Clam Production Research Articles

Incidental production of tetraploid Manila clams, Tapes philippinarum (Adams and Reeve)

For about 10 years the quest for tetraploid bivalves has eluded researchers leading to doubts that tetraploids were viable. Recently two reports of tetraploid shellfish have led to increased optimism. We document a third species in which tetraploids are viable. Newly fertilized eggs of the Manila clam, tapes philippinarum (Adams and Reeve), were treated with cytochalasin B from 20–40 min post-fertilization to induce triploidy. Following standard nursery culture, juveniles were planted at two locations for analysis of growth differences between diploid and triploid clams. Samples from the planted populations were taken at regular intervals from March, 1993. Of 200 clams sampled from the Welsh site, 1 was unequivocally tetraploid; of 120 clams sampled in Israel, 2 were tetraploid, and 2 were hypotetraploid. Tetraploids formed gametes with eggs about 41% larger than diploids. Tetraploids were produced as an artifact of triploid induction, either from inhibition of the first polar body alone or in combination with the second, or by inhibition of the first mitotic division. Production of artifact groups like tetraploids suggests that development of the eggs was asynchronous. The major implication of these findings is that tetraploidy can be tolerated in T. philippinarum and apparently at least two other species, and some evidence exists for gamete production, therefore offering encouragement for continuing research on induced tetraploidy in shellfish.

Optimal Harvest Age for Giant Clam,Tridacna derasa: An Economic Analysis

This paper analyzed the optimal harvest time (replacement cycle) to maximize economic returns for the giant clam, Tridacna derasa. The optimal harvest age ranged from no production to 13.2 years if only shell was assumed to be saleable, from no production to 9.2 years if only adductor muscle was assumed to be saleable, and from no production to 10.5 years if only other meat was assumed to be saleable. If all clam products were assumed to be saleable, the optimal harvest age ranged from no production to 10.7 years. If only meat was assumed to be saleable, the optimal harvest age ranged from no production to 9.0 years. The results indicate that clam farming can be profitable if prices of clam products are high or juvenile and annual operating costs are low. If all clam products can be sold, clam farming is generally profitable. If only meat can be sold, clam farming is profitable only if the high production costs are low and the prices of the meat products are high. Marketability of shell is an important fa...

Effects of graveling on the primary productivity, respiration and nutrient flux of two estuarine tidal flats

This study was conducted in Puget Sound, USA, and investigated the effect of graveling intertidal mud and sandflats to enhance clam production on the benthic assemblage structure, primary productivity, respiration, and nutrient flux. The study was conducted between spring and autumn (1991), the period of greatest productivity and plant standing stock in Puget Sound. Graveled and control plots were established in the low intertidal zone on a mud flat in a protected embayment (Chapman Cove) and an exposed sandflat (Semiahmoo Bay). Gravel altered benthic assemblage structure, respiration, and nutrient flux rates. Graveled plots contained more surface coverage of sessile animals and seaweeds. Net productivity (NP), which differed relatively little between graveled and control plots, was negative for all but one sampling at the protected embayment plots. In contrast, NP was always positive at the exposed sandflat plots. The respiration rate was 13 to 57% greater in the graveled plot at Chapman Cove than in the adjacent control plot, and 7 to 54% greater in the gravel plot than the control plots at Semiahmoo Bay. Heterotrophy was greater in the graveled plots, as reflected by a lower net productivity to respiration ratio. Effects of graveling on water quality parameters such as dissolved oxygen and inorganic nitrogen concentrations were not detected. Graveling sandy and mud beaches increases secondary productivity, which is associated with increased rates of remineralization and release of dissolved nutrients to the water column.

Effect of Size-Selective Predation by Muskrats (Ondatra zebithicus) on a Population of Unionid Clams (Anodonta grandis simpsoniana)

SUMMARY (1) The effect of size-selective predation by muskrats on the population size, sizestructure, and growth of clams was evaluated in a small lake in central Alberta. Clam shells were collected at regular intervals over 1 year from muskrat middens found along the shoreline of the lake. The number, biomass, size distribution, and age distribution of clams eaten were compared with the number, biomass, and age and size distributions of clams in the lake. (2) Muskrats ate almost 37000 clams, or 3% of the population in the study area in I year. When expressed as biomass, muskrats ate 677 kg of clams, which was equivalent to 5 8% of the biomass and 31% of the annual production of clams in the study area. (3) Muskrat predation was highly size-selective-86% of the clams eaten were > 55 mm long compared to 37% for clams in the lake-and had a strong effect on the size structure of the clam population. When all age-classes were combined, the biomass of clams > 70 mm eaten exceeded the annual production by an amount equivalent to 9 5% of the estimated biomass in the lake. Muskrats ate the fastest growing clams between 3 and 9 years old. Mean length at annulus did not increase for clams older than age-8 in the lake. The biomass of large clams in the lake will therefore decline if the observed predation continues. (4) Muskrats ate a large fraction of the reproductive output (glochidia) of the largest clams. Only 6% of the glochidia of clams 70 mm long was eaten. Genetic selection for slow growth has not occurred.

The Mariculture of Giant Clams

Recent research has been directed at methods for farming giant clams to meet the demands for giant clam products and to replenish overfished populations. Giant clams differ from other bivalves not only in their large size but also in their dependence on symbiotic algae for nutrition. Typical bivalve mariculture methods are thus not necessarily applicable to giant clams, and innovations have been required. We describe mariculture research and methods that have been developed to culture giant clams, in particular for the largest and fastest-growing species, Tridacna gigas.

The Mariculture of Giant Clams

AbstractRecent research has been directed at methods for farming giant clams to meet the demands for giant clam products and to replenish overfished populations. Giant clams differ from other bivalves not only in their large size but also in their dependence on symbiotic algae for nutrition. Typical bivalve mariculture methods are thus not necessarily applicable to giant clams, and innovations have been required. We describe mariculture research and methods that have been developed to culture giant clams, in particular for the largest and fastest-growing species, Tridacna gigas.

Effect of mining activities on the clam fisheries and bottom fauna of Goa estuaries

Comparison of two clam beds and associated benthic fauna, in Mandovi and Cumbarjua canal estuarine system of Goa, severely affected by massive inputs of mining rejects, revealed that, in less than 10 years (1972–73 to 1982–83), high biotic variability induced by increasing environmental stress has caused irreversible ecosystem instability. Reduced dissolved oxygen concentration; high suspended solids and blanketing of bottom deposits by mining rejects, has resulted in more than 70% reduction in clam production; near extinction of resident fauna and the appearance of a low diversity bottom fauna, comprising of tolerant but vagrant species. Ever increasing entry of mining rejects, which has reduced the healthy and highly productive estuarine environment of 1972–73, into an impoverished biotope, in less than 10 years, unless prevented will result in the total extinction of estuarine life in the near future.

Recent innovations in cultivation of edible molluscs in Taiwan, with special reference to the small abalone Haliotis diversicolor and the hard clam Meretrix lusoria

Annual production and culture area of small abalones and hard clams have surprisingly increased in recent years. The factors contributing to this increase, aside from heavy demand and high price, include new techniques in catching the tiny clam seed (0.5 mm in size), success in mass production of seed of both species in hatcheries, better understanding of the molluscs' biology as well as their response to environmental changes, improved management of water quality to provide the optimal growth conditions, cheap and easy supply of Gracilaria for abalone feed, use of powdered formulated feed for hard clams, application of chicken manure and rice bran to enrich the water, intermediate harvest of animals of marketable size, and control of predators. However, major constraints to further development and expansion of molluse cultivation remain, including water pollution from industrial wastes, limited area with suitable water supply, strict regulations on the use of seashore land and littoral zone, and the lack of export market. Other species of molluscs, including the oyster Crassostrea gigas, the short-necked clam Comphina veneriforms, the purple clam Soletellina diphos, the cockle anadara granosa, and the small freshwater clam Corbicula, have potential for further expansion if problems of seed supply, water pollution, control of disease, and market price are properly solved.

Survey of the U.S. Atlantic Coast Surf Clam, Spisula solidissima, and Clam Products for Anisakine Nematodes and Hyperparasitic Protozoa

Surf clams, Spisula solidissima, and processed surf clam products (that may also contain meat of the mahogany clam, Artica islandica) were examined for their parasite content. Of 3,358 raw clams collected on the U.S. Atlantic Coast from July 1976 through September 1977, 771 (23%) contained ani- sakine nematodes classified as a Sulcascaris sp. Of the 1,696 recovered anisakine nematodes, 15 (<1%) had discernible numbers of spores of a protozoan, Urosporidium spisuli; this represented a large decrease in the hyperparasitism observed during the previous 3 yr. The average number of nematodes per 100 clams was 51. Small clams contained fewer nematodes than large clams, and there was no seasonal fluc- tuation of incidence, indicating that the nematodes persist and accumulate in the clams. Southern clams contained more nematodes than northern clams; this geographical distribution corresponded to the dis- tribution of sea turtles, Chelonia mydas and Caretta caretta, believed to be definitive hosts of the ani- sakine nematode. The nematode was found in all parts of the clam, and its numbers in the edible and nonedible portions of clams were similar. Although most clams showed no gross tissue reactions to the nematode, there was thickening of clam tissue surrounding approximately 60 recovered nematodes; 16 nematodes were in individual cysts of clam origin. In 226 portions of 100-g amounts of canned clams, 10 nematodes or nematode fragments were recovered from 3.5% of the portions; nine nematodes or fragments were recovered from five of 229 portions (2.2%) of 100-g amounts of frozen, breaded clams. These figures represent a decrease in nematode contamination of processed clam products from preceding years.

The Analysis of Surf Clam Production Using an Exhaustible Resource Model

The surf clam (sp. Spisula solidissima) is the predominant clam species harvested in the United States. Starting from modest beginnings as a New England bait clam fishery, surf clam meats are now used in virtually all processed clam products, having crowded out other species from their bate or processed goods markets over the past 20 years. This has been due primarily to the surf clams’ greater availability, high meat yield and low cost of harvesting by mechanical means. In 1974, the surf clam fishery produced landings of 96 million pounds of meats, approximately 80 percent of the total catch of all species. The value of landings in 1974 was $12.2 million, over 30 percent of the value of all U.S. landings. Unfortunately, this trend is not expected to continue unabated. Catch in 1975 decreased by 10 percent to 87 million pounds. Current stocks available for harvesting are one-fourth of what they were in 1970 (Chang, Ropes, and Merrill). Due to the increased effort being applied to the remaining populations, it is doubtful that the resource will be able to reach previous harvest levels.

Reduced growth and survival of clams transplanted to an oil spill site

Production of soft clams fell by 20% in two years following oil pollution although in adjacent mudflats production of them increased by 250%. Transplanting uncontaminated clams into this area confirmed poorer survival and slower growth in polluted mud.

Possible Effects of Passamaquoddy Power Project on Clams, Scallops and Shipworms in Canadian Waters

Only three molluscs in the Passamaquoddy region of the Bay of Fundy have economic significance. The annual soft-shell clam (Mya arenaria) production now at 4 million lb will drop 2 million and fishermen's earnings will drop to $100,000 because the area of clam flats in the High Pool will decrease 95%. Annual scallop (Placopecten magellanicus) production, now at 60,000 lb, will increase to about 90,000 lb and fishermen's earnings will increase to about $36,000 because reduced water exchange in the High Pool will favour survival of larvae and recruitment of scallop stocks. The shipworm (Teredo navalis), no problem now, will become an expensive pest in the Low and High Pools because higher water temperatures will encourage the build-up of heavy breeding populations.

The Effects of Temperature and Predators on the Abundance of the Soft-Shell Clam, Mya Arenaria, in New England

Abstract Explanation of the recent decrease in production of the soft-shell clam in New England is the objective of the Fish and Wildlife Service Clam Investigations. Experimental clam farms failed unless protected from green crabs, Carcinides maenas. Laboratory experiments proved that Carcinides is a serious predator. Clam abundance decreased 50 percent in 4 years in study areas even though fishing intensity was very low and reproduction was adequate. Population decrease occurred principally in places where green crabs had become abundant, and in size groups which are attacked by crabs. Clam population on commercial flats decreased from 1949 to 1954 and recent year classes are scarce or absent. Green crabs became abundant during this period. Scientific records and interviews of fishermen indicate a northeastward extension of the range of the green crab since 1940 and great increase in abundance in Massachusetts, New Hampshire, and Maine. This extension was accompanied by decrease in clam production. An i...