It seems like every day there’s another article proclaiming that oysters are having a good year. Yes, we’ve had a few years where the number of oysters in the Bay has been increasing and where wild oyster reproduction has been strong. However, a lot of this has to do with environmental conditions favoring oyster reproduction and survival. So while these numbers are good, the goal is to continue to see improvements, even in years when environmental conditions aren’t as favorable.
We’ve discussed this in newsletter articles in years past but as a reminder, the Maryland Department of Natural Resources does an annual fall oyster survey. They sample a variety of oyster bars to get an idea how many oysters there are, how many oyster spat there are, and what disease prevalence and intensity looks like across different oyster bars in Maryland. This yearly data helps us understand how oyster populations and disease levels are changing over time.
And while the published report of last fall’s oyster survey isn’t out yet because they’re still processing and analyzing some pieces, a preliminary report was made to the Oyster Advisory Commission on January 9th, 2024. The full powerpoint that was presented is available online here.
The first thing to note from this preliminary report is that streamflow was favorable for oysters again this year. From 2020 to 2022 we had normal streamflow (which is a proxy for rainfall and freshwater flow into the Bay) and in 2023, we actually had below normal streamflow. This means less fresh water and therefore higher salinities, especially in northern parts of the Bay. This is important for oysters because they need salinity of at least 10 parts per thousand (ppt) to reproduce successfully and salinity of greater than 5 to feed and survive. Adult oysters can close their shells and stop feeding for several days if salinities drop below 5 but they will eventually die if those conditions persist.
We can see some of the impact of salinity in the figure above, which shows oyster spatfall in 2022 (left) compared to 2023 (right). There’s more oyster spat in many areas in 2023 and some areas that did not have any spat in 2022 had spat in 2023. However, the problem with counting the oysters in areas that did not have spat in 2022 is that if we get a year with higher than normal streamflow, many of these oysters may die and we will not see spat return to these areas until we return to normal or below normal streamflow. Basically, the oyster populations in these areas will fluctuate based on environmental conditions and there’s not much we can do about it. In the meantime, they are out there filtering water and providing habitat but that may not always be the case. For example, the DNR presentation specifically mentions finding a large amount of spat in the Potomac, in an area where oysters had basically been wiped out due to extreme freshwater input in 2018/2019.
Overall oyster biomass, which measures both the number of oysters and their size, is above the 30 year average. However, oyster biomass peaked in 2021 and we saw a decrease in 2022 and again in 2023. The things that remove oysters from the population are harvest, disease, and natural mortality. The recent decreases in oyster biomass are likely due to harvest and disease. Oyster harvest was up last year (2022/2023 season). In fact, it was the largest oyster harvest in 36 years. This isn’t necessarily a bad thing as oyster restoration involves planting in oyster sanctuaries (no harvest) and public oyster bars (harvest allowed). In those public areas, watermen assist with the planting and practice rotational harvesting to allow oysters time to grow to market size. This helps reduce harvest pressure on oysters produced from wild spat. In addition, we’ve discussed this before as well, but there is evidence that harvest of older oysters can help reduce disease prevalence as older oysters tend to have higher intensity disease, which means they are more likely to spread disease to other oysters. All this is to say that reducing oyster biomass due to harvest isn’t always a bad thing, especially as the 2023 fall survey saw some major increases in disease across Maryland oyster bars.
This increase in disease may be due to the lower than normal streamflow and therefore higher salinities. As a reminder, there are two oyster diseases of concern in the Chesapeake Bay; dermo and MSX. MSX tends to be less of an issue in the mid-Bay as it prefers higher salinities. Dermo is the big concern in the mid-Bay as it prefers mid range salinities, above 12 ppt. That’s one of the trade-offs with oysters; they do best at 10 ppt and above so we see increasing reproduction and survival in years with less freshwater input but we’re also likely to see increases in dermo disease. Likely because of the lower than normal streamflow in 2023, we saw the highest disease prevalence and intensity of dermo since 2017. Unfortunately, we also saw an increase in the other disease, MSX. There’s not much other information available on MSX from the fall survey yet as these samples are still being processed.
Overall, a strong spatfall this year is very good but it doesn’t mean we’re in the clear and that oyster populations have recovered. One bad year could have drastic impacts on oyster biomass in the Chesapeake Bay. We need to keep an eye on harvest and disease to make sure oyster biomass isn’t decreasing too quickly. There’s still work to breed disease resistance oysters at hatcheries which can then be planted in the Bay, which we hope will help reduce disease prevalence and mortality long term but it’s a slow process. We need to continue to make sure there’s lots of mature oysters out there reproducing each summer so that we continue to see strong spatfall, even in years when streamflow and salinity aren’t in the oysters’ favor. Finally, we need to continue to support monitoring efforts so that we know what is going on in the population and so that we can make adjustments to harvest levels based on that information.