Lobsters may dive deeper as climate changes

           Wahle, R.A.; Dellinger, L.; Olszewski, S.; and P. Jekielek. 2015. American lobster nurseries of southern New England receding in the face of climate change. ICES Journal of Marine Science.  doi: 10.1093/icesjms/fsv093

Introducing the American Lobster

Summer in New England means lobster season. Natives will gladly tell you their favorite recipes for this delicacy, and whether they prefer Maine, Massachusetts, or Rhode Island caught crustaceans.  According to an old salts’ tale, it is still legal in Massachusetts for a lobsterman to shoot at someone he catches pulling his pots. Each year, thousands of pounds of shellfish are removed from New England coastal waters and sold commercially, according to the National Marine Fisheries Service.  The American lobster is iconic and economically relevant, but it may be at risk due to human induced rapid climate change.  Lobsters are particularly vulnerable to changing ocean temperatures because they spend a long time in the larval stage which is sensitive to extreme conditions.

Figure 1: Homarus americanus, the American lobster, in its rocky ocean habitat

Looking at Lobster Abundance over Time

Data on commercial lobster catches provide anecdotal evidence that lobster populations are indeed threatened by extreme events thought to be tied to rapid climate change.  The American lobster’s abundance peaked during the mid-1990s in southern New England.  Commercial lobster harvests were at historical highs during that period; however, shell disease, extremely warm summer temperatures, mixing of ocean layers from extreme storms, and heavy rainfall have all contributed to a dramatic decline in lobster landings.  During the summer of 2000, there was a 75% drop in Long Island Sound lobster populations which collapsed the local commercial fishery.  Further north, where summers remain cooler, sudden and short-lived heat waves in 2012 actually led to an over-supply of landed lobsters.  Processing factories that year were overwhelmed by the catch, causing the price of Maine lobsters to drop.

Many researchers believe that lobsters, like other temperature sensitive species, will shift their range northward in the coming years to seek out a more suitable, cooler habitat.  This study instead examined lobster populations at a range of depths, asking whether lobsters will also be shifting their habitat into deeper ocean, which can be cooler than coastal waters.

Survey Methods

This study evaluated shifts in the number of lobsters surviving to adulthood at several sites with different depths around Narragansett Bay, Rhode Island, using a previous study as a baseline for comparison.  The authors visited sixteen sites located along a range of depths in 1990 and again in 2011 or 2012.  Furthermore, a single deep water site was sampled once in 2012; this site was 17 meters deeper than the average depth of the other sites. The data on lobster abundance at each site was collected in three ways: 1) visual surveys, 2) suction sampling, and 3) passive post-larval collectors (Figure 2). 

• Visual surveys, performed in July and early August, were conducted by divers. The divers carefully overturned rocks and searched through crevices in boulders, working slowly to avoid clouding the water with sand and other sediment that would make it difficult to see the crustaceans.  They counted and measured the length of all the lobsters they found.
• Suction sampling occurred from late August until the first week of September. Diver pairs worked together to vacuum small, randomly placed patches along the sea floor.  Twenty of these identically-sized patches were sampled at each site, and post-larval lobsters were again counted and measured.
• Passive collectors like the one pictured in Figure 2 were placed at the sites to check whether the diver-based methods were accurate. These traps are large enough to support living organisms for several months, as they are mostly open to the elements.  Unlike traditional lobster pots, these allow the animals to move freely both in and out of the trap. Collectors were dropped at the sites in late May and retrieved at the same time as the suction sampling, in late August.

Figure 2: A passive post-larval collector like the ones used in this study

Results

The results showed a dramatic decline in lobster populations and sizes between 1990 and 2012 – there was nearly a ten-fold decrease in population density between the twelve years (Figure 3).  Animals were smaller and found at fewer sites in 2012 than in 1990.  Most of the lobsters found in the 2012 data were collected at the deepest site, which was 17 meters deeper than the average depth for the other sites.  Unfortunately, the deep site was not surveyed in 1990, so it is impossible to know whether lobsters were found there abundantly during that time.  It certainly is reasonable to conclude that lobsters are moving deeper in search of cool summer habitat.

Figure 3: Lobster density at the study sites over the three years sampled, showing a higher density during 1990 compared to the early 2000s.  Asterisks (*) indicated sites that were only sampled once.  Fort Wetherill is the deepest site, located 17 meters deeper than the average depth of all other sites.

Another interesting finding from the study involves a highly invasive species called the Asian shore crab (Figure 4).  Asian shore crab populations have exploded in recent years from southern New England down to the Chesapeake Bay.  This species thrives in many different coastal habitat types, breeds quickly, and eats a variety of different foods.  Native crab populations have dwindled in the face of this invader.  While this data was not quantified in the current study, the authors noted that Asian shore crabs were found at all of the sites except the deepest site surveyed.  It is possible that the Asian shore crabs are out-competing lobsters in shallow waters but are limited by extreme depth.  Certainly, further study about the effect of Asian shore crab on lobster populations is needed.

Figure 4: Hemigrapsus sanguineus, the Asian shore crab, with a penny for size comparison

Conclusions

This study shows that coastal lobster populations have all but collapsed in southern New England.  While lobster ranges may be shifting northward, they are also likely shifting into deeper waters, which will make catching them more dangerous and difficult for lobstermen.  While this shift is probably due primarily to climate change, an invasive species called the Asian shore crab may also be influencing lobster populations.

Questions?  Comments?  Please sound off below!  I’d love to hear from you :)

Dina Navon

I am a doctoral candidate in the Organismic and Evolutionary Biology program at the University of Massachusetts Amherst. I’m interested in how an individual’s genes and the environment in which it grows come together to determine its physical traits. I study a group of closely related freshwater fish called cichlids which live in the African rift lakes like Victoria, Malawi, and Tanganyika.