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Intraspecific life history variation in the southern oyster drill, Stramonita haemastoma: patterns and causes.

ABSTRACT Along the Louisiana coast of the Gulf of Mexico, the southern oyster drill, Stramonita haemastoma (Gray), reached peak densities at an exposed, coastal location, with lower but more constant densities in an estuarine oyster reef. Average oyster drill size in field samples was largest at the estuarine, intertidal reef and smallest at the exposed location. Caged oyster drills grew more rapidly and were more fecund at the subtidal, estuarine oyster reef than at the exposed location, resulting in higher biomass and production estimates. Life histories thus changed from slower growth, and delayed and semelparous reproduction at the exposed location to rapid growth, and earlier and iteroparous reproduction at the subtidal, estuarine oyster reel When oysters were added to cages at the exposed location (where they are rare), oyster drill growth and reproductive rates increased dramatically. Trapping revealed no obvious differences in the abundances of crab predators between the estuarine and exposed site, nor were there any differences in gastropod shell thickness (e.g., vulnerability to predators). We conclude that the availability of profitable prey like oysters plays a greater role in explaining oyster drill life history variation than predation risk, and that wave exposure has only indirect effects, by limiting the availability of oysters or forcing oyster drills to feed on smaller prey.

KEY WORDS: environmental causes, life history, oyster drill, population ecology, Stramonita haemastoma

INTRODUCTION

The southern oyster drill, Stramonita (= Thais, Kool 1987) haemastoma Gray, an important predator of oysters (Brown & Richardson 1987), is common throughout the Gulf of Mexico (Butler 1985). Along with the black drum (Pogonias cromis), it restricts the distributions of oysters in the Gulf of Mexico to es tuarine areas where salinities are too low for most predators (Chatry et al. 1983, Wilber 1992), or to the high intertidal at coastal locations (Bahr & Lanier 1981, Brown 1997. Brown & Stickle 2002, Roegner & Mann 1995). Recent research on oyster drills has concentrated on tolerance and capacity adaptations to salinity, temperature, and hypoxia (Brown & Stickle 2002, Stickle 1985, Stickle 1999), population structure (Liu et al. 1991), and foraging behavior (Brown 1997, Brown & Alexander 1994, Brown & Richardson 1987, Richardson & Brown 1990, Richardson & Brown 1992). Relatively little is known, however, about intraspecific variation in oyster drill life history characters such as growth or fecundity.

This study documents variation in density, growth, and reproduction between oyster drill populations in coastal and estuarine locations, and at a smaller scale, between an intertidal and a nearby subtidal oyster reef at the estuarine location. Contrasting coastal and estuarine sites is important because the lowered levels of wave activity (Richardson & Brown 1990) and salinity (Stickle 1985) at estuarine sites alter oyster drill feeding rates. Comparison of intertidal and subtidal sites is important because aerial exposure decreases oyster drill feeding rates (Brown & Stickle 2002). Specifically, we quantitatively sampled and measured oyster drills for a year at three locations (one coastal and two estuarine) and tagged snails at each location and caged them to follow growth in two seasuns and reproductive activity during the egg-laying season in 1 yr. To integrate these life history data, we calculated rates of secondary production at each site, which combines in a single measure life history parameters like density, biomass, individual growth, development time, recruitment, and mortality (Benke 1993, Taylor 1998).

We also discuss what environmental factors may cause the variation, and provide information on two possible causes, food availability and predation risk. Prey abundance and quality have been shown to affect the ecology of thaidid snails on other coast lines (Palmer 1983, Spight 1982). The degree of predation risk can reduce feeding rates (Richardson & Brown 1992), lower growth rates, and result in increased shell thickness as a defensive mechanism (Palmer 1990). To evaluate the relative importance of prey availability and predation risk for Stramonita haemastoma, we (1) added oyster prey at a site where only barnacle prey were available, and (2) assessed predation risk by estimating the abundance of the stone crab (Menippe adina) and looked for differences among oyster drift populations in shell thickness.

MATERIALS AND METHODS

Site Descriptions

The three locations [for a map of locations, see Brown and Swearingen (1998)] are approximately 140 km south of New Orleans (29[degrees]10'N, 90[degrees]05'W). Caminada Pass (the exposed site, = CP) is the narrow mouth of Barataria and Caminada Bays, an extensive estuary system southeast of New Orleans. Strong currents occur during tidal flows, and CP is fairly exposed to wave action from the Gulf. Using plaster casts, Richardson and Brown (1990) found that over three-quarters of the mass was lost over a 2-day interval at this site (Table 1), indicating much greater levels of exposure and flow than at the estuarine site, where only one-fifth of the mass was lost on average. Average temperatures vary from 12[degrees]C during winter months to 30[degrees]C during summer months (Fig. 1), and salinities average 26 psu (practical salinity scale, Table 1). By far the most common prey at this site are barnacles (Balanus eberneus) (Table 1).

[FIGURE 1 OMITTED]

The other two locations, near the Louisiana Universities' Marine Consortium laboratory at Port Fourchon, Louisiana, about 20 km west of Caminada Pass and 10 km inland from the mouth of Bayou Fourchon, are more estuarine (Table 1). The first location is a an intertidal oyster reef (= Laboratory Int) in a Spartina alterniflora marsh, and the second site is a subtidal reef (= Laboratory Sub) 50 m from the intertidal reef. Both sites are heavily colonized by oysters, with densities over a 5-yr period (pooled over both tidal heights) 50 times greater than at the exposed site (Table 1). Barnacle density, in contrast, is greater at the exposed site. The data in Table 1 were collected over a 5 yr period using quadrat sampling for oysters and colonization plates for barnacles (for more details, see papers given in Table 1).

Sampling

We quantitatively sampled all three locations (Table 2) on a monthly basis from August 1998 to October 2000 by haphazardly placing out 10 16 [m.sup.2] quadrats during low tide. Each quadrat was first searched visually to remove larger oyster drills, and oyster shell was then rinsed over a sieve to locate smaller drills (although we undoubtedly missed small, recently recruited individuals). All snails were measured to the nearest 0.1 mm. A two-way analysis of variance (ANOVA) was used to contrast densities and shell sizes among locations and through time. Locations were considered a fixed effect because they were selected to contrast different levels of exposure or tidal height, and dates were considered fixed effects as they were selected so that each month was represented to allow calculation of monthly production estimates. Density data were log-transformed to improve normality.

To assess differences in predation risk among sites we (1) trapped crabs at Caminada Pass and the subtidal, estuarine reef, and (2) assessed differences in shell lip thickness of oyster drills at both sites. We fished six, bailed crab traps at each site for two nights, on three dates (summer through fall 2001) and pooled all crabs collected per date at each location. Crab catch per unit effort (CPUE) data were log-transformed and subjected to a two-way ANOVA (two crab species x two locations with the three dates as replicates), and data were also analyzed in a nonparametric analog of the two-way ANOVA, the S. R. H. modification of the Kruskal-Wallis test (Sokal and Rohlf 1995).

Secondary Production

To estimate biomass from shell length, 100 snails from Caminada Pass and the subtidal, estuarine oyster reef were measured to the nearest mm, and tissue was dried at 60[degrees]C for 24 h. Dry tissue mass was estimated using the following equation ([R.sup.2] = 0.95, P < 0.0001):

g dry mass = (1.8 x [l0.sup.-6])[(shell length).sup.3.46].

Monthly, size-specific production (g dry tissue mass/[m.sup.2]) at each site was estimated using the instantaneous growth method (Benke 1996, Waters 1977). The size-specific instantaneous growth rate was estimated from the growth of similar-sized snails in experiments, and monthly, size-specific biomass estimates (g dry tissue mass/[m.sup.2]) from field samples. For each time interval, growth was averaged for all individuals within each millimeter size class and applied to size-specific, mean monthly biomass estimates. Production was calculated for each millimeter size class, then summed to estimate monthly production.

Growth Experiments

Two experiments were conducted from December 1998 to April 1999 and from June 1999 to August 1999 (Table 2) to assess seasonal differences in growth rates. Snails from Caminada Pass and the subtidal, estuarine oyster reef (but not the intertidal reef because of low snail abundance, see "Results") were mixed to produce a size range, and 10 individuals in each of four different size categories (averaging 30, 40, 50, and 60 mm mean shell length [+ or -] 1 into SE) were placed separately in cages at all three locations. Cages were plastic trays (60 cm x 75 cm x 15 cm high) covered with 3-mm vexar mesh, and placed on the bottom and held fast by cable ties to PVC poles embedded 1 m in the sediment at all four corners. There were four replicate cages for each size class at each location. Snails were tagged with numbered bee tags attached with super glue and were measured (tip of spire to tip of aperture) to the nearest 0.1 mm with a Vernier caliper and weighed to the nearest milligram with an analytical balance at the start and monthly through the end of the experiment.

The most common prey at Caminada Pass were barnacles that rapidly recruit to subtidal surfaces (see discussion above), and we simply allowed prey to colonize cages. At the two estuarine locations, we added 20 oysters (from 50 to 150 g wet total mass per oyster) to each cage to approximate the abundance and size range of oysters available at the locations (Brown 1997). As cages were checked monthly, oysters were added to replace those consumed (at the estuarine locations), and the mesh was cleaned to facilitate water circulation.

Gastropod growth rates are dependent on size, and we used Ford-Walford plots (Fig. 2) to illustrate how shell growth increments were related to initial shell length (Etter 1989). Plotted data are the total shell growth increment over the experiment versus initial size for each snail surviving the experiment. However, because 10 snails were placed in each cage, using individual snails as replicates in statistical analyses could be considered pseudoreplication. We therefore also used the average shell length per cage as the experimental unit and performed a repeated measures ANOVA to test for time effects (e.g., change in size since oyster drills were measured monthly), as well as the location effect and the location times time interaction (Proc GLM, Statistical Package (SAS) Inc., 1988).

[FIGURE 2 OMITTED]

Reproduction Experiments

These experiments (Table 2) were conducted in April 1999 through May 1999, during the peak of the oyster drill egg-laying season, at Caminada Pass and the subtidal, estuarine oyster reef (experiments were not replicated at the intertidal site because of low ambient snail densities). The same four size classes, with three replicate cages for each size class, each with 10 oyster drills, were deployed at each location in the same fashion as in the growth experiments. Cages were checked monthly, and all egg capsules removed. A randomly selected subset of five capsules from each cage was preserved in 70% ethanol and counted at 7x magnification to estimate embryo number. Southern oyster drill eggs hatch in approximately 2 wk (Roller & Stickle 1988), but capsules continue to adhere to surfaces, so monthly counts were not underestimates. Preliminary data indicated counting the embryos in five egg cases gave an accurate estimate of the average number of embryos per capsule. We used two-way ANOVAs to test for significant differences among locations and oyster drill shell lengths in number of capsules produced and average number of embryos per capsule. Data were checked for normality with procedure. Univarlate (SAS Institute 1988) but transformation was not necessary. Tukey's a posteriori tests were used to determine which means differed.

Prey Addition Experiment

To determine if reduced growth and fecundity at Caminada Pass were explained by the absence of high-quality prey, we added 20 oysters (wet total mass from 50 to 150 g per oyster) to cages and compared growth and fecundity to control animals in cages with "ambient" prey. On March 28, 2000, six control and six experimental cages were placed at Caminada Pass at the same site and in the same fashion as in earlier experiments. Each cage had 10 marked oyster drills (mean shell length [+ or -] SE, control = 44.8 [+ or -] 1.2, experimental = 45.9 [+ or -] 1.2) and was retrieved after 2 mo. Two control cages were damaged, but increments in average shell length and total number of capsules produced were contrasted between the tour remaining control and six experimental cages in a one-way ANOVA.

RESULTS

Density, Size, attd Secondary Production

Oyster drill densities varied significantly among the exposed and two estuarine locations (F = 130.0, P < 0.0001), months (F = 22.3, P < 0.0001), and a significant interaction occurred bclwcen location and month (F = 15.7, P < 0.0001). Overall, densities were highest at Caminada Pass, intermediate at the subtidal, estuarine oyster reef, and lowest at the intertidal oyster reef (Table 3). However, densities fluctuated considerably through time (Fig. 1), explaining the significant interactions between location and month.

There were also significant differences among the three locations (F = 365.3, P < 0.0001) and through time (F = 160.1, P < 0.0001) for average shell length as well as a significant interaction (F = 127.2, P < 0.0001). Tukey's a posteriori tests indicated oyster drills were largest at the intertidal estuarine oyster reef (Fig. 3, Table 3), intermediate in size at the subtidal, estuarine oyster reef, and smallest at Caminada Pass.

[FIGURE 3 OMITTED]

Unlike density, average biomass was higher at the subtidal estuarine oyster reef than at Caminada Pass (Table 3), reflecting the greater number of large snails at the estuarine, subtidal location. Mean monthly biomass at the intertidal reef location was much lower. Peak biomass among all locations was observed in April 1999 at the subtidal oyster reef, when biomass exceeded 20 g/[m.sup.2]. The higher bimnass and markedly greater individual growth rates at the subtidal, estuarine oyster reef (see next section) resulted in a 10-fold higher average monthly production in comparison to Caminada Pass (Table 3).

Predation Risk

Crab CPUE values differed among the two crab species, with blue crabs much more abundant than stone crabs (Table 4). The species effect was significant (F = 20.0, P < 0.001), unlike the location effect (F = 0.01, P = 0.98) and interaction (F = 2.5, P = 0.12). The nonparametric test also indicated a significant species effect (H = 6.9, P < 0.05), and insignificant location (H = 0.2, P > 0.05) and interaction (H = 3.7, P > 0.05) effects. Blue crabs had somewhat larger carapace widths than stone crabs (Table 4), but stone crabs with a carapace width of 9 cm are readily capable of feeding on oyster drills (Brown & Haight 1992). There were no significant differences in oyster drill shell lip thickness among locations (Table 4, F = 0.5, P = 0.62), although total shell length had a highly significant effect on shell lip thickness (F = 18.3, P < 0.001). The lack of a significant interaction (F = 0.4, P = 0.7) indicates no difference in slopes of lines relating lip thickness to shell length among locations, and the slightly greater lip thickness of oyster drills at the estuarine, subtidal oyster reef there fore appears primarily due to their greater average shell length (Table 4).

Growth Experiments

There were significant differences in growth among the three locations in both experiments, with higher growth rates at the subtidal, estuarine oyster reef. Although the repeated measures ANOVA of the winter 1998-1999 data detected no time (e.g., month) effect (Wilk's [lambda] = 0.53, F = 1.1, P = 0.45), there was a significant interaction between time and location (Wilk's [lambda] = 0.062, F = 3.8, P < 0.05), and a significant difference among locations (F = 37.9, P < 0.001). The covariate, initial size, also had a significant effect on growth (F = 53.1, P < 0.001). Portraying the results of this winter growth experiment as Ford-Walford plots (Fig. 2A) clearly illustrates the greater growth rates of smaller oyster drills at the subtidal oyster reef, as well as the significant effect of the covariate. The same basic pattern occurred in the summer 1999 experiment, although growth rates overall were somewhat lower (Fig. 2B). The repeated measures ANOVA again detected no time effect (Wilk's [lambda] = 0.79, F = 0.57, P = 0.65), a significant interaction between time and location (Wilk's [lambda] = 0.066, F = 5.8, P < 0.01), and a significant difference among locations (F = 33.9, P < 0.001). The covariate, initial size, again had a significant effect on growth as well (F = 40.3, P < 0.001).

Reproduction Experiment

There were clear differences between the two locations (with values again greater at the subtidal, estuarine oyster reef than at the exposed location) and oyster drill size classes in number of egg capsules deposited per cage (Fig. 4A). Both the location (F = 29.7, P < 0.0001), shell length (F = 5.2, P < 0.01), and location times shell length interaction terms (F = 5.2, P = 0.01) were significant in the ANOVA. Comparing Tukey's a posteriori tests among size classes within locations, there were no significant differences among size classes at Caminada Pass. However, at the subtidal, estuarine oyster reef, the largest snails (60 mm) over lapped with the next largest (50 mm) but not the two smaller size categories of snails, whereas the snails averaging 50 mm overlapped with the 40-mm snails but not the 30-mm size category, and the 40-mm- and 30-mm-sized snails overlapped as well. Comparing locations within size classes, oyster drills laid more egg capsules at the subtidal, estuarine oyster reef than at Caminada Pass in both of the larger size classes, but not in the smaller size classes.

[FIGURE 4 OMITTED]

Average number of embryos per capsule also varied significantly among locations (again greater at the subtidal, estuarine oyster reef, F = 5.6, P = 0.03) and size classes (F = 31.1, P < 0.0001), without any interaction (Fig. 4B). Oyster drills at the subtidal, estuarine oyster reef produced on the average 2,021 embryos per capsule versus 1,598 at Caminada Pass. A posteriori tests indicated the largest oyster drills produced significantly more embryos per capsule than the other three size classes, while the two intermediate classes overlapped, as did the 30- and 40-ram shell-length oyster drills. These values compare with average embryo numbers per capsule of about 3,200 for snails greater than 40-ram shell length held under laboratory conditions (Roller & Stickle 1988). Multiplying average numbers of egg cases times average embryo counts, and assuming five of the snails were females, per capita fecundity at Caminada Pass was approximately 35,000 versus 1,020,000 at the estuarine oyster reef.

Prey Addition Experiment

Addition of oyster prey to cages at the exposed site (= CP) resulted in both increased growth in oyster drill shell length (F = 7.4, P = 0.02) and higher production of egg capsules (F = 11.5, P = 0.01) in comparison to control cages. Growth in shell length was greater in experimental treatments by a factor of eight, and production of egg capsules increased by a factor of four (Fig, 5).

[FIGURE 5 OMITTED]

DISCUSSION

In general, intraspecific life history variation in intertidal gastropods is usually explained as the result of three factors acting singly or in combination: (1) differences in prey quantity or quality (Palmer 1983, Palmer 1984, Spight 1982), (2) differences in wave action (Brown & Quinn 1988, Denny et al. 1985, Richardson & Brown 1990), or (3) variation in predation risk (Palmer 1990, Richardson & Brown 1992). In Stramonita haemastoma, a combination of these factors probably explains variation among locations in life history patterns. The dominant prey at Caminada Pass, the barnacle Balanus eburneus, may not be as conducive to producing growth as oysters, the most common prey at the estuarine locations, explaining the increased growth and fecundity of the oyster drills in cages with added oysters at the exposed site. Although there is yearly variation in abundance, especially for barnacles, oysters were consistently more abundant at the estuarine sites over time in this study, and this same trend occurs at other locations along the Gulf of Mexico coast, and the lower Atlantic coast of the United States as well (Bahr & Lanier 1981, Brown & Swearingen 1998, Chatry et al. 1983, Roegner & Mann 1995, Wilber 1992). However, the increased tidal flows and wave action at the exposed site could also reduce growth (Richardson & Brown 1990), and the effects of food value and wave action are thus likely confounded. Increased wave action can also directly reduce average sizes in exposed populations by dislodging the largest individuals (Denny et al. 1985).

Although we worked with only one location within each exposure or tidal height category, we consider our results to have some generality because these trends to a large extent agree with other studies on this and other intertidal snail predators. Brown and Richardson (1987) recorded higher densities and smaller-sized individuals of Stramonita haemastoma at exposed locations in comparison to protected locations, and others have also found considerable variation in intertidal gastropod densities and size distributions among locations and seasons (Carroll & Highsmith 1996, Navarette 1996, Petraitis 1998). The greater growth at the subtidal, estuarine oyster reef than at the exposed location is similar to earlier studies of Nucella lapillux (Etter 1988, Etter 1989, Etter 1996) and Nucella emarginata (Brown & Quinn 1988). However, Etter (1989) also found increased fecundity at exposed locations. Differences in growth rates in N. lapillus are the result of ecophenotypic variation, not genetic differences among populations (Etter 1988, Etter 1996). Stramonita hemastoma has a relatively long pelagic larval stage, lasting 90 days, and there is considerable gene flow among populations in the northern Gulf of Mexico (Liu et al. 1991), arguing against high levels of genetic variation among these populations, which arc only separated by tens of kilometers. The prey manipulation experiment also suggests that oyster drill growth and reproductive rates are plastic and will respond to increased food availability.

Oyster drills in the northern Gulf of Mexico reach 30 mm in shell length and usually reproduce in their first year (Butler 1985). However, the slow growth and small sizes attained by the oyster drills at Caminada Pass (average oyster drill size was only 35 mm) may mean they do not reproduce until their second spring, and senesce shortly afterwards. This could explain the peak recruit mental Caminada Pass in only 1 yr of the study, as a cohort matured, reproduced, and senesced. In contrast, at the subtidal protected oyster reef, rapid growth may have resulted in reproduction in the first year, and individuals probably survive and reproduce for several years (average size was around 50 mm, and iteroparity may also explain the bimodal size distribution at this site shown in Fig. 3). Such iteroparous reproduction has also been reported before in southern oyster drills (Butler 1985).

Our results indicate that wave exposure along the relatively protected Louisiana Gulf of Mexico coastline does not have as extreme an effect on life histories as it does in thaidid snails along other coastlines. Wave exposure may have a more indirect effect, by altering the abundances of prey organisms (oysters again recruit more readily at estuarine locations) or forcing oyster drills to consume smaller, suboptimal prey (Richardson & Brown 1990). Nor does predation risk appear as important as for thaidids along other coastlines, as we could detect no differences in the abundances of stone crabs among locations: in fact stone crabs were relatively rare in comparison to blue crabs, which are effective predators only on small, recently settled oyster drills (Butler 1985). If anything, predators like stone crabs or black drum are probably more abundant in the northern Gulf of Mexico at higher salinity coastal sites (Brown et al. 2003), and the increased predation risk seen at protected sites for thaidids on other coastlines either does not occur in this system or may even be reversed, with higher predation rates at coastal sites. The lack of any difference in shell lip thickness among locations also provides indirect support for the lack of any differential in predation risk among sites.

We conclude that prey value and availability appear most important in explaining intraspecific life history variation in southern oyster drills, as both growth and reproductive rates responded with four to eight-fold increases when oyster prey were provided to snails held at exposed sites. This result is certainly consistent with other studies suggesting that prey value and availability may determine thaidid growth tales (Palmer 1983) and recruitment (Spight 1975, Spight 1982).
TABLE 1.
Comparison of two locations along the Louisiana coast as to salinity
(psu, range), degree of wave exposure (percent of mass loss from
plaster casts), and abundances and sizes of prey items of oyster
drills.

 Location Salinity Exposure Prey

Fourchon Lab 22 (16-28) 21% Numbers/[M.sup.2]
 Size
Caminada Pass 26 (18-34) 79% Numbers/[M.sup.2]
 Size

 Location Prey Oysters

Fourchon Lab Numbers/[M.sup.2] 222 [+ or -] 9.5
 Size 79.2 [+ or -] 56.7 g
Caminada Pass Numbers/[M.sup.2] 4.0 [+ or -] 0.3
 Size 10.6 [+ or -] 3.1 g

 Location Prey Barnacles

Fourchon Lab Numbers/[M.sup.2] 13,885 [+ or -] 1,273
 Size 11.0 [+ or -] 3.0 [mm.sup.2]
Caminada Pass Numbers/[M.sup.2] 65,215 [+ or -] 8,310
 Size 9.0 [+ or -] 1.2 [mm.sup.2]

Standard errors are given for prey abundances and sizes. Oyster sizes
refer to total wet mass (including shell). Barnacle sizes are basal
areas. Data on prey abundance taken from Brown (1997), Brown and
Swearingen (1998), McCoy and Brown (1998), and Banks and Brown (2002).

TABLE 2.
Summary of what sampling and experiments were conducted at each
of the three locations, along with the types and periodicities
of data collected.

 Field Growth
 Location Sampling Experiments

Lab Int. X X
Lab Sub. X X
Caminada Pass (=CP) X X
Data Density, size Individual growth
 distribution, (mm)
 production (g dry
 mass/[M.sup.2])
Periodicity and length Monthly for 1 yr Monthly for 3 mo

 Reproduction Prey Addition
 Location Experiment Experiment

Lab Int.
Lab Sub. X
Caminada Pass (=CP) X X
Data Numbers of egg Individual shell
 capsules, numbers growth (turn) and
 of embryos per capsules per cage
 capsule
Periodicity and length Monthly for 2 mo Monthly for 2 mo

TABLE 3.
Average oyster drill density per [M.sup.2], shell length (mm),
biomass (g per [M.sup.2]), and production (g per [M.sup.2]),
[+ or -] SE (n = 12 mo) at three locations along the Louisiana
Gulf of Mexico coast.

 Location Density Size

Lab Sub. 6.25 [+ or -] 2.52 49.6 [+ or -] 1.0
Lab Int. 0.04 [+ or -] 0.02 77.2 [+ or -] 2.7
Caminada Pass 11.40 [+ or -] 4.10 34.6 [+ or -] 2.0

 Location Biomass Production

Lab Sub. 8.01 [+ or -] 1.36 1.46 [+ or -] 0.20
Lab Int. 0.74 [+ or -] 0.49 0.02 [+ or -] 0.01
Caminada Pass 4.04 [+ or -] 0.85 0.14 [+ or -] 0.03

TABLE 4.
Crab catch per unit effort for two species of crabs, crab carapace
widths (in cm), and oyster drill shell lip thickness and total shell
length (both in mm), at two locations along the Louisiana Gulf of
Mexico coast.

 Location Trait Blue Crab

Fourchon Lab CPUE 60.7 [+ or -] 46.6
 Carapace width 12.8 [+ or -] 0.3 (182)
Caminada Pass CPUE 45.7 [+ or -] 15.5
 Carapace width 13.8 [+ or -] 0.1 (137)

 Location Trait Stone Crab

Fourchon Lab CPUE 2.3 [+ or -] 1.5
 Carapace width 9.4 [+ or -] 0.2 (7)
Caminada Pass CPUE 1.0 [+ or -] 0.2
 Carapace width 8.5 [+ or -] 0.1 (3)

 Drill Shell
 Location Trait Thickness

Fourchon Lab CPUE 0.75 [+ or -] 0.04
 Carapace width
Caminada Pass CPUE 0.57 [+ or -] 0.03
 Carapace width

 Drill Shell
 Location Trait Length

Fourchon Lab CPUE 55.2 [+ or -] 0.9
 Carapace width
Caminada Pass CPUE 41.9 [+ or -] 0.5
 Carapace width

All values are means and standard errors. Thirty oyster drills were
measured at each site, and numbers in parentheses are the number of
crabs measured. CPUE, crab catch per unit effort.


ACKNOWLEDGMENTS

Scan Keenan, Patrick Banks, and Susan Bolden helped with field work and Dr. James Geaghan consulted with us on statistical analyses.

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KENNETH M. BROWN, *, (1) MICHAEL MCDONOUGH AND TERRY D. RICHARDSON

Department of Biological Sciences, Louisiana State University, Baton Rouge, Louisiana 70803; Department of Biology, University of North Alabama, Florence, Alabama 35632

* Corresponding author. E-mail: kmbrown@lsu.edu
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Publication:Journal of Shellfish Research
Date:Apr 1, 2004
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