LOBSTER BIOLOGISTS ON LOBSTER
continued from Home Page
I think warming explains
the very large increase
in settlement and
recruitment in eastern Maine
and in the Bay of Fundy.
– Bob Steneck
Fishermen’s Voice editor, Mike Crowe, not to be out-done, came up with seven more questions, which Fishermen’s Voice asked lobster biologists Richard A. Wahle, Ph.D., Research Associate Professor, School of Marine Science, University of Maine (richard.wahle@maine.edu); Bob Steneck, Ph.D., Professor of Oceanography, Marine Biology and Marine Policy, School of Marine Sciences, University of Maine, (steneck@maine.edu); and Fisheries and Oceans Canada lobster biologist Michel Comeau, M.Sc., (michel.comeau@dfo-mpo.gc.ca).
Professor of Marine Biology at Boston University lobster biologist and adjunct scientist at the Woods Hole Oceanographic Institution, Jelle Atema, Ph, D. (atema@bu.edu), reached via e-mail while traveling, responded to Wilson’s and Crowe’s questions with the forthright, “I find the questions quite important and at the same time have no answers. I have come to think that we have a pretty primitive concept of lobster biology with many observation points but few connections to see the whole picture. The specific point you address seems to be what the consequences may be of a sustained 20% annual increase in catch. (He wrote, ‘An occasional increase is normal, but sustained annual increases would be of concern; it would signify an unbalanced ecosystem and we would not really know the causes. Several have been suggested.) In most populations it will lead to a crash at some point. A likely candidate would be a disease or parasite; another may be a new predator, but these are speculations on my part.” Although Atema said he was willing to discuss his answers in more detail on his return, he insisted, “I am most sure that we do not have the correct answers,” followed by, “Quite unsatisfying.” He then explained, “I mean that we lobster biologists are only beginning to get some idea of lobster life and its population fluctuations. Some ideas may be wrong. Other facts may be right, but cannot be placed in the proper context.”
Asked how readers could see some of the documentation, Wahle attached a paper from the scientific journal Marine Ecology Progress Series that he co-authored with Mark Gibson (RI Dept. Environmental Management), and Michael Fogarty (NMFS) entitled: “Distinguishing disease impacts from larval supply effects in a lobster fishery collapse” about shell disease in southern New England. (Vol. 376: 185-192, 2009). He also attached the 2011 Update of the American Lobster Settlement Index, and wrote, “We’re also developing an interactive link to the data on the Gulf of Maine Council’s EcoSystem Indicator Partnership website: There’s a link under Fisheries where you can map and graph the Lobster Settlement data. (http://www2.gulfofmaine.org/esip/reporting/gmap.php).”
Comeau attached an extended abstract or executive summary of a Science Advisory Report (http://www.dfo-mpo.gc.ca/csas-sccs/publications/resdocs-docrech/2008/2008_054-eng.htm). He also wrote, “ First, increased catches have been reported (by harvesters and Provincial colleagues) for all our spring fisheries (LFA 23-24-26A and 26B) in 2012. Secondly, DFO do not assess landing fluctuations on a yearly basis, but long trends within our reported landings.”
Crowe’s questions:
#1. Why were there landings increases in 2002 and 2004? He added, “The 2012 increase is being attributed, at least in part, to earlier large recruitment and warmer water.”
Wahle responded, “Several factors may be at work in explaining landing fluctuations in any given year. They include the strength of the upcoming year classes, temperature, the molt, and fishing effort. Fishing effort is influenced by the economics of fishing and changes in regulations. That’s why by looking at time trends in fishery independent data, like the Maine near-shore trawl survey, permits you to rule out changes in fishing effort as a factor contributing to variability.
Steneck answered, “Warm water can affect molt timing. If molt (shedding) is early, then there is a good chance that lobsters will shed twice in the year. When that happens you get higher landings. Often the later, second shed may delay the next year’s shed, so for 2002: two shed, 2003: one shed, 2004: two shed. That’s speculation - I don’t think we have data on this, but some of the lobstermen I’ve been talking to recently agree with me.
“Recruitment is greater in warmer years because more of the sea floor is above the ‘thermal threshold’ that lobsters need to settle to the sea floor.
“Carl may have also pointed out that the brood-stock has also been growing, so more larvae may be in Maine’s waters. Sadly, no one monitors lobster larvae anymore. The last surveys were about a decade ago.”
Comeau replied, “Recruitment indices within the southern Gulf of St Lawrence (sGSL) have been positive for quite a few years. At this moment, it is difficult to link this wave of good recruitment to bottom water temperature, as we do not see a clear relationship.”
#2. Is there a pattern in the years of these landings increases? If so, what does the pattern suggest?
Wahle wrote, “Rather than focus on any given year, such as 2002 and 2004, the real question is: What is controlling the long term trends? Landings in Maine have been generally increasing since the late 1980s with a few ups and downs so that now Maine is landing about 5 times what it had been landing then. As I stated above, you always have to be cautious about using landings as an indicator of abundance because changes in regulations and economics can also influence landings. But the long-term upswing in lobster landings definitely correlates with fishery-independent surveys that also show an upturn. There is some pretty strong evidence that the increase in lobsters in the Gulf of Maine can be explained at least partly by the depletion of their groundfish predators. But it’s also true that warming temperatures especially in eastern Maine may be creating more favorable conditions for settling larvae there. The lobster settlement index reveals growing densities of young-of-year lobsters in nurseries Downeast since the early 2000s. Back in the 1980s there were very few young-of-year lobsters in eastern Maine; now the nurseries are chocked full!”
Steneck responded: “I think I covered that with my first paragraph above.”
Comeau said, “ We are monitoring trends over a long period, not on a yearly basis. We are presently analyzing our landings trends (we will present the results at the Regional Assessment Processes in March 2013), but preliminary analysis seems to indicate an increase in landings for the majority of our fisheries in the last 5 years. [It is] important to note that a significant increase in landings was observed starting in the mid-1970’s (see Figure 2 of SAR).”
Atema stated, “The first two questions are better answered by Carl who has all the population data.”
#3. Is there any evidence that these increases will become the norm, given the probable permanence of warmer ocean water?
Wahle replied, “Sometimes I wonder if there is a norm!”
Steneck wrote: “There is excellent evidence that Maine’s surface waters are warming. Look at DMR’s records going back nearly a century. There is also a suggestion that the colder Maine Intermediate Water is shrinking due to warmer-than-average winters. That may have triggered the early shed, especially in western Maine.
Comeau responded, “The answer to this question would be highly speculative. We know water temperature is an important factor in the life cycle of lobster; however, it is not the only factor. The environment cannot be summarized to only temperature.”
#4. At what water temperature/point in time will lobster migrate north, making Canada the new Maine?
Wahle wrote: “Well, that’s a complex picture. Eastern ME and SW Nova Scotia seem to be seeing some of the greatest increases in numbers, suggesting favorable conditions, relative to the south. On the other hand, the southern Gulf of St. Lawrence, particularly Northumberland Strait gets stressfully warm in the summer, much like Long Island Sound, and there are signs lobsters are either dying or moving away from those hot spots. So it’s not a simple north-south, US to Canada story. Northumberland Straight is shallow and semi enclosed, not unlike Long Island Sound. So, in summer it warms up quickly and in the fall it cools down quickly. Unlike LIS, it freezes over. (Sometimes it’s the extreme events that have the greatest impact: Long Island Sound had a mass mortality of lobsters in the late 1990s during an extremely warm summer from which it has not recovered.)”
Steneck wrote: “ I doubt they will migrate north, but new recruits may be more successful farther to the north (and east in Maine). I think warming and possibly slowing of the Eastern Maine Coastal Current explains the very large and well documented increase in settlement and recruitment in eastern Maine and in the Bay of Fundy.” He then added, “It is unclear to me that lobsters will “migrate north.” Very few tagging studies have ever recorded a net northward migration. I think larval settlement success can increase to the north and disease related mortality will increase to the south. So, as a result, the demographic center of gravity (so to speak) will shift northward, but I doubt you’ll see lobsters migrating up there.
“The southern populations will more likely suffer disease as sea temperatures increase and lobsters become stressed. This has already happened in Rhode Island. It never recovered from the 1998 shell disease.
“Canadian landings already surpass Maine and New England’s. So Canada is doing fine and. of course, because they only fish three months of the year in any sector, they have product year round, so they are able to be the distributor of lobsters to the world. Maine floods the market and then runs dry of lobsters other months.”
Comeau responded, “[I have] no information [to offer] on that issue. It is, however, interesting to note that the southern Gulf of St. Lawrence landings started to increase in the mid-1970s (see figure 2 of the SAR), which is somewhat similar to what was observed in Maine and along the coast of Nova Scotia.”
Atema replied, “In a way you can say, we see that in all sorts of animal and plant populations over the last decades, so in a way you could predict that would be the case. What we do not know is the consequences. For instance, question #5 is one of those [consequences].”
#5. What effect does water temperature have on recruitment?
Wahle replied, “Favorable if it’s not too warm. 20°C (about 68F) seems to be a physiological threshold for lobsters. They thrive below that temperature, but start to show signs of metabolic stress above it. Southern New England and the Northumberland Strait are seeing summer temperatures above that threshold with increasing frequency.”
Steneck said, “ Lobsters recruit in waters warmer than about 12 or 13 degrees C (53.6 or 55.4 F). More of Maine’s sea floor is in that category.”
Comeau wrote, “ See Answer #3.”
Atema said, “#5 is an example of what we do not know about the consequences of water temperature on recruitment. [It] is dependent on so many factors that I don’t think we can honestly predict [the consequences]because there will be effects on predators as well when the water temperature increases. There will be an effect on the entire eco-system. To what degree that will benefit or reduce recruitment, we really don’t know.”
#6. Are there any scientific records of water temperature, recruitment, shedder numbers, and the lobster population declines seen in southern New England?
Wahle wrote, “Yes - See the attached Settlement Index update for 2011 and my 2009 publication in Marine Ecology Progress Series.”
Steneck responded: “Sea temperatures, recruitment and lobster population declines in southern New England are well documented by Rick Wahle and others.”
Comeau replied, “The question should be addressed to lobster biologists from southern New England.”
Atema said, “Of course, I’ve been involved with this shell disease project that has just been published in the Journal of Shellfish Research. The whole issue is devoted to lobster shell disease research spearheaded by Kathy Castro in Rhode Island. (Vol. 31, #2. June 2012).
“What this group of scientists seriously considered, but with no conclusive answers, is that shell disease was in part caused by stress, which was caused by abnormally high temperatures. We don’t know if that affects the settlement of the larvae, but it certainly shows its effect on the animals, particularly some of the mature females, which spend more time in their old shells than males because of egg bearing. So there is a clear indication that it could be an effect.
“The other causes people have brought out are water pollution, some chemical pollution, oil spills, the estrogen mimics that are used in so many compounds—these have all been proposed without having conclusive answers. It’s not that we can say, This is the culprit. So we cannot say, therefore, whether the effect of temperature is directly affecting the population because it really has crashed: the population—landings are way down—or that [temperature] has an indirect effect through shell disease, for instance.
“And you could also argue that because the lobster population as we know it does not go much further south than New Jersey, maybe; so that seems to indicate that it is really a cold water species. That’s just a platitude. I think we all know that.”
#7. What effect does shedding early have on the rest of the lobster’s annual life cycle?
Wahle responded, “Shedding early means the lobster is growing faster. In much of Maine lobsters of legal size molt once a summer, but in warmer years, there may be a second molt. In general, warmer temperatures mean faster growth rates.”
Steneck wrote: “Other than what I say above, early shedding likely translates to higher growth rates. There are lots of possible consequences, but this is tough to predict.”
Comeau responded, “This response is strictly for stocks in [southern Gulf of St. Lawrence] sGSL based on our observations (see Comeau et al. 2001 Journal of Crustacean Biology, 21(4): 923-936 for more details). Early molting could indicate that some lobsters would molt twice during that year (mainly lobsters <83mm of carapace length), otherwise there are no signs of a different life cycle.”
Atema said simply, “Again, I have no answer to that. One could speculate: there could be pros and cons to that. It might be that animals would grow more because they could shed twice in one season if they have enough food and can grow fast enough, but again, I think that this is not really scientific data.”
Notes/Glossary:
Young-of-year (aka settlers) are the baby lobsters that enter the population by larval settlement to rocky cobbled nursery grounds. They usually settle in late summer, which is why lobster scientists do their counts in early fall.
Wahle explained, “A little warming in the north where waters are cold, actually promotes lobster growth and larval settlement, but additional warming in the south crosses a physiological maximum temperature threshold, making it stressful.
“Recruits are lobsters that enter the fishery by growing into the legal size. With the Settler-to-recruit analysis, we’re basically trying to follow year classes through time. So we compare our counts of young-of-year to the counts of older lobsters from trawl surveys about 5-6 years later when we would expect them to be of an age just about to enter the fishery.
“The spawner-to-recruit analysis relates the abundance of parents (spawners) to the numbers of offspring they produce that enter the fishery.”