Stock mapping, size structure and biological parameters of the clam Polititapes aureus in the shellfish production area of the southern Tunisian waters (Central Mediterranean)

Shellfish production is a major contributor to the Tunisian fishery economy. Bivalves can play an important role in addressing the scarcity and rising prices of protein sources in Tunisia. The Tunisian coast stretches over long distances, so it becomes necessary to look at the exploitation of bivalves occurring in these areas. Intertidal bivalves are conspicuous members of productive areas. Unfortunately, shellfish exploitation has so far focused only on the clam Ruditapes decussatus (Linnaeus, 1758), which is extensively harvested from natural populations in Tunisia. This species is important to the economy, especially in terms of employment and the trade balance of the country through its exports.

The venerid Polititapes aureus (= Venerupis aurea) (Gmelin, 1971) is endemic to the Mediterranean Sea (Fouda & Abou-Zied 1990). It has migrated through the Suez Canal and successfully colonized the Tunisian coasts. The clam P. aureus belongs to the most common marine mollusk species found in the intertidal zone on the southern Tunisian coast including vast mudflats and sandy beaches favorable for infaunal bivalves (Ben Salem et al. 2002). Natural populations of P. aureus occur in sandy and silty-sandy sediments in bays, estuaries, coastal lagoons and other sheltered environments, living on tidal flats and below the low-tide level to a depth of 1 m. The species also plays an important role in ecosystems. It is a filter feeder pumping water through its siphons at the sediment level (Vilela 1950).

Despite its considerable economic importance, no studies have been carried out in Tunisia on the spread of the mollusk P. aureus. Consequently, the status of this species is still poorly understood and there is a knowledge gap in stock assessment. The literature review clearly indicates a lack of studies focusing on the stock size and biology of P. aureus in southern Tunisia. Stock mapping is necessary to indicate the location of major bivalve beds along the southern coastal areas, which may have potential commercial value. This study can provide basic knowledge in different fields of environmental research and constitute a first step for any future work and can be useful as a reference for research on in marine invertebrates. It presents a method for quantitative assessment of the bivalve species conducted in the area with the highest shellfish production. The species may be important due to its high exploitation potential. Therefore, this paper is the first attempt to obtain basic information on P. aureus by investigating its population structure, sexuality, geographical distribution and stock size in relation to the effects of certain abiotic factors.

The current study reports for the first time the spread of the clam P. aureus in the largest shellfish production area in the south of Tunisia by investigating the stock levels, population distribution, size structure and sex ratio. The findings of this study indicate an exceptional stock of this species, resulting in a remarkable biomass of 201 t of the total fresh weight and a high level of abundance, reaching over 91 million individuals. In general, this study was aimed at ascertaining whether the stock of P. aureus is large enough to start management of its stocks. Stock assessments of soft-bottom clams are scarce in the literature. The available information on the population stock assessment and distribution is insufficient. What is known about this species is limited to preliminary studies conducted in the Bou Ghrara Lagoon (64 t and a high level of abundance reaching over 156 million individuals; Derbali 2006). A similar gradient can be observed for another clam species (R. decussatus) with a total fresh weight of about 891 t and a high level of abundance reaching over 261.7 million individuals estimated in the same study area. Thus, our results showed that the variance within and between the sampling sites was large and confidence limits of biomass and density were correspondingly wide.

Stock mapping showed a fluctuation of P. aureus habitats in the colonized area. The stock appears to have varied considerably between the strata. The population seems to have been influenced by strong interactions at different levels (e.g. physicochemical, edaphic and hydrological factors operating in the study area). Stergiou et al. (1997) clarified that temperature and food resources are the most important factors affecting phenotypic differences in growth patterns and maximum size in different marine organisms. Elevated water temperatures and phytoplankton levels occurring in the sampling area may promote rapid growth rates in bivalve species. Due to the high water temperatures (12–27°C) and shallowness, salinity remains fairly stable throughout the year. The highest salinity was during summer (48 PSU) and often in winter (36 PSU). This result indicates that salinity remain high throughout the year. The maximum value recorded in this study is much higher than that recorded in Lake Vouliagmeni where salinity varied between 17 and 18 PSU (Chintiroglou et al. 2000). The increased salinity appears to have a major effect on the composition of the fauna and has reduced their standing crop (Fishar 2000; El-Shabrawy 2001).

The clam population appears to have been strongly affected by abiotic factors. During the present study, hydrodynamic conditions were found to be similar within the sampling area and it can be assumed that the relative population growth is affected by other environmental parameters such as sediment type, organic matter content, burrowing behavior of bivalve species and their subsequent strategies to counter dislocation and avoid predation. In fact, some interesting connections were detected between environmental conditions and bivalve behavior. Elevated water temperatures and phytoplankton levels may contribute to rapid growth rates in many local bivalve species (Dridiet al. 2007; Enríquez-Díaz et al. 2009; Sobral & Widdows 2014).

Additional mechanisms structuring clam populations include soft bottoms and organic matter content. The high diversity of shellfish species is particularly interesting when considering the relative organic matter content (11–22.13%; Table 1), depth (0–1 m) and silty-sand bottom. These factors can provide ecological conditions that are able to maintain highly diverse reef communities in the colonized area. Indeed, the absence of the clam P. aureus in the area characterized by sandy substrate (site 3) prevented the species from reproducing. Except for site 3, the clam P. aureus is omnipresent throughout the study area. The colonized sites maintain a high diversity compared to other areas of southern Tunisian waters, where only a few species (Gibbula ardens, Cerithium vulgatum, Cerithium scabridum and Bittium reticulatum) were found on rock and hard substrates. This low diversity seems to be attributable primarily to seabed characteristics. This hypothesis may be supported by the relationship between the sedentary nature of some shellfish species such as P. aureus and the substrate.

As regards faunal diversity and substrate types, this species was more frequent and abundant in areas sheltered by seagrass C. nodosa and Z. noltei, covering more than 65% of the silty-sandy seabed. Heterogeneity of the geographical distribution of P. aureus was found to be significantly correlated with the distribution of seagrass. This positive correlation was probably related to the main organic source provided by C. nodosa. The C. nodosa detritus was the richest in organic carbon and was the dominant source of primary organic matter. This is a result indicating that the colonized sites currently provide good conditions for the proliferation of clams. It can thus be concluded that this species improves the food resources and dissolved oxygen levels. The same conclusion was reached by Vilela (1950) for natural populations of R. decussatus in Portugal. Several studies have confirmed the correlation between patterns of community structure and primary production. In particular, local abundance and biomass of filter-feeders was correlated with both intertidal productivity and nearshore primary productivity (Menge & Olson 1990; Bustamante et al. 1995). Other parameters such as light and tides may also contribute (Drummond et al. 2006; da Costa et al. 2013). Seasonal metabolic activities in bivalves are usually the result of interactions between food availability, environmental conditions, growth and gametogenic cycles (Gabbott 1983; Derbali 2011).

The results on the distribution of clams may explain the dominance of this species in these particular water bodies. They also show the importance of environmental factors in controlling the density and subsequently the biomass of shellfish assemblages. This study showed that the clam population was not uniformly distributed: smaller specimens are dispersed in a relatively heterogeneous pattern throughout the study area, while larger individuals were more geographically restricted to sites 4 and 10. The mean sizes ranged from 18.41 ± 1.14 mm at site 5 to 26.10 ± 0.43 mm at site 2. The size range in populations varied between the sites. Size ranges of some bivalve species vary greatly across studies, complicating comparisons between species from different geographical areas. Size fluctuations may be related to abiotic factors that vary from one habitat to another. The growth, development and survival of bivalves are generally controlled by physical and chemical parameters, temperature in particular. This conclusion is the same as that given by Laing et al. (1987) and Jara-Jara et al. (1997) for R. decussatus from Conwy (UK) and Galicia (NW Spain), respectively.

In this study, the clam population exhibited characteristics similar to those described for other bivalve species. The condition factor estimated from the length–weight relationship can provide an indication of the “well-being” of a given species and can be an indicator of food abundance for a species in a given area or time (Mzighani 2005). For many bivalve species, several environmental factors are known to affect shell morphology and relative proportions, such as latitude (Beukema & Meehan 1985), depth (Claxton et al. 1998), shore and tidal levels (Franz 1993), water currents and turbulence (Hinch & Bailey 1988; Fuiman et al. 1999), wave exposure (Akester & Martel 2000), type of bottom (Claxton et al. 1998) and sediment type (Newell & Hidu 1982). Thereafter, Tlig-Zouari et al. (2009, 2010) and Derbali et al. (2011, 2012) from Tunisia argued that these parameters are known to be potentially related to environmental conditions and vary within bivalve species and between localities.

Sexual maturity of bivalves is classified according to microscopic features, such as the presence of gametes in gonads and the degree of their development, though sometimes it can be determined from macroscopic features, i.e. the appearance and colour of gonads (Lucas 1965). Similar trends were observed in other bivalve families, such as Pectenidae, Mytilidae, Limidae and Cardiidae, both sex and the degree of gonad development can be determined based on the color of gonads (Lubet 1959; Derbali et al. 2009a,b; 2012, 2014). Accordingly, the overall sex ratio (F:M = 1.26:1) significantly deviate from parity (F:M = 1:1; p < 0.001). The same sequence of events was reported for the surf clam Mactra stultorum from the Gulf of Gabes (south Tunisia). On the other hand, different trends were found for the clam R. decussatus from the present study area. Elsewhere, similar trends were observed for P. aureus from Lake Qarun in Egypt (Kandeel 2017).

Finally, this study provides crucial information on the spread of potentially commercially valuable P. aureus in the major shellfish production area in Tunisia by investigating the stock size, distribution, size structure and sexuality. In addition, this paper is the first attempt to provide new information on its stock levels and parameters responsible for its variability. The distribution of P. areus was found to be strongly affected by many abiotic factors. For any production purposes, further work is required to investigate other biological parameters to accurately monitor the exploitation of clams. The adoption and implementation of rules, such as limiting the size of clams, will be required to protect this new exploitable fishery resource similar to R. decussates natural populations.