Exploring the Potential of Indigenous Species for Aquaculture in Bahia Magdalena:
Growth Rate of Cultivated Concha Nacar (Pteria sterna)
Exploracion del Potencial de Especies Nativas para Acuacultura en Bahia Magdalena:
Tasa de Crecimiento de Concha Nacar (Pteria sterna)Cultivado
Previous attempts at aquaculture in the Bahia Magdalena region of Baja California Sur, Mexico, have included non-indigenous species such as the Japanese oyster and have not proven very successful. The concha nacar pearl oyster (Pteria sterna) is native to the area and was cultivated briefly at the turn of the century, but the wild population is now nearly gone due to 400 years of harvesting. Recent research on the concha nacar has focused primarily on spat collection techniques, in the hopes of helping to restore the wild population and reopening the pearl and shell market. This study compared the mortality and growth rates of concha nacar in captivity, in three size classes and three different culture structures. Mortality rate was very low; less than 0.5% of the oysters overall died during the study period. The smaller oysters grew significantly faster than the medium oysters, which in turn grew significantly faster than the large oysters. Those kept in canasta trays grew significantly faster overall than those kept in black or green bags, suggesting that canastas may be the best culture structure for these oysters. But a linear regression relating water temperature and oyster growth showed a positive correlation between water temperature and growth of oysters raised in canastas, suggesting that oysters grown in canastas may be more sensitive to changes in water temperature.
Key words: aquaculture, bivalve, pearl oyster, concha nacar, winged oyster, Pteria sterna, Bahia Magdalena, Baja California Sur
Ensayos anteriores de acuacultura en la region de Bahia Magdalena, Baja California Sur, Mexico, se han incluido a especies extranjeras, como el ostion japones, y no han tenido buen exito. El ostion de perla "concha nacar" (Pteria sterna) es un especie indigena y fue cultivado brevemente a principios de este siglo. Pero la poblacion silvestre es casi terminada a causa de 400 años de la recolecion. Investigaciones recientes sobre la concha nacar se han enfocado primeramente a la colecion de la semilla, esperando restaurar la poblacion silvestre y tambien reabrir el marcado de perlas y conchas. Este investigacion comparo las tasas de crecimiento y mortalidad de concha nacar cultivada en tres categorias de tamaños y tres estructuras de cultivo. La tasa de mortalidad fue muy baja: menos que 0.5% de los ostiones se morieron durante el periodo de estudio. Los ostiones mas pequeños crecieron significativemente mas rapidemente que los medianos, que crecieron significativemente mas rapidemente que los grandes. Los que estaban en canastas crecieron significativemente mas rapidemente en todo que los que estaban en costales negros o bolsas verdes. Esta diferencia sugiere que las canastas podrian son las estructuras mejores por estos ostiones. Pero una retroceso lineal de la temperatura del agua y el crecimiento de los ostiones demuestra una correlacion positiva entre la temperatura del agua y el crecimiento de los ostiones en canastas, que sugiere que los que estan en canastas podrian estar mas susceptible de cambios en la temperatura del agua.
Palabras Claves: acuacultura, bivalvo, ostion de perla, concha nacar, Pteria sterna, Bahia Magdalena, Baja California Sur
Advocates of aquaculture tout it as an answer to all the problems faced in fisheries today. According to the National Research Council (1992), total world harvest from capture fishing has decreased since its peak productivity around 1960 and now nearly all of the established major world fisheries are presently being fished at or beyond their sustainable yield. Aquacultural industries can not only reduce pressure on these declining harvest fisheries, but also provide an economic alternative for skilled workers living in communities that are highly dependent on those resources. Aquaculture can also be used to supplement food production, provide products for export and enhance wild fish stocks (NRC 1992, Tiddens 1990).
Cultivation of many species of fish, crustaceans, mollusks, amphibians and aquatic plants has indeed proven successful throughout the world. In 1990, aquaculture generated 15% of the 102 million metric tons of world fisheries output, and since then aquacultural production levels have continued to rise (SEPESCA 1990). Asia and the United States lead the world's aquacultural efforts, but Mexico is becoming an increasingly dominant contributor: in 1990, Mexico ranked fifth worldwide in aquacultural production of mollusks (SEPESCA 1990).
Bahia Magdalena, in Baja California Sur, is one area of Mexico that could benefit from aquacultural development. Currently 70% of the population in the Bahia Magdalena region is dependent on fisheries for its economic livelihood, and the fisheries harvests have decreased over the last decade (SEMARNAP, Puerto San Carlos, unpublished data). Illegal fishing has increased as regulations have been tightened to protect the bay's resources. Since few alternative jobs are available, the decline of the fisheries of Bahia Magdalena has had negative impacts on the quality of life in the area (Gloria Espinoza, resident, Puerto San Carlos, personal communication, 1999).
The state government of Baja California Sur has recently begun to encourage more aquacultural development by increasing funding for and public availability of existing aquaculture techniques and farming methods. Although several government-supported farms have attempted to cultivate shrimp, fish and oysters, all of these projects have unfortunately failed due to some combination of poor management, insufficient funding, and low production success rates (Samuel Chavez-Rosales, Center for Coastal Studies, Puerto San Carlos, personal communication, 1999).
Several private farms have been initiated on the peninsula. The Mazabe aquaculture farm, located in Estero San Buto (Figure 1), is a private company with ownership based in La Paz. Researchers there have tried to cultivate the Japanese oyster (Crassostrea gigas) in hanging racks. These attempts have been commercially unsuccessful, which they attribute at least in part to the local water temperature being warmer than that of the native habitat range of the Japanese oyster. They are now exploring the possibility of farming native mollusk species, hoping to have a higher success rate. In particular, they have been able to collect a sizeable amount of wild Pteria sterna juveniles, locally known as "concha nacar," in Estero San Buto (Francisco Sinsel, Mazabe Aquaculture Farm, Estero San Buto, personal communication, 1999).
The pearl oysters Pteria sterna and Pinctada mazatlanica are distributed in the Pacific from Baja California to Peru (Brusca 1980). The Spanish first began to harvest the oysters for pearls and shells during their initial explorations of the Baja California peninsula in the 1530's. The fishing increased over the following four hundred years until 1940, when the stocks were almost completely depleted and the Mexican government banned pearl oyster fishing (Monteforte et al. 1995, Caceres et al. 1997). Despite this prohibition, Monteforte et al. (1995) have observed continued clandestine fishing around La Paz, which they hypothesize is impeding the natural recovery of the oyster populations in that area. While the natural populations were being fished out, pearl oysters were also being successfully cultivated; Gaston Vives's company in La Paz (Criadora de Concha y Perla de Baja California, S.A.) cultured Pinctada mazatlanica for pearls and shells between 1903 and 1914, but closed during the Mexican Revolution (Caceres et al. 1997).
Recent research on these oysters has focused primarily on techniques of spat collection, in the hopes of recovering the natural populations and reopening the pearl market (Monteforte et al. 1995). Apart from Gaytan-Mondragon et al.'s study (1995) on the growth of both oyster species in different culture structures, little is known about the growth rate of these oysters. This study investigated mortality and growth rates of Pteria sterna in three different size classes and three different hanging culture structures provided to Mazabe by the state government.
Materials & Methods
Study Site & Culture System
The study was conducted at the Mazabe aquaculture farm in Estero San Buto, Baja California Sur, Mexico (N 24.772°, W 112.038°) from September to November, 1999 (Figure 1). Water temperature varied over the study period between 23 and 25° C. Water depth was always shallower than secchi depth, and pH remained constant at 8 ± 0.5.
The concha nacar used for the study were collected in the estero by researchers from the farm and until the start of the study were all stored in canastas: square plastic trays (0.6 x 0.6 x 0.1 m) which are stacked vertically in groups of five and buoyed with a large styrofoam slab to make a single hanging system. These hanging systems were tethered to lines situated about one hundred meters from the sandy mangrove shore where the water depth varied with tides from 1 to 3 meters deep. The canastas were changed about once a month before and during the study to remove buildup of algae and other planktonic growth.
Mortality and Growth Rates in Different Size Classes
Individual mortality and growth rates were compared between oysters in three initial size classes (small: 60-70 mm, medium: 80-90 mm, and large: 95-115 mm), each containing 40 individuals. Oyster size was determined by measuring to the nearest mm at the widest point of the wing at the base of the shell using manual calipers. Each oyster was numbered with a plastic tag tied loosely with string just above the valve hinge. The members of each size class were distributed randomly between five canastas that made up one hanging system. Individuals were measured and mortality noted every 7-10 days. A multiple range test was used to compare individual total growths between each size class over the study period and a linear model was fit to the data to assess growth rates over time.
Mortality and Growth in Different Culture Structures
Average mortality and growth rates were compared between groups of oysters in three hanging culture structures: (1) canastas, (2) large black bags (1 x 0.5 m2) of stiff plastic 1.5-cm mesh and (3) small green bags (0.25 x 0.25 m2) of flexible nylon 0.5-mm mesh. Thirty-five oysters, initially between 60 and 80 mm, were placed in five replicates of each culture structure for a total of fifteen containers. The containers were distributed randomly along a single buoyed line, roughly 100 meters from the shore. Each group of oysters was measured and mortality was noted every seven to ten days. A multiple range test was used to compare average total growth between each culture structure and a linear model was fit to the data to assess growth rates over time.
Water Temperature and Growth Rate
Water temperature was measured at every data collection session using an alcohol thermometer. Linear regressions were run relating water temperature and average growth per week in each size class and culture structure. A rejection rate of p < 0.05 was used for all statistical analyses.
Mortality and Growth in Different Size Classes
None of the tagged size-class oysters died during the study period. Total growth of the individual oysters differed significantly between the three size classes (Figure 2). The small oysters grew an average of 9.5 mm over the study period, and the linear model indicated a growth rate of 1.5 mm per week. The medium oysters grew an average of 4.9 mm with a linear indicated growth rate of 0.9 mm per week. The large oysters grew an average of 0.1 mm over the study period and the linear model also indicated a growth rate of 0.1 mm per week.
Mortality and Growth in Different Culture Structures
Three of the 525 oysters (0.5%) died during the study period. All of these mortalities occurred in black bags. Total average growth of the oysters differed significantly between the culture structures (Figure 3). The oysters in black bags grew an average of 2.8 mm over the study period, and the linear model indicated a growth rate of 0.6 mm per week. The oysters in green bags grew an average of 5.2 mm, with a linear indicated growth rate of 0.9 mm per week, and the oysters in canastas grew an average of 7.7 mm, with a linear indicated growth rate of 1.4 mm per week. The growth of the oysters in canastas decreased by 89% over the last measuring period (Figure 4).
Water Temperature and Growth Rate
The linear regression showed a positive correlation between water temperature and growth rate of oysters in canastas (R2=0.72) (Figure 5). Water temperature was not as strongly correlated with growth rate of oysters in black or green bags (R2=0.32 and 0.29, respectively). Nor was water temperature as strongly correlated with growth of the tagged oysters in any of the three size classes growing in canastas, although it was more strongly correlated with growth of the medium oysters (R2=0.66) than with that of the small (R2=0.45) or the large (R2=0.18) oysters (Figure 6).
The remarkably low mortality rate across all the oysters used in the study suggests that neither size class nor these three culture structures have much effect on mortality in these oysters. It indicates too that the concha nacar is indeed a better choice for local cultivation than the non-native Japanese oyster (Crassostrea gigas), which had mortality rates sometimes higher than 50% under similar conditions (Francisco Sinsel, Mazabe Aquaculture Farm, Estero San Buto, personal communication, 1999). This difference in mortality rates is despite the fact that the concha nacar grow byssal threads attaching themselves to the surrounding material which are severed each time they are moved or cleaned, while the Japanese oysters do not form any such attachment and so do not undergo similar trauma. The low number of mortalities also suggests that further cultivation of indigenous species would be more likely to meet with success than would cultivation of imported species.
Growth Rates in Size Classes
Although the total average growth and the linear indicated growth rate in the large oysters were the same (due to their minute total growth and ill fit of the linear model), the larger oysters clearly grew less and more slowly than did the smaller oysters. These growth patterns came as no surprise, since most aquatic organisms grow faster when young and grow more slowly as they age (Everhart et al. 1981). If raising the oysters for meat, the slower growth rate of the larger oysters may make it worthwhile to harvest smaller oysters (presumably with less meat) earlier, rather than waiting a long time for only a minute increase in size. For example, at the growth rates estimated for these three size classes, it would take about 30 weeks (over six months) to grow an oyster from 60 to 95 mm, but more than that amount of time again to grow it from 95 to 100 mm. These calculations, of course, assume a constant rate of growth within those size classes.
Further research of growth rates among different size classes could be used to calculate a maximum sustainable yield curve for the concha nacar if the species was ever opened for harvest again. Since cultivation and harvest in the past of concha nacar was primarily for pearls rather than meat, further research might also examine the relationship between size of oyster and presence, size and quality of pearls.
Growth Rates in Culture Structures
The overall average growth rates of the oysters in different culture structures suggest that concha nacar grow much faster in canastas than in either of the two other culture structures. This finding concurs with that of Gaytan-Mondragon et al. (1993), who found that the oysters grew faster in submerged cages than hanging baskets or nets. But the stronger correlation between water temperature and oyster growth in canastas suggests that those in canastas could be more vulnerable to changes in temperature, or grow more slowly in colder temperatures. Previous studies have shown a correlation between water temperature and oyster growth (Grant 1993).
The tagged size class oysters, though, were stored in canastas as well, and their growth was not as strongly correlated with water temperature. This inconsistency might be due to the fact that the sample size for the oysters in size classes (n = 40) was smaller than that for the oysters in culture structures (n = 175). Longer-term research comparing culture structures over several seasonal changes and over temperature events like El Niño and La Niña might serve to clarify this question further, and could lead to practices such as changing culture structures by season or year. Seasonal or yearly changes in growth rate would also change the estimates of time needed to grow concha nacar to certain sizes.
One clear drawback to canastas is that they are at present more labor intensive than either of the two bags. All of the culture structures become fouled with planktonic organisms, but canastas must be brought to shore to be dried and cleaned, so every oyster must be removed and placed into a new canasta at every cleaning, whereas the bags can be cleaned from the boat. Researchers at the Mazabe farm are presently conducting studies to determine how long the canastas can go without cleaning before oyster growth is negatively affected. They are also attempting to look into biological methods of reducing planktonic growth, such as biculturing Murex or other motile planktonic feeders along with the oysters, instead of scraping the growth off by hand or using anti-fouling chemicals.
In addition, if, like Gaston Vives, one is growing concha nacar primarily for pearls and shells and not for meat, maximizing growth rate and size may not be the most important factor in cultivation. Oysters that grow the fastest may not produce the best pearls. Further research might be well put to investigate culture structures and conditions that are most conducive to satisfactory pearl formation.
This study would have been impossible without the help of many people. I couldn't have asked for better partners in crime: James Benazzi kept us laughing and Janneke Volkert kept us sane. Kat(ita) Waters took the best notes I have ever seen. Susan Gardner advised us calmly and constructively. Francisco Sinsel's work and ideas provided the foundation for this study, and he made a knowledgeable co-worker. Pedro who passed away during the study and Jose-Jesus provided field assistance. And last but not least, Julio, Joel and Norberto the pangueros brought us through the choppy waters safely every week and made delicious ceviche to boot.