Population dynamics of two oyster species pinctada capensis (Sowerby, 1872) and saccostrea cucullata (Born, 1778) with potential for aquaculture in Maputo Bay

Abstract

Oysters are important resources for the daily diet, a source of economic income for many coastal communities and a delicacy for the tourism industry. In Maputo Bay, these invertebrates species are intensively exploited for food consumption or as ornamental objects for peoples inhabiting coastal areas, tourists and other resident and non-resident communities. The high demographic pressure and easy access of the coastal communities to the oyster banks, allows high captures of these organisms, causing intense exploitation in the wild banks. This practice can lead to over-exploitation and eventual risk of depletion in some areas around Maputo Bay. Thus, the goal of this thesis is to evaluate the population dynamics with a focus on the oyster’s exploitation, reproductive activities and larvae settlement as input for management measures to prevent over-exploitation and as a baseline for future cultivation of oyster’s species in large scale at Inhaca Island, southern Mozambique. The thesis includes four (I-IV) studies that culminated with 5 published papers. Study I (Paper I) aimed at assessing the catch per unit effort, density and size composition of oysters Pinctada capensis and Saccostrea cucullata. The study was conducted over a three-year period on eight locations at Inhaca Island. Transects, quadrats and daily fisheries catches approaches were used. Results show that P. capensis is the most exploited species on the island. Oyster densities, sizes and catches per unit effort were higher in the less accessible areas only for P. capensis. Study II (Paper II) describes reproductive aspects of these two species on Inhaca Island. Adult oysters were collected monthly over two years period within seagrass banks for P. capensis and rocky shore habitats for S. cucullata. Animals were evaluated using biometric and histological analysis of the gonads. Females were predominant among larger individuals (>55 mm) while males were more dominant among smaller individuals (<55 mm) for both species. Five gonad maturation stages were identified: indifferent, developing I, developing II, ripe and spent. The size at first maturity was 27 mm and 26.2 mm for pearl oyster females and males, respectively, whereas for rocky shore oysters was 32.8 and 28.3 mm for females and males, respectively. Overall, the reproduction of S. cucullata and P. capensis occurs mainly in summer, with a short resting period in winter. Study III (Paper III) the combined effects of temperature and salinity on the embryonic and larval development of the rocky oyster were investigated in laboratory conditions. A factorial experimental design tested three temperatures (24, 30 and 34 oC) and three salinities (30, 35 and 40 parts per thousand) over a seven-day period. Larval survival and growth were assessed by regular sampling by counting and measurement of larvae under an optical microscope equipped with a micrometric scale. Significantly higher larval survival was observed at the combination of 30 °C and 35 ‰ salinity. The lowest temperature (24 oC) negatively affected growth regardless of salinity level and survival decreased linearly with increasing salinities. Study IV (Paper IV and V), spat of Pinctada capensis and Saccostrea cucullata and fouling fauna were collected using artificial substrate from four (4) selected sites at Inhaca Island, during a field experiment of 8 months to assess the effect of different factor (season, location, surface and duration) on spat settlement and abundance of fouling fauna. Ceramic tiles were deployed from surface to 3 m depth and replaced monthly. Overall, settlement in ceramic tile were higher for S. cucullata than pearl oyster P. capensis. There was a tendency of higher number of spat settled during summer than winter and on rough surface than smooth surface for both species. The results have also indicated that the ceramic tiles could be used as substrate for rocky shore oyster S. cucullata settlement in later summer (Paper IV). Five groups of fouling vfauna were recorded on the oyster collectors: barnacles, gastropods, mussels, Polychaeta and algae. The diversity of fouling fauna was higher on collectors deployed in seagrass habitat than on rock shores. Barnacles dominate among fouling fauna on collectors. For both rocky shore and seagrass habitat, there was a tendency to higher numbers of fouling organism in summer and in rough surface. Artificial tiles used as oyster collectors, have also shown to attract a variety of fouling fauna when deployed in rocky and seagrass habitats (Paper V).