Success of concrete and crab traps in facilitating Eastern oyster recruitment and reef development

Background Abundance of the commercially and ecologically important Eastern oyster, Crassostrea virginica, has declined across the US Eastern and Gulf coasts in recent decades, spurring substantial efforts to restore oyster reefs. These efforts are widely constrained by the availability, cost, and suitability of substrates to support oyster settlement and reef establishment. In particular, oyster shell is often the preferred substrate but is relatively scarce and increasingly expensive. Thus, there is a need for alternative oyster restoration materials that are cost-effective, abundant, and durable. Methods We tested the viability of two low-cost substrates—concrete and recycled blue crab (Callinectes sapidus) traps—in facilitating oyster recovery in a replicated 22-month field experiment at historically productive but now degraded intertidal oyster grounds on northwestern Florida’s Nature Coast. Throughout the trial, we monitored areal oyster cover on each substrate; at the end of the trial, we measured the densities of oysters by size class (spat, juvenile, and market-size) and the biomass and volume of each reef. Results Oysters colonized the concrete structures more quickly than the crab traps, as evidenced by significantly higher oyster cover during the first year of the experiment. By the end of the experiment, the concrete structures hosted higher densities of spat and juveniles, while the density of market-size oysters was relatively low and similar between treatments. The open structure of the crab traps led to the development of larger-volume reefs, while oyster biomass per unit area was similar between treatments. In addition, substrates positioned at lower elevations (relative to mean sea level) supported higher oyster abundance, size, and biomass than those less frequently inundated at higher elevations. Discussion Together, these findings indicate that both concrete and crab traps are viable substrates for oyster reef restoration, especially when placed at lower intertidal elevations conducive to oyster settlement and reef development.

154 (Gainesville, FL) and consisted of 12 notched, interlocking forms that assembled into a 155 rectangular structure. The modular design facilitated transportation and deployment, as each 156 piece weighed 7 to 20 kg, and provided stability against moderate wave action. The concrete mix 157 design was standard, with an approximate ratio of 1:2:5:3 for water, cement, course aggregate, 158 and fine aggregate by weight (see Supplement for construction details). When assembled, the 159 structure exterior measured 42 cm H x 96 cm L x 57 cm W (0.23 m 3 ; Fig. 1a). 160 The plastic-coated wire crab traps were collected from Florida Fish and Wildlife 161 Conservation Commission after their 2015 derelict trap clean-up, cleaned to remove any fouling 162 organisms, and secured to the substrate using 2 m rebar poles whose ends were bent into hooks. 163 Four crab traps were 42 cm H x 61 cm L x 61 cm W (0.16 m 3 ; Fig. 1b), and the fifth was 26 cm 164 H x 120 cm L x 61 cm W (0.19 m 3 ). The one trap with alternative dimensions was used in the 165 experiment at the request of restoration practitioners we work alongside, to evaluate whether this 166 trap type might also perform well in oyster restoration; however, because the mesh size was the 167 same for all traps (5 cm x 2 cm mesh) and the majority of oysters settled on the bottom 25 cm of 168 the traps, we do not distinguish this trap from the others in our analyses. We closed all crab trap 169 openings using cable ties to prevent traps from capturing terrapins, blue crabs, and other larger 170 species; no dead animals were observed in the traps over the experiment's duration. In addition, 171 we recorded no visible deterioration of the crab traps; however, one concrete structure had one 172 piece knocked askew, likely by a boat, during the experiment. Data were collected onsite on 20 September 2015 (1 month after deployment), 24 May 175 2016 (9 months after deployment), 11 August 2016 (12 months after deployment), and 1 June 177 establishment on the structures, we monitored the percent cover of oysters by haphazardly 178 positioning replicate 15 by 15 cm frame quadrats on the seaward-and landward-facing sides of 179 each structure, between 5 and 15 cm from the sediment surface. We used six quadrats on each 180 side (seaward and landward) of each concrete structure, and four and two quadrats on each side 181 of the 0.16 m 3 and 0.19 m 3 crab traps, respectively. Within these same quadrats, we also 182 monitored the percent cover by barnacles to evaluate its impact on oyster recruitment and 183 settlement and to compare barnacle establishment rates between the two treatments. Other 184 potentially fouling species known to hinder oyster restoration success, such as sponges and 185 ascidians, were never observed on the restoration structures. In addition, predatory whelks and 186 conchs are common predators of oysters in the region but were not observed on the restoration 187 structures over the course of the experiment so were not monitored.

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On the final observation date, we measured the dimensions of the oyster reef established 189 on each structure. Specifically, we took ten measurements of oyster reef height (i.e., the distance 190 between the benthos and the top of the highest live oyster) and eight measurements of each reef's 191 length and width at heights between 5 and 30 cm above the benthos. We estimated the percent 192 change in the volume of each reef by subtracting the initial structure volume (product of length, 193 width, and height) from the total volume of each reef (product of mean oyster reef length, width, 194 and height measurements) and dividing this difference by the initial structure volume. Structures 195 were then removed from the field, wrapped in tarps to protect the oysters established on them, 196 and transported to University of Florida's Coastal Engineering Lab. In the lab, we used 12 197 replicate 15 by 15 cm quadrats per structure to measure the percent cover of oysters and 198 barnacles and, within each, counted the number of spat (<2.5 cm), juvenile (2.5-7.5 cm), and 223 the logarithm of the odds ratio for a regressor, and from we estimate the probability of an event 224 occurring due to a unit increase in the regressor (or, in the case of a categorical variable, due to 225 switching from one factor level to another). That is, from it follows that . = ln ( 1 -) = 1 + 226 For instance, if =2, the probability that the response occurs increases by an estimated 88% 227 given a unit increase in the regressor.

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To test for significant differences (α=0.05) in oyster abundance by size class (spat, 229 juvenile, market-size, and total), reef biomass, and reef volume between substrate types, we 230 specified generalized additive models (GAMs) that assumed the response variables exhibited a Manuscript to be reviewed 269 (individuals per 100 cm 2 ) observed on concrete structures were 7.8, 2.4, and 3.0 times higher 270 than those observed on crab traps (Fig. 3, Table 2). Further, barnacle cover had a negative effect 271 on the final abundance of spat, juvenile oysters, and total oysters, but only on concrete structures. 272 The abundance of market-size oysters (i.e. those >7.5cm) was relatively low (mean±SE: 273 0.27±0.06 individuals per 100 cm 2 across all structures) and did not differ between substrates.
274 Oyster reef volume 275 The percent change in oyster reef volume was significantly higher on crab traps 276 (mean±SE: 46.7±7.6% increase in volume) than on concrete structures (mean±SE: 22.4±7.5% 277 increase in volume) and decreased with increasing elevation (Fig. 4a; Table 2).

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Our results reveal that concrete structures and crab traps facilitated oyster reef development 284 over the experiment's almost two-year period, suggesting that both substrates offer promising 285 solutions for Eastern oyster restoration. In line with our first hypothesis, oysters colonized the 286 concrete structures more quickly than the crab traps (Fig. 2a), and the more rapid establishment 287 resulted in higher densities of oyster spat and juveniles on this material by the experiment's 288 conclusion (Fig. 3). Oysters also established on crab traps and exhibited higher lateral growth on 289 this substrate than on the concrete (Fig. 4a)  In monitoring oyster cover over time, we discovered that the broad, flat surfaces of the 301 concrete were particularly conducive to oyster settlement relative to the crab traps, whose mesh 302 provided far less surface area for establishment (Figs. 1, 2). The greater availability of settlement 303 surface enabled the concrete to support high oyster recruitment through the 2015 reproductive 304 season (August through October). It is important to note that these results may differ for concrete 305 riprap or crushed concrete, which would have relatively less exposed surface than our interlocking, 306 erect concrete structures. However, we also observed fairly rapid increases in oyster cover on the 307 crab traps during the second summer, likely because those oysters engineered and expanded the 308 available surface area and emitted chemical cues to conspecifics. As a result, there was a near 309 convergence in oyster cover between treatments by the experiment's end (Fig. 2a)  It is important to note that we only present coverage data from the structures' exterior 328 surfaces, due to the difficulty in quantifying interior oyster and barnacle cover without destroying 329 the structures and the organisms settled upon them. However, we observed significant oyster 330 colonization on the mesh that bisected the interior of the crab traps as well as on the broad interior 331 surfaces of the concrete structures. Inclusion of data from these interior surfaces would certainly 332 alter the inferences drawn from this work to some extent; however, we anticipate that the main 333 factors influencing oyster and barnacle cover-substrate type and elevation-would not change 334 with these interior data. That is, the trends reflected in Fig. 2 are consistent with our observations 335 of the structure interiors: We observed rapid oyster colonization of the interior surfaces of the 336 concrete and more gradual, but substantial, oyster colonization of the interior mesh of the crab 2 Note: Coefficient estimates are log-odds ratios. P is the probability of an increase in the response 3 variable given a unit increase in the regressor. Negative estimates are associated with low 4 probabilities, indicating high probabilities (1-P) of negative effects on the response. Significant 5 fixed effects (α=0.05) are indicated in bold.