Crassostrea virginica dredge efficiency in Texas estuaries

Abstract

Quantifying and comparing stocks of oysters (Crassostrea virginica) within and among estuaries across the Gulf of Mexico is difficult because the sampling equipment used is either inconsistent among studies, or inefficient. In Texas, USA, stock assessments of oyster populations are made using an oyster dredge, which is an inefficient sampling tool. We compared sampling densities estimated by oyster dredges with more accurate estimates taken by diver-quadrat samples to determine a dredge efficiency rate. Our calculated efficiency rate (0.125) was negatively affected by the number of dead oysters, and the number and volume of total oysters in an area, but not affected by sediment grain size, water quality, and other oyster metrics. The dredge efficiency rate calculated in this study can be applied to past and future dredge-collected oyster quantity data to provide more realistic estimates of oyster densities and allow more accurate stock assessments and comparisons among studies and regions.

Introduction

An ongoing challenge to managing Crassostrea virginica oyster populations is the lack of standardized metrics for quantifying and comparing stock abundance within and among estuaries, states, and across the Gulf of Mexico. In Texas, stock assessments of subtidal oyster populations are conducted using an oyster dredge, which cannot provide oyster density or shell volume as a quantitative, comparable metric (e.g. per unit area). It is recognized that oyster dredges are inefficient samplers; they do not collect everything in their path and can vary widely across replicate samples (Chai et al., 1992). Dredge results can also be affected by towing speed, frequency of dredging, bottom characteristics, and length of towline (Meyer et al., 1981; McLoughlin et al., 1991; Powell et al., 2002). Resulting data may underestimate oyster densities (Chai et al., 1992) and limit the ability to estimate stock abundance and assess populations across states and/or regions.

Objective

The objective of this study is to determine a gear efficiency ratio for oyster dredging in shallow, subtidal estuaries that occur in the northern Gulf of Mexico. The purpose of determining this ratio is to allow the conversion of dredge-derived oyster data, such as that collected since 1984 by the Texas Parks and Wildlife Department fisheries-independent monitoring program (Martinez-Andrade 2018), to quantitative, areal oyster densities to facilitate accurate stock assessments. Oyster densities as collected by dredge were compared with concurrent diver-collected quadrat surveys to determine dredge efficiency for oyster collection because quadrat surveys can accurately quantify oyster densities.

Methods

Dredge efficiency comparisons were conducted at 19 locations within four Texas estuaries from July to October 2018 (Figure 1). The locations encompassed a range of environmental conditions and oyster densities within and among estuaries. Oysters were sampled in triplicate using quadrat samples taken by scuba divers (0.5 m²), and then dredge tows (14.625 m²: 0.375-m wide dredge for 30 s at 1.3 m s⁻¹) at each location. Sediment grain size composition was approximated by visually observing and manipulating the substrate by hand to estimate the proportion of mud, sand, shell hash (<25 mm) and whole shell (live or dead) in the top 10 cm of each quadrat sample. After this grain size analysis, the abundance and volume of live and dead oyster shells (>25 mm) were standardized to unit area, and all live oyster heights were measured (+/− 1 mm) in each sample. Shell volumes (dead and live) were determined by water displacement in a measuring bucket. Ancillary salinity and depth data were measured with a YSI ProDSS sonde at each location.

Figure 1. Sampling locations.

Results

There are positive linear relationships (Pearson correlations) between oyster metrics sampled by diver quadrat and dredge for live oyster density (n m⁻², r = 0.78, p < 0.0001; Figure 2), live oyster volume (L m⁻², r = 0.67, p ≤ 0.002), and dead oyster density (r = 0.51, p ≤ 0.03). The gear efficiency of the dredge as determined by linear regression on mean oyster densities (n m⁻²) collected by dredge and by quadrat at each station are 0.125 (SE 0.016) and 0.044 (SE 0.008) for live and dead oysters. Dredge efficiency (density_dredge/density_quadrat for each station) for live oyster density is negatively correlated (Pearson correlation) with dead oyster density in quadrats and total oyster density and volume (r < −0.49, p < 0.04), but no other oyster variables, sediment grain size components or water quality variables (p ≥ 0.10). The sediment composition of the sampling locations are generally dominated by shell hash but vary among stations (μ = 79%, σ = 32%). The size distributions of live oysters are similar in quadrat and dredge samples where densities are high (Figure 3). Data generated in this study are available at GRIIDC (Beseres Pollack et al., 2019). Detailed results are in the Supplementary Material.

Figure 2. Oyster abundance as estimated by quadrat and dredge sampling. Error bars represent standard error of three replicates at each location.

Figure 3. Histograms and kernel density curves of oyster heights in each estuary as sampled by dredge and diver quadrat.

Discussion

Dredges provide a low-cost, rapid method for assessing oyster populations, but dredge-estimated oyster densities can be affected by several factors (e.g., tow speed, line length, dredge width, etc.), leading to dredges being labelled as unreliable tools for accurate oyster surveys (Schulte et al., 2018). However, dredge-estimated oyster densities were often less variable than quadrat-estimated densities in this study, which can be attributed to the fine-scale (< 10 m) spatial homogeneity that is not detected by the larger area sampled by an oyster dredge. The lower efficiency of the dredge at higher shell densities is similar to that observed by Morson et al. (2018). But despite the perceived and observed sampling variability, there is still a strong linear correlation between dredge-based and quadrat-based oyster density estimates over a range of sediment types.

Conclusions

Despite its inefficiency, the oyster dredge is a useful sampling tool that can be used to estimate subtidal Crassostrea virginica densities after applying a correction factor. If the area sampled by oyster dredges used in monitoring studies can be estimated, then the efficiency ratio can be used as an estimate of actual oyster density for historical and future density assessments. Further sampling and statistical analyses, such as those in Powell et al. (2007), could tease out more accurate gear efficiency ratios. However, the simple ratio developed in this study is useful and applicable to Gulf of Mexico estuaries for assessment and management of oyster populations.