Wahle takes multifaceted approach to studying lobster populations in the Gulf of Maine
South of Cape Cod, in Rhode Island, Connecticut and as far south as Long Island, lobster fisheries have virtually collapsed. This is due, in part, to stressfully warming temperatures, low oxygen conditions, and the spread of shell disease that have triggered mass die-off of lobster populations, according to Richard Wahle, a research professor in the University of Maine School of Marine Sciences.
The Gulf of Maine’s waters, by contrast, have historically been on the colder end of the lobster comfort zone, compared to warmer waters in southern New England, so the temperature increase in ocean waters has favored not only an increase, but an unparalleled boom, in lobster populations in Maine over the same time.
This boom has elevated the US lobster fishery to its current status as the most valuable single fishery in the United States, Wahle says. Ninety percent of that value now comes from the Gulf of Maine.
“In the 2000s into the 2010s, we’ve seen the lobster population here explode just as things are collapsing in southern New England,” says Wahle. “And research shows it’s likely the boom was assisted by the overharvesting of predatory groundfish, such as cod and flounder, that prey on juvenile lobster.”
Either way, the lobster benefitted, and the Maine lobster harvest increased about sixfold, from 20 million pounds a year in the late 80s and early 90s to over 120 million pounds a year in between 2013 and 2016.
As Maine’s chief export commodity, lobster is nothing short of an economic driver of Maine’s coastal economy. “Whether you’re a restaurant owner, a trap maker or a boat builder, or a tour boat operator, in some way, you’re probably connected to the lobster fishery here,” Wahle says.
Wahle is director of the University of Maine’s Lobster Institute, which promotes, conducts and communicates research focused on the sustainability of the American lobster fishery in the United States and Canada.
Wahle and his collaborators have a long-standing track record of trying to understand the drivers of lobster population change by following the fate of lobster year classes from hatch to catch. Doing that requires detective work and a long-term commitment. It takes a newly hatched lobster about 5 to 10 years to grow large enough to be legally harvestable, depending on where you are.
Their work has revealed some surprising indirect connections to climate change.
In the late 1980s, Wahle developed a method to census baby lobsters newly settled from the planktonic larval stage to the rocky seabed in coastal nurseries. “This census employs a diver-based methodology that involves an underwater vacuum that allows you to suck up these little babies from between cobbles where they take shelter,” Wahle says. From their fall surveys, they could distinguish the “young-of-the-year” by their small size from older, larger, juveniles still sheltering in the same nurseries.
Taking all the counts together from dozens of sites up and down the coast over many years, Wahle’s team discovered the size of baby populations can be used to predict the size of the harvest some six years later. Settlement predicted the boom that peaked between 2013 and 2016. Despite the harvest boom over the past decade, declines in settlement since about 2012 portend a downturn to come. Indeed, since the 2016 peak, Maine lobster landings have fallen off by more than 25%. “The next frontier is really the Gulf of St. Lawrence and Newfoundland… those areas are seeing increasing trends in harvest now,” Wahle says.
“Even as Maine’s broodstock has been near an all-time high, our larval settlement indices were near historic lows,” said Wahle. “When we started to look at what might explain the downward trends in larval settlement, we were astonished to find it to be strongly correlated with what we think might be the key larval food – a tiny planktonic crustacean no bigger than the head of a pin called Calanus finmarchicus.”
Calanus finmarchicus, is a foundational species in the food web of the North Atlantic ecosystem, including the Gulf of Maine, at least for now. Despite its small size, it is packed with fats that are well known to be high-energy food for herring and the endangered North Atlantic right whale to give just two noteworthy species. As a cold-water-loving species it is also at the southern end of its range in the Gulf of Maine, and its population there has been on the decline, as its distribution has been receding northward over the past decade.
It is already well documented that herring populations have been on the decline in the Gulf of Maine, and harvest quotas have followed suit. Right whales have been tracked to be moving out of the Gulf of Maine and into richer feeding grounds in the Gulf of St. Lawrence. Wahle sees the northward shift in herring, right whales and lobster to be at least partly linked to the shift in Calanus finmarchicus.
But correlation does not necessarily mean causation. With support from Sea Grant and the National Science Foundation, Wahle and his team are also conducting intensified analyses that they hope will better test whether Calanus is a limiting food source for larval lobster. Collaborators on the study include David Fields and Peter Countway of Bigelow Laboratory for Ocean Sciences and Rachel Lasley-Rasher, of the University of Southern Maine.
Intensive field sampling is giving a better picture of the distribution and abundance of lobster larvae and their planktonic prey. From these samples UMaine Marine Science graduate students, Alex Ascher and Evelyn Layland, are conducting a comprehensive gut content analysis of wild-caught larval lobsters.
Larval lobsters are about the size of a peppercorn, and the food that they eat is barely visible to the naked eye, so the analysis takes place on a microscopic and molecular level. To date, microscopy has revealed lots of tiny fragments of crustaceans in the larval gut, but it’s hard to know what species they are. DNA sequencing methods now being developed are likely to unlock the identity of larval prey.
“We also want to know how selective larvae are with respect to different prey available at that time of year. Do they selectively feed on Calanus, or are they indiscriminate?” Wahle explains. “We’re conducting a whole battery of laboratory-based feeding experiments that will provide a new line of evidence that will inform our understanding of climate impacts on the lobster population and its ecosystem.”
Written by Ali Tobey