So scientists and fishermen and everyone else rely on computer models that mimic what is known about fish. Into the models goes information like size, age, growth rate, how many fish will die of natural mortality (predation, disease, moving away from the area) and how many are taken in the fishery.
But lobsters, Maine’s largest and most economically important fish, aren’t fish, and they present a number of challenges to modelers. Genny Nesslage, a stock assessment scientist with the Atlantic State Marine Fisheries Commission, points out that this is what makes modeling lobsters so tricky. “For one, they don’t just age and get bigger like finfish-they have molting events (when they shed and build a new shell). If females are egg-bearing generally they don’t molt, if they aren’t bearing eggs, than they do.”
Lobsters lack body parts like ear bones that help to reveal the age of other species; as a result, modelers use lobster size instead. But even size can be misleading, because lobster growth rates also vary with temperature: the warmer the water, the faster they grow. And since most lobsters move around throughout the year and over the course of their lives, their growth rate does not stay the same.
Variable growth rates can have surprising effects. According to Dr. Yong Chen, a fisheries scientist at the School of Marine Sciences at the University of Maine, the lobsters that grow into the size that can be legally harvested during any given year can include individuals born over a seven-year time span. In other words, the lobster on your plate could be four years old and the one on your friend’s plate could be eleven years old, even if they are both one-pound lobsters that were caught in the same trap.
These facts complicate computer models. Throughout the 1990s and early 2000s, the National Marine Fisheries Service model consistently underestimated the number of lobsters in the sea, and therefore overestimated the percentage being caught in the fishery each year, leading federal scientists to believe that overfishing was occurring in the Maine lobster industry. Yet year after year, catches went up and research surveys recorded higher and higher numbers of lobsters. Clearly the model wasn’t working.
Fast forward to 2008: The Atlantic States Marine Fisheries Commission and the National Marine Fisheries Service officially adopted a completely new model that estimated the lobster population as “not overfished.”
This transformation occurred largely thanks to the talent and tenacity of Dr. Yong Chen.
“In fishery science you only see parts of the real story-like the five blind men and the elephant: you feel the tail, the leg, the trunk, and each give you part of the information about what is really there,” said Chen. “With the computer model we essentially create a model story, trying to get as close to the real story as we can on the number of lobsters in each size class, how fast they are growing, how often they molt, how many are caught in the fishery, how many die from other causes.”
According to Chen, there are four main areas where his model improved on the prior version. “We included the inshore trawl data from Maine and other state surveys, in addition to federal survey data; we had better catch data to work with than before; we had more realistic biology built into our virtual lobsters; and we used a statistical approach that incorporates margins of error in our inputs (this approach uses Bayesian statistics),” he said.
Nesslage noted that one of the major benefits of Chen’s model is that it accurately characterizes-using data from the Maine Department of Marine Resources’ sea sampling program-the conservation measures associated with minimum and maximum length rules, and v-notching protection for egg-bearing females.
In Chen’s view, these practices give the Maine lobster industry an insurance policy by maintaining a high number of female lobsters old enough to reproduce. His group has shown that without these conservation practices, the number of new lobsters that recruit into the virtual fishery declines. In a sense, he helped prove what Maine lobstermen had been saying for years: V-notching makes a difference, and the men and women on the water have knowledge that can inform science and management.
Today, concerns for the future of the fishery are much more focused on price than volume, although fisheries scientists continue to have concerns about the large percentage of lobsters that grow into minimum legal size each year that are harvested. But for the time being, Nesslage said the data confirms what lobstermen observe on the water, “Enough lobsters make it through the gauntlet of the fishery during their recruitment year, and become larger than maximum size, shoring up the ability of the population to maintain itself and a large fishery at the same time.”
This article is made possible, in part, by funds from Maine Sea Grant and the Oak Foundation.
Heather Deese holds a doctorate in oceanography and is the Island Institute’s director of marine programs. Catherine Schmitt is communications coordinator for Maine Sea Grant.