Repopulation and culture of the Pacific Calico scallops in Bahia Concepcion, Baja California Sur, Mexico

The Pacific Calico scallop, Argopecten ventricosus (Sowerby), is the most important commercial species in Bahia Concepcion. Catches declined steadily from 1991 to 1993 as a result of overexploitation, forcing the adoption of management measures. The Government has now established regulations for exploitation and scallop culture development. In 1994, Centro Interdisciplinario de Ciencias Marinas (CICIMAR) and the Fisheries Department of the Mexican Government initiated research on the feasibility of increasing scallop biomass through a reseeding programme to protect artisanal fishing. The use of onion bags to collect spat in Bahia Concepcion (1988-1994) demonstrated that settlement was cumulative from January to March. At Punta Coloradito, there were two peaks, in January and early March. Commercial collectors were deployed in February at El Indio and in April at El Remate. In June, about one million juvenile scallops were placed in an enclosure, held until mid-August and then released. Mass mortality (>80%) of the scallops was observed in September, increasing to 99% in October. The high water temperature and predators may be responsible for the mortalities.     

Effect of immersion time of cultch on spatfall of the scallop Argopecten purpuratus (Lamarck 1819) in the Marine Reserve at La Riconada, Antofagasta, Chile

A pilot program to evaluate the feasibility of collecting juvenile (seed) scallops using Japanese technology was carried out from October 2001 to April 2002 in the Marine Reserve at La Rinconada, Antofagasta, Chile. The continuous presence of larvae in waters in the reserve allowed for collection of 400–15,340 spat per collector and showed the high reproductive capability of local scallops and the relationship between numbers of larvae in the water and amount of spatfall. Highest rates of settlement were proportional to high numbers of umbone larvae in the water at >5000 larvae m⁻³ when collectors were first immersed. Absence of a relationship between larval numbers and postlarval settlement with trophic factors such as chlorophyll and particulate organic matter in the water suggests that productivity was not limited by these factors. Harvest of spat from collectors immersed for four and five months had recovery efficiencies of 5.5% and 59.4% compared with settlement in collectors after immersion for one month. Cohort analyses of recovered spat suggested that settlement occurred over the lengthy immersion period and was due to the continuous presence of larvae in the water. The loss of a high percentage of spat over the 4–5 immersion period compared with settlement in the first month can be attributed to intraspecific competition of spat density and growth as well as clogging of the collectors by sediment and marine fouling that impeded a flow of water through the nets.     

提高海湾扇贝虾池保苗成活率技术研究

利用６５亩虾池进行提高海湾扇贝保苗成活率技术研究，入池稚贝２．１亿粒，经４３天暂养，出池商品苗９７２０万粒，成活率达４６．３％，纯益３５．８４万元。而海上保苗成活率仅为１０％左右。１９９４年２月，专家鉴定认为，本研究居国内先进水平。     

Influence of season, initial size, depth, gear type and stocking density on the growth rates and recovery of sea scallop, Placopecten magellancius, on a farm-based nursery

Hatchery-reared sea scallop (Placopecten magellanicus) spat weremonitored for growth and recovery in three experiments to determine themost suitable system for nursery culture. In Experiment I, four size classesof nursery-sized spat held at two depths from October to July exhibiteddeclining growth rates over the winter period and increased growth ratesin the spring. Overall, season, depth and initial size had a significantinfluence on the absolute and specific growth rates of scallops. Recovery,defined as number of scallops remaining after mortality and loss of spatthrough gear mesh, was influenced by season and initial size, but notdepth. Scallops in the 3.0 mm⁺ size class had higher growth rates andrecovery than those in the 1.4–1.6 mm, 1.7–1.9 mm and 2.0–2.9 mm sizeclasses. In Experiment II, two gear types containing similar size spat werecompared. Growth rates were significantly higher in 3.0 mm pearl nets thanin 3.0 mm collector bags, although recovery was similar between the twoequipment types. Experiment III, two stocking densities of nursery-sizedspat were compared in collector bags. Neither growth rate nor recoverywere significantly different for the two densities (2600 and 5200spat/collector bag) tested. Overall, these studies indicated that importantparameters for optimizing the growth and recovery of scallops in a farm-based nursery system include season, initial spat size, deployment depthand gear type.     

Post-transport recovery of cultured scallop (<Emphasis Type="Italic">Pecten maximus</Emphasis>) spat,juveniles and adults

High mortality associated with transport operations in scallop culture has been a major problem faced by European farmers. Simulated transport with Pecten maximus L. spat <2 mm, spat 15–30 mm, juveniles 30–50 mm and adults >100 mm were carried out in Spain, Ireland and Norway. Different time and temperature combinations were studied in order to maximise post-transport survival and establish best practices. Out-of-water transport could result in 100% survival if conditions were right, but the response to emersion stress depended on size, season and location. Post-transport recovery decreased with emersion time and was strongly influenced by temperature. Air exposure was tolerated for a longer time by adult scallops than spat and juveniles, but the results differed among trials in the different countries. The maximum emersion time that gave post-transport survival ≥80% was 12 h for the smallest spat, 18 h for larger spat and 24 h for juvenile and adult scallops. Adults were less affected by transport temperatures that deviated from ambient seawater temperature than spat and juveniles. In general post-transport recovery was high when sea temperature was <10°C, but during warm-water seasons special care should be taken to avoid stressful and lethal transport conditions. A transport temperature <12°C was recommended, though not more than 10°C below ambient culture temperature. A maximum transport time of 9 h was suggested for spat and juveniles to attain post-transport survival close to 100%, but 12–24 h was feasible during the cold-water season or at favourable transport temperatures.     

Survival and growth of hatchery-produced postlarvae and spat of the Caribbean scallops Argopecten nucleus and Nodipecten nodosus

A study was made of the growth and survival of postlarvae and spat of Argopecten nucleus and Nodipecten nodosus produced in the laboratory and exposed to different conditions of settlement and culture. An evaluation was made of the different types of collection substrates (onion-bags, artificial-grass, fibreglass-plates), two types of settlement inducers (epinephrine, low temperature), and two culture environments (sea, laboratory). The results showed that larger numbers of postlarvae were obtained in artificial-grass collectors than on fibreglass-plates or in the onion-bags. The growth of the A. nucleus postlarvae was not affected by the collector type, while that of N. nodosus was higher on fibreglass-plates, and lesser in the onion-bags. Both thermal and chemical stimulation of the competent larvae resulted in recovery of a greater percentage of postlarvae. The growth of postlarvae of A. nucleus was better in groups exposed to the two methods of settlement induction, while those of N. nodosus remained unaffected by this treatment. The survival of the postlarvae and spat was greater in the laboratory than at sea except in the case of A. nucleus postlarvae attached to collectors, whose survival was similar in both environments. The growth of postlarvae and spat was greater in the sea than in the laboratory.     

Suspended Culture of Doughboy Scallops Mimachlamys asperrima (Lamarck)

To evaluate the effects of suspended culture on the wet meat weights, reproductive condition and fecundity of doughboy scallops Mimachlamys asperrima , scallops held either in suspended cages or collected from wild stocks on the sea bed were compared over a year. Changes in gonosomatic index (GSI) and macroscopic staging of gonads were similar in both populations; however, after one month, soft body, muscle and gonad weights were heavier in suspended scallops and, with the exception of gonad weight in early summer, remained heavier throughout the year. Fecundity following serotonin-induced spawning of suspended Mimachlamys asperrima was on average almost twice that of similar sized scallops grown on the sea bed (1.58 and 0.84 million eggs/female, respectively). Suspended scallops surpassed all previous maxima for tissue weights, GSI and fecundity recorded for wild Jervis Bay Mimachlamys asperrima . This indicated that farmers could obtain greater meat yields from suspended rather than reseeded or wild Mimachlamys asperrima and that broodstock to be used in hatcheries may be best obtained from suspended culture.     

Spat Collection and Growth to Commercial Size of the Tehuelche Scallop Aequipecten tehuelchus (D'Orb.) in the San Matias Gulf, Patagonia, Argentina

Studies on larval settlement and growth of the commercial scallop Aequipecten tehuelchus were carried out over a 2-yr period in San Matias Gulf, Argentina. Gonadal index was used to indicate spawning and spat collectors were deployed when spawning began. Artificial spat collectors (mono-filament bags filled with thin shrub branches) were placed at a depth of 25 m in two areas of the Gulf during two seasons (1989–1990 and 1991–1992). Maximum settlement occurred by mid-February during the first season and by mid-January in the second season. Larger numbers of spat were found in collectors placed near the sea bottom. After settlement, spat were removed and placed in cages for growout. Scallops reached the average commercial size (60 mm shell height) 16 mo later. Mortality rate during the entire period was 10%.     

Experimental suspended culture of mussels (Mytilus edulis L.) in Wales using spat transplanted from a distant settlement ground

Spat of Mytilus edulis L. were transferred 300 km by road on collectors from an area of high and regular spatfall, but unfavourable culture conditions, to a raft at a site deficient in mussel settlement. Spat were collected on coir ropes and fibrous rubberized matting staked to intertidal settlement grounds in Morecambe Bay, northwest England. During January to April, 80 mm circumference ropes could collect 3 000–7 000 spat/m/week, with smaller numbers in other months, and fibrous matting sometimes also caught many spat.Spatted collectors were suspended in the Menai Straits, North Wales. Despite heavy losses from overcrowding, fast currents and turbulence, growth was sufficiently good for some rope populations to produce marketable crops after about 16 months. Mean lengths increased from 1.5–5.0 mm to 47.5 mm in the first year, and to 60 mm after 1.5–2 years. At about 55 mm, attained at ca 16 months, the best crops yielded 10–15 kg/m live weight, corresponding to 2–3 kg of cooked meat/m. Percentage meat yields from 40–50 mm mussels were equally good, suggesting that crops could be harvested below the present marketable size.Economic systems for collecting spat and for on-growing at controlled densities remain to be developed for different hydrographic conditions and scales of commercial operation in Britain.     

Comparison of early life history stages of the bay scallop, Argopecten irradians: Effects of microalgal diets on growth and biochemical composition

The culture of bay scallops, Argopecten irradians, is limited by a reliable and affordable supply of spat and the ability to ensure that animals attain market size within a single growing season. The main goals of our study were thus: (1) to develop growth-optimizing algal diets for implementation in hatcheries, and (2) to identify and compare bay scallop postlarval and juvenile dietary requirements, especially of lipids and fatty acids, which if met may enhance production. Nutritional needs of postlarval bay scallops (present study) are compared with those of sea scallops, Placopecten magellanicus, offered the same diets in a previous companion study. To this end, postlarval (initial shell height, SH = 240 μm) and juvenile (initial SH = 10 mm) bay scallops were offered 6–7 microalgal diet combinations at 20 °C, for 3 weeks. A similar growth ranking among diets was observed between the two developmental stages. A combination diet of Pavlova sp. (CCMP 459) and Chaetoceros muelleri was far superior to any other diet tested, yielding growth rates of 58 and 357 μm day^− 1 which were 65% and 25% higher than the next highest performing diet of Tetraselmis striata/C. muelleri in postlarvae and juveniles, respectively. The T. striata/C. muelleri diet, which is limited in the n-3 fatty acid docosahexaenoic acid (DHA), yielded very poor growth of sea scallop postlarvae in a prior study, indicating that bay scallops may have less stringent requirements for DHA than sea scallops. The Pav 459/C. muelleri diet, which also supported the highest growth of sea scallop postlarvae, is characterized by elevated levels of the n-6 fatty acids, arachidonic (AA) in C. muelleri and 4,7,10,13,16-docosapentaenoic (DPA) in Pav 459. The two diets deficient in AA and n-6 DPA, Pavlova lutheri/Thalassiosira weissflogii and P. lutheri/Fragilaria familica, yielded the lowest growth rates in both bay scallop postlarvae and juveniles. Tissue enrichment of these two fatty acids relative to the diet, as well as overall enrichment in ∑n-6 fatty acids was observed across developmental stages and dietary treatments. A similar pattern has previously been observed in sea scallop postlarvae, suggesting a dietary requirement for n-6 fatty acids in pectinids that has often been overlooked in the past.     

Dynamic models applied to giant barnacle culture

In this study, a dynamic model is applied to giant barnacle (Austromegabalanus psittacus) spat collection from artificial substrates located in the wild. Semi-industrial culture of the giant barnacle, A. psittacus “picoroco” in southern Chile is an interesting option for aquaculture diversification. The model establishes relationships between variables and carries out simulations to determine their effects on spat provision. The dynamic hypothesis proposes that the number of giant barnacle spat obtained from the wild is influenced by competent larval abundance (cyprids) over time, substrate availability and mortality after larval settlement. In the conceptual model, 15 variables were selected and related, establishing the polarities of each causal relationship and of each feedback loop. The Stock & Flow diagram was undertaken using STELLA 9.0 simulation software. Simulation tests were carried out to establish the consistency of the model using the empirical background obtained from semi-industrial cultures in southern Chile. The model establishes relative quantity of competent larvae, substrate area and the number of spat, as key variables. Synchrony between level of cyprid abundance and location of artificial substrates in the water is critical to achieve maximum collector efficiency. A difference of less than 1 week out of synchronization produces significant losses (60–70%) in spat production. When the deployment of collectors and maximum quantity of competent larvae are synchronized, sensitivity analysis establishes an increase of up to 49.4% in the number of spat, as a result of the collector area released by spat early mortality. The application of dynamic models in aquaculture constitutes a useful tool for optimizing the process. The model proposed enables us to understand the processes associated with obtaining seed from the environment and can be applied to other similar processes, such as mytilid cultures.     

Effects of water depth on survival and growth of <Emphasis Type="Italic">Argopecten purpuratus</Emphasis> (Lamarck, 1819) spat in northern Chile

Collection and suspended culture of Argopecten purpuratus spat in Japanese-type collectors was undertaken in the Rinconada Marine Reserve (Antofagasta, Chile) to determine growth variations between surface and bottom waters. Scallop spat was collected at 16-m depth and grown at 1- and 16-m depths. An initial settlement of ca. 13,000 post-larvae per bottom collector (2 cohorts) was observed on 3 February 2001. Two new cohorts settled in bottom collectors on 3 March, while no spat settlement occurred in surface collectors. The four cohorts exhibited substantial and different mortality rates over the study period, depending on the cohort and on-growing depth. The first and second cohorts exhibited mortality rates of 80.3 and 53.1% in bottom collectors, respectively, versus 29 and 43.6% in surface structures. The third and fourth cohorts exhibited mortality rates of 21.4 and 59.7% in bottom collectors, respectively. Mean shell height of scallop spat after 88 days of culture was 6.46 mm for the first cohort and 4.37 mm for the second cohort in bottom collectors, versus 10.33 and 7.31 mm in surface collectors, respectively. Mean post-larval growth rate in bottom collectors (86 μm day⁻¹) was significantly lower than growth rate in surface collectors (146 μm day⁻¹). Factors explaining the depth-related mortality and growth rates are discussed to improve scallop culture in the Reserve. It was concluded from the results that culture improvement in Antofagasta Bay would require collecting spat in bottom waters and raising it near the surface during initial cultivation stages.     

Interactions between predatory sea stars (Asterias amurensis and Distolasterias nipon) and Japanese scallops (Mizuhopecten yessoensis) and implications for scallop seeding in mariculture

Sea stars are a key group of species in benthic marine ecosystems and represent the primary cause of mortality for cultured scallops. In the mariculture grounds of Japanese scallops (Mizuhopecten yessoensis) in north‐eastern Japan, two sea star species, Asterias amurensis (AA) and Distolasterias nipon (DN), are serious predators. However, the predatory impact of these sea stars on scallops remains unquantified, with limited information being available on the biological characteristics of DN. We compared the predatory impact of both sea star species by evaluating their predatory behaviour (i.e. probability of predation and number of captured scallops) on scallops in laboratory experiments. Both sea star species preferred smaller scallops over large ones, with the probability of predation and the number of scallops being preyed on increasing with increasing water temperature. However, DN had a greater predation impact than AA. For instance, the probability of predation by DN was higher, with more scallops being consumed. In particular, compared to AA, DN continuously captured small scallops and fed on larger scallops. Furthermore, the predatory impact of both sea star species (particularly DN) on small scallops peaked at high temperatures. In conclusion, our results present information that could help improve the survival rate of seeded scallops in mariculture systems.     

Experimental Collection of Pearl Oyster, Pinctada mazatlanica and Pteria sterna, Spat on a Filament Substrate

Juveniles of Pinctada mazatlanica and Pteria sterna were collected with artificial collectors from Falsa Bay, Baja California Sur, Mexico during a 12 month experimental period. Filament substrate artificial collectors contained in 2 mm plastic mesh bags (25 ± 25 cm) were deployed from the surface to 17 m of depth, and replaced monthly. Pinctada mazatlanica had the greatest recruitment (12 juveniles/cm²) in late summer and P. sterna (5 juveniles/cm²) in winter. Maximum spat collection from September 1986 to April 1987 occurred at 10 m depth for P. mazatlanica and at 4–7 m depth for P. sterna . However, peak spatfall under 11m depth occurred from late July to August 1987. The mean prodissoconch shell height was 254 μ m (SE = 0.003; N = 50) for P. mazatlanica and 313 μm (SE = 0.004; N = 50) for P. sterna . The growth equations for the first five weeks of both species, based on the prodissoconch shell height and the maximum spat height (measured from the umbo to distal edge) are presented.     

Improving the efficiency of wild Pecten maximus spat collection: potential effects of spat bag design and of species temporarily settling in spat bags

The single and combined effects of external bag size and external bag mesh size upon the collection and retention of wild King Scallop spat (Pecten maximus L.), other scallop species (Aequipecten opercularis) and other animal groups were investigated in a collection area off the west coast of Scotland. Collection was assessed after 5 weeks and at harvest (after 20 weeks). After 5 weeks, 90% of collected animals consisted of scallops (P. maximus and A. opercularis), Mytilus edulis (Mollusca: Bivalvia) and Lacuna vincta (Mollusca: Gastropoda). One-way analysis of variance on count data obtained after 5 weeks showed that the larger bag and mesh size treatments had collected significantly greater numbers of scallop spat when numbers collected were compared to the quantity of material used for each treatment (P<0.05). This was partly attributed to significantly greater numbers of L. vincta crawling juveniles temporarily recruiting in the smaller bag and mesh treatments (P<0.05). Similar trends of spat abundance in treatment bags were observed at harvest, although all treatments had scallop spat retention efficiencies of less than 12%. The importance of the structural design of spat bags for spat collection and the effects of incidentally collected non-predatory species upon the collection of scallop spat are discussed.     

A method to eliminate self-fertilization in a simultaneous hermaphrodite scallop. 1. Effects on growth and survival of larvae and juveniles

Argopecten purpuratus, as several other scallops of commercial importance is a functional hermaphrodite. The usual methods for breeding cause an unavoidable degree of self-fertilization with the subsequent deleterious effects on survival and growth rates. Some researchers have unsuccessfully attempted to eliminate self-fertilization in these bivalves. Applying our knowledge of the cellular mechanisms of spawning in this species we have developed a method by which self-fertilization was eliminated. By this in vitro fertilization method, oocytes were obtained by scraping the female portion of the gonad, meiosis resumption was induced by serotonin, and then the oocytes were fertilized with sperm from different individuals. The progeny obtained by this method were compared with others obtained by cross-fertilization and self-fertilization. In all these procedures, the oocytes were fertilized at a ratio of 60 sperm per oocyte. The fertilization rates and D larvae survival were determined at 2 and 48 h, respectively. After the D larval stage, the hatchery culture of larvae continued as usual with sampling for survival and growth rates every three days. When larvae looked ready to settle and metamorphose, netlon collectors were introduced into the tanks. After 12 days the collectors were placed in 1 mm mesh collection bags and transferred to the sea. Juveniles were brought back to the hatchery every 40-70 days, to be counted, measured, and placed in mesh pearl nets. The assay of the three procedures for fertilization showed similar fertilization rates but low survival for D larvae obtained from the in vitro method. However, this procedure resulted in the highest size and survival rates of larvae. Survival of juveniles coming from this fertilization procedure was also the highest. The growth of juveniles obtained by in vitro fertilization was also higher than that of those obtained by self-fertilization. We conclude that this in vitro fertilization procedure provides a methodology that results in higher survival and growth rates of juveniles than obtained by procedures involving any degree of inbreeding. This in vitro procedure should be applicable to other simultaneous hermaphroditic scallops.     

Potential predation rates by the sea stars Asterias rubens and Marthasterias glacialis, on juvenile scallops, Pecten maximus, ready for sea ranching

The potential for predation by the sea stars Asterias rubens and Marthasterias glacialis on seed-size (41 ± 3 mm shell height) juvenile scallops (Pecten maximus), ready for seeding in sea ranching areas, was investigated in a 30-day laboratory predation experiment. There was no significant difference (P > 0.05) in predation rate of large A. rubens (95–115 mm radium) and large M. glacialis (120–164 mm radius), which averaged 0.88 and 0.71 scallops individual⁻¹ day⁻¹, respectively. Maximum rates of predation were 2.44 scallops individual⁻¹ day⁻¹ for large A. rubens and 3.00 scallops individual⁻¹ day⁻¹ for large M. glacialis. Small M. glacialis (76–87 mm radius) had a significantly lower predation rate than large individuals of either species (average 0.13 scallops individual⁻¹ day⁻¹, P < 0.05). Small A. rubens (50–80 mm radius) only began to prey on scallops when average scallop size was reduced to 35 mm. Based on estimated density of sea stars at a Norwegian sea ranching site and average predation rates, a population of scallops seeded at 10 m⁻² would be reduced by between 0.5 and 11% in 1 month. Furthermore, using the highest observed predation rate, the degree of loss of scallops indicated that scallop culture via sea ranching would not be economically viable and thus methods for reducing scallop predation by sea stars are necessary.     

Recruitment of catarina scallop (Argopecten ventricosus) larvae on artificial collectors off the NE coast of the Gulf of California

We evaluated recruitment of larvae of catarina scallop, Argopecten ventricosus, in the area of Puerto Peñasco, NE Gulf of California. We moored artificial collectors in six sites from June 2007 to August 2008 and replaced them every 2 months. We used monthly (July 2002–September 2011) sea surface temperature (SST, °C) and surface chlorophyll‐a concentration (SSChl, mg m^?3) Aqua/MODIS satellite data to describe seasonal environmental behaviour study area. Also, we recorded bottom temperature at each site every 4 h, and every 2 months measured sea surface salinity, temperature and dissolved oxygen. We used a repeated measures anova to evaluate differences in the number of recruited spat between main factors, and analysed the presence of multimodal spat shell size frequency distributions. Overall, spat recruitment was negatively correlated with seawater temperature and showed higher spat recruitment abundances throughout winter, which is the season with the highest surface chlorophyll a concentration. We estimated multimodal shell size frequency distributions characterized by more than one modal size. The natural collection of A. ventricosus spat on artificial collectors in the area can be successfully performed over a protracted period (November–December to May–June). Our results extend the area where collection of A. ventricosus spat can be successful.     

A preliminary study of the behaviour and vitality of reseeded juvenile great scallops, of three sizes in three seasons

In order to have a better understanding of recessing in great scallop, Pecten maximus and consequently the causes of mortality at reseeding, this study has monitored, at different seasons, the dispersion and recessing of different sizes of juveniles (about 15, 30 and 45 mm, called small, medium and large) after seeding. Moreover, the aim was to see when small spat (15 mm) could be seeded, and thus reduce the costs of intermediate culture.Three monitoring approaches were used together: (1) continual observations by remote video camera, of a defined area (less than 1 m²) containing 10 scallops from each size group; (2) daily monitoring of behaviour with divers along three bottom lines, with 20 × 1 m² plots each and nine marked scallops per plot; and (3) the biochemical content of the muscle: adenylic energetic charge and storage of energy reserves (glucides, proteins, lipids).The video monitoring identified but did not quantify predator behaviour, particularly at night. The role and behaviour of spiny crab, Maia squinado, and of small predators has clearly been shown, such as: (a) small crustaceans, Inachus sp., breaking the edges of scallop valves; and (b) small gobies, Pomatoschistus pictus, pecking the tentacles of the scallop mantle.For the monitoring by divers, filtering appeared much too difficult to look at for it was very disturbed by divers, and anyway the resumption of filtering came immediately after seeding. On the other hand, diver monitoring of dispersal and recessing was quite easy to do with a minimum of practice. On the basis of dispersal, the best seasons for seeding appear to be spring or summer. In autumn, two-thirds of small and medium juveniles are missing 3 days after seeding, but we could not observe whether they had been eaten by predators or had just moved and recessed farther. There was no experiment in winter owing to adverse conditions for scallop seedings.Biochemical analyses confirmed the unsuitability of autumn for scallop seeding, because of very low glucide content in this season.The adenylic energetic charge in the smooth part of the muscle showed that stress before seeding (aerial exposure, handling), and post-seeding behaviour (swimming, recessing) have a high energetic cost for scallops. In summer and autumn, 3 days after seeding, none of the three size batches recovered their initial vitality.     

Spatfall of pearl oyster, Pteria sterna (Gould), on experimental collectors at Bahía de La Paz, South Baja California, Mexico

Spatfall of pearl oyster, Pteria sterna (Gould), was analysed at two sites, stations Gaviota and Merito, at Bahía de La Paz, South Baja California, Mexico, during an annual cycle in 1991-1992. and from surface to 15 m depth. Sampling was carried out monthly using experimental collectors which remained immersed for 60-65 days. Four different substrates were tested for spat settlement preference: a native bush called ‘chivato’, dry palm leaves, and red and yellow plastic onion bags: temporal variations, and horizontal and vertical spatfall distribution patterns were evaluated between and within stations. Spat settlement showed significant differences depending on the structure and texture of the substrate type. Chivato bush was in general the best spat collector, but was difficult to handle for large-scale operations; plastic onion bags were more suitable and spat yield was adequate; palm leaves were poor spat collectors. There was no evidence for colour preference of spat settlement between red and yellow onion bags. Pteria sterna spat were present throughout almost all of the study period. Main spatfall was detected in winter-spring, coinciding with low temperatures; most spat then occurred from surface to 4-5 m depth. Few spat were present in summer and most then occurred below 7-9 m depth. The distribution patterns of spat within substrate replicates were apparently influenced by the abundance of spatfall. Aggregated spatial patterns were evident in shallow water during months of maximum spatfall but not when spat was less abundant. These events did not occur in the same substrate type, and differences were also detected between stations. Trends were related to oceanographic conditions prevailing at each site. Two patterns of spatfall abundance and vertical distribution were evident, one for winter-spring, and the other for summer. Samples at station Gaviota were grouped by spatfall abundance and spat vertical distribution patterns, while the latter defined better the grouping of samples at station Merito. Geographical variations of spatfall characteristics for Pteria sterna itself, and in comparison with Pinctada mazatlanka (Hanley) in the Gulf of California, are discussed.