous lacing sections. The spat was
transferred into a 20-L tank filled with sea water and brought to the laboratory.
Monthly mussel density on the ropes was determined by counting spat on the
40 cm sections of each filamentous lace. The settlers of each species were
identified by shell morphology, counted and preserved in 70% alcohol.
Temperature and salinity were measured monthly during the
experimental periods using a probe (6600 V2 YSI). Seawater samples were
collected at the experimental site from a 5 m depth using a Niskin bottle of 3
L. Three replicas were considered. The water samples were transferred to a
laboratory and filtered onto Whatman GF/C filters after acetone extraction to
determine chlorophyll a (μg L-1).
Spatial and temporal trends in spat abundance were analysed using
ANOVA. A series of ANOVAs were therefore used on balanced subsets of the
data to assess settlement variability. Two-way ANOVA was performed to test
the influence of season and depth on the settlement of both species. Where
significant differences (p≤0.05) were recorded, post-hoc comparisons were
done using Tukey’s test. We used a three-way balanced factorial ANOVA
(n=4) to examine the effects of season, depth, and species on the settlement.
The temperature ranged between minimum values of 15.7°C in winter
and 21.1°C in summer (Table 1). Salinity varied within a narrow range
(35.335.9). Chlorophyll-a concentration displayed significantly marked seasonal
changes. Chlorophyll-a concentration presented minimum value (0.77±0.32
μg L-1) in winter and maximum value (1.95±0.23 μg L-1) in summer. There
was a significant correlation between chlorophyll-a and temperature (r=0.87,
Settlement of Perna perna
In winter, settler abundance of P. perna on collectors ranged between
52±21 (10 m) and 108±35 (1 m) per meter rope (Fig. 2). The settlers were
more abundant in spring and summer whatever the depth. The number of spats
settled in spring, ranged between 286±50 (10 m) and 462±88 (5 m) per meter
rope. In summer, the abundance of settlers fluctuated between 406±43 (10 m)
and 594±65 (1 m) per meter rope. The settlers were more or less abundant in
autumn, with a mean maximum value of 272±53 settlers.m-1.
Two-way ANOVA was performed to test the influence of season and
depth on the settlement of P. perna. The results revealed that their effects were
highly meaningful (ρ<0.0001, Table 2), but not with a significant interaction
among them (ρ=0.202). Therefore, the effects of season and depth were
evaluated separately without considering their interaction effect. In winter, the
settlement of P. perna was significantly lower than those recorded in the other
seasons (Fig. 3, post-hoc comparison ρ<0.01). The settlement occurred mainly
over two seasons: spring and summer, with no significant difference among
them (post-hoc comparison ρ=0.185). Furthermore, the number of settlers
decreased significantly from summer to autumn (post-hoc comparison
The depth affected highly the variability of the settler abundance
(ρ<0.0001, Table 1). However, similar abundances were found at 1 m and 5 m
depth whatever the season (Fig. 4, post-hoc comparison ρ=0.982 in winter,
ρ=0.819 in spring, ρ=0.911 in summer and ρ=0.991 in autumn). The settlement
of P. perna decreased significantly from 5 m to 10 m depth both in winter and
spring (post-hoc comparison ρ<0.05).
Settlement of Mytilus galloprovincialis
The settlement of M. galloprovincialis was extremely low in winter,
ranging between 12±7 (10m) and 39±11 (1 m) per meter rope (Fig. 5). By
cons, the settlers were more abundant during the other seasons. In spring,
settler abundance ranged between 272±34 (10 m) and 414±51 (1 m) per meter
rope. In summer, this abundance fluctuated between 315±52 and 488±60,
recorded at 5 m and 1 m depth respectively. The settlement in autumn reached
at 5 m depth, a maximum value of a398±49 settlers.m-1.
The settlement of M. galloprovincialis was drastically lower in winter
whatever the depth, in comparison with the other seasons (post-hoc
comparison ρ<0.0001). There was not a significant difference in abundances
between spring and summer whatever the depth (post-hoc comparison
ρ=0.841(1 m), ρ=0.981 (5 m), and ρ=0.103 (10 m). In autumn, the numbers of
settlers decreased significantly at 1 m and 10 m depth (ρ<0.01 and ρ< 0.05
respectively). However, similar abundances were found at 5 m depth whatever
the season, except in winter (post-hoc comparison ρ=0.919 for spring-summer,
ρ=0.317 for spring-autumn, and ρ=0.730 for summer-autumn).
Numerous studies on the spat settlement of mussels indicate spatial
and temporal variability over a wide range of scales. Our results reported that
settlement of P. perna was highly patchy in time. Indeed, the effect of season
on settlement of P. perna was consistent, with significantly higher settlement
both in spring and summer. This strongly implies meaningful differences in
delivery of spat among seasons. However, the presence of settlers throughout
the year reflects the occurrence of different spawning periods for P. perna.
Peak settlement of spat occurred in April and August whatever the depth. The
temporal differences could be assigned to several biotic and abiotic factors
involved in larval dispersion and settlement. The timing and magnitude of spat
(Porri, McQuaid & Radloff 2006)
, algal coverage
Crowe & McGrath 2006)
and microbial coverage
(Hunt & Scheibling 1996)
are the most evident biotic factors. Abiotic factors combined the local
hydrographic regimes implicated in nutrient and spat dispersion
Pechenik 1987; Dobretsov & Miron 2001)
, physico-chemical substratum
(Pulfrich 1996; Alfaro et al., 2006)
Garland et al., 2002)
, orientation and daylight (Bayne 1964). In this study, the
spawning of P. perna was stimulated by increased temperature in spring,
which is the initial spawning period in the region. The peak settlement of P.
perna occurred both at 1 and 5 m depth, which suggests that it is quite possible
that we have a homogeneous distribution of settlers between 5 m and 1 m
depth. Thus, no depth preference can be inferred until 5 m from the data. Seed
densities were relatively lower at 10 m throughout the year. Mussel seeds
settled on the ropes near the surface because of the high temperature,
abundance of light and available food (Karayücel et al., 2002).
The settlement of M. galloprovincialis was less patchy in time in
regard to P. perna. Indeed, the effect of season on settlement of M.
galloprovincialis was inconsistent from spring to autumn. Overall, this implies
no meaningful differences in delivery of spat among these seasons. However,
the settlement of M. galloprovincialis was extremely low in winter whatever
the depth. The vertical variation of M. galloprovincialis did not follow the
same pattern as that of P. perna. Indeed, the settlement of M. galloprovincialis
was continuous from spring to autumn until 10 m depth. Peak settlement of
spat occurred in May and September at 1 m depth.
It has been proved that especially artificial collectors with filamentous
structure are effective in the settlement of mussel seeds
Lekang et al., 2003)
. Despite the preferable settlement of spats on filamentous
structures using the same laces, reported in many studies (Lutz & Kenish,
1992; Ramón et al., 2007), our results showed lower settlement densities. This
suggests that larval supply has been the limiting factor in settlement success,
but not the lack of suitable substrates. Cáceres-Martínez et al. (1994) obtained
highest settlement in the open sea with almost 60.000 mussels m-2 after five
months. Okumuş (1993) reported that density of spat on the collector rope had
to be at least 1200 ind. m-1 with commercial mussel farming. Thus, the
settlement densities obtained in this study seem to be not suitable for
commercial mussel farming. This inappropriate spat settlement revealed that
the study area had not suitable hydrological conditions and as a result, it not
facilitated mussel larvae settlement on the collectors. Thus, the local
oceanographic conditions were reported to play an important role in larval
(Blanchette & Gaines, 2007; Blanchette et al., 2007)
. The basic
requirement of all mussel culture practices is secure supply of spats and that’s
why big culture operations around the world have been located in areas
traditionally as where natural seed mussels are readily available every year. Id
halla et al. (2017) obtained better productions in this area and suggested
offshore cultivation as an opportunity for commercial enterprises without
worrying about the shortage of seed. In this case, it is necessary to collect or
purchase seed from distant areas which may involve expensive transport costs.
However, transferring seed from one site to another, however, results in extra
seed loss (Paul, 1987) and could be very labour intensive. Therefore, there is
no doubt that a site suitable for both good settlement and growth is always
The mussels exhibited rapid growth under cultivation in the offshore
area, hence marketable size (≥ 60 mm) was reached after 10 months of culture.
However, considerable effort is required to obtain sufficient seed to develop
the mussel farming in offshore. The spat collectors must be installed close to
the rocky shores, as the ropes tested did not produce enough seed in offshore
for new ropes (or thinning out ropes) where mussels remain until they reach
marketable size. If there is not enough spat on the spat collectors, seeds will
be collected from natural beds and transferred to the longlines after tubing in
bio-degradable socks. However, it must be taken into account that seed density
might change year to year depending on environmental factors at the spat
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