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This project has demonstrated the effectiveness of environmentally benign methods for biofouling control. There are many methods of biofouling control which have been suggested, with varying degrees of effectiveness and commercial applicability. Some may be highly species-specific, while others may target a range of species. In order for a biofouling management technique to be widely adopted, it must have the potential to be applied commercially without adding unreasonable labor demands, and it must be effective in controlling biofouling with disrupting the crop of interest, the oysters. While not the only suitable option for biofouling control, this project primarily sought to delve into an understanding of the use of desiccation, or periodic aerial exposure, in controlling biofouling. As a secondary component to this work, methods to control fouling among juvenile oysters were also assessed.
Over the course of two field seasons (2018 and 2019), the biofouling community of four areas in the Chesapeake Bay was characterized. Biofouling is known to demonstrate variability, so this multi-year, multi-site approach was deemed necessary. An Upper-Bay site (Chester River) provided an understanding of biofouling in low salinity oligohaline conditions (2018). A site in the Mid-Bay (Choptank River) provided an understanding of biofouling in the mesohaline (2019). Two sites in the lower-mid Bay (Honga River) provided an understanding of biofouling in the polyhaline portion of the Bay (2018). The Choptank River site monitoring took place at the University of Maryland Center for Environmental Science, using a custom hoist system (Figure 1). The sites in the Chester and Honga Rivers utilized miniature versions of commercially available flippable floating cage systems (Figure 2). Both epibionts, or species which colonize the external portion of the shell, and bioexcavators, or species which bore into the shell, were monitored. Species richness and relative composition were monitored on oysters and cages subject to each of three desiccation regimes, as well as a subtidal control which experienced no desiccation. Oyster performance, measured as oyster growth, mortality and condition, was monitored to understand treatment effects on the crop of interest.
The species documented in this project include: Alitta succinea, Balanus improvisus, Conopeum tenuissimum, Diadumene leucolena, Ischadium recurvum, Mytilopsis leucophaeata, Polydora websteri, Ulva intestinalis, and Victorella pavida. While these do not represent the totality of species which utilize or interact with oyster cages, they are the sessile or excavating species which were found to colonize the oysters and/or cages. Mobile species (fish, crabs, amphipods, etc.) were not included in these assessments. Additionally, other species are known to be problematic in different areas of the Chesapeake Bay, but were not found at the sites included in this project. The project could only ascertain the effectiveness of this antifouling treatment among the species documented, and results are limited by the species present at these sites during the years of study.
Total biofouling species composition demonstrated variability in both space and time. Seasonal effects (temperature changes) were associated with alterations in the biofouling species composition and colonization intensity. Additionally, species composition and coverage varied with each site, with total biofouling accumulation increasing with decreasing salinity. Overall, greater biofouling accumulation was identified among lower salinity sites when compared with higher salinity sites.
Desiccation as a management practice demonstrated a significant impact on the biofouling community, with each of the tested desiccation intervals (4, 8 or 24 hours weekly) leading to a significant reduction in total biofouling (both epibionts and bioexcavators). For example, at the site in the Choptank River (HPL), the non-desiccated oysters were, on average 64.3 percent covered in biofouling (Figure 3), with oysters desiccated for 4, 8 and 24 hours being 23.7, 18.4 and 12 percent fouled respectively (Figures 4-6). Worm infestation followed similar trends, with non-desiccated oysters hosting, on average, 55 worms per oyster. Oysters desiccated for 4, 8 and 24 hours weekly hosted, on average, 31, 12 and 3 worms per oyster respectively.
These gross averages over time are useful as a long term management strategy, but belie the variability that occurs throughout the season. For example, during the July peak fouling period, V. pavida required 8 or 24 hours of desiccation to be controlled, while just four hours of weekly desiccation proved effective during other times of the year. These species- and season-specific responses demonstrate an opportunity for an oyster farmer to target their fouling control strategy to match the fouling pressure at a given time of year, or to prescribe one standard strategy with widespread applicability.
Increasing duration of desiccation was associated with reduced coverage by the species recorded, but was also associated with a growth penalty (measured via change in shell height). Oysters desiccated for longer time intervals, notably 24 hours weekly, grew less than non-desiccated oysters. The oysters in this treatment also experienced higher mortality compared to non-desiccated oysters.
It must be noted that the prescriptive frequency of application of these desiccation intervals (weekly) ensured that fouling organisms were exposed to the air <7 days post-settlement. Results cannot be expected to transfer to organisms which have been allowed to set and establish for a longer period of time. Many common biofouling organisms would be expected to tolerate lengthy desiccation intervals if given an opportunity to develop to an adult form before the desiccation practice were applied. The frequency of desiccation is critical in the successful application of this practice as a fouling management strategy.
While desiccation has proved to be an effective strategy to control biofouling among adult oysters, this practice is not recommended for juvenile seed. However, these seed oysters can still suffer from the same effects of biofouling as their adult counterparts, particularly given the small mesh used to containerize these juvenile oysters. To understand methods to control fouling among juvenile oysters, the research team employed a series of antifouling treatments applied bi-weekly. Treatments included a 4 hour desiccation, acetic acid dip and a brine dip. All treatments were compared to a subtidal control group of juvenile oysters. Of these treatments tested, no treatments demonstrated a negative impact on the growth or mortality of the oysters. However, only the bi-weekly acetic acid dip yielded a significant reduction in biofouling composition. This treatment also yielded a significant reduction in Atlantic mud crab (Panopeus herbstii) infestation, a common challenge in juvenile oyster culture.
Outcomes demonstrate that biofouling, while a major challenge to oyster production in Maryland and many regions, can be successfully controlled. Diligent application of desiccation intervals can reduce fouling colonization and the many negative impacts caused by excessive fouling.