11/14/2023
Some facts from Oyster Annie:
An article about farmed oysters and their impact on clams was recently published in the Mount Desert Islander and Ellsworth American. I am writing to provide more details that add depth and context to those articles. In particular, I want to make clear that (a) oysters farmed in Maine are a native species (American/Eastern Oyster– Crassostrea virginica) and are not invasive, (b) evidence suggests that green crabs are the primary culprit for the declining wild clam population, and (c) coexistence between Soft Shell Clams, Eastern Oysters and Quahogs is well documented. Now that you've read the highlights, here is the detail.
A. In the articles, a great resource was mentioned about the history of oysters called “A History of Oysters in Maine” that was compiled by Randy Lakovic, the now-retired librarian for the Ira C. Darling Marine Center at the University of Maine. Very few records exist about pre-colonial abundance of oysters, but one quote in this document suggests oysters were in existence around Acadia and harvested by Native Americans prior to the development of modern oyster farming, which arose hundreds of years later: "In the 18th century Charlevoix wrote, ‘Oysters are very Plenty in Winter on the Coasts of Acadia, and the Manner of fishing for them is something singular. They make a Hole in the Ice, and they thrust in two Poles together in such a Manner, that they have the Effect of a Pair of Pincers, and they seldom draw them up without an Oyster.’"
B. This evidence highlights a crucial point about Maine’s ecological history: there was a point in time where both oysters and clams lived in Maine’s waters in much larger numbers than we currently see. This reality is also evidenced by historical data including the presence of shell middens (shell pilings created by indigenous communities before European colonization) that contain shells of both clams and oysters. It is well documented that healthy estuarine habitats show a pattern of coexistence between soft shell clams, Eastern oysters and Quahogs, and archaeological evidence points to this being the case in the greater Acadia area , as well.
C. In fact, oysters farmed locally in Maine are the native species commonly known as the Eastern oyster, or Crassostrea virginica, that is found in the wild along the east coast from Canada to the Caribbean. This species has occupied this native range for millenia.
So what has happened between the time of historical clam and oyster coexistence and today? The short answer is that just as there have been losses of mussel beds and clam beds, there have also been losses of oyster beds. The losses of all these wild shellfish beds can be attributed to ecological changes caused by a variety of human activities, including the introduction of European Green Crabs– which have moved north as coastal waters have warmed:
D. The biggest threat to clam species is not oysters, it is green crabs – which is demonstrated by substantial scientific data and experiential evidence from clammers. I won’t reiterate all of that evidence here, but I would like to highlight two quotes. First, from the website of the Downeast Institute: “Repeated independent field trials have found that predation is the primary factor affecting soft-shell clam survival (Beal et al. 2018), and that warming water in the Gulf of Maine has created an environment that favors predators such as the invasive green crab and milky ribbon worm, which have decimated soft-shell clam populations.” Second, from an article in Down East Magazine: “‘Everything is being systematically eaten [by the green crabs],” [president of the Maine Clammers Association, Chad] Coffin says. “All the shellfish resources disappeared based on proximity to the deepwater channel” [where the large male green crabs live]. These quotes provide both quantitative and qualitative evidence for green crabs being the biggest factor influencing clam population decline, not oysters.
E. What is more, oysters live in a different substrate from clams, and therefore are not competing with them for space. Specifically, adult oysters live on rocks, and adult clams live in mud.
So the species of oysters farmed in Maine are in fact native to the Maine coast, and they are not the primary factor causing declines in the clam population. Nevertheless, one point I’ve heard several times this year is that oysters are eating clam larvae. This is an oversimplification that is frequently taken out of context. Here I will elaborate with detail, as detail is necessary to address misunderstandings which are stemming from this oversimplification.
Generally speaking, oysters mostly eat phytoplankton –microscopic algae that float in the water column. Phytoplankton are the food that oysters and other bivalve shellfish species (clams, mussels, scallops etc.) need in order to thrive. The nature of bivalve biology is such that sometimes things other than phytoplankton can get drawn into the bivalves. To expand on this, let me provide a quick description of bivalve feeding: bivalve shellfish eat by drawing water over their gills ( similarly to finfish, shellfish have gills, too). Some particles in the water are too big to be processed by the gills and are immediately washed out again. Meanwhile, other particles are drawn into a gill structure that filters water– hence the name ‘filter feeders,’ which we often use to refer to bivalve shellfish. It is through this process of filtering water that shellfish extract phytoplankton and other small particles from their environment.
On the occasions that microscopic shellfish larvae are drawn into the gills of an oyster or another ‘filter feeder,’ they can either be (1) digested or (2) wrapped into mucus and expelled as pseudo f***s. A shellfish is capable of filtering and digesting (or filtering and expelling) larvae of other species or of their own species. To summarize, adult shellfish are able to consume larval shellfish. This is true for all filter-feeding bivalve shellfish– clams, mussels, oysters, etc. Adult clams can eat larval clams. Adult oysters can eat larval oysters. And so on.This is all a natural part of a healthy, dynamic ecosystem.
So, why has this concern about adult oysters eating larval clams arisen? Bear with me while I get a little nerdy…
In 1996 there was a study done by Tamburri and Zimmer-Faust in which several larval shellfish (including hard clams) were fed to Eastern oysters. This study found that Eastern oysters are physically able to digest some shellfish larvae and that they prefer them over inert particles– even when other food (phytoplankton) was available. (Inert particles are tiny pieces of things that are not food at all - in this study, scientists used foam and glass). So what does the Tamburri and Zimmer-Faust 1996 study tell us?
Oysters prefer to eat larvae over inert particles (foam and glass) - i.e. Eastern oysters prefer to eat larvae over things that aren't food.
Oysters ate larvae even when phytoplankton was present. This conclusion is what is being used as evidence that oysters eat clam larvae at high enough levels to cause declines in wild clam populations.
However, what is not mentioned in these conclusions is that the Tamburri and Zimmer-Faust study involved placing hard clam larvae directly inside the oysters in close proximity to their gills, which meant the larvae had no chance to swim away as they might in the wild. What is more, since these oysters were being intentionally fed clam larvae, they were able to consume an artificially high quantity of larvae that may not be possible in natural environments. Therefore, the Tamburri and Zimmer-Faust 1996 study does not give any evidence that oysters would prefer clam larvae over phytoplankton in the natural environment.
Additionally, the Tamburri and Zimmer-Faust study does not look into whether clam larvae can swim away from oysters to avoid being consumed – more research and evidence are needed to make a conclusion about this one way or the other. Furthermore, Tamburri and Zimmer-Faust do not demonstrate that oysters are actually able to consume soft-shell clam larvae at a level that would negatively impact clam population sizes. In fact, we do not have any research that supports oysters eating larval clams at a volume large enough to impact wild clam populations. What we do have, however, is abundant historical evidence that these two species have existed in abundance for millenia.
A 2010 Canadian literature review by Marie-Josee Abgrall and others looked at interactions between soft-shell clams and Eastern oysters. This review was commissioned because this same issue was raised by clam harvesters in eastern Canada. This review came to the same conclusion that I have outlined here– although additional field-based and laboratory studies would improve our understanding of interactions between populations of shellfish in the marine environment, the historical coexistence of Eastern oysters and soft shell clams is well established and beyond doubt.
