36 JUNE 2024 • WORLD AQUACULTURE • WWW.WAS.ORG observed during an extreme low salinity event. Qualitatively, the observed mortality often leads to the affected grower reaching out for assistance to determine the cause. • ‘Syndrome’ — Frustratingly, these mortalities have been considered to be unexplained given the lack of relationship to the ‘usual suspects’, such as disease, extreme environmental conditions, culture methods, and so on. To date, SUMS events have been characterized by the described set of signs that occur together during these events. The reports of mortality are usually from intensively cultured oysters, or those grown in containers, though there is some evidence that even extensively planted oysters may also be affected. There is no evidence that wild oysters experience this mortality, but the question remains as to whether they might exhibit the same syndrome if subjected to the potentially more stressful conditions associated with aquaculture — a question that awaits experimentation. There appears to be no parasitic or other infectious agent associated (Figure 2), although work on “summer mortality” in other systems, for example, Pacific oyster culture in Europe and the Pacific Northwest, has revealed underlying bacterial etiologies that may be more widespread. Whatever the causes of this syndrome, the reality is that it has been documented to cause sudden mortality spikes at farms widely spanning the Atlantic and Gulf coasts and is an urgent concern for eastern oyster aquaculture interests at the national level. Due to its rising significance [with over 150 cases alone submitted to the Virginia Institute of Marine Science (VIMS) from Maryland to Texas since 2012], the mortality syndrome has become a prominent area of focus for research at numerous institutions in the eastern and southern U.S. (see recommended reading below for examples) and a regular topic of presentations and panel discussions at the annual Oyster South Symposium. These efforts have resulted in a steady increase in the amount of published scientific literature on the subject each year, contributing to a better understanding of the syndrome. Despite these efforts, the industry has emphasized the urgent need to accelerate research focusing on the syndrome to move more For an oyster grower, it’s a sickening feeling: dumping a bag of oysters out on the culling board and hearing the distinctive clatter of the empty shells of dead oysters. A grower finding bag after bag of nearmarket size oysters that had been fine the week before, now full of mostly empty shells and a handful of live oysters, immediately asks, “why?,” and, “how can I prevent these losses?” When samples are collected and analyzed, and environmental conditions and culture methods are reviewed, no clear causative agent is identified. What’s going on? Initially, dubbed “triploid mortality,” as many early significant reports were observed in the triploid crops that most farmers were growing, we name this current phenomenon ‘sudden unusual mortality syndrome’ or SUMS. SUMS events in eastern oysters on the U.S. Atlantic and Gulf coasts have been observed since at least 2012 (when it was first reported by Virginia commercial shellfish growers) and have been reported to affect oyster farms throughout the southeastern U.S. These unpredicted spikes in mortality have threatened the financial survival of the affected farms, particularly in areas like North Carolina, where severe mortality events have been observed annually at some sites. At the 2024 Oyster South Symposium, 62% of the sixty growers surveyed during a presentation on the mortality had experienced at least one SUMS event. At the 2024 North Carolina Aquaculture Development Conference, 89% of eighteen growers surveyed reported experiencing at least one SUMS event, with 39% experiencing three or more such events. Not surprisingly, those surveyed at these presentations declared that SUMS is a ‘mission critical’ research priority. What characterizes a SUMS event? • ‘Sudden’ — Typically, these mortality events occur in the spring and/or summer of the crop’s second year (Figure 1), a time of elevated air and water temperatures and peaking oyster reproduction, with mortality occurring in a sudden, relatively short period (i.e., weeks). • ‘Unusual’ — The observed mortality rates are unusually high and well beyond the expected, typical, low-level, chronic mortality that many farmers observe. These mortalities are also not readily explained by an obvious environmental challenge, such as might be Sudden Unusual Mortality Syndrome (SUMS) in Eastern Oysters William C. Walton, Tal Ben-Horin, Jessica M. Small, Adriane K. Michaelis and Ryan B. Carnegie FIGURE 1. Oysters near market size, such as these, are most often involved where SUMS events are observed. Photograph by Bill Walton.
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