Nasty neighbors, bad food and lousy accommodations

The complete version of the article was first presented at the “American Lobster in a Changing Ecosystem” symposium, held on Prince Edward Island in November, 2015. Reprinted in Landings, December, 2015, with permission.

Anyone working in the North Atlantic recognizes its dynamic nature. For scientists trying to understand lobster biology, the strong seasons and complex life cycle, they must deal with the added challenge in trying to evaluate how ongoing ocean warming, acidification, and other human-induced changes may alter future lobster populations. Changing ocean conditions across the entire range of American lobster could mean nasty neighbors, bad food, and lousy accommodations. In other words, these changes may favor invasive species, alter food supply for lobster, and change environmental conditions.

“Holding Ground,” by Peter Ralston. Image courtesy of the Ralston Gallery, Rockport, Maine.

“Holding Ground,” by Peter Ralston. Image courtesy of the Ralston Gallery, Rockport, Maine.

Because lobster larvae have a swimming larval stage, changes on the seafloor as well as in the water above it could be significant; it’s a top-to-bottom problem in the coastal ocean. Indeed, the swimming larval stage lobsters interact with a completely different set of species than in their juvenile and adult stages. Warmer waters, changes in ocean currents, acidification, pollution, and changes in food webs could all affect the swimming larval stages. Similar factors could affect juveniles and adults on the seafloor, but with the added pressure of fishing.

Changes on the seafloor

Lobster are part of a complex, interlinked food web of many species that has changed even within our lifetimes, as cod stocks have declined dramatically with resulting complex effects on their prey. Declines in cod in Canada coincided with increases in lobster to the south and snow crab and shrimp to the north. One study in the New England region shows that the distances over which adult lobster move contract when cod are present. There’s clear evidence of cannibalism in lobster, so even high abundances of lobster have ramifications for lobster survival.

The arrival of invasive species, most noticeably green crab, adds to the mix. Memorial University PhD student Melanie Rossong Barrett found relatively little habitat overlap between green crab and lobster in southern Newfoundland and only modest habitat overlap between small green crab and small rock crab, a major prey item for lobster. But the overlap was greater for larger individuals of both species raising concerns about food web effects. Her laboratory studies also found that small lobster fed less and were less likely to leave their shelters in the presence of green crab. Furthermore, the species occupying the seafloor changed in the presence of green crab, whether in mud, sand, or eelgrass. Thus, green crab may change the availability of food for juvenile and adult lobster.

Juvenile and adult lobster require specific habitats that range from cobble shelter for juvenile lobster to muddier seabed for burrowing as lobster get older. But we know that ocean warming has resulted in northward shifts in species distributions in this hemisphere already, and several studies point to the importance of temperature for adults and the depths and locations where they occur. The temperatures experienced from 10 to 25 meters depth play a significant role in defining adult lobster distribution; indeed, adults move around much more outside their ~ 14-16 °C. (57-61°F.) “comfort zone.” Seafloor habitat does not shift with temperature, which raises the question: As warming oceans slowly push lobster northward, will they find suitable habitat? We generally lack sufficiently detailed data to answer that question. Several studies suggest we can expect modest effects of ocean acidification on adult lobster, though some of their potential prey may not fare as well. We also know that lobster often use kelp as habitat, and native kelp abundance is generally declining because of competition with invasive kelps and attachment by bryozoans that destroy kelp blades. In all likelihood, warming oceans, increased acidification and invasive species will change adult lobster accommodations, with poorly understood consequences for future populations.

Changes above the seafloor

We know even less about what happens during the lobster larval stages or even where those larval stages occur. We do know that most larvae die, and stomach contents of schooling fish show they feed on larval lobster. We know almost nothing about what larval lobster eat. A few studies point to small crustaceans as their preferred food, mostly based on laboratory observations. Some evidence also suggests that diet may change as larvae develop. But we really have no idea how lousy neighbors and bad food might become a problem for larval stage lobsters.

Our knowledge gaps on where larvae actually live complicate the question of the effect of changing “accommodations” because the water, ocean currents, temperature and food all contribute to what defines larval habitat. Memorial University PhD student Ryan Stanley found significant differences in the swimming behavior of larvae spawned from different geographic regions as a function of water temperature. Using models of ocean circulation he also found that the timing and location of spawning had the greatest effect on where larvae were transported, but once again these findings must be set against a backdrop of potential changes in water temperature and circulation as warming continues. Though not well studied, evidence to date suggests that ocean acidification will delay larval development, which could increase dispersal and exposure of vulnerable larval stages to predators.

What can we do about changing oceans?

The short answer on changing oceans is that we cannot do much as individuals other than try to ensure robust lobster populations and minimize human impacts, including recognizing and trying to do our bit to slow climate change. But we can work to mitigate the impacts of forthcoming changes through better planning and better understanding. In coastal Newfoundland, lobstermen in the town of Eastport developed their own Marine Protected Area for lobster, where local fishermen agreed to set aside several small areas where nobody could fish. Such efforts can potentially create a buffer to change by maintaining a healthy, reproductive subset of the population that could potentially seed adjacent areas and enhance the catch.

Another important strategy in preparing for change is to understand what really matters for lobster. Despite impressive new science tools to study ocean environments, numerous important gaps remain for lobster. What do larval lobster eat, and are they fussy? Who eats larval lobsters and how do these predators contribute to fisheries production? Are some population sources for lobster recruits more important than others? How can we develop better habitat maps for different life stages of lobster?

The North Atlantic and other oceans are changing, and predicting what these changes will mean for the lobster fishery of tomorrow remains a great challenge. Nonetheless, we do have tools to increase understanding and to minimize that change. Application of those tools to address the lobster fishery of tomorrow will require cooperation and exchange of ideas.