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The reproductive performance knowledge of economically important bivalves is essential to establish better shellfishery management measures and aquaculture programs. The reproductive cycle, as well as the nutrient storage and utilization of two Portuguese Cerastoderma edule populations from Óbidos and Ria Formosa lagoons were characterized by evaluating gonadal development stages, gonadal and condition indexes, as well as glycogen and total lipid content, over several consecutive years. The relationship between reproduction and environmental parameters (sea surface temperature and chlorophyll a) was assessed. The reproductive cycle of both populations of C. edule followed a clear seasonal pattern. The onset of gametogenesis took place in early autumn for both populations, and its evolution coincided with a temperature decrease which progressed throughout the winter. The mature stage was reached in spring, followed by a spawning period during spring and summer, triggered by a rise in temperature. After spawning, both populations progressed to a prolonged inactive stage. The condition index reflected the accumulation and utilization of glycogen, which was considerably higher in the Óbidos population. However, no relationships were found in the reproductive cycle of the species. Total lipids remained similar not only during the gametogenesis period, but also during the spawning period, which may be related to successive and simultaneous gamete production and release.

Cockles are amongst the most exploited bivalve species in Portugal, playing an important ecological and socioeconomic role in coastal ecosystems. Two sympatric species of cockles, Cerastoderma edule (Linnaeus, 1758) and Cerastoderma glaucum (Bruguière, 1789–1792) may co-occur in estuaries and coastal lagoons in mixed populations along the European Atlantic coast, namely in Portugal, France and the United Kingdom. The increasing importance of shellfish harvesting in Portugal requires a good knowledge of cockle stocks and temporal variability in stock levels to better inform sustainable management practices. Therefore, this study aimed at assessing spatial and temporal variations in cockle populations in two Portuguese estuarine systems where the species are exploited at low levels. Sampling was carried out using a clam dredge, covering the entire potential area of occurrence of cockles in the Tagus and Sado estuaries at around the same time of the year in each of the three sampling years (2015, 2018, and 2019). The abundance, spatial distribution and population structure of cockles were examined at each system. Moreover, several water and sediment parameters were measured to understand the influence of environmental conditions on the spatial distribution and abundance of cockles. The results obtained showed that cockles occur mostly in the intermediate areas of both estuarine systems and are more abundant in the Tagus estuary. Depth, average sediment grain size and the species Ruditapes philippinarum were the factors that better explained the probability of species occurrence. The population structure analysis indicated that natural mortality is constraining the cockle communities given the low abundance of adult individuals with marketable size in both estuaries. This study highlights the need for appropriate management measures to ensure the sustainability of these bivalve population stocks that have significant socioeconomic importance for local populations.

Understanding how long water is retained in an estuary and how quickly it is completely flushed is essential to estimate an estuary’s health in areas with significant pollutant loadings. The present study analyses the effect of five different Tagus River discharge scenarios ranging from low to extreme on residence time (RT), exposure time (ET) and integrated water fractions inside pre-established Tagus estuary areas, to identify its most vulnerable areas to pollution. The 3D version of the MOHID hydrodynamic model coupled to a lagrangian tool was used. The increase of the river discharge generated high current velocities which, in turn, led to an increased rate of tracers leaving the estuary. As a consequence, RT and ET decreased from 59 to 3.5 days under a low and extreme river discharge scenario, respectively. Under a low river discharge, significant differences were observed between RT and ET in the areas located in the main body of the estuary and in the bays. As river discharge increased, RT and ET decreased in all areas of the estuary and those differences faded, with the greatest differences observed in the areas situated along the south margin. In general, results showed that with high river discharges the tracers released in the upper estuary are spread throughout the estuary, but mainly in downstream areas. However, when the river discharge reached exceptionally high values, local eddies were formed, leading to the retention of the tracers in the estuary’s south margin and inner bays. The results in this study allowed to identify the most vulnerable areas within the estuary as a function of the river discharge.

The present study assessed the influence of sex, size and reproductive status on the meat yield (soft tissues proportion) of the purple dye murex (Bolinus brandaris) from the Ria Formosa lagoon (southern Portugal). During one year of monthly sampling (October 2008 - September 2009), average meat yield of B. brandaris was 40.5±6.1% (range: 25.8-56.1% wet weight), with no significant differences between sexes. Relationships established between specimen size and soft parts weight indicated that both shell length and total weight are excellent indicators of meat yield. Significant differences in meat yield between size classes further reinforced the trend of increasing meat yield during ontogeny. Meat yield exhibited significant monthly variation and a similar temporal trend in both sexes, which were directly related to the reproductive status. Meat yield of B. brandaris was compared with that of other muricid species and the marked influence of the reproductive status on meat yield prompted a comparative assessment of the spawning season and peak of three sympatric muricids (B. brandaris, Hexaplex trunculus and Stramonita haemastoma). Overall, these findings have implications at diverse levels, including the management, regulation and inspection of this fishing/harvesting activity and the commercialization and consumption of this seafood product.

The present study reports the growth rate of the purple dye murex, Bolinus brandaris (Gastropoda: Muricidae), estimated from mark-recapture experiments. A total of 1067 specimens (shell length = 43.4±8.1 mm, range = 14.6−78.4 mm) were marked with Dymo® tape tags and released in a semi-intensive fish culture earthen pond. After a period at liberty ranging from almost two months to around two years, 288 individuals were recaptured (shell length = 67.4±6.2 mm, range = 45.3−88.6 mm), which corresponded to a recapture rate of 27.0%. At recapture, only one specimen had lost the tag (tag loss rate < 0.1%) and all remaining tags were intact and legible. Mean monthly growth rates were 0.9±1.0 mm in shell length, 0.4±0.5 mm in shell width and 0.7±0.7 g in total weight. Growth rates showed high inter-individual variability and an evident decreasing trend with specimen size. Comparison of growth rates with similar information available for other muricids confirmed that B. brandaris is a relatively slow-growing species. This provides valuable information for both fisheries management and for assessing the potential of B. brandaris as a candidate species for molluscan aquaculture.

Ecological aspects of 75 megabenthic bivalve species found on the south coast of Portugal (from Vila Real to Sagres) were studied based on the information provided by seasonal bivalve surveys from 2000 to 2007 (8600 hauls, simultaneously using a razor shell dredge and a clam dredge). Of the 75 species identified, 36 occurred less than 30 times, and thus were considered rare in the area. The remaining 39 bivalves were analysed according to their occurrence, bathymetric distribution, geographic preferences, seasonal preferences, burying-depth, beach exposure and river mouth proximity. All species belonging to the Pharidae and Solenidae families (razor shells) and most Tellinidae were significantly more frequent in the razor shell dredge (indicating deeper burying in the sediment), whilst the families Cardiidae and Donacidae were significantly more frequent in the clam dredge (indicating that these species are probably closer to the sediment surface). As for the season, most bivalve species occurred similarly in both seasons (19 sp; 49%), but a large proportion were more abundant during the autumn surveys (17 sp., 44%), whereas only three species were commoner during the spring surveys. Most species belonging to the families Cardiidae and Mactridae were commoner in the autumn surveys. The spatial distribution differed between species and cluster analysis identified four communities with greater geographic affinity. Species belonging to the family Cardiidae were preferably found in the western part (WB) and the eastern part (S) of the study area, whereas the families Donacidae, Mactridae and Tellinidae occurred mainly in the central area (EB) and the eastern (S) coastal sectors. Overall, shallower species (modal depth at 3-6 m) showed greater occurrences and abundances than the deeper ones, and the depth pattern observed did not change between seasons. Donacidae and Mactridae (except Mactra glauca) were represented essentially by shallow species, whereas Cardiidae (except Acanthocardia tuberculata and Cerastoderma edule) included mostly species occurring in deeper waters. Bathymetric sequence of species distribution within some families was observed.

The artisanal fishery for muricid gastropods in the Ria Formosa lagoon (Algarve coast, southern Portugal) is a locally important fishing activity because the banded murex (Hexaplex trunculus) and the purple dye murex (Bolinus brandaris) are greatly appreciated seafoods with high commercial value in the Portuguese seafood market. An integrated study was implemented to monitor the muricid gastropod fishery with the artisanal fishing gear (“wallet-line”) through monthly experimental fishing operations carried out during one year. The aim was to describe the fishing operations and fishing gear, to estimate the fishing yield, to characterise the target species catch composition, and to identify by-catch species and discards. The “wallet-line” is neither a species-specific nor a size-selective fishing gear, because the catches comprised a variety of by-catch species and a high proportion of commercially under-sized target species. The vast majority of the by-catch is discarded immediately on board, so mortality is presumably negligible. The CPUE of both target species and by-catch species decreased during consecutive fishing days, mainly due to declining bait attraction. Considering the overall information gathered on this fishing activity, some management measures are proposed for this artisanal fishery, which might ultimately contribute to the long-term sustainable exploitation of the fishing resource.

During 1998, a study of the European hake (Merluccius merluccius) static gear fishery was undertaken in southern Portugal. This study aimed to compare long-line and gill net catches within the same area in terms of catch composition, fishing yield, by-catch and discards, and specifically for hake, catch size frequency distribution and quality of the fished product. At least 35 species of fish and invertebrates were caught, with hake dominating the catches in terms of both weight and number. The by-catch represented 23% and 15% of the catch in weight for long-lines and gill nets respectively. No illegal sized hake (under 27 cm total length) were caught in either gear during the study period. Discards of non-commercial species were similar for both gears and considered to be negligible (< 3% by weight). Hake discards were significantly different between gears (7% for long-lines and 42% for gill nets, by weight), due to fish deterioration related to soaking time. Higher daily yields were obtained for long-lines (258.37 kg / 9,000 hooks) compared with those for gill nets (127.12 kg / 10 km net). These findings will help to improve the management and conservation of this valuable hake fishery.

A study was undertaken to evaluate the existence of depth segregation between size classes of Donax trunculus populations that occur along the Algarve coast (southern Portugal). Five collecting areas were sampled in the Ria Formosa barrier islands, namely in the Culatra, Barreta and Armona Islands, between March and June 1998. The study was based on size frequency distribution analysis and shell biometric relationships. The size frequency distribution showed the predominance of smaller individuals (< 25 mm) in the renewal area (0.5-1.0 m depth) and the prevalence of larger individuals (? 25 mm) at greater depths (4.0-6.0 m depth). The biometric study revealed the existence of significant differences in the type of growth between size classes in relation to depth distribution and burrowing activity. On the basis of these results, some specific management measures were proposed for this important bivalve fishery, taking into account the distinct fishing strategies and operations (manual and boat-operated dredging) performed at different depths.