Cownose Ray Predation on Chesapeake Bay Oysters by Esra Gokturk

     Cownose rays have recently received a spike in media attention for concerns of hindering oyster restoration efforts in the Cheaspeake Bay. The Bay is heavily integrated into human society and economy, but currently has some major environmental concerns, specifically the depletion of oyster populations. While this has been a concern over the past 40 years, predation of rising numbers of the cownose ray, Rhinoptera bonasu, has recently gained attention due to increased conflict between the rays and restoration efforts in the Bay (Fisher et al. 2013). In order to determine the impact of the cownose rays on oyster populations, it is necessary to look at the life history, trophic cascades, dietary preferences as well as the other ecosystem factors and explore management strategies accordingly. Many other factors are at play with this issue, and while the cownose rays may not be helping, it is highly unlikely that predation is a major factor impeding oyster restoration.    

The native Eastern Oysters, Crassostrea virginica, of the Chesapeake Bay play an essential role in the ecosystem. They are largely responsible for water filtration, and many plants and animals rely on oyster beds for habitat use. Oysters also graze on phytoplankton, which allows more light through the water for benthic plants (NOAA 2013). Outside of ecosystem benefits, oysters have a high commercial value. Commercial fishing providing jobs for over 17 million people living in the Chesapeake Bay watershed, making it important to sustainably harvest shared resources (Chesapeake Bay Program n.d.). Unfortunately, native oyster populations have declined approximately 99.7 percent between 1980 and 2009 (Wilberg et al. 2011).

Population losses in native oysters has been occurring for several decades before the effect of rays became an issue of public interest, making it difficult to measure the level of impact the cownose rays are actually having. Historically, oyster populations were at a peak in the late 1800s, with all time lows in the late1900s after uncontrolled harvesting. Populations have gradually begun to recover since, however the numbers are still nowhere near that peak (University of Maryland Center for Environmental Science 2015).  So while predation of rays may contribute to population declines, there are many other factors that play a role. Some of these factors include habitat destruction, overfishing, agricultural pollution, as well as disease.

       Cownose population numbers in the Bay have always been abundant, but have recently experienced a greater increase, possibly resulting from several factors. For example, a decline in commercial fishing rigs in the Chesapeake Bay over the last 50 years is hypothesized to contribute to the reduced mortality and increased abundance of Cownose Rays (Merriner and Smith 1979) . Furthermore, The issue between cownose rays and native oysters in the Bay is not an isolated event. A cascading effect is occurring beginning with the loss of apex predatory sharks in the oceans. Overfishing and consequent decline of these sharks that prey on species like the cownose ray have led to an increase in population size (Bade et al. 2014). This has indirectly resulted in an increased predation of rays on their bivalve prey. This trophic cascade could potentially reach even further and negatively impact seagrass habitat, damaging the benthic environment in the Bay (Myers et al. 2007). 

Trophic cascade in the Chesapeake Bay (Ferretti et al. 2010)

 In addition to the top-down effects of predation on oyster populations, the impacts of cownose rays can negatively affect the entire ecosystem. They are durophagous rays, meaning they crush the shells and hard external parts of prey, which they excavate from the benthic environment (Bade et al. 2014)^.  This excavating can lead to habitat disruption, such as replacing habitat with unstable sand in eelgrass beds (Curran and Cross 2008). Benthic environment and native plant disruption can lead to decreased biodiversity and species density (Pierce 2005). However, it could also be argued the disruptions are habitat modification by the rays. After causing a depression pit from feeding, sediment disturbance rapidly accumulates organic material, increasing food availability and consequently attracting new invertebrates to recolonize (Pierce 2005).

Part of the issue with Cownose Ray abundance is that they inhabit the Bay in such large schools during migration. They have a large distribution and inhabit open waters, ranging from the Western Atlantic and southern New England up to Northern Florida and the Gulf of Mexico, with migrations down to South America (Monterey Bay Aquarium Foundation 2015).  This species of ray partake in cyclic migration, and migrating schools can be over 10,000 individuals. This is potentially problematic in the Chesapeake Bay, where rays are known to migrate in summers, the same season Cownose Rays pup (Bade et al. 2014). Before oysters became a topic of concern, prey depletion by Cownose Rays has been seen in other prey species. Using controlled ray-exclusion experiments, since 1996 it has been seen that Cownose Rays cause almost complete mortality of bay scallops by early fall during migration (Myers et al. 2007). As with the scallops, Cownose predation occurs before the spawning periods its prey, causing a greater impact. Other studies have shown a correlation between the increase in Cownose Rays and decline in oysters ‘without substantial recovery, as opposed to areas beyond the ray’s northernmost limits where populations were stable or showing increasing numbers from recovery efforts (Myers et al. 2007).

The primary diet of Cownose Rays consists of bay scallops, clams, oysters as well as a few other noncommercial bivalves. A scientific study using genetic techniques to identify the gut contents of cownose rays in the Bay found that the rays were not consuming significant portions of oysters or commercially important bivalves (Bade et al. 2014). Another study indicated the preferred food of the cownose rays is the soft-shelled clam, which is has been severely depleted for the past few decades (Merriner and Smith 1979). Other studies support findings that weak-valved bivalves are preferred while hard shelled-bivalves and oysters are not a part of their natural diet (Fisher et al. 2013)The decline in preferred prey species may have increased predation on native oysters, causing them to select more for a food they don’t necessarily seek out. However, based on these studies there is strong evidence that oysters are not a food source that Cownose Rays select for.

Rhinoptera bonasus feeding on bivalves (photo: Chesapeake Bay Program)

 Given the size and importance of the Chesapeake Bay to so many people, there are many proposed ideas and public opinions on management strategies. Many fisherman and aquaculturists that consider Cownose Rays a nuisance called for directed fishing and culling (Bade et al. 2014).  In response, mitigation attempts have resulted in unregulated fishery of Cownose Rays, which risks overfishing (Fisher et al. 2013). This false assumption of Cownose Rays as a nuisance is very risky as management rays can be difficult due to their life history. Similar to most elasmobranchs, these rays have late maturity and low fecundity (Merriner and Smith 1979). So while population levels are abundant presently, recruitment cannot compensate for potential overexploitation, which needs to be considered when looking at potential solutions.

It needs to be recognized that this issue is not a simple cause and effect, where the rays are solely causing the decline of oysters. This is a complex system with many interactions of biotic and abiotic factors, so the most effective solutions may lie outside of Cownose Ray management. However, if you were to explore Cownose Ray management, establishing a regulated fishery could help offset some of the impact predation. An issue with this method is finding a market and having enough demand, as well as needing strict regulation due to the susceptibility elasmobranchs to overexploitation. Strategies to decrease Cownose Ray predation outside of reducing stocks include physical barriers such as mechanical fences around shellfish beds (Merriner and Smith 1979).    However, this may not be cost-effective and is limited to intertidal and shallow sub-tidal beds.

There are other management strategies to be explored as alternatives to managing Cownose Ray populations. The top-down effects resulting from the loss of apex predators opens up discussion for management focusing on conservation of coastal predatory sharks in order to offset the impact of Cownose Rays. It seems the loss of other prey species was a major contributor to the diet shift to oysters, so restoration efforts in prey species may offset population declines of oysters. Additionally, management of the other factors contributing to oyster declines may be a more effective strategy, like focusing on overfishing or habitat restoration. There is no single solution, but rather a combination of strategies implemented to help stabilize oyster populations. Public awareness is a major step in this process, so that people can be more informed in order to have an educated public opinion on the issue.

The effects from this controversy extend far beyond the Chesapeake Bay, which is why it is important to consider all factors in conservation. While certain studies indicate predation by Cownose Rays contributes to the decline in oyster populations, this does not mean it is a major factor. Looking at historic oyster trends as well as the many contributing factors, we cannot assume that Cownose Rays are driving factor in oyster declines. These rays are vital to ecosystem balance and the overall health of the Chesapeake Bay, an important role that has been undervalued with the negative media attention on this issue.  Restoration efforts for native oysters are important for ecosystem health and human economy, however there are more practical solutions for restoring oyster populations that do not result in decimating the ray populations. Due to the complex system of interactions affecting the Chesapeake Bay, it is unrealistic to assume simply decreasing Cownose Ray predation will directly result in increasing oyster populations.

References

Bade, L. M., Balakrishnan, C. N., Pilgrim, E. M., Mcrae, S. B., & Luczkovich, J. J. (2014). A genetic technique to identify the diet of cownose rays, rhinoptera bonasus: Analysis of shellfish prey items from North Carolina and Virginia. Environmental Biology of Fishes, 97(9), 999-1012.

Chesapeake Bay Program.(N.D.). Facts & Figures. Discover the Chesapeake.  

Curran, M. C., & Cross, R. E. (2008). Effects of ray sediment disturbance on meiofauna in tidal and intertidal zones. Georgia Journal of Science, 66(2), 50-60.

Ferretti F, Worm B, Britten GL, Heithaus MR, & Lotze HK (2010). Patterns and ecosystem consequences of shark declines in the ocean. Ecology letters, 13 (8), 1055-71 PMID: 20528897

Fisher, R. A., Call, G. C., & Grubbs, R. D. (2013). Age, growth, and reproductive biology of cownose rays in chesapeake bay. Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science, 5(1), 224-235.

Merriner, J. V., & Smith, J. W. (1979). A report to the oyster industry of virginia on the biology and management of the cownose ray (Rhinoptera bonasus, Mitchill) in lower chesapeake bay.

Monterey Bay Aquarium Foundation. (2015). Animal Guide: Cownose ray.

Myers, R. A., Baum, J. K., Shepherd, T. D., Powers, S. P., & Peterson, C. H. (2007). Cascading effects of the loss of apex predatory sharks from a coastal ocean. Science, 315(5820), 1846-1850.

NOAA. (2013). Oyster Restoration.  

Pierce, Simon (2005). The Role of Rays in the Ecosystem. School of Biomedical Sciences, University of Queensland. Marine Education Society of Australia.

 University of Maryland Center for Environmental Science. (2015). Oyster History. Horn Point Lab Oyster Hatchery.

Wilberg, M. J., Livings, M. E., Barkman, J. S., Morris, B. T., & Robinson, J. M. (2011). Overfishing,     disease, habitat loss, and potential extirpation of oysters in upper Chesapeake Bay. Marine Ecology  Progress Series, 436, 131-144.

Working with Cownose Rays over the summer is what got me interested in this topic!