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The Impact of Crown Conch on Intertidal Oyster
Populations in Mosquito Lagoon

By: Casey Craig, Courtney Buck, Chelsea Landau, and Jordan Filipponi
Mentor: Dr. Linda Walters

Discussion

Oyster harvesters attribute oyster population declines in Mosquito Lagoon to predation by M. corona. To test this, we conducted a three–part experiment in CANA to determine the impact of M. corona on intertidal oyster populations. We found that individuals tested in this study were not selective when feeding and consumed all sizes of oysters presented to them. Conch traveled approximately 0.5 meters in a 10–minute interval or 5 cm/minute. Although harvesters insist M. corona are abundant enough to cause declines in oyster reef populations, this study revealed that M. corona can only be found in detectable numbers in a few hotspots, but are absent otherwise or their shells are occupied by hermit crabs (Table 2).

Our data suggest that M. corona likely did not decimate large numbers of oysters in CANA. Garvis et al. (2015) reported that there are an average of 100 live oysters per square meter in Mosquito Lagoon. As there are 2,802 oyster reefs in Mosquito Lagoon (Garvis et al. 2015), and if one in 18 reefs are hotspots, then 156 reefs should be hotspots. Based on our observations and the number of reefs, it is estimated that 5137 live M. corona were present in Mosquito Lagoon during fall at peak density. Using this density and the aerial extent of oysters in Mosquito Lagoon, we calculated that there are approximately 14,800,000 live oysters in the waterbody. If 50% of conch eat an oyster in three days, this equates to one conch eating five oysters in a 30–day period. If approximately 25,000 oysters are consumed per month by M. corona (5,137 M. corona x 5 oysters per month), then their total consumption of oysters equates to only 0.17% of the oyster population consumed monthly, or 2.09% annually, by M. corona in Mosquito Lagoon. This conclusion assumes the oyster population is static. It is not, and there are new recruits entering the system each year from April to December (Garvis et al. 2015). In 2014, the mean was 222 new oysters per square meter (L. Walters, unpublished data). Combined, the data suggest that the M. corona have very little impact on the oyster population.

Most of the M. corona (61 %) were found on one of the surveyed reefs in fall 2014. It was a restored reef adjacent to a boating channel. Thus, we predict that in Mosquito Lagoon, M. corona is patchily distributed, and most obvious when located in hotspots, where higher densities could lead to misinterpretation of overall numbers in the lagoon. Abiotic factors, such as water flow or turbidity, or biotic factors, such as recent mating aggregations or recent hatching events, could also impact distributions.

Oyster harvesters likely mistake empty M. corona shells or shells occupied by the striped hermit crab, C. vittatus, for live conch. This mistake is easy to make, unless you examine each shell individually. There were approximately 2.4 C. vittatus occupying M. corona shells for every one conch in Mosquito Lagoon. Empty M. corona shells were often disintegrating or acting as substrate for oysters or sessile fouling organisms, such as barnacles and anemones. Shells were often empty if there was any visible damage to the shell.

The results from our tracking experiment also illustrate M. corona is capable of traveling among oyster reefs that are considerable distances apart. After averaging the distance traveled per 10 minutes for each reef type, the values were scaled up by a factor of 6 to acquire an hourly locomotion rate, and then by 24 to determine how far M. corona are capable of traveling in one day. In a 24–hour period, our results suggest that M. corona is capable of traveling 63.5 meters. However, no conch were observed leaving the reefs during the tracking experiment. If resources were not sufficient on a particular reef or abiotic variables were problematic, an individual should be capable of relocating to a more hospitable reef.

In summary, our results suggest that any observed declines in Mosquito Lagoon oyster populations are not primarily the result of ecological stress associated with M. corona predation. While this is true for Mosquito Lagoon, live conch numbers need to be assessed location by location. A question is now raised for Mosquito Lagoon resource managers: what is responsible for oyster population declines? Garland and Kimbro (2015) suggest that high salinity, due to a prolonged drought, is to blame because disease and predator populations flourish in these conditions. Brown tide (Aureoumbra lagunensis) has also been linked to lower oyster growth and survival (Gobler et al. 2013; Makris 2016). In addition, with oyster harvesting being on record for having negative impacts on oyster reefs in national surveys, we recommend reviewing existing harvesting regulations and licensing laws to better determine the extent of this anthropogenic influence.

Appendix A>>