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Assessment of the potential for culture of the Chinese clam (Chione gnidia) in the estuary of La Cruz, Sonora, Mexico
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Assessment of the potential for culture of the Chinese clam (Chione gnidia) in the estuary of La Cruz, Sonora, Mexico
Aubert, Hernan, M.S.
The University of Arizona, 1990
UMI 300 N. Zeeb Rd. Ann Arbor, MI 48106
1
ASSESSMENT OF THE POTENTIAL FOR CULTURE OF THE CHINESE CLAM (CHIONE GNIDIA) IN THE ESTUARY OF LA CRUZ, SONORA, MEXICO.
by Hernan Aubert
A thesis submited to the faculty of the SCHOOL OF RENEWABLE NATURAL RESOURCES In partial fulfillment of the requirements For the Degree of
MASTERS OF SCIENCE WITH A MAJOR IN WILDLIFE & FISHERIES SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA
19 9 0 2 STATEMENT BY AUTHOR This thesis has been submited in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the university library to be made available to borrowers under rules of the library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in partmay be granted by the head of the major department or the Dean of the Graduate College when in his or her judgement the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author
SIGNED:
APPROVAL BY THESIS COMMITTEE This thesis has been approved on the date shown below:
/a/zpAo 4L0. Eugen (jtywAiUo Q. Charles D. Ziebell, Assistant Unit Date Leader, Arizona Cooperative Fish and Wildlife Research Unit William Associate Date Professor of Fisheries Science 3 ACKNOWLEDGEMENTS I would like to thank Dr. Eugene Maughan, Dr. Charles Ziebell and Dr. William Matter for the invaluable assistance, cooperation and good will shown throughout this project. Also, I would like to thank the above mentioned persons whose editing skills made this thesis possible. In the preparation of the tables drawings and charts, Francisco Ornelas gave freely of his time for review and discussion, and to him I am truly indebted. Ernesto Meza Vega deserves a special mention; without his dedication cooperation and helpful suggestions the present study would not have been developed. Finally, and more specially, I would like to acknowledge the tremendous support I received from my wife, Marcela Vasquez, who was always at my side helping and encouraging me during moments of total desperation. 4 TABLE OF CONTENTS Page LIST OF FIGURES 5 LIST OF TABLES 8 ABSTRACT 9 INTRODUCTION 10 Status of aquaculture in Mexico 12 BACKGROUND INFORMATION 15 Estuary Huivuilai 15 Estuary La Cruz 20 DESCRIPTION OF STUDY AREA 24 MATERIALS AND METHODS 26 RESULTS 34 DISCUSSION 44 CONCLUSIONS 49 APPENDIX A Residual and Full Normal plots, statistical analysis 51 LIST OF REFERENCES 68 5 LIST OF FIGURES Figure Page 1. Map of Huivuilai coast 16 2. Map of Kino Bay 21 3. Diagram of experimental units 27 4. Mean temperature variation at Estuary La Cruz.. 29 5. Cummulative growth of clams in study sites in La Cruz and Huivuilai (experiment 1) 36 6. Cummulative mortality in the pool and the canal site in La Cruz (experiment 1) 38 7. Cummulative growth of clams on several types of sediment (experiment 2) 40 8. Cummulative growth of clams in pool and canal environments (experiment 4) 41 9. Cummulative mortality of clams in pool and canal environments (experiment 4) 43 6 la. Full normal plot for total mortality (experiment 1) . 52 2a. Residual plot for total mortality (experiment 1) 53 3a. Full normal plot for growth periods (experiment 1) 54 4a. Residual plot for growth periods (experiment 1) 55 5a. Full normal plot for growth over the total periods of experiment 1 56 6a. Residual plot for growth over the total periods of experiment 1 57 7a. Residual plot for total mortality (experiment 2) 58 8a. Full normal plot for total mortality (experiment 2) 59 7 9a. Full normal plot for growth periods (experiment 2) 60 10a. Residual plot for growth periods (experiment 2) 61 11a. Full normal plot for growth periods (experiment 3) 62 12a. Residual plot for growth periods (experiment 3) 63 13a. Full normal plot for total mortality (experiment 4) 64 14a. Residual plot for total mortality (experiment 4) 65 15a. Full normal plot for growth periods (experiment 4) 66 16a. Residual plot for growth periods (experiment 4) 67 8 LIST OF TABLES Table Page 1. Summary of environmental conditions for the pool and canal habitats in the Estuary La Cruz... 25 2. F values of the 2x2 factorial anova (growth)... 35 3. F values of the 2x2 factorial anova (mortality) 37 9 ABSTRACT Growth and mortality of the Chinese clam (chione gnidia) were examined in the Estuary of La Cruz, Sonora, Mexico from 1987 to 1989. Clams were reared in trays in two habitats of the Estuary of La Cruz, the pool and the canal, where four experiments were carried out. In experiment 1 growth and mortality under two densities and two environments was assessed. Experiment 2 yield information on growth and mortality of clams growing on four different substrates and at two densities. Experiment 3 assessed growth and mortality in high density conditions, and experiment 4 assessed growth after a period of growth stagnation. In all cases, highly significant differences in growth and mortality were observed between environments. The pool was found to be the place where higher total growth and least mortality occurred. However, the sizes achieved by clams in the pool suggest that the Estuary of La Cruz is not a suitable environment for the cultivation of this clam, and that the introduction of C. gnidia in this area is not feasible under present habitat conditions. 10 INTRODUCTION Clams are generally classified in the order Heterodonta. All clams are characterized by heterodont or schizodont hinge dentition, similarity of adductor muscles, equality of size of valves, and shells smooth or concentrically ribbed and with an elongate sub-rectangular, or ovoid shape (Newel 1969). Clams support commercial, sport and subsistence fisheries throughout the world and are in high demand, especially in the U.S. (Nanzi et al. 1980). Unfortunately, the harvest of most clams is declining. According to Manzy (1989), declining clam populations are caused by a number of factors, including pollution and overfishing. In response to declining natural populations, interest in aquaculture of commercial clam species has grown, especially in the last decade. Basic biological research has yielded knowledge of environmental requirements, ecological interactions, and, to some extent, diseases and disease treatment for some commercial species (Rhodes 1985, Winter et al. 1984). Culture techniques for hatchery, nursery, and field growout have also been developed for several commercial clam species. Furthermore, commercial facilities have been constructed to produce and supply seed and to grow out seeds to market size. Despite these 11 efforts, clam culture produced only 15% of the world's total landings in 1989 (Manzy et al. 1989). By the turn of the century, predictions are that clan culture will account for 25 % of the total production (Parker 1986). Even though clam aquaculture is not as widely practiced as oyster and mussel culture, it comprises a significant part of worldwide aquaculture production. In 1975, world clam culture produced 6 million metric tons, 9 million metric tons in 1979, and 10.5 million metric tons in 1985 (FAO 1987). Yield was estimated to be 13 million metric tons in 1986 (Parker 1986). This increase in cultured clams accounted for about 14 % of the total world commercial catch in 1985 and 15 % in 1986. These data reveal a relatively constant increase in clam production of about 0.5 metric tons per year from 1975 to 1983. Total commercial production of cultured clams has thus increased nearly 70 % since 1975. The relatively low percentage of the world's clam catch produced by aquaculture occurs because clams are cultured in only certain regions of the world; many countries depend only upon natural production. In Asian countries, particularly those around the South China Sea, over 20 % of the molluscs harvested involve some aquaculture prior to marketing. Almost all cockle and clam landed in this area require some growout (SEAFDEC 1980). In North America, 12 where the technological infrastructure for the rearing of marine bivalves is better than in many other countries, wild fisheries still account for 90 % of the production (Davy and Graham 1982). Low production in countries with substantial technology is normally related to extreme hydrographic conditions, pollution, siltation, red tides, naturally occurring mercury contamination, poor sanitation, low nutrient content of water and poor productivity (Manzi 1985). These aquacultural constraints might be overcome with additional technology and development of environmental policies (Castagna 1984). STATUS OF AQUACULTURE IN MEXICO Although clams are highly prized in Mexico, little clam culture has developed. The lack of interest in clam culture is at least partly due to the abundance and diversity of species found along its coasts (Baqueiro et al. 1982). At present there is only one commercially exploited species in the Gulf of Mexico, one in the Caribbean and nine in the Pacific and the Gulf of California (Baqueiro 1989, in Manzi and Castagna 1989). Most landings of clams occur on the Pacific coast. While commercial landings declined in other regions, the Pacific coast's total yield has increased from 54% to 81% in only four years (1979-1983). This increase in 13 production in the Pacific region is directly related to the exploitation of natural beds since aguaculture produces only a negligible percentage of the total fishery (Baqueiro 1984). The development of clam culture has been slow on the Pacific coast of Mexico because of the focus on better known species like lobsters, shrimp, oysters and fish. In addition, although fishery regulations encourage the culture of commercially exploited species for economic reasons, they also constrain and discourage private involvement and investment in clam culture. High value species are given to cooperatives to culture, and complicated bureaucratic processes are involved in getting water concessions and culture permits (Baqueiro 1989 in Manzi & Castagna, 1989). Consequently, bivalve culture is poorly developed except for efforts by local cooperatives to rear Pacific oysters Crassostrea gigas. Lack of biological information on other species with the potential for cultivation increases the risks of failure and discourages their production. One of the most commonly harvested groups of clams in Mexico are species of the genus Chione. There are only a few studies on this genus, including a population study of Chione sp. (Aguilera & Mathews 1974); a study of population and reproduction of C. undatella (Baqueiro & Masso 1983); a 14 technical report on C. fluctifraga and C. gnidia (Martinez 1986); and a brief (two lines) description of the occurrence of Chione spp. spat in Bahia de La Paz, Baja California Sur (Caceres & Ruiz 1987). Martinez (1986) suggested that C. gnidia beds had once been abundant in La Cruz but were overexploited and depleted and that the species had become locally extinct. The general objective of my study is to provide a better understanding of the biology and ecological requirements of Chione gnidia (i.e., optimum density, preferred substrate 'type/ growth and mortality rates in different environments.). The second objective of my study, is to assess the feasibility of culture and reintroduction of this species to the Estuary of La Cruz. 15 BACKGROUND INFORMATION Martinez (1984) stated that Chione gnidia was extirpated from the estuary of La Cruz and that the fishery might be re established. To test this assertion, I surveyed the estuary of La Cruz and characterized the available habitats. I also surveyed the estuary of Huivuilai, where an extensive fishery exists for C. gnidia, and characterized habitats available to and used by C. gnidia. - Estuary Huivuilai: The estuary of Huivuilai (Tobari), located at 27 degrees 05 minutes North and 109 degrees 55 minutes East, is part of the coast of Ciudad Obregon, Sonora, Mexico (about 100 km southwest of Ciudad Obregon). The mainland forms a 10-km long bay, along which lie the villages of Paredon Colorado and El Paredoncito (Figure 1). These villages, located in the central and southern part of the bay, were established in the 1930's and have a combined population of about 2,500 people. About 1.5 km east of the bay, Huivuilai Island lies parallel to the coast. This 8.5-km long island has a central chain of hills with elevations from 50 to 100 m. Most of these hills are sand dunes (particularly in the northern part), but some hills in the central chain are formed of granitic rocks and soil. Island vegetation is predominantly mangroves, U.S.A. • T • Harmoiillo •mo Eitare Punt* ChiTuahu* MEXICO iil«ro Tokari Nrtden Gol f e or California Ul« figure 1 Hap of Huivuilai coast. I) Northen estuary, II) Eastern coast of Huivuilai Island, III) Southern estuary, IV) Bay's coastal region, and V) Central Fringe. . 17 halophytes and, in the northern part where a natural fresh water spring emerges, palm trees. Huivuilai Island reduces the impact of waves from the Sea of Cortez on Paredon's Bay; the only entrance to the bay is through two narrow (about 500- m wide) "mouths" located at each end of the island. Paredon bay is divided into two parts (north and south) by a broken concrete bridge that once connected the island with the mainland. Although the bridge is no longer functional, it still restricts water movements to three 150-m long breaks in the bridge supports. The restricted water movements and geological events cause the bay's sediments to range from highly anoxic to highly aerobic. Paredon Bay can be divided into five distinct regions according to the type of sediment, tidal range, and occurrence and relative densities of clam species. These regions are: I) the northern estuary, II) the eastern coast of Huivuilai island, III) the southern estuary, IV) the bay's coastal region and V) the central fringe of Paredon Bay (Figure 1). The most northerly part of the bay (region I) is characterized by finger-like projections from the mainland which form the Punta Verde (green point) estuary. This area is heavily forested with mangroves; water levels generally are less than 1.5 m mean depth at high tide, and the bottom sediments are muddy, rich in nutrients and anaerobic. The 18 sediments are extremely black, oily and have a strong smell of hydrogen sulfide. Several species of clams inhabit these nutrient rich sediments: almeja ronosa (Chione fluctifraga), pata de mula (Anadara grandis, and A. multicostata) and, in association with mangrove roots, mangrove oyster (Crassostrea sp.). Along the eastern coast of Huivuilai Island (Region II) a predominantly sandy beach extends toward the middle fringe. There are areas near the southern part of the island with mud and gravel bottoms. Samples from the southern area show enormous quantities of crushed clam shells of the genus Chione and some Mitilidae. The northern sandy beaches support a large population of Chione californiensis together with small gastropods and crabs. Region III has a superficial resemblance to the northern Punta Verde estuary. Broad finger-like projections extend into the sea from the mainland. These projections are heavily forested with mangroves and form an estuary called El Tobarito. The sediment and clam composition is similar to that at Punta Verde. Region IV runs parallel to the bay's coast extending west into the sea for approximately 600-700 m. It is a shallow region in which an almost imperceptible slope toward the sea enhances the tidal influence. The sediment is mostly mud and 19 mud/sand mixture which is almost entirely exposed at low tide. Few species of clams are present, but the most common ones are Chione fluctifraga and A.grandis. Their densities are much lower than in the Punta Verde estuary. Region V is the largest region; about 1 km wide (east-west) and 5 km long (north-south). This central fringe has a mean depth of 2.0 m at high tide. Its sediment is almost exclusively composed of highly aerobic mud; sea grasses and aquatic vegetation are rare. This region is almost exclusively inhabited by C.gnidia. These clams are about 9-10 cm in shell length and bury themselves within the top 5 cm of sediment. Growth seems to be continuous throughout the year, but accurate growth rates are lacking for this species. Villagers sell this clam in the local markets. The estimated harvest per person per day is 20 to 40 kg live weight or 600 clams (mean weight per clam is about 40-60 g). Since an average of 10-15 people harvest clams, a considerable number are taken each year. According to local fishermen, C. gnidia is and has been very abundant in these waters for the past 30 years (Danton, pers. comm. and pers. obs.). The prospect for the success of a commercial aquaculture enterprise in this region is very good because: a) C. gnidia is widely accepted in the larger markets (Hermosillo, Ciudad Obregon, Navojoa), b) the growth of the species seems to be 20 faster than that of. other commercial species (8-10 cm/year), and c) there is a seasonally plentiful supply of natural clam seed. Furthermore, C. gnidia seeds have been experimentally obtained using the same technique for obtaining Crassostrea spp. seeds (these were produced at the research laboratory of Fomento Pesquero in Kino Bay; M. A. Robles pers. comm.). - Estuary of La Cruz (Kino Bay): The estuary of La Cruz is located at 111 degrees 55 minutes east and 28 degrees 47 minutes north, about 100 km southwest of Hermosillo (capital of the state of Sonora). This estuary, considered a coastal lagoon since the impoundment of the Sonora River in Hermosillo, is about 52 Km2 in area. The closest town is Kino Bay (Kino Viejo), located about 1 Km north west of the estuary with a population of about 1,500 people (Figure 2). The coasts of La Cruz are thickly covered with mangroves (Rizopkora manglae) and inland vegetation is dominated by halophytes, saguaros (Canegia gigantea), cardons (Pachycereus pringlee) and Opuntia spp. Lowlands, usually flooded during the rainy season (October-January), extend north, south and east of La Cruz. The west coast of the estuary is composed of a series of low hills, dunes (about 8-10 m high) and higher mountains (300-500 m). Water currents enter and leave the 21 U.S.A. .KINO OIBIOON Kino BAHIA KINO ESTERO LA CRUZ 2 Map of Kino Bay. Region I composed. of fine coarse sand and II sand mud mixtures. A) location of C.Z.C.T.U.S research station. 22 estuary through a narrow, 300-m wide mouth (Figure 2). North of this entrance, low sand dunes extend parallel to the coast forming a long and narrow extension that separates the bay and the estuary. South of the estuary's mouth is the region of high mountains, the tallest (approximately 500 m) being San Nicolas. The sedimentology of the La Cruz estuary is complicated by the heavy load of allochthonous materials brought in by the currents and by bank erosion. The shore extending from Punta Santa Cruz along the estuary is composed of sand-mud mixtures. The sediment in the rest of the estuary of La Cruz is entirely composed of nutrient rich anaerobic mud. Major nutrient contributions come from the decomposition of mangrove leaves, nutrients washed from the granitic San Nicolas hill complex and peripherals, and decomposition of large seagrass beds of the genus Gracilaria. This rich anaerobic substrate is inhabited by three commercially valuable species of clam: the pata de mula clam, Anadara multicostata, A. grandis, and the almeja ronosa, Chione fluctifraga. Chione subrugosa, occurs in the middle sandy parts of the estuary, but does not have commercial potential due to its small size. Nevertheless, it is occasionally collected for personal consumption. This fishery is not protected by regulations at present. Exploitation is 23 determined by the tidal regime, the severity of the winter and the degree of knowledge about the location of existing clam beds. According to local fishermen, there are five or six families involved in the harvest of C. fluctifraga and A. grandis. Mean yield is 30 Kg of C. fluctifraga per day per fisherman. Each 30 Kg contains from 2,800 to 3,000 clams averaging 3.5 to 4.0 cm in shell length. Clams are collected three to four times a week during the summer and spring. Little harvest occurs during the winter. Clam size drops below average during the last part of the fishing season, but average size is re-established by the following spring. Market pressure constitutes an important fishery regulation and price drops with size. Little clam culture occurs in La Cruz, but Piedras Pintas, a local cooperative, grows the Pacific oyster, Crassostrea gigas to marketable size. 24 DESCRIPTION OF STUDY AREA Two experimental sites were selected in the Estuary of La Cruz for rearing of C. gnidia. The "canal site" was a small arm of the estuary of La Cruz (Figure 2 "A") about 6 m wide and 100 m long. The northeast side was covered with mangroves and the other side, by an experimental aquaculture unit of the University of Sonora. This canal provides water to the aquaculture unit and was chosen for study because it was a) accessible; b) representative of the substrates in most parts of La Cruz; and c) close enough to the laboratory to discourage vandalism (theft). The "pool site" was a man-made reservoir about 100 m long and 6 m wide and located about 75 m from the canal. This site was accessible, secure, and had physical characteristics that were substantially different from those of the canal (Table 1). This pool is filled with estuarine water from the canal through a shallow, narrow connecting ditch. The higher elevation of this ditch relative to the canal, allows only fine sediments to be transported to the pool. Tidal ranges were only 20-30 cm, and the maximum depth was 0.6-1.0 m. These differences between the pool and the canal provide distinct environments for comparing growth and mortality of C. gnidia under different densities and substrate conditions 25 Table 1. Summary of environmental conditions for the pool and canal habitats in the Estuario La Cruz. VARIABLES CANAL POOL SEDIMENT GRANULOMETRY COARSE FINE ORGANIC MATTER CONTENT HIGH LOW CONSISTENCY STICKY COLOUR BLACK LIGHT BROWN SMELL STRONG (H2S) NO H2S ODOR CONSTITUENTS SAND/MUD SLIME SALINITY (mean)1 42.9 p.p.t. 42.3 p.p.t. TEMPERATURE RANGE (°C)2 11.9-32.07 13.3 - 32.07 INTERTIDAL RANGE (cm) 75 -90 22-30 DEPTH RANGE(m) 2 - 1.3 1-0.5 AQUATIC VEGETATION Gracilaria sp. 1 Salinity measurements were taken during November, 1989. 2 Temperature measurements were taken for a 7-month period for the pool and canal. 26 MATERIALS AND METHODS Seeds of Chione gnidia were collected at the end of June, 1989 in the estuary of Huivuilai by scuba diving. They were transported to Kino Bay in moist sediment without water. At the CICTUS research station, seeds were kept in fiber glass tanks 10 m long X 2 m wide X 1 m deep. Water was pumped from the canal, and oxygenated with a 1 HP aerator. Seeds. were picked at random from the tanks, sanded on one side and individually numbered. The numbered clam seeds were measured to the nearest 0.01 cm at their widest point using a Mitutoyo 505-636-50 caliper. The clam seeds were placed at different densities in covered square perforated plastic shellfish rearing trays, 60 cm long and 5 cm deep. Two "beds" capable of holding eight trays each were made of a wood frame and plastic screen mesh bottom. These beds held the trays together and, avoided dispersion of the seeds. The experimental units (trays) were then submerged in the pool and the canal. The trays were anchored with lead weights. These weights were firmly affixed to the adjacent lips of the top and bottom tray (Figure 3). The weights also attached the trays to one another, avoided dispersion of seeds and excluded predators (crabs and shell perforating snails). 27 TRAY 1 TRAY 2 Figure 3. Diagram showing experimental units. Lid tray (tray 1) is attached to the one containing the clams (tray 2) by means of lead rings. 28 The trays were allowed to accumulate sediment carried by the currents for 2 months. At this time, all the trays were taken from the beds, the accumulated sediment dislodged and the clams washed. Individual measurements for growth and mortality were made. Water temperature and salinity were measured in the two environments twice a day (4 PM and 4 AH); temperature was taken for 13 months (Figure 4), and salinity for 1 month (November). Repeated collections of clams at Huivuilai estuary, allowed me to estimate the growth rate of C. gnidia. Collections of more than 1,000 individuals were made in the same location during each collection period. These data were used as a baseline for comparison with growth rates experimentally obtained in La Cruz. The general method explained above, was common to all four experiments outlined below. All data were analyzed at a confidence level of alpha=0.01. Different ANOVA procedures were used for each experimental design. A) Experiment 1 A total of 480 C. gnidia seeds were placed into the shellfish rearing trays at two densities, eight trays of 20 and eight of 40 clams per tray. In July of 1989, four trays 29 Figure 4. Mean temperature variation at Estuario La Cruz. 40 Aug. '89 Canal Pool O o 3 % a a. E a> H 20- Dec. '88 Mar. '90 30 of each density were placed in the bed in the canal at a depth of 1.5-1.8 m. The trays contained no sediment and the seeds were clean. At the same time, an equal number of units of each density were placed inside another bed located in the pool. The experiment was completed at the end of March 1990. Growth data were transformed into square root functions and analyzed by 2x2 factorial ANOVA (Sokal and Rohlf, 1969) under the following null hypotheses: A) Ho: there is no difference in mean growth rate at either density; and B) Ho: there is no difference in mean growth rate between the two locations. Mortality was analyzed using the same procedures as above under the following null hypotheses: A) there is no difference in total mortality at the two levels of density; and B) there is no difference in the total mortality at the two locations. B) Experiment 2 At the end of October 1989, 1,000 juvenile clams (mean size 3.9 cm in shell length) were collected from Huivuilai estuary. The juveniles were taken from the same location where the seeds were collected in July. Clams (480) were numbered using the same procedure as above and placed in 31 trays containing different sediment types at two densities (20 & 40 clams/tray). Four substrates were used in this experiment: crushed oyster shells, sand, small (< 0.5 mm) pebbles, and sediment carried in by the currents. The sediment treatments were allocated randomly to four experimental units (two of each density). The trays were filled with 2 to 3 cm of sediment, and the clams were placed on top of the sediment. Each tray was covered with a lid tray and placed directly in the pool. Growth and mortality were measured every 2 months for a period of 5 months (November-March). The canal was not used in this experiment because mortality there had been very high in the first experiment. Growth data were analyzed using a 4x2 factorial ANOVA under the following hypotheses: A) Ho: there is no difference in mean growth on the four sediment types; and B) Ho: there is no difference in mean growth rate at the two densities. The 4x2 factorial ANOVA for the mortality data was conducted to test the following hypotheses: A) Ho: there is no difference in total mortality among clams on the four types of sediment; and B) Ho: there is no difference in total mortality at the two densities. Experiment 3 32 Juvenile clans (480) collected from Huivuilai in October 1989, were marked and placed in trays at two densities, 40 and 80 clams/tray. A total of eight trays, (four of 40 and four of 80) were prepared as described in experiment 1 and placed in the pool. Growth and mortality were recorded every 2 months from November 1989 to March 1990. The data were analyzed using One Way ANOVA with the following null hypotheses: A) Ho: there is no difference in mean growth rate between the two densities; and B) Ho: there is no difference in mortality between the two densities. Experiment 4 In November of 1989, half of the clams that were placed in the canal in July 1989 (where growth had stagnated) were transferred to the pool. One hundred and fifty four clams were available from experiment 1, and 86 clams were taken from another study being conducted in the canal. The seeds previously used in experiment 1 were randomly divided into two groups and enough clams were added to bring the numbers of each experiment to 120 individuals/group. Clams were randomly assigned to different density levels. Four trays (two of 20 clams and two of 40) were transferred to the pool, and replicate groups were kept in the canal. This experiment was 33 carried out from November 1989 to March 1990. The growth data were analyzed by a 2x2 factorial ANOVA with the following hypotheses: A) Ho: there is no difference in mean growth rate between clams in the two locations; and B) Ho: there is no difference in mean growth rate at the two densities. Mortality data were analyzed using the same procedure as above under the following null hypotheses: A) Ho: there is no difference in total mortality between clams in the two locations; and B) Ho: there is no difference in total mortality at the two densities. 34 RESULTS Residual and full normal plots (appendix A) indicated that the assumptions of equal variance and normal distributions, needed for the ANOVA procedures, were met. EXPERIMENT 1 -two densities (20 and 40) and two locations (pool and canal)- A) Growth: Clams in the canal showed significantly lower growth rates than those in the pool (Table 2 and Figure 5) only for the first two periods (Jul-Sep and Sep-Nov). The interaction between location and density was not significant nor were growth differences between densities. B) Mortality: Total mortalities were not significantly different for either density (Table 3) nor was interaction between location and density. There was a highly significant difference in mortality between locations being the canal the place with highest mortality (Figure 6). EXPERIMENT 2 -four sediment types and two densities- (A) Growth: The interaction between sediment type and density was Table 2. F values of the two by two factorial ANOVA. The analyses include growth data for all experiments. 1 = July to September, 2= September to November, 3 = November to January, and 4 = January to March. T = Total growth. ECOLOGICAL VARIABLES EXPERIMENT 1 DENSITY LOCATION INTERACTION GROWTH PERIOD 1 3.360 60.56** 3.30 2 0.003 104.31** 2.08 3 1.167 7.59* 0.16 4 0.616 3.29 0.23 T 2.216 133.76** 1.28 EXPERIMENT 2 DENSITY SEDIMENT TYPE INTERACTION GROWTH PERIOD 3 1.990 351.23** 0.59 4 0.017 581.41** 0.38 T 4.630 532.09** 1.83 EXPERIMENT 3 DENSITY GROWTH PERIOD 3 0.250 4 0.570 T 0.100 EXPERIMENT 4 DENSITY LOCATION INTERACTION GROWTH PERIOD 3 0.610 412.36** 0.38 4 3.510 228.41** 0.31 T 1.921 388-34** 0.01 * Significant at alpba= .05 ** Significant at alpha= .01 36 Figure 5. Cummulative growth of clams in study sites in La Cruz and Huivuilai (experiment 1). 10 n Hulvullal E o 8 - * o O 6 - 0) > E E a 4 - O pool "eanal' • i • i • I Jul. Sap. Nov. Jan. Mar. 37 Table 3. F values of the two by two factorial ANOVA. The analyses include mortality data for all experiments. ECOLOGICAL VARIABLES EXPERIMENT DENSITY LOCATION SEDIMENT INTERACTION NUMBER TYPE 1 0.68 54.35** _ 2.20 2 0.06 _ 0.77 0.95 3 4 3.31 46.29** 1.55 ** Significant at an alpha* .01 i 6 Cummulative mortality in the pool and canal site in La Cruz (experiment 1). 100' pool 90 • 'canal" 80' 70' 60 < 50' 40 30 20 10 0 Jul. Sep. Nov. Jan. Mar. 39 not significant, nor were the differences in growth between the two densities (Table 2). Clams on sand and no substrate (control) grew significantly faster than did those on gravel and crushed oyster shells (Figure 7). B) Mortality: F-values for the mortality data (Table 3) indicate that there were no significant differences among the assorted treatments. Mortality was virtually negligible (< 1%) in this experiment. EXPERIMENT 3 -two high densities (40 and 80 clams/tray)- Growth and mortality: Growth of clams held at two relatively high densities was not significantly different (Tables 2 and 3). Growth rates were negligible (less than 0.025 mm/month) and survival was 100%. EXPERIMENT 4 -growth of clams held at two densities after growth stagnation- A) Growth: There was a highly significant difference in the growth of clams transferred to the pool compared to those that 40 Figure 7. Cummulative growth of clams on several types of sediment (experiment 2). 3.806 -i 3.804 - no sediment E o sand 3.802 - o 0 > (0 3.800 - shells 3 E E 3 o 3.798 - gravel 3.796 Nov. Jan. Mar. Figure 8 Cummulative growth of clams in pool and canal environments (experiment 4). 'canal 1 0.1 • i 1 • 1 I —* 1 Sep. Nov. Jan. Mar. 42 remained in the canal (Table 2 and Figure 8). The mean growth rate of clams tranfered to the pool was 10 to 20 times greater than those in the canal. The interaction of the two factors (density and location), as well as differences in growth at different densities, were not significant. B) Mortality: Mortality was significantly higher for clams in the canal than for those in the pool (Table 3 and Figure 9). The Interaction between density and location was not significant; nor were the differences between the two densities. 43 Figure 9. Cummulative mortality of clams in pool and canal environments (experiment 4). 20 pool "canal" >» •MM ~a 10 o > E E January March 44 DISCUSSION There were highly significant differences in growth and mortality of C. gnidia between the pool and the canal habitats. Clams in the canal habitat showed reduced growth rates and higher mortalities than those in the pool. Many of the areas in the estuary and the canal were anoxic. Large decomposing beds of Gracilaria sp. and other organisms within the sediment resulted in low levels of benthic dissolved oxygen (DO). In addition, algal blooms, especially brown algae, common in La Cruz during the summer and early winter, further lowered the oxygen levels in the hypolimnion by reducing the photic zone. Other factors, such as a mean depth of 1.3-2.0 m, poor light penetration (Secchi disk depth about 35-40 cm) caused by large amounts of suspended silt probably also contributed to the anoxic benthic conditions. The sediment that accumulated in the trays provided further evidence of hypoxia in this environment. This sediment was very dark and had the strong smell of hydrogen sulfide, characteristic of anoxic sediments. Benthic anoxia occurred to a far lesser extent in the pool. The pool was shallow, clear (Secchi disk depth of about 80-100 cm), and was devoid of seagrasses. Although the lower 5-6 mm sediment in the bottom of the trays was 45 anoxic, the upper 2-3 cm containing the clams was "aerobic". The rapid growth of seeds previously stunted in the canal (Figure 8) could be related to the higher 00 levels in the pool. There is widespread evidence that: oxygen levels are crucial to benthic organisms. For instance, seasonal hypoxia or anoxia in estuaries and coastal waters have been implicated in mass mortalities of commercial shellfish. An estimated 62% of the total biomass of New Jersey surf clam (S. solidissima), equivalent to more than 100,000 metric tons, was lost during 1976 due to anoxic events (Ropes et al. 1979). Even though many bivalves are facultative anaerobes, and can withstand low DO levels for extended periods of time (Hochachka and Mustafa 1972, Rossi and Reish 1976, Famme et al. 1981), they cannot remain in this anaerobic cycle indefinitely. Anoxia may not lead to massive mortalities, but could reduce the energy available for activity (Brafield 1963, Bayne et al. 1976), growth or even cause indirect or delayed mortalities. For example, under laboratory conditions (10 and 20 °C), S. solidissima could live indefinitely at DO levels down to 2.1 ml/1, tolerate DO levels down to 1.4 ml/1 for extended periods of time, but died if levels dropped below 1.4 ml/1 (Thurberg and Goodlett 1979). Other environmental factors can heighten 46 the effects of low dissolved oxygen and may have accentuated the difference between my two study areas. Survival at low DO levels is critically dependent on temperature; at high temperatures survival capacity is notably lower than at low temperatures (Theede et al. 1969, Shumway et al. 1983). High temperatures (30-32 °C) plus low DO could explain the high mortality recorded in the canal during the summer months. Differences in the amount of silt in the two environments also could have contributed to the difference in growth and survival among clams. The amount of silt is much higher in the canal than the pool. Low silt levels in the pool probably occur because its water inlet slows the rate of incoming water and particles settle out before they pass into the pool. The canal has a large amount of seston; a large percentage is inorganic silt. The presence of silt particles can dilute the amount of algae consumed by clams, thereby reducing growth rates and increasing mortality (Bricelj and Malouf 1984). Growth is negatively correlated with the silt content (Appeldoorn 1983). However, "siltiness" is also correlated with other environmental variables, especially water velocity, that may independently influence clam growth. Other studies indicate that silt produces branchial obstructions (Russel- Hunter 1983). Suspended sediments inhibited growth and 47 energetics of Spisula subtruncata (Mohlenberg and Kiorboe 1981) and M. mercenaria (Bricelj and Malouf 1984, Bricelj et al. 1984). Siltation is probably of secondary importance in the Estuary of La Cruz. If siltation had diluted the amount of algae consumed by the clams, I would have expected lower growth rates in clams reared at higher densities. Consequently I should have observed a direct relationship between mortality and clam density and an inverse relationship between growth and density. Such relationships did not occur; higher densities in the canal did not result in higher mortalities or slower growth (Tables 2 & 3). High salinity may also be a factor in the canal. High salinity heightens the effect of low DO; hard clam and oyster larvae have a narrower range of temperature tolerance if they are also subjected to sub-optimal salinity (Davis and Calabrese, 1964). Differences in clam growth and mortality indicated that the pool provided better conditions for C. gnidia than the canal. However, even in the pool the maximum growth of clams was far below the growth of clams in Huivuilai and it is highly probable that DO is the key factor in determining survival and maximum clam size in the pool. Clams in experiment 1 grew quite rapidly up to November but very slowly 48 thereafter (Figure 5). Poor growth also occurred for larger clams in experiments 2 and 3. Poor growth during this period might be attributable to seasonal conditions (winter), but the renewal of growth when clams were transferred from the canal to the pool during the winter would seem to discount this possibility (Figure 8). 49 CONCLUSIONS Field observations and experimental data suggest that environmental conditions in the estuary of La Cruz are not adequate for the growth and development of C. gnidia. Local fishermen and villagers who have been in the area for 40-50 years do not recall ever exploiting this clam. Therefore, overexploitation, as suggested by Martinez (1984), has probably not occurred. My conclusion is that C. gnidia may have been absent from the Estuary of La Cruz by the time Kino Viejo was established. Low dissolved oxygen in the sediments and high silt content in the waters probably limit growth and increase mortality of this clam in this estuary. The growth and mortality problems encountered by C. gnidia in the estuary of La Cruz, are not density dependent nor related to substrate. Clams can be cultivated in the pool, at densities of up to 80 clams per tray (160 clams/m2) with no effect on growth or mortality. Density did not affect growth or survival in the canal either. Environmental factors in both the pool and the canal limit the maximum size clams will achieve; they get much larger in the pool. This limit does not seem to be overcome by placing clams on different substrates (crushed shells, sand, small pebbles). Under 50 current conditions, the clam does not grow to harvestable size in either habitat and suffers high mortalities in the canal and presumably in the estuary. Aquaculture of C. gnidia on a commercial basis in the estuary of La Cruz currently does not seem to be feasible. 51 APPENDIX A RESIDUALS AND FULL NORMAL PLOTS, STATISTICAL ANALYSIS Figure 1a Full normal plot for total mortality (experiment 1). T -I -2 -1 0 2 Normal Scores Figure 2a Residual plot for total mortality (experiment 1). • • • • • 0 1 n 1 r I 1 2 3 4 Predicted Values 54 Figure 3a Full normal plot for growth periods (experiment 1). 0.2- 0.1 - D 0 B S8 0.0- bbsS • • :8bbb$ J2 -0.1- • (0 3 • XI « -0.2- 4) Growth Period QC -0.3 - • First • Second -0.4- • Third -0.5- O Fourth -0.6 T ~i -2 0 2 Normal Scores 55 Figure 4a Residual plot for growth periods (experiment 1). 0.2- 0.1 ° H mpB 0.0- B ©Og D Ip B m O • « .0.1 - 3 T3 CO ® -0.2 QC Growth Period -0.3- • First • Second -0.4- • Third -0.5- O Fourth -0.6 —I— —I— —I— 0.0 0.2 0.4 0.6 0.8 Predicted Values 56 Figure 5a Full normal plot for growth over the total period of experiment 1. 0.1 n • • • • o.o- M <0 3 TJ 3) • • 0) OC -0.1 - -0.2 T -I -1 0 2 Normal Scores 57 Figure 6a Residual plot for growth over the total period of experiment 1. 0.1 -i • • 0.0- w (0 • 3 •o *55 B • 0) AC -0.1 - • -0.2 H 1 1 1 1 1 > 1 1 0.4 0.5 0.6 0.7 0.8 0.9 Predicted Values Figure 7a residual plot for total mortality (experiment 2). • ° D • • • 1 1 1 1 -2 -1 0 1 2 Normal Scores 59 Figure 8a Full normal plot for total mortality (experiment 2). 1 - V) "3 3 • • •o o - '35 .• ° o oe -1 - —r- —r -I I -1 0 1 2 Normal Scores 60 Figure. 9a Full normal plots for growth periods (experiment 2). 0.1 • • tn • • 3 |0 M 0> 0.0- • •• £E • • • • Growth Period • First • Second -0.1 ~T~ -r -2 -1 o 2 Normal Scores 61 Figure 10a Residual plot for growth periods (experiment 2). S ^ dP D D s• S Rb Growth Period • First • Second ' 1 1 1 > 1 1 .01 0.02 0.03 0.04 0.05 0.06 Predicted Values 62 Figure 11a. Full normal plot for growth periods (experiment 3). 0.2-1 0.1 tn ca 3 |0 o o.o- • H DC Growth Period -0.1 • First • Second -0.2 -l r- -I -1 2 Normal Scores 63 Figure 12a. Residual plot for growth periods (experiment 3). 0.2n 0.1 m CO CO a> 0.0- EC b I Growth Period -0.1 - • First • Second -0.2 —T— —I— —I -0.1 0.0 0.1 0.2 0.3 Predicted Values 64 Figure 13a. Full normal plot for total mortality (experiment 4). M CO 3 2 °1 M 0) tr •2 ~r -I -2 -1 0 2 Normal Scores 65 Figure 14a. Residual plot for total mortality (experiment 4). tn a 3 2 0 "I m 0) £E —1 10 20 30 Predicted Values 66 Figure 15a. Full normal plot for growth periods (experiment 4). 0.2- • • 0.1 - Growth Period • First • Second -0.2 I » i • 1 • 1 • 1 -2-10 1 2 Normal Scores 67 Figure 16a. 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