Grass Island Reef: A Restoration Shuck-ccess Story

Evan Pettis | Habitat Assessment Team Lead, TPWD Coastal Fisheries
Grass Island Reef: A Restoration Shuck-ccess Story

Fig 3. Market-sized oysters collected at the Grass Island Reef restoration site just nine months after cultch placement.

The eastern oyster is a culinary favorite that provides significant economic and cultural benefits to Texas’s coastal residents. As a unique keystone species, they also support coastal resiliency and the overall health of the bays and estuaries. A single oyster can filter up to 50 gallons of water per day, playing a critical role in nutrient cycling and maintaining water quality. Though they begin life as tiny, free-swimming larvae, they soon attach themselves to hard substrates (often other oyster shells) and become “oyster spat.” As they grow and recruit additional spat, they gradually form consolidated structures known as “oyster reefs,” which provide several ecological services by reducing wave energy, preventing erosion, and protecting other submerged and shoreline habitats. Moreover, these sprawling complex habitats provide critical foraging and nursery grounds for many commercially and recreationally important finfish and invertebrates.

Unfortunately, oyster populations have been declining globally for decades. Today, Texas’s oysters face ongoing threats of coastal development, destructive storm events, poor water quality, and heavy fishing pressure. One of the major factors limiting the sustainability of oysters in Texas bays is a lack of hard structure for larvae to settle on. One strategy Texas Parks and Wildlife (TPWD) implements to deal with the shortage of suitable substrate is oyster reef restoration, through which degraded reefs are supplemented with “cultch material” (shell, rock, or concrete). Since 2009, TPWD has enhanced over 600 acres of oyster habitat through contracted cultch placement and routinely monitors those reefs to evaluate their success and inform future restoration efforts.

For example, TPWD recently restored 34 acres on the Grass Island Reef complex in Aransas Bay during the summer of 2020. This site was identified as an ideal location for restoration based on a thorough evaluation of available ecological and environmental data sets, pre-restoration bathymetric (depth) surveys, and input from local resource managers and other stakeholders. Grass Island Reef presented a unique challenge for restoration in that water depths at the site ranged from 4 to 9 ft, necessitating multiple cultch deployment strategies to avoid creating a navigational hazard. Because of this, the final site design included a mix of continuous, 3-inch layers of cultch (“flats”) placed in shallower areas and arrays of cultch piles (“mounds”), 2-ft in height in deeper areas. This layout provided an opportunity to assess different restoration approaches based on reef context and compare the efficiency and resiliency of alternative cultch deployment strategies.

Reef health at Grass Island was monitored using the same methods as TPWD’s longstanding, coast-wide Oyster Monitoring Program. Standardized oyster dredges were towed at 3 mph for 30 seconds, and the catch for each sample (including live and dead oysters of all sizes) was counted and measured. Sampling was conducted at the site immediately prior to cultch placement (“pre-restoration”) to establish baseline conditions and then twice per year after placement to track the reef’s progress. A nearby, unrestored (“reference”) section of Scotch Tom Reef was also concurrently monitored to evaluate restoration success and account for broad-scale environmental changes in Aransas Bay.

Monitoring results indicate that the Grass Island restoration effort was an immediate success by producing more adult and sub-adult oysters. Within just eight months of construction, TPWD detected significant spat recruitment and the catch rate for adult oysters increased to over 14.5 times greater than pre-restoration rates. In fact, many of the oysters that settled on the cultch had already grown to market size (3”) by the time of TPWD’s first post-construction sampling (Fig. 3). Due to persistently good recruitment and growth, the restoration reef continued to produce increasingly more oysters and even outperformed the natural reference reef through spring of the following year. While both cultch deployment strategies were successful at enhancing oyster habitat, preliminary data suggest that placement as mounds results in higher oyster abundances within the footprint of the restored bay bottom.

TPWD’s monitoring data demonstrate that multiple restoration strategies can be employed successfully to fit localized environmental conditions. Supplementing degraded reefs with thin, flat layers maximizes the spatial extent that can be enhanced with a limited amount of cultch and is suitable for shallower sites that could otherwise become a hazard to navigation. Cultch mounds, on the other hand, cover less area with the same amount of material but may provide better habitat for oyster recruitment and growth when water depths allow. The additional vertical relief that mounds provide may also be more resilient to storm events, reef subsidence, sedimentation, and low-oxygen conditions along the seafloor. Moreover, there is evidence that the ‘complexity’ of reefs built using mounds provides greater benefits to recreationally and commercially important finfish and their prey. TPWD plans to continue monitoring this site periodically to further assess its long-term progress and to evaluate how costs scale with oyster production for both deployment strategies. The lessons learned from TPWD’s success at Grass Island Reef are instrumental in planning future restorations and adaptively managing Texas’s oyster resources for the benefit of current and future generations.