Population dynamics of Donax trunculus on the sandy beach of Taghazout (southern Morocco)

Journal Pre-proof Population dynamics of Donax trunculus on the sandy beach of Taghazout (southern Morocco) Imane Lamine, Aicha Ait Alla, Mohamed Ben Hadad, Hammou El Habouz, Meryam Nadir, Abdellatif Moukrim

PII: DOI: Reference:

S2352-4855(19)30346-9 https://doi.org/10.1016/j.rsma.2019.100912 RSMA 100912

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Regional Studies in Marine Science

Received date : 13 May 2019 Revised date : 21 October 2019 Accepted date : 28 October 2019 Please cite this article as: I. Lamine, A. Ait Alla, M. Ben Hadad et al., Population dynamics of Donax trunculus on the sandy beach of Taghazout (southern Morocco). Regional Studies in Marine Science (2019), doi: https://doi.org/10.1016/j.rsma.2019.100912. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier B.V.

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Population dynamics of Donax trunculus on the sandy beach of Taghazout (southern Morocco) Imane LAMINE1, Aicha AIT ALLA1(*) , Mohamed BEN HADAD1, Hammou EL HABOUZ2, Meryam NADIR, Abdellatif MOUKRIM3 1

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Laboratory Aquatic Systems: Marine and continental ecosystems; Faculty of Sciences, Ibn Zohr University,

Agadir (Morocco) 2

National Institute of Fisheries Research, Regional Centre of Agadir, Morocco

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Faculty of Sciences, Abdelamalek Essadi University, Tetouan (Morocco)

* Correponding author : Pr. Aicha AIT ALLA, e-mail : [email protected]

Abstract

The study of the structure and the dynamics of populations exposed to environmental pollutants

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can be a useful investigative tool for finding the connections between different levels of biological organization.

Population dynamics on wedge clam, Donax trunculus is still unknown in Morocco. FiSAT software (FAO-ICLARM Stock Assessment Tools) was used for calculating population parameters

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of this species. The specimens of D. trunculus (Linnaeus, 1758) were collected monthly in the Agadir Marine Bay (South of Morocco) for two years (January 2016- December 2017) from three stations (S1,S2 and S3), which are directly exposed to eventual pollutants owing to the developing tourism resort Taghazout Bay.

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The study of the population dynamics of D. trunculus shows that the annual mean of density, biomass and condition index, presents significant seasonal variations depending on the physicochemical parameters and the animal's life cycle. The monitoring of age-class frequency distributions of D. trunculus individuals revealed a slighter difference at the three study stations. Asymptotic length (L∞) was 37.96 mm and growth coefficient (K) was estimated at 1.93 year -1. Total mortality (Z) for D. trunculus was 3.30 year -1. Natural mortality (M) and fishing mortality -1

respectively. Exploitation level (E) was computed as 0.48,

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(F) were 1.73 year-1 and 1.57 year

indicating an optimum level of exploitation of D. trunculus in the coastal waters of Taghazout. It is well demonstrated that bimodal recruitment takes place twice a year; one in late summer-early autumn and the other in spring. The results of this study provide information on the health status of this ecosystem receiving Taghazout Bay project. Keywords: D. trunculus, Dynamics, Density, Biomass, Condition index, Growth.

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1-Introduction The marine environment has always played a major socio-economic and ecological role in the region of Agadir. Recently, the regional development strategy includes several investment projects along the shoreline of the Agadir bay. If these projects are an advantage for socio-economic development, their installation requires good management of the environmental aspect in the vision of sustainable development. Our research is part of this

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approach; since it provides the necessary data for the management of the environmental aspect of Taghazout coastal ecosystem that is hosting the large tourist complex Taghazout Bay covering an area of 615 ha in front of a coastline of 4.5 km.

This paper provides the state of health of the Taghazout coastal ecosystem before the implementation of the tourist-project.

It is worth noting that the sector concerned by this project has been the subject of several studies, but these have mainly concerned the estimation of pollution levels by determining the

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concentrations of certain contaminants: heavy metals (Banaoui et al, 2004), HAP (El Hamidi et al., 2003), pesticides (Agnaou et al., 2014) or monitoring the impact of wastewater on the receiving environment (Touyer et al., 1996; Kaaya et al., 1999; Id Halla et al., 1997; El

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Hamidi et al., 2003; Ait Alla et al., 2006a; Ait Alla et al., 2006b; Mouhani et al., 2011; Nadir et al., 2015). Meanwhile, our work focuses on the evolution of the ecosystem following the implementation of a touristic project.

To carry out the study of the state of health of the ecosystem (baseline state) we have opted

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for the species Donax trunculus. Several studies regarding different aspects of D. trunculus have been carried out in the Atlantic (Ansell & Lagardère, 1980; Guillou & Le Moal, 1980; Bayad & Guillou, 1985; Bayed, 1991, 1998; Guillou & Bayed, 1991; Usero et al., 2005; Boussoufa et al., 2012; Tlili et al., 2010, 2011,2013) and Mediterranean coasts (Mouëza, 1972; Mouëza & Chessel, 1976; Ansell & Bodoy, 1979; Bodoy & Massé, 1979; Ansell et al., 1980; Costa et al., 1987; Neuberger-Cywiak et al., 1990; Ramón et al., 1995; Fishelson et al.,

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1999).

This mollusk has been also selected as a sentinel species in several investigations in our laboratory in different ecosystems of the Agadir Bay. The various studies revealed keen interest of this bivalve as a sentinel species for the assessment and monitoring of the state of the sandy beaches health (El Hamidi et al., 2003; Moukrim et al., 2004; Idardare et al., 2008; Nadir et al., 2015). These researchers have also reported the importance of studying population dynamics using D. trunculus as a sentinel species, both to provide information

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Journal Pre-proof about the population in the ecosystem and to understand population dynamics for interpreting natural variation of different biological parameters used as biomarkers of pollution. However, in Moroccan ecosystems, except for the information provided by Bayed (1991) and Lagbouri (1997), the population dynamics of D. trunculus and its response to pollution is poorly known all over Moroccan area of distribution. Then, our investigation constitutes a contribution of making the “state zero” of Taghazout

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ecosystem and should be linked to long-term further surveys in order to provide basic information for the development of strategies for the protection and monitoring of the coastal ecosystem of Taghazout.

For this study, our main objective is to study the population dynamics of the species D. trunculus, characterizing sandy ecosystems and known for its role as a sentinel species, in Taghazout sandy beach ecosystem receiving the touristic complex Taghazout Bay. It should also be noted that this species is abundant in the region, studying its biology and dynamics is

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very useful for maintaining its stock. Our investigations on biological study of the mollusk will be reinforced with the study of the influence of some physico-chemical parameters in the

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field (seawater and sediment).

2-Materials and methods

2-1-Study area and climatic characteristics

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From a geomorphological point of view, the Bay of Agadir (South-West / North-East orientation) is characterized by wide morphological and geological diversity. It has an alternation of rocky and sandy beaches. It is open to the plain of Souss that is a welldeveloped agricultural region.

Bounded by Cap Ghir and Cape Juby, the trade winds are more regular and less strong; the continental shelf is wider, shallower and its slopes are lower than the one on the north of Cap

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Ghir; the direction of the coast is clearly deflected towards the West; this situation generates an important upwelling because of the constant regularity of the winds and the shallow depth of this zone (Moujane et al., 2011). From a climactic point of view, the region is characterized by an arid to semi-arid climate influenced by relief, ocean and Sahara, with a warm summer from June to August and a rainy winter from December to February (annual rainfall of about 300 mm).

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Journal Pre-proof Our study site is located 15 km from the north of Agadir City with a latitude and longitude respectively of 30° 30'40" North and 9° 40'33" West, hosting an important tourist project with large natural diversity and surfing spots known internationally. Therefore, tourism in this site has grown steadily.

2-2-Field Sampling and Analysis

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Samples of D. trunculus were collected at monthly intervals for a period of 2 years from January 2016 to December 2017 at low tide, from three selected stations on the coastline of Taghazout (Fig. 1). The first station is located on the south of the touristic complex (30° 30' 59,2" N 9° 41' 22,3" W), the second one in front of the station (30° 31' 07,4" N 9° 41' 26,0" W) and the third one in the north of the station (30° 30' 21,4" N 9° 41' 32" W).

The choice of these three sampling stations was based on their exposed position to the eventual contamination that can be caused by the touristic project. The three stations cover the

limit and S3 as the northern limit.

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distribution area of D. trunculus at the level of the studied ecosystem, with S1 as the southern

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2-2-1. Environmental parameters and sediment characteristics Due to the importance of the influence of environmental parameters on the biology and the population dynamics of mussels (Bayed, 1991) particularly filtering bivalves, we have studied several physico-chemical parameters in the three stations, during the study period from

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January 2016 to December 2017.

To assess the variation of the physico-chemical parameters of seawater during the sampling period, these parameters were measured in situ: Temperature (T°) and salinity (SAL) were measured using the Thermo Scientific Orion StarTM A222, and dissolved oxygen (DO) content by the Thermo Scientific Orion 3-Star plus device. For pH, we used the pH meter HI9024 Hanna instruments which can be read to the nearest 1/100th of a unit. For organic matter, the top 1 cm of sediment was sampled every month and the organic matter

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content of the sediment was determined by the loss of weight on ignition (4 h at 550 °C) after drying to constant weight (72 h at 65 °C), an homogenized portion of about 100 g of sediment (CEAEQ, 2003).

To measure particles grain size, additional sediment samples (100 g) were taken from the zone where the specimens were collected, at each station, these samples were dried for 48 hours at 60°C (Bergayou, 2006). The remaining sediment was dried again at a temperature of 60°C, and all samples were then sieved through an AFNOR series of meshes (500, 315, 250, 4

Journal Pre-proof 200, 160, 125, 100 and 63μm). The following fractions were observed: medium sand (MS) (grain size: 0.5–0.25 mm), fine sand (FS), (grain size: 0.25–0.125 mm) and finest sand (FTS) (grain size: 0.1-0.05 mm).

2-2-2. Population dynamics In each station, three quadrats were established (0.25 m²) using a shovel. According to

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Menesguen (1980), this surface allows to sample suitably the fauna whatever its regular method of distribution: random or contagious, the sediment was taken to a depth of 10 cm and then sieved using a 1 mm mesh size sieve.

A total of 3866 individuals of D. trunculus were collected and analyzed over the whole study period. After collection, D. trunculus samples were transported alive to the laboratory. The antero-posterior length of each individual was measured using a digital caliper to an accuracy of 0.01 mm. The animals were grouped into size classes based on their shell length with an

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interval of 1 mm (Mouëza, 1971; Guillou and Le Moal, 1980; Bayed & Guillou, 1985; Gaspar et al., 2002). Thus, the frequency distribution of these different classes is determined monthly. At each sampling site, D. trunculus density was estimated in terms of number of individuals

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per square meter (ind.m−2) and biomass was expressed as the total fresh weight of clams per square meter (g.m-2).

2-2.3. Condition index

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Additional samples comprising 50 individuals (randomly taken) were collected from the three stations during the two studied years for condition index (CI) analyses. Bivalves were individually measured for shell length (maximum antero-posterior shell length), accurately to 0.01 mm using a digital caliper. Total weight was measured and tissues were removed from the shell and weighed separately. Then each shell weight was recorded. Both tissue and shell was dried for 48 h at a temperature of 55 °C, and then weighed precisely

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to 0.001 g. Among the measurements carried out; the weight of the fresh tissue, so that we can control if the weight is constant after 48 hours. Dry condition index was estimated according to Beninger (1984) using the following formula:

CI: [(dry tissue weight÷ dry shell weight) ×100]

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2-2-4. Data analysis The modal progression method (Battacharya, 1967), was adopted to identify the modal length of the cohorts. Several authors have used this method in their studies on Donacidae (Bodoy, 1982; Ramón et al., 1995; Manca-Zeichen et al., 2002; Marcano et al., 2003). For estimating Von Bertalanffy growth parameters, asymptotic length (L∞) and growth

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coefficient (K), the length measurements of two-year data were pooled month-wise and grouped into length classes by 1 mm intervals, and analyzed using the ELEFAN (Electronic Length Frequency Analysis) of FiSAT software (Gayanilo et al., 1996).

The total mortality (Z) was estimated by a length converted catch curve method (Pauly, 1984). The natural mortality coefficient (M) was determined using M ≈ K approximation (Gayanilo & Pauly, 1997). However, the fishing mortality coefficient (F) was estimated by

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subtracting M from Z. The exploitation level (E) was obtained from the relationship of Gulland (1965). For other parameters, statistical analysis was done using Statistica v6 with the single classification ANOVA and Student–Newman–Keuls test for multiple comparisons

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among means. The correlation was also performed with Statistica v6 at a significance level of p<0.05 to test the relationship between environmental parameters and biological parameters of the species.

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3-Results

3-1. Environmental parameters and sediment characteristics Fig. 2 shows the temporal variations (mean ± standard deviation) of temperature, pH, salinity and dissolved oxygen during the sampling period in the three stations. Water temperature showed seasonal variations values with fluctuations ranging from 14.9 °C in winter (February

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2016) to a high of 26.6 °C in summer (August 2017). These variations are related to the meteorological conditions recorded during the study period (Fig. 2a), in fact, the climate of the region is arid to semi-arid with a warm thermal variation, and the average temperatures are relatively high with a moderate summer (26.7°C) and a very mild winter (14°C). Concerning salinity, fluctuations among years were observed with the lowest values of 26.9 and 31.26 ‰ recorded respectively in winter (February 2016 and 2017), while the maximum

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Journal Pre-proof value was observed in summer (August 2016 and 2017) with values of 35.2 and 37.9 ‰ respectively (Fig. 2b). The seasonal evolution of dissolved oxygen shows highest value in March 2017 with 8.5 mg/l, while the lowest value was recorded in winter (November 2016 and October 2017) with 6.9 mg/l (Fig. 2c). The values of pH varied between 7.04 in winter (February 2016) and 8.17 in summer (August 2017) (Fig. 2d).

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Our results show that there is no significant differences in physico-chemical parameters between stations (p>0.05) using Student–Newman–Keuls test.

The variation of sediment contents in organic matter is illustrated in Fig. 3. During the year 2016, the mean annual value of organic matter was 0.94 ± 0.15% against 1.65± 0.46 % during the year 2017. The Student–Newman–Keuls test showed no significant variation among stations, however, a highly significant difference was observed between years (p<0.001).

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Fig. 4 shows the variation of sediment grain size. During the study period at the three stations the medium sand class (0.1 to 0.5 mm) occupied the majority of the sediment. It had an average percentage of 72.4%, followed by the fine sand (0.1 to 0.2 mm) with 27.52% and the

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finest one (0.1 to 0.05 mm) with a percentage of 0.074%.

The variation of granulometric composition was highly significant between months for the medium sand class and for the fine sand class (p<0.001), while for the very fine sand class,

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the difference was significant (p<0.05). However, there were no significant differences between the three stations (p>0.05) (Fig. 3). 3-2. Density, biomass and condition index of D. trunculus The monthly variations of the density and biomass of D. trunculus in the Taghazout sandy ecosystem are presented in Fig. 5. During the two years, the mean annual density of D. trunculus was 50±15 ind.m-² with a maximum (113 ind.m-²) in July 2016 and a minimum (24

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ind.m-²) in February 2017. The mean annual biomass was 1.05±0.21 g.m-² with a minimum (0.40 g.m-²) in September 2016 and a maximum (1.56 g.m-²) in November 2017. In general, an increase in the density is observed between late spring and early autumn, followed by a decrease in winter. These results show that, there is a significant difference among months (p<0.05) in both density and biomass of D. trunculus. However, there is no significant difference between the three stations (p>0.05).

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Journal Pre-proof Fig. 6 shows the monthly evolution of D. trunculus condition index values during both cycles of study. At the three studied stations, the condition index showed a highly significant increase during the spring time, specifically in April 2016 and March 2017 (65.17 and 67.2% respectively) and in November 2016 and 2017 (64.3 and 61.7% respectively). On the other side, a highly significant decrease (p<0.001) was observed in summer, more exactly in July

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2016 and 2017 (33.4 and 30.2 % respectively) using Student–Newman–Keuls test. 3-3.Relationship between density, biomass, condition index and environmental parameters

Correlations between density, biomass and condition index (CI) of D. trunculus and the physico-chemical parameters of seawater and sediment (Temperature, pH, salinity, dissolved oxygen grain size and organic matter) are given in Table 1. The study showed a limited range of significant correlations between the means of biomass, density, condition index of D.

three stations selected in this study.

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trunculus and the means of the physico-chemical parameters of water and sediment in the

At station S1, biomass was not significantly correlated with any of the environmental parameters, while density had a significant negative correlation with dissolved oxygen

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(r=-0.4247, p=0.039), another negative and significant correlation was observed between condition index and temperature (r=-0.5983, p=0.002) as well as pH (r=-0.5442, p=0.006). At station S2, biomass is correlated negatively and significantly with pH (r=-0.4654, p=0.022), while density is correlated positively with temperature (r=0.414, p=0.044); for the condition

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index, it has a significant negative correlation with temperature (r=-0.5106, p=0.011) and pH (r= -0.5442, p=0.006). At station S3, biomass and condition index were not significantly correlated with any of the environmental parameters, while density has a significant positive correlation with temperature (r=0.5511, p=0.033). 3-4. Size frequency distribution of D. trunculus

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The monthly evolution of the populations of D. trunculus (Fig. 7) shows a slight difference between the three studied stations during the two-year period (2016-2017). When the study began in January 2016, the population in the three stations was formed by five cohorts, C1, C2, C3, C4 and C5. Three newly recruited cohorts were detected in 2016, in S1, the cohorts C6 and C7 were observed in August 2016 and C8 in November 2016. In S2, the cohort C6 was found in August 2016, C7 in October and C8 in November 2016, while in S3, the cohort C6 was detected in April 2016, C7 in July 2016 and C8 in December 2016. In 8

Journal Pre-proof the second year (2017) three newly cohorts were also detected, C9 in March 2017, C10 in August 2017 and C11 in October 2017 for both stations S1 and S2, while in S3 the cohort C9 was found in April 2017, C10 in July 2017 and C11 in September 2017. In total, eleven cohorts were observed along the study period. These results showed that recruitment of juveniles of D. trunculus takes place twice each year (spring and late summer – early autumn) on Taghazout coast.

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The average length for the new cohorts that were recruited within the study period, varied between 3.5 and 6.5 mm (Fig. 7). The cohorts that were followed from recruitment to extinction grew from a length of 5.5 to 25.5 mm in 18 months (C5) and from 7.5 to 30.5 mm in 18 months (C6). From these results, it seems that the life span of D. trunculus does not exceed one year and half in the coast of Taghazout. 3-5. Recruitment pattern

Whole recruitment pattern of D. trunculus was continuous throughout the study period with

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two major peaks, one in spring and the other in summer for both years (Fig. 8). The percent recruitment varied from 0.31 to 16.10 during the present study. 3-6. Age and Growth

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The growth rate and the absolute increase in age are presented in Fig. 9. The Von Bertalanffy growth parameters obtained are as follows: L∞ =37.96 K=1.93 t0=0. The value of the third parameter of the Von Bertalanffy growth function was supposed to be zero (Newman, 2002). The life cycle of D. trunculus proved to be 1.5 years. This result is validated by the analysis of

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the size frequency distribution of this species (cohort C3 - C4). 3-7. Mortality and exploitation

From Fig. 10, the total mortality (Z) was estimated as 3.30 year using the length converted catch curve. The natural mortality (M) and fishing mortality (F) were 1.73 and 1.57 year-1 respectively based on the mean habitat temperature (26.2 °C). An exploitation level (E) of 0.48 was obtained and seemed to be closer to the expected optimum level of exploitation (E =

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0.50) of D. trunculus at the study area. 3-8.Virtual population analysis

Length structured virtual population analysis of D. trunculus is shown in Fig. 11. It indicates that the maximum fishing mortality was 22.1765 year-1 at the mid length of 28 mm, whereas the minimum fishing mortality was 0.0338 year-1 at the mid-length of 2 mm. The fishing mortality (F) was comparatively higher over the mid-length from 20 to 29 mm. 9

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4-Discussion

4-1.Environmental parameters and sediment characteristics During the study period, the seawater temperature shows a similar annual variation in

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the three stations with a minimum recorded in winter and a maximum in summer. The pH also shows an increase in summer, it coincides with the periods of phytoplankton blooms in the region (Regragui, 1991); however, the decrease in pH corresponds to the period of high rainfall (about 300 mm), so a massive influx of water continental loaded with organic matter. Low pH values can cause significant changes in the running of the oestrous cycle of many species in the ecosystem (Id Halla, 1997).

Salinity also shows a significant seasonal variation with an increase in summer and a decrease

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in winter. The same result was reported by Guerimej (1989) and Naciri (1990) in the region of Temara (Morocco). Thus, the rainwater carried by the wadis and discharged into the sea are the origin of seawater dilution and consequently the reduction of the salinity. On the other hand, the rise in atmospheric temperature causes evaporation of the water, which then causes

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an increase in salinity (Ben Charrada et al., 1997).

The annual cycle of dissolved oxygen concentration is similar in the three studied stations with a decrease in autumn and an increase in winter. Id Halla, (1997) has reported similar results at Cap Ghir (Agadir Bay). Indeed, Guerimej (1983) reports that the oxygen

sea.

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concentration of the seawater in Temara (Morocco) depends mainly on the agitation of the

Concerning the sediment, it shows a significant increase of the organic matter content in the second year of study; this would result from the deposit of re-suspended sediment, in addition to the high rainfall recorded in 2017 (260 mm) and the continental contribution resulting from

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the construction works of the tourist resort.

4-2.Population Dynamics 4-2-1. Density and biomass In the study area, D. trunculus was present in high densities during the summer months. A decrease in this parameter was observed in winter suggesting a decrease of water temperature; it is verified through the correlation Table 1. Indeed, the temperature is a crucial factor in determining such wide fluctuations in population density, as also reported by Neuberger10

Journal Pre-proof Cywiack et al., (1990). These seasonal fluctuations are linked to the species reproductive cycle. In the literature, higher densities coincide with the period of larval recruitment (Neuberger-Cywiak et al., 1990; Le Moal, 1993; Gaspar et al., 1999, 2002; La Valle, 2006), as confirmed by the present study. The peaks of highest densities coincide with the emergence of young individuals in the population, which confirms that recruitment takes place twice a year, one in the spring and the other one in the autumn. During the two years, the mean annual

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density of D. trunculus was 50 ± 15 ind.m-2. Moreover, the abundance and distribution of this species are also influenced by the grain size of the sediments (Bally, 1983; Alexander et al., 1993; de La Huz et al., 2002; Nel et al., 2001; La Valle, 2006; La Valle et al., 2007). D. trunculus is commonly found on well sorted sandy beaches ranging from medium to fine sand (Bayed and Guillou, 1985; Guillou and Bayed, 1991; Dhaoui-Ben et al., 2003). However, in our study, there is no correlation between the density of D. trunculus and the sediment grain size as shown in Table 1.

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The results of Lagbouri (1997) concerning the population density of D. trunculus in Aghroud (30 km North of Agadir), show that this one is variable among years. However, the highest densities (820 ind.m-²) were recorded in the same period between April and July 1995 and the

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lowest ones (51 ind.m-²) were observed in autumn and winter 1996. A study carried out on several beaches on the Atlantic coast of Morocco showed inter-annual variability in density (Bayed, 1991).

Similar results were detected in Spain (Delgado et al., 2017) with a mean population density

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ranging from 30.6 ± 20.7 ind.m-² on Doñana and 52.9 ± 33.9 ind.m-² on Isla Canela. On the South Adriatic Coast of Italy (Manca Zeichen et al., 2002), D. trunculus was present in high densities ranging between 131 ind.m-² and 460 ind.m-². However, the densities of this species along the Latium coasts of Italy were lower about 40 ind.m² during winter (La Valle et al., 2011).

The mean annual biomass (total weight) of D. trunculus in the present study was 1.05 ± 0.21 g.m-² with a minimum of 0.40 g.m-² in September 2016 and a maximum of 1.56 g.m-² in

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November 2017. Different results were recorded in the Gulf of Annaba (in Northeast Algeria) where the biomass varied between 0.42-2.75 g.m-2 (between July-February) at Sidi Salem and 1.19-3.32 g.m-2 (between July-January) at Echatt (Hafsaoui et al., 2016). Nevertheless, the biomass of D. trunculus was much higher in Spain. It varied between 54.5 ± 39.6 g.m-2 on Doñana and 42.9 ± 29.6 g.m-2 on Isla Canela (Delgado et al., 2017).

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4-2-2.Recruitment period The temporal monitoring of the population structure makes it possible to study growth through the determination of recruitment periods. This corresponds to the appearance of the new individuals from the distribution histograms of the size classes of the antero-posterior

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length (Bayed and Guillou, 1985). Concerning the length-frequency distributions of D. trunculus, two recruitment periods were identified each year in the littoral of Taghazout, one in spring and the other in late summer – early autumn.

Spring recruitment would be the result of spawning in August of the year preceding recruitment, while the one observed in the end of the summer or early autumn may have come from spawning in May-June of the same year. This is confirmed by condition index results

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that show higher values in spring, when environmental conditions were favorable, rather than in summer. The increase in CI values corresponds to gonad development and/or somatic tissue growth, while the reduction of this parameter relates to the emission of gametes that may be combined with a growth arrest due to the use of reserves during unfavorable periods.

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With regard to D. trunculus from the Azure Coast, the same results have been described with a maturation period coinciding with the periods of increase in weight and the periods of spawning that correspond to the lowest weights (Ansell et al., 1980). This is consistent with the research of Beldi (2007) in the Gulf of Annaba.

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Similar results (two recruitments) were reported by Lagbouri, (1997) in the Bay of Agadir, in the Mediterranean coast of France (Bodoy and Massé, 1979), in Spain (Ramon et al., 1995) and in Algeria (Moueza and Frenkiel-Renault, 1973). A bimodal recruitment seems to be a characteristic of the Mediterranean populations (Mouëza and Frenkiel-Renault, 1973; Ansell and Bodoy, 1979; Ramon et al., 1995). This type of recruitment characterizes the populations of D. trunculus when latitude decreases (Lagbouri, 1997). This is confirmed by the research

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of (Bayed and Gillou, 1985) for the populations of D. trunculus of Mehdia. On the other hand, several studies report the presence of unimodal recruitment for populations of D. trunculus in the Mediterranean (Neuberger-Cywiack, 1990; Voliani et al., 1997; MancaZeichen et al., 2002; Sifi, 2009) and the Atlantic (Ansell and Lagardère, 1980; Guillou and Le Moal, 1980).

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The values of the different parameters of this equation for other populations of D. trunculus are presented in Table 2. The comparison of K values allows us to say that the growth rate of D. trunculus in Taghazout coast is high compared to other studies both in the Mediterranean and the Atlantic coasts. The lowest growth rate (K = 0.30) encountered on the Italian Coast

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(Zeichen et al., 2002), while it reaches 0.97 on the Spanish Coast (Fernández et al., 1984). This variation of the growth rate of D. trunculus between populations from different regions may be linked to the variation of temperature (Thippeswamy and Joseph, 1991; Gosling 1992; Singh, 2017). The effect of temperature is usually combined to that of food (Wilson, 1977; Thompson, 1984). Indeed, several authors have reported that growth varies with environmental conditions and population density (Neuberger-Cywiak et al., 1990; Zeichen et al., 2002). It should be mentioned that other factors such as pH, light, emergence time, age

growth (Page and Hubbard, 1987).

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and population structure can all have an important role on filtration and consequently on

According to the Von Bertalanffy’s equation, the maximum length that D. trunculus can reach on Taghazout coast is L∞ = 37.96 mm. However our results show that this length is only 34

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mm. The maximal value of L∞ (52.84 mm) was observed in the Atlantic Coast (Mazé and Laborda, 1988), while the minimal value (35.9 mm) in the Mediterranean Sea (Bodoy, 1982). The current study indicates that D. trunculus from Taghazout coast with maximum length (34

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mm) has a shorter lifespan of approximately around 1.5 years. In contrast Lagbouri (1997), reported that the longevity of D. trunculus in Aghroud Beach (Agadir Bay) is 3 years, the same thing was found in the south areas of D. trunculus distribution where it can live between 2 and 3 years (Moueza, 1972; Bayed and Guillou, 1985; Mazé and Laborda, 1988; Ramon et al., 1995) but in the northern region its longevity can reach 5 years (Gillou and Moal, 1980). Thippeswamy & Joseph (1991) reported that the life span of Donax incarnatus was above 1 year (less than 15 months) at Panambur. Like Donax incarnatus, the small-sized clam, Donax

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denticulatus has a shorter life span of 18 months (Marcano et al., 2003), even Donax hanleyanus showed a lifespan of about 17 months (Cardoso and Veloso, 2003). Most wedge clams (Donax spp.) have a relatively short life span of 1–2 years (Ansell, 1983; McLachlan, 1979).

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Journal Pre-proof 4-2-4. Mortality The total mortality (Z) of D. trunculus shows that it is higher in winter, during the spawning periods and the establishment of juveniles (Lagbouri, 1997). Similar results were reported for Donax incarnatus (Thippeswamy and Joseph, 1991), Donax hanleyanus (Penchaszadeh and Olivier, 1975) and Donax sordidus (McLachlan, 1979), indicating a 50% decrease in the juvenile cohort two months after recruitment. In the present study, the total mortality (3.30

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year -1) was higher when compared with the low values recorded in the bay of Agadir, as demonstrated by Lagbouri (1997), with a mortality rate ranging from a 0.658 to 1.861 year -1. This result could be due to the recreational fishing practiced by tourists frequenting this area.

5-Conclusion

This study presents a first overview of population dynamics of D. trunculus on the Taghazout coast; site selected for the installation of the tourism project. It provides clear data of the

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current exploitation of this species as well as length-frequency distributions, population density, biomass and condition index. The results of our study were in the moderate-low range reported for this species and were similar to those obtained elsewhere. The seasonal

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variations shown in this study depend mainly on the physico-chemical parameters and the animal's life cycle. This study will be complemented by other surveys to get an idea of this ecosystem including the study of population structures, the assessment of multi-marker responses in D. trunculus, the analysis of trace metals accumulation in this environment and

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others. All of these studies will also be done on the ecosystem after the starting of the resort in order to follow its evolution and the possible impact of this project on the marine environment of Taghazout.

6-Acknowledgements

Our gratitude is expressed to H. El Habouz for his statistical advice. The author also thanks the

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PhD students who helped in the bivalve collection.

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References

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73. TLILI, S., MINGUEZ, L., GIAMBERINI, L., GEFFARD, A., BOUSSETTA, H. ET AL., 2013. Assessment of the health status of Donax trunculus from the Gulf of Tunis using integrative biomarker indices. Ecological Indicators, 32, 285-293. 74. TLILI, S., METAIS, I., AYACHE, N., BOUSSETTA, H., MOUNEYRAC, C., 2011. Is the reproduction of Donax trunculus affected by their sites of origin contrasted by their level of contamination? Chemosphere 84:1362–1370

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75. TOUYER, O., ID HALLA, M., ZEKHNINI, A., MOUKRIM, A., 1996. Etude physico-chimique des eaux usées des trois principaux rejets du grand agadir. Journées Francophones des Jeunes Physicochimistes, juillet, Lille, France 76. USERO, J., MORILLO, J., GRACIA, I., 2005. Heavy metal concentrations in molluscs from the Atlantic coast of southern Spain. Chemosphere 59: 1175-1181. 77. WILSON, J.H., 1977. The growth of Mytilus edulis from Calingford Lough. Ir. Fish. Invest. Ser. B (Mar.), 17: 1-15.

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78. VOLIANI, A., AUTERI, R., BAINO, R., & SILVESTRI, R. 1997. Insediamento nel substrato ed accrescimento di Donax trunculus L. sul litorale Toscan settlement and growth of Donax trunculus L. along the Tuscanian coast. Biologia Marina Mediterranea 4, 458–460.

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Fig. 1. Map of the study area. The position of stations is indicated.

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Fig. 2. Temporal variations in water temperature, dissolved oxygen, salinity and pH in the three stations (S1, S2, S3)

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Fig. 3. Temporal variations of the organic matter content in sediment in the three stations (S1, S2, S3) throughout the study period (2016-2017).

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Fig. 4. Variations of granulometric composition expressed as percentages in sampling stations (S1, S2, S3) throughout the study period (2016-2017).

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Fig. 5. Density and Biomass of D. trunculus in the three stations (S1, S2, S3) throughout the study period (2016-2017).

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Fig. 6. Seasonal changes in the condition index (CI) of D. trunculus at the three stations (S1, S2, S3) throughout the study

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period (2016-2017).

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Fig. 7. Size frequency distribution of D. trunculus in the three stations (S1, S2, S3) throughout the study period (20162017).

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Fig. 8. Recruitment patterns of D. trunculus for two years (2016-2017) in Taghazout Coast

Fig.9. Plot of age and growth of D. trunculus based on computed growth parameters.

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Fig. 10. Mortality estimation of D. trunculus using Pauly’s linearized length converted catch curve method and estimation of exploitation.

Fig. 11. Length-structured virtual population analysis of D. trunculus

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Table 1: Correlations between biomass, density, condition index and environmental parameters at the study stations T°

pH

Salinity

Dissolved oxygen

Grain size

Organic matter

-0.0499

-0.0597

0.0642

-0.0424

-0.0352

0.004

p=0.817

p=0.782

p=0.766

p=0.844

p=0.870

p=0.985

0.3982

0.2438

0.1049

-0.4247

0.1588

-0.0707

p=0.054

p=0.251

p=0.626

p=0.039

p=0.458

p=0.743

-0.5983

-0.5442

-0.1067

0.4903

-0.083

-0.1469

p=0.002

p=0.006

p=0.620

p=0.015

p=0.700

p=0.493

-0.3283

-0.4654

-0.2433

-0.1601

0.0302

-0.1568

p=0.117

p=0.022

p=0.252

p=0.455

p=0.888

p=0.464

0.414

0.2244

0.2193

0.2666

0.0112

0.1343

p=0.044

p=0.292

p=0.303

p=0.208

p=0.958

p=0.531

-0.5106

-0.5636

-0.1031

-0.2305

0.1334

-0.1949

p=0.011

p=0.004

p=0.632

p=0.279

p=0.534

p=0.361

0.2031

-0.0068

0.0785

-0.0597

-0.1133

-0.139

p=0.468

p=0.981

p=0.781

p=0.833

p=0.015

p=0.621

0.5511

-0.0011

0.33

0.2182

-0.0353

-0.1389

S1

pro of

Biomass

Density

Condition index

Density

Condition index

Biomass

Density

urn a

S3

Jo

Condition index

31

lP

S2

re-

Biomass

p=0.033

p=0.997

p=0.230

p=0.435

p=0.901

p=0.622

-0.4484

-0.1548

-0.0463

-0.3658

-0.0304

0.3159

p=0.094

p=0.582

p=0.870

p=0.180

p=0.914

p=0.251

Journal Pre-proof

Table 2: Growth parameters of the Von Bertalanffy equation for D. trunculus populations in different areas

L∞

K

Authors

Atlantic (Morocco) (Taghazout)

37.96

1.93

This study

Atlantic (Morocco) (Aghroud)

40.33

0.48

Lagbouri (1997)

Mediterranean (France)

35.99

0.96

Bodoy (1982)

Atlantic (France)

38.22

0.70

Ansell & Lagardère (1980)

Atlantic (Spain)

43.80

0.97

Fernández et al. (1984)

Atlantic (Spain)

52.84

0.55

Mazé & Laborda (1988)

Mediterranean (Spain)

41.80

0.58

Ramón et al. (1995)

Atlantic (Portugal)

47.30

0.58

Gaspar et al. (1999)

Mediterranean (Italy)

47.56

0.30

Zeichen et al. (2002)

Marmara Sea (Turkey)

44.10

0.76

Çolakoğlu (2014)

re-

lP urn a Jo 32

pro of

Area

Journal Pre-proof Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

re-

pro of

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Jo

urn a

lP

79.

33