Distribution of Quahog Larvae Along a North-South Transect in Narragansett Bay

A study on the distribution of quahog (Mercenaria mercenaria (Linnaeus 1758)) larvae in Narragansett Bay, Rhode Island was conducted during a single summer season in 1995. Samples of the larvae were collected weekly using an electric bilge pump and a 60 ~m mesh plankton net at two depths (0.3 m and 1.6 m) at five sampling stations distributed landward along the West shore, 12.6 km to 30.6 km from Rhode Island Sound. Three stations were located in the upper estuary, i.e., Upper Bay, and two stations were in the lower estuary, i.e., Upper


LIST OF TABLES
Temperature (OC) and salinity (ppt) at five sampling stations in Narragansett Bay during the summer 1995. T, temperature; S, salinity; N/A, data not available; Numbers Page in parentheses denote Julian day. 20 Table 2. Densities of quahog larvae (per 100 I) at two sampling depths in five stations of Narragansett Bay during summer 1995. T, total larvae; E, early stage larvae (see text); L, late stage larvae (see text); N/A, data not available; Numbers in parentheses denote Julian day. 22 Table 3.
Densities of other bivalve larvae (per 100 I) at two sampling depths in five stations of Narragansett Bay during summer 1995. T, total larvae; E, early stage larvae (see text); L, late stage larvae (see text); N/A, data not available; Numbers in parentheses denote Julian day. 25 Table 4. Rate of mortality of quahog larvae in Narragansett Bay during summer 1995. Cohort is defined as a group of larvae at an early stage developing into the late stage within 11 -17 days. 28 Table 5. Non-parametric procedure to determine whether there is a difference between abundances of quahog larvae during neap (N.T) and spring (S.T) tides in Narragansett Bay in the summer of 1995. 29 Table 6. Non-parametric test, using Kruskal-Wallis method, to determine whether there are differences of quahog larval abundances among five stations. 31 vi Table 7. Table 8. Table 9.  The quahog (Mercenaria mercenaria) fishery in Rhode Island is of major economic importance. It is considered to be the largest inshore fishery within the state (Pratt, 1988;Lazar et aL, 1995). The fishery began in pre-colonial times (Rice, 1996), and has gradually become more important following the decline of the oyster fishery between 1930s and 1950s, primarily due to pollution (Pratt, 1988;Bean, 1990). The commercial landings of quahogs depend almost entirely upon the stock in Narragansett Bay (Lazar et aI., 1995;Rice and Goncalo, 1995), the principal estuary in the state of Rhode Island.
The harvest reached a peak of 5 million pounds in 1955, after which catch declined (Lazar et aL, 1995). A second peak of 4 million pounds occurred in 1983.-However, commercial landings in 1994 ranked second to Connecticut among other states in New England (Rice, 1996).
The harvestable quahog stocks in Narragansett Bay are not distributed uniformly (Pratt, 1988). There is a tendency of decreasing numbers from upper to lower estuary (Pratt, 1988). In the upper estuary, the Providence River area and Upper Bay (Figure 1), which are prohibited areas for shellfishing (Bean, 1990), have the highest densities of adult quahogs, and these quahogs constitute a potential spawning stock (Pratt, 1988). In contrast, the West Passage in the lower estuary, an open area for shellfishing (Bean, 1990) has a much lower quahog density (Rice, 1993).
Despite many studies focusing on the adult quahog populations (Stickney and Stringer, 1957;Saila et aL, 1967;Jones et aL, 1989;Rice et aI., 1989), there have been few studies on quahog larvae in Narragansett Bay. Landers (1954)  studied the abundance of bivalve larvae -assuming them to be quahogduring three consecutive years (1950, 1951, and 1952) at two Narragansett Bay sites, in Greenwich Bay and off Wickford ( Figure 1). No other study directed attention at the distribution of bivalve larvae in Narragansett Bay for about 40 years, until Rice and Goncalo in 1993 conducted their study at seven stations in Greenwich Bay. Therefore, to establish a better understanding of the ecology of early life stages of quahogs, this study attempts to describe patterns of quahog larval distribution in the western part of Narragansett Bay.

Literature Review
The northern quahog is a filter feeder inhabiting shallow coastal waters from the Gulf of St. Lawrence in Canada to Florida (Rice, 1992). Like many other bivalves, young quahogs are typically males. In successive years they may change sex and produce eggs, a characteristic called protandric hermaphrodism (Rice, 1992). Both eggs and sperm of adults are expelled in the water column, thus, fertilization proceeds externally (Rice, 1992).
Spawning normally begins in the late spring as water temperature increases (Menzel, 1989;Rice, 1992), and it lasts into the summer months (Bricelj, 1993;Rice 1993). Therefore, the existence of the larvae of quahog in the water column is considered temperature -dependent.
Larval quahogs pass through a series of well-defined, recognizable stages characterized by appearance and dimension (Carriker, 1961;Loosanoff et aI., 1966;Chanley and Andrews, 1971). According to Carriker (1961) (Manning and Whaley, 1954) and Greenwich Bay, Rhode Island (Rice and Goncalo, 1995) tended to remain in the upper water column, while late stage larvae stayed close to or on the bottom. The difference in depth distribution may be a function of stage -dependent specific gravity of the larvae. Younger larvae have lower specific gravity than seawater; the specific gravity of the older larvae is greater (Manning and Whaley, 1954). Rice and Goncalo (1995) speculated that the effect of light was responsible for depth distribution; early stage larvae were considered positively phototactic as they occurred near the surface, while late stage larvae were considered negatively phototactic because they occurred away from the surface.
According to Wood and Hargis (1971) the selective swimming process (i.e., sinking and rising throughout the tidal cycle) within the water column is stimulated by tides. The late stage bivalve larvae in a New Jersey estuary dropped on or close to the bottom during the period of slack water and remained there through the falling salinities of ebbing tide. They were apparently stimulated to rise by the increasing salinity of early flooding tides (Carriker, 1951). In contrast, early stage larvae showed uniform vertical distribution throughout the tidal current cycle (Carriker, 1951;Kunkle, 1958).
Larvae inhabiting upper water column during ebb tides can be dispersed seaward, and the densities were lower than those during flood tides (Carriker, 1961;Andrews, 1983). Nelson (1955) studied the distribution of oyster larvae in Barnegat Bay, New Jersey and found that the early stage larvae were carried as far as 3.4 km seaward from spawning beds. In order to maintain their existence in the estuary, the larvae should, therefore, swim bottomward during ebb tides as lower layer gravitational circulation may carry the larvae landward (Wood and Hargis, 1971;Sandifer, 1975).
On the studies of oyster larvae near 06sterschelde, Holland (Korringa, 1947) and quahog larvae in Little Egg Harbor, New Jersey (Carriker, 1961), those researchers found that major peak of larval abundance occurred 8 -10 days after full or new moon, concomitant with neap tides. The abundance of the larvae showed minor peaks in the intervening periods.
Wind induced circulation is another influential factor on the dispersion of bivalve larvae, besides tidal circulation. Prevailing winds blowing from one direction may induce currents (Perkins, 1974), which produce a net -horizontal transport of water on the surface (Ippen, 1966;Perkins, 1974;Tett, 1987). In St. Mary's River estuary, Maryland, according to Manning and Whaley (1954), the pattern of longitudinal circulation is strongly influenced by prevailing southerly winds and provides a means of slow up-estuary transport and insures retention of the larvae in the upper estuary. The St. Mary's River is characterized by having low riverine discharge and high spatfalls on the upriver beds. Additionally, wind driven circulation has the potential for dispersing larvae from the spawning beds. In the Choptank River system, Maryland, prolonged down -estuary winds dilute out the retained larvae of two tributaries constituting major spawning areas, and carry the larvae downestuary (Seliger et aI., 1982).
In respect to temporal distribution, Landers (1954), Carriker (1961), and Rice and Goncalo (1995) established the fact that the peak of bivalve larval abundances occurred as water temperature reached about 20°C, as summer progresses. During three consecutive years (1950, 1951, and 1952)  However, in his study of oyster larvae near 06sterschelde, Holland, Korringa (1947) attempted to show but failed to prove that the peak of larval abundance depends upon water temperature.
Additional knowledge of distribution and abundance of laNae might explain their ability to maintain adult bivalve populations within estuaries (Haven and Fritz, 1987). In this study I tested hypotheses designed to enhance knowledge of Narragansett Bay quahog populations.

Hypotheses
The hypotheses evaluated for early stage and late stage quahog laNae in this study are: 1. Daily water temperatures affect the temporal distribution of the laNae ; 2. Spring and neap tides affect the abundances of quahog laNae; 3. The abundance of quahog laNae vary spatially due to variations of adult quahog density; 4. LaNai abundance varies between two sampling depths.

Study Area
Narragansett Bay is a major estuary in Rhode Island; and considered as the largest one in southern New England ( Figure 1). Its location is surrounded by four coastal areas: Long Island Sound, Block Island Sound, Rhode Island Sound, and Buzzard's Bay (Spaulding, 1987). This system "... is composed of three north-south interconnected passages: a) West Passage, b) East Passage and the Providence River, and c) the Sakonnet River... " (Hicks, 1959).
Narragansett Bay is 40 km in length (Gordon, 1982); the surface area, shoreline length, and mean water volume are about 328 km 2 , 412.5 km, and 2,724 km 3 , respectively (Chinman and Nixon, 1985;Spaulding, 1987). Mean water depth is 8.3 m, however, the depth of East Passage tends to be greater by a factor of two to West Passage and Sakonet River (Spaulding, 1987). There are three major rivers discharging into Narragansett Bay: a) Blackstone River, b) Pawtuxet River, and c) Taunton River and Seekonk River (Spaulding, 1987).
The first two rivers discharge into the Providence River, while the last one empties into Mount Hope Bay (Spaulding, 1987). The mean discharge of the Blackstone, Pawtuxet, and Taunton are 20, 11, and 19 m 3 /s, respectively (Spaulding, 1987). The mean residence time of Narragansett Bay water is about 26 days (Pilson, 1985).
Circulation in Narragansett Bay is due mainly to strong tidal currents (Levine and Kenyon, 1975), with gravitational induced by horizontal pressure gradients and wind-driven currents superimposed (Hess, 1976). This estuary is classified as a partially-mixed estuary, which means that the halocline dissipates and salinity increases gradually from surface to bottom (Pilson, 1985). Salinity decreased from the entrance to the head of the Bay; 31 ppt at the entrance, while at the head values in the range of 10-20 ppt (Hicks, 1959). A salinity difference of only 2 ppt from surface to bottom is common (Gordon and Spaulding, 1987).
Temperature increased from the entrance to the head with the difference of about 3°c from surface to bottom (Hicks, 1959).

Field sampling
This study was conducted at five sampling stations in Narragansett Bay Passage. Those five sampling stations were selected with regard to a declining trend of adult quahog densities from upper to lower estuary (Pratt, 1988;Kremer, 1975). The sampling was carried out every two days per week during summer of 1995, from May 26 to August 30. Due to strong winds and wave on particular days, samplings were postponed for safety reasons.
Water samples were collected haphazardly with respect to tides, using a 12 volt battery electric bilge pump, at two depths, 0.3 m and 1.6 m, at each station, similar to the method of Landers (1954). Prior to sampling collections, the electric bilge pump flow rate was calibrated by counting the amount of time required for pumping a 100 liter of water sample. The water samples were then filtered with a 60 /-lm mesh plankton net. Retained plankton samples were fixed with 95 % ethanol. Both surface water temperature and salinity in each station were measured. Tides and tidal current charts (Spaulding et aI., 1990) were used to provide an information on the height of tides and velocity of tidal currents at the sampling stations.

Laboratory Analyses
In the laboratory, retained plankton samples were transferred to 30 ml mixture of 25 % ethanol and 75 % seawater. Three replicates of 1.0 ml subsamples of the preserved plankton were pipeted into a Sedgewick Rafter counting chamber. These subsamples were observed and counted using a stereoscopic microscope. To assist the progress of the identification, larval dimensions such as total, height, and hinge length were measured by using an image analysis (Optimas 4.0 software). Shape and dimension of the larvae were compared with photomicrographs provided by Loosanoff et al. (1966) and Chanley and Andrews (1971). The quahog larvae were categorized into two developmental stages. The early stage or straight-hinge larvae were characterized by a length of the hinge at least half of the total length. The late stage or umbonate stage was characterized as having hinge less than half of the total length and umbo begins to form.

Enumeration of Larvae
Numbers of larvae per 100 liter water sample (N) were calculated: N=nxv (1) where n =average number of larvae per 1.0 ml subsample, and v =volume of mixture 25 % ethanol and 75 % seawater =30 ml.

Estimation of Mortality Rate
Rate of mortality of the quahog larvae was estimated by first calculating the rate of survival using the following method (Royce, 1984) : where S =survival rate, N 1+1 =number of larvae at late stage, N 1 =number of larvae at early stage. A cohort, herein, was defined as a group of larvae at an early stage developing into the late stage within two weeks. Thus, rate of mortality (M) was obtained from : Statistical Analyses To investigate the relationship between larval abundance (Y) and water temperature (X), a linear correlation analysis was used. where In addition, a qualitative evaluation of the water temperature required to trigger quahog spawning in Narragansett Bay was made by comparing the temporal variation in temperature with the temporal variation in larval abundances.
A Wilcoxon Rank-Sum Test was used to examine: (1) the abundances of quahog larvae during neap tides vs spring tides; (2)  Since the sample sizes were larger than 8, the null distribution of the ranksum statistic is approximately normal and the Wilcoxon test can therefore be performed using the normal table (Johnson and Bhattacharya, 1992). Under H o , the distribution of WA has mean = 1/2 {nA (nA +nB +1)}, and variance = {nA nB (nA +nB +1)} / 12 Z = (WA -mean) /~variance Therefore, the rejection region to be : R: To determine if the abundance of quahog larvae varies among stations due to variations of adult quahog, the Kruskal-Wallis method was used because there are more than two stations as independent variables. This test is based on the ranks of the observation.
2. H1 :~1 :j; ... :j;~5 3. All observations are ranked from 1 to n. Let Ri be the mean of the ranks for the ith factor level and R.. the overall mean rank.

Test statistic:
12 5. R : X 2 KW > X 2 (1 -a; r-1) Multiple regression analysis was used for the purpose of constructing a model for prediction in this study. The dependent variable to be assigned in this study is the abundance of quahog larvae (Y), and the independent variables are depths (X1), moon phase (X2) in respect to spring and neap tides, water temperature(x3), and stations (X4). A full model of multiple regression is : where Stepwise regression analysis was also performed, using SAS software, to examine a better linear regression model.

Physical Environment
Data of surface water temperature and salinity are presented in Table 1.
Water temperature measured during the summer of 1995 ranged from 16.0 to 26.5 0 C. During the early summer, the average water temperature was 19.7°C. The average percentage of quahog larvae relative to other bivalve larvae was 40 %. Concentration ranges of total, early stage, and late stage quahog larvae were 0 -19,740 larvae, 0 -19,680, and 0 -420 larvae per 100 I, respectively.  respectively. The larvae at Wickford, however, showed different pattern. The maximum abundance (3,775 larvae per 100 I) occurred 11 days earlier (June 9) than that at other stations, where water temperature was only 19.5°c when the peak occurred. Abundances, afterward, were less than 325 larvae per 100 I.
Maximum numbers of late stage veligers were observed on July 7 (188th Julian day) with abundances 165, 60, 230, and 65 larvae per 100 I in respect to Conimicut Pt., Rocky Pt., Warwick Pt., and Mt. View. At Wickford, the peak of 40 larvae per 100 I on July 7 was not considered to be the maximum concentration since it was only at the factor of 1.5 lower than the peak on June 9 -160th Julian day -(65 larvae per 100 I).
In respect to lunar phase, the peak abundances of quahog larvae generally occurred 6 to 12 days after new moon or full moon, and coincided with. neap tides (Figure 12). During the intervening periods, the quahog larvae showed lower abundances. The evidence of major peak abundance of June 20 fell within a period of neap tide.

Mortality
As it is assumed that larval duration is about 2 to 3 weeks, the rate of survival of quahog larvae was estimated by calculating the ratio of late and early stage larvae with a time lag of 11 -17 days. The rate of mortality was obtained by subtracting the survival rate from 1. Table 4 shows the rate of mortality of quahog larvae in Narragansett Bay which ranged from 80 to 99 %. Detailed calculation for mortality rate is presented on Appendix A.

Statistical Analyses
This study evaluated four hypotheses. Linear correlation analysis was performed to detect an association between water temperature and abundances of quahog larvae. The correlation coefficients (r) were relatively small, ranging between -0.28 and -0.04 (Figure 13 -17). These small numbers indicated that the relationship between water temperature and larval abundance is weak or there is almost no distinct relationship between these two variables, thus, the linear regression model does not seem to give a good fit to the data (Johnson and Bhattacharya, 1992).
A non parametric procedure utilized for comparing larval abundances in neap and spring tides demonstrated a significant difference (Table 5). This study, therefore, supports previous studies (Korringa, 1947;Carriker, 1961) which reported that the abundances of bivalve larvae were different during neap and spring tides. Another test proved that there was a significant difference among the abundances of quahog larvae in five sampling stations (Table 6).
There is enough evidence to claim that the larval abundances were different among five sampling stations. However, this procedure failed to reject the nullhypothesis for testing the effects of depths on the larval abundances (Table 7).
The present study failed to support the result of the Rice and Goncalo study (1994) that early stage larvae were found in higher number at upper water column (0.3 meter).
Coefficient of determination (r 2 ) in multiple regression models determines how much variation of dependent variable can be accounted for by the model. Table 8 presents the result of multiple regression analysis between larval abundance and independent variables, i.e., water temperature, moon phase, stations, and depths. This full model, which included all variables, provided a very low r 2 (0.20). Thus, only 20 % of the variation in the abundances of quahog larvae in Narragansett Bay can be explained by all of those independent variables.
A partial analysis of multiple regression (stepwise method) showed that the simpler model, which included only moon phase, had small r 2 of 0.09 (Table 9).

Distribution-of Quahog Larvae
The temporal distribution of quahog larvae may describe the events of the spawning pattern. This study found that quahog larvae at early stage along a North -South transect occurred weekly from May to August. The larvae reached peak abundances on June 20 at all sampling stations, except Wickford. These results are consistent with the results of Landers (1954) and Rice and Goncalo (1995) who reported that peak abundances of bivalve larvae in Wickford and Greenwich Bay occurred on June 11 and on June 14, respectively. The patterns of summer larval abundances indicated that Narragansett Bay quahogs commenced spawning throughout summer months once water temperature rises, and released gametes at least once a week. Intense spawnings, which might relate to major peak abundances of the swarming larvae, could be triggered by increase water temperature to about 20°C, in mid June.
The reproductive cycle in Narragansett Bay quahogs has been addressed by Diamond (1981). She discovered that two cycles of reproductive activity in quahog population occurred in summer and fall, however, rapid maturation of the gonads which subjects to intense spawning was between April and June.
Individuals of this population appear to spawn partially. According to Loosanoff (1937a), an individual quahog does not discharge all of its eggs or sperm at one time, but it continues at intervals of a few days or perhaps weeks to complete the spawning. The male usually spawns first, then stimUlates other males and later the females also to spawn (Carriker, 1961). Increased water temperature, coupled with phytoplankton blooming, is considered to be of primary importance in controlling the spawning (Loosanoff, 1937a(Loosanoff, , 1937bLoosanoff and Davis, 1951;Carriker, 1961;Nelson, 1987).
Diffe-rences of the temporal pattern of spawning at different sites in a certain estuary may be due to the ve..riations of water temperature. Landers (1954) reported that quahogs inhabiting a shallow water spawned earlier than those in a deeper one. Warming of the exposed bottom in the shallower water triggers release of gametes. This hypothesis may apply in the present study in According to Loosanoff and Davis (1951), at the age of approximately 12 days or at pediveliger stage, the quahog larvae become competent and ready to settle.
We, therefore, can deduce that quahog larvae in Narragansett Bay are ready to settle at least 2 -3 weeks after fertilization. An intense settlement can occur in the middle of summer as the late stage larvae reached the major peak mostly in July. It appears that once intense settlement occurs, the rate of settlement then gradually declines.
Spawning of bivalves may also coincide with lunar cycles. Intense spawnings of oysters and quahogs and peak abundances of the larvae in Little Egg Harbor occurred concurrently with neap tides (Loosanoff and Nomejko, 1951;Carriker, 1961). During neap tides as tidal amplitude is low, exchange with cooler ocean water results in warmer bay water temperature, thus, inducing intense spawning. On the other hand, high tidal amplitude during spring tides give rise to high tidal exchange which probably accounts for the loss of the larvae. The result of the present study appeared to be consistent with those previous studies that more larvae were observed in the water column during neap tides than during spring tides. Peak abundances of early and late stage larvae occurred coincidentally in neap tides, therein, tidal amplitudes were between 0.55 and 2.2 feet lower than those in spring tides (Chinman and Nixon, 1985;Spaulding et aL, 1990).
The second aspect of larval distribution to be discussed is spatial distribution which tends to be patchy and shows a tendency of decreasing number from upper to lower part of Narragansett Bay. In previous studies on plankton dynamics, Smayda (1988) and Durbin and Durbin (1988) (Carriker, 1951).
According to Okubo (1980Okubo ( , 1994 and Scheltema (1984), the dispersion of the larvae is affected by not only tidal and non-tidal currents but also by turbulent diffusion. Tidal and non-tidal currents, on one hand, are capable of advecting the larvae horizontally from the parent beds. During transport, some of the larvae are trapped in the boundaries (Andrews, 1983;Scheltema, 1984;Okubo, 1994). At the same time, turbulent flow causes the larvae to disperse with respect to one another, giving rise to a distribution in which there is a concentration gradient decreasing from the center of a patch of the larvae outward (Okubo, 1980;Scheltema, 1984). The implication is that fewer numbers of larvae are apparent at increasing distance from the spawner stock beds. Stoner et al. (1996) noted that more queen conch larvae were concentrated in the area near the center of the source of the larvae.
By assuming that those concepts of centroid-like dispersion can be applied to the present study on the distribution of bivalve larvae, the closer the location of the swarming larvae to the spawner stock area, the more the larvae are concentrated in that location. In contrast, the longer the radius of the location of larvae from the parent bed, the less the larvae are found in that  (Sparsis et aI., 1993).
Mytilus edulis is also one of the bivalves distributing more in the lower part of the Bay; and the area near Prudence Island is the northernmost region of the occurrence -of this species (Nelson, 1984). Therefore, the distribution of bivalve larvae along a North -South transect in Narragansett Bay appears to coincide with adult distribution.
In addition to that, tidal-induced current dominating water circulation is a potential means to displace the larvae from the parent beds, and may contribute to the patchiness of larval dispersion along a longitudinal axis of the estuarine system. Tidal excursion of approximately 1 -4.4 km in the Bay (Turner, 1984;Spaulding et al. 1990;Turner et aI., 1991Turner et aI., a, 1991  The results of those hydrodynamic studies seem to imply that larvae from Providence River are mostly retained in Upper Bay, and only small fraction of the larvae can be transported into West Passage. In conclusion, it is not surprising that samples of quahog larvae collected during summer 1995 were patchy and more concentrated in upper portions of the Bay. The effects of filter feeding of adult quahogs may also give a considerable contribution to the patchiness of larval distribution. As a filter feeder, quahogs are able to filter their own larvae (Rice and Goncalo, 1995). As a consequence, densities of the larvae were low in the location in which close to the area of dense adult quahogs (Rice and Goncalo, 1995), and might lead to a relatively high mortality. However, the present study seems not to support their evidence.
The possible explanation is that the former study dealt with the area having a shallow depth, while the latter dealt with the area having a deeper depth. The differences of the topography may partly contribute to describe the patchiness of larval distribution due to the effect of benthic predation. Larvae close to or on the bottom are subject to greater predation by filter feeders than those staying in the upper layer (Carriker, 1961). Thus, a study conducted in an area with shallow depths may better represent the real condition of biological interaction on the bottom habitat than the one conducted in the area with deeper depth in which samples collected from upper layer. The depth of the study area within which Rice and Goncalo (1995) conducted sample collections is about 2.0 m. In their study, samples were collected from 0.3 and 1.6 m which are relatively close to the bottom and might well explain the condition of the relation between adult assemblages and larval supply. The average depth of the present study area is approximately 5.6 m (Chinman and Nixon, 1985), and samples were taken from 0.3 and 1.6 m which are considered to be a surface layer.
MortalitÕ n the basis of the ratio of earl~and late stage larvae from the same cohort -assumed that the planktonic life is about 2 to 3 weeks-, the mortality of quahog larvae would be between 80 and 99 % (Table 4). This finding seems to be reasonable. Carriker (1961) found only about 2 % of quahog larvae in Little Egg Harbor recruited into the settlement phase and metamorphosis, thus becoming juvenile clams. In Greenwich Bay, Rice and Goncalo (1995) calculated that 95 % of bivalve larvae were lost due to natural mortality.
High mortality at early life stages is common since the animal at this stage is prone to environmental changes and predation. However, quahog larvae are tolerant to a wide range of physical conditions (Carriker, 1961). The quahog larvae may survive in the salinity ranging from 15 to 35 ppt, and in the water temperature between 10 and 30°c (Carriker, 1961;Davis, 1969). Thus, in a favorable environment which physical condition meets the requirements for survival, the source of quahog larval mortality seems to be due to the predation.
The larvae of brachyuran crabs are a potential predator of bivalve larvae (Sastry, 1983;McConaugha, 1985). Pagurus longicarpus prey upon the oyster veligers (McConaugha, 1985). Laboratory observation showed that Neopanope texana is a ferocious predator of young hard clam, however, the predation is considered to be size-dependent (Landers, 1954). In Narragansett Bay, larvae of Neopanope texana, Neopanope sayi, and Cancer spp. were abundant in summer months (Hillman, 1964;Trifan, 1987). During the plankton sampling of summer 1995, there were significant amounts of unidentified crab larvae in the Bay. Therefore, it is possible that high mortality of quahog larvae during the COurse of sample collection was partially due to the predation by crab larvae. §..tatistical Analyses Lack of a linear relation between water temperature and larval abundance may be better explained by looking at the nature of the diagram of those variables. A qualitative evaluation shows that the densities of the larvae were low at water temperatures of less than 20°c. Intense spawning occurred as Water temperature rose to 20°c and was characterized by major peaks of larval abundances. However, the densities then declined following the peak abundances as water temperature increase to above 20°c. It appears that a water temperature of 20°c is a threshold to stimulate the spawning, thus, levels of Water temperature beyond the threshold give rise to less intense spawning and low larval densities.
Multiple regression analysis showed that only 20 % of the variability of larval abundances can be attributed to the linear model. A large portion of the variability is still left unexplained. Therefore, result of linear correlation and mUltiple regression analyses suggest that linear model does not seem to be relevant for evaluating the temporal and spatial variations of quahog larvae due to the given physical factors.