UMass studies document scallop die-off and predator invasion
Monitoring seen as critical to rotational fisheries management
New research by University of Massachusetts scientists underscores the crucial role of monitoring in rotational management, a common strategy for fisheries managers. In a pair of articles published in the October and November issues of the journal Marine Ecology Progress Series, Professor Kevin Stokesbury and Research Associate Michael Marino, both of the UMass Dartmouth School for Marine Science and Technology (SMAST), and their co-authors focused on changes in the Atlantic sea scallop fishery under a regime of rotational management.
"The two studies examine different types of natural mortality in sea scallops," said Stokesbury, who chairs the Department of Fisheries Oceanography at SMAST, "but they both highlight the critical importance of frequent, quality observations to the success of rotational management."
Together, the papers document greater concentrations of sea scallop predators (sea stars) on closed areas of Georges Bank as compared to areas open to fishing, and a mass mortality-or "die-off"-of scallops in the Nantucket Lightship Closed Area (NLCA), a Marine Protected Area. The die-off, which occurred between summer 2004 and summer 2005, was estimated by Stokesbury and his co-authors to have removed about 100 million dollars in scallop meat from the fishery at then-current ex-vessel prices.
The terrestrial analog of a Marine Protected Area is a farmer's field left fallow for a period to recoup its nutrients. In rotational fisheries management, fishing is restricted in closed areas so that target species populations can rebound. In the absence of external factors, such as fishing, a depleted population is expected to grow toward the level that the ecosystem can support. However, even with a relatively sedentary species like the sea scallop, the reality is not so straightforward.
The process is complicated, said Stokesbury, by natural population cycles that are as yet poorly understood. Based on five years of survey results, he and his co-authors postulate that newly settled scallops that entered the NLCA population around 1994, when the area was first closed, grew to maturity in numbers undiminished by fishing. But the density of scallops on the sea floor may affect settling of the young, as subsequent year classes were much smaller. So when the 1994 generation reached old age a decade later, they became susceptible to parasites and disease and a large number died in a very short time, leaving a population deficit that the generations behind them were not sufficient to offset.
What happened in the NLCA, Stokesbury pointed out, highlights the nature of the monitoring/management relationship. "We don't yet have the knowledge to predict reliably what will happen in a closed area on a scale of years or decades, so we need to adjust our management plans in the light of regular observations in order to avoid both overfishing, on the one hand, or underutilization, on the other, which can be nearly as costly."
The study led by Marino, as part of his master's research, hypothesized that sea stars would thrive where their prey was abundant, which should be observable as more and larger sea stars in closed areas than in areas where the scallop population was being reduced by active trawling. And in fact, according to Marino, "In the summer of 2003, we observed a significant sea star predation event in the southern portion of Closed Area II. What seemed to be a highly dense wave of sea stars moved across the area preying on the scallops in its path." Detection of this high natural mortality event led to the area being opened to fishing earlier than scheduled.
"SMAST's annual video survey of the Atlantic sea scallop resource, made possible by the support of the scallop industry itself, has opened up new research opportunities in fisheries science," said Stokesbury. "These two studies demonstrate how critical it is to continue tracking these marine resources."
"The two studies examine different types of natural mortality in sea scallops," said Stokesbury, who chairs the Department of Fisheries Oceanography at SMAST, "but they both highlight the critical importance of frequent, quality observations to the success of rotational management."
Together, the papers document greater concentrations of sea scallop predators (sea stars) on closed areas of Georges Bank as compared to areas open to fishing, and a mass mortality-or "die-off"-of scallops in the Nantucket Lightship Closed Area (NLCA), a Marine Protected Area. The die-off, which occurred between summer 2004 and summer 2005, was estimated by Stokesbury and his co-authors to have removed about 100 million dollars in scallop meat from the fishery at then-current ex-vessel prices.
The terrestrial analog of a Marine Protected Area is a farmer's field left fallow for a period to recoup its nutrients. In rotational fisheries management, fishing is restricted in closed areas so that target species populations can rebound. In the absence of external factors, such as fishing, a depleted population is expected to grow toward the level that the ecosystem can support. However, even with a relatively sedentary species like the sea scallop, the reality is not so straightforward.
The process is complicated, said Stokesbury, by natural population cycles that are as yet poorly understood. Based on five years of survey results, he and his co-authors postulate that newly settled scallops that entered the NLCA population around 1994, when the area was first closed, grew to maturity in numbers undiminished by fishing. But the density of scallops on the sea floor may affect settling of the young, as subsequent year classes were much smaller. So when the 1994 generation reached old age a decade later, they became susceptible to parasites and disease and a large number died in a very short time, leaving a population deficit that the generations behind them were not sufficient to offset.
What happened in the NLCA, Stokesbury pointed out, highlights the nature of the monitoring/management relationship. "We don't yet have the knowledge to predict reliably what will happen in a closed area on a scale of years or decades, so we need to adjust our management plans in the light of regular observations in order to avoid both overfishing, on the one hand, or underutilization, on the other, which can be nearly as costly."
The study led by Marino, as part of his master's research, hypothesized that sea stars would thrive where their prey was abundant, which should be observable as more and larger sea stars in closed areas than in areas where the scallop population was being reduced by active trawling. And in fact, according to Marino, "In the summer of 2003, we observed a significant sea star predation event in the southern portion of Closed Area II. What seemed to be a highly dense wave of sea stars moved across the area preying on the scallops in its path." Detection of this high natural mortality event led to the area being opened to fishing earlier than scheduled.
"SMAST's annual video survey of the Atlantic sea scallop resource, made possible by the support of the scallop industry itself, has opened up new research opportunities in fisheries science," said Stokesbury. "These two studies demonstrate how critical it is to continue tracking these marine resources."
