Population dynamics and secondary production of Donax trunculus (Mollusca, Bivalvia) in the Gulf of Annaba (Northeast Algeria)

The population dynamics and secondary production of the wedge clam Donax trunculus were studied in the Gulf of Annaba (Northeast Algeria) monthly for one year at a site close to the Annaba port and the Seybouse river, affected by industrial and agricultural pollution (Sidi Salem), and at a site more distant from major pollution sources, but exploited by fishery (Echatt). The number of individuals (N) was lower at Sidi Salem (36–148 ind m-2) than at Echatt (63–272 ind m-2) in most sampling dates, while the biomass was more variable from one date to another, with no consistent pattern of differences between sites. The condition index (CI), although slightly higher at Sidi Salem than at Echatt, showed at both sites a major increase in March/April, June/July and October, reflecting two main periods of gonads development and increase in the level of stored reserves at the end of the reproductive period. Consistently, the recruitment of D. trunculus ran from April to October with a major peak of abundance in spring and a minor one in early fall. The maximum age of D. trunculus was 3 years and the growth rate was highest in the first year. Annual somatic production (P) was lower at Sidi Salem (0.773 g AFDM m-2 yr-1) than at Echatt (1.262 g AFDM m-2 yr-1), possibly reflecting a lower mean annual biomass at Sidi Salem (1.642 AFDM m-2) than at Echatt (3.046 AFDM m-2), while the annual P/B ratio was similar between the two sites (i.e. 0.471 and 0.414 yr-1, respectively). Lower N and P at Sidi Salem compared to Echatt are consistent with the proximity of Sidi Salem to the industrial port of Annaba and low hydrodynamic conditions which may favor the accumulation of pollutants, such as hydrocarbons. On the other hand, moderate secondary production of D. trunculus at Echatt compared to other Mediterranean sites may be due to excessive harvesting. We suggest that the low secondary production described in this study should be taken into account for the development of sustainable strategies of clam exploitation in the Gulf of Annaba. In particular, effort should be made to reduce land-based pollution and to regulate the collection of D. trunculus according to the life cycle and production potential of this species

Dynamique de la coque Cerastoderma glaucum (Mollusca, Bivalvia) dans la lagune Mellah (Algérie Nord-Est)

Le travail présenté étudie sur un cycle (mars 2010 – février 2011) la densité, la structure des tailles et la croissance du mollusque Cerastoderma glaucum dans trois stations sur les berges (profondeur < 1.50 m) de la lagune Mellah. Les densités de C. glaucum varient entre 22 ± 9 ind.m-2 (station 1, janvier) et 176 ± 22 ind.m-2 (station 3, avril). Le cycle biologique de cette espèce est caractérisé par un recrutement moins abondant en début du printemps, alors que celui de l’été est plus important et s’étale de juin à septembre. L’étude de la croissance de ce bivalve en utilisant la méthode indirecte, montre une croissance rapide en longueur durant la première année, où les jeunes recrues gagnent jusqu’à 21.33 mm. Durant la deuxième année, l’allongement de la taille des individus diminue progressivement pour augmenter de 5.57 mm seulement. La relation entre la taille et le poids chez la coque du Mellah est proportionnelle, où le coefficient de corrélation est élevé (r = 0.90). Par ailleurs, le coefficient d’allométrie (b = 2.62) chez ce pélécypode est inférieur à 3, traduisant ainsi une allométrie minorante, pour laquelle la taille croît plus rapidement que le poids.Mots clés: Bivalve – Cerastoderma glaucum – Abondance –Croissance – Lagune Mellah – El-Kala. This work studies on a cycle (March 2010 - February 2011) the density, size structure and growth of mollusc Cerastoderma glaucum in three stations on the shore (depth <1.50 m) of the Mellah lagoon. Densities of C. glaucum varies between 22 ± 9 ind.m-2 (station 1, January) and 176 ± 22 ind.m-2 (station 3, April). The life cycle of this species is characterized by less abundant recruitment in early spring, but the summer one is more important and lasts from June to September. The study of the growth of this bivalve using the indirect method shows a significant growth in length of recruits (21.33 mm) during the first year. Then, the annual extension progressively decreases as the size of individuals increases of only 5.57 mm in the second year. The relationship between length and weight for the cockle of the Mellah, is proportional, where the correlation coefficient is high (r = 0.90). Also, the allometric coefficient (b = 2.62) for this pelecypod is less than 3, reflecting a lower allometry which than the length grows faster than the weight.Keywords: Bivalve - Cerastoderma glaucum - Abundance - Growth – Lagoon Mellah- El-Kala