Module 6: Crab Case Study
Crabs are crustaceans with soft bodies protected by a calcified exoskeleton. A variety of marine crab species are harvested for consumption across the globe, with approximately one-third of annual US crab catches coming from the Bering Sea, Aleutian Islands, and the Gulf of Alaska. Of the ten species of crab that are commercially important in Alaska, ocean acidification researchers have focused their efforts on these six species: red king crab, blue king crab, golden king crab, Tanner crab, snow crab, and Dungeness crab.
Red king crab in the Bering Sea
Snow crab in the Norton Sound
After hatching, crab species in Alaska spend two to three months in larval stages, in which they survive by feeding off plankton for the next several months. When the larvae begin to settle to the bottom of the water column, the molt cycle begins. Because a crab's skeleton is its shell, crabs must molt their shells in order to grow. Crab shells are made of the minerals calcite and calcium carbonate, and crabs shed their shell by absorbing water and cracking it. Red king crabs can live up to 20 - 30 years! Tanner crabs, snow crabs, and red, blue, and golden king crabs are seldom found co-existing with one another even though the habitats and depth ranges where they live may overlap.
Crabs are a major part of Alaska's economy. Crabs provide jobs for commercial fishermen, processing line workers, and the many trades that maintain our fishing boats and crab processing plants, while supporting our coastal communities. Alaska crab fisheries are currently managed according to the “three S’s”: size, sex, and season. Only male crabs of a certain size may be harvested, and fishing is not allowed during mating and molting periods. These measures help ensure that crabs are able to reproduce and replace the ones that are harvested. Every year, managers set the harvest limit for the next fishing season using the most recent estimates of crab abundance.
Watch this 7-minute video that investigates how the chemistry of the North Pacific Ocean is changing in ways that pose serious threats to Alaska's two signature crab species: the red king crab and the snow crab. Although this video is 10 years old, the issues it raises about ocean acidification remain relevant.
A general pattern has emerged from the research conducted on these crab species in Alaska thus far:
Crab survival decreases at most life stages when they are exposed to more acidic water.
Crab have been the longest-studied species for OA in Alaska. There are many different ways to study a species of crab, including looking at different life stages, different lengths of time submerged in water, different generations, factors of growth, calcification, survival, and reproduction. Remember: like salmon, the results of lab studies on crabs often reflect a worst-case scenario. This is because the artificial environment restricts crabs’ ability to use the adaptive strategies they might use in the wild (relocating, changing foraging habits, etc). With these factors in mind, let’s take a look at a short selection of recent OA research on Alaska crab species.
Red king crab and Tanner crab are most sensitive to more acidic (higher CO2) water. Studies consistently show decreased growth and increased mortality at multiple life-history stages in both species. The most likely reason for this is that the crabs have to spend a lot of energy on basic life-sustaining functions in more acidic water. This leaves less energy for other critical activities like growing and fighting disease. While blue and golden king crabs are still sensitive, they seem to be less sensitive than red king and Tanner crabs.
Snow crabs are the only species that appear resilient to more acidic water. Since Tanner and snow crabs are closely related, NOAA’s Kodiak Laboratory is planning to conduct more experiments to understand why Tanner crabs are sensitive and snow crabs are resistant. This work will include looking at how their blood chemistry changes in acidic waters and measuring gene expression.
There are currently no Alaska-based Dungeness studies underway. To learn more about Dungeness crab OA response in the lower 48, visit NOAA’s Dungeness Crab Case Study.
Juvenile crab being exposted to different CO2 levels in the lab.
Close-up of crab experiment at the Kodiak NOAA Lab.
NOAA’s Kodiak Laboratory has been conducting research on the effects of ocean acidification on commercial crab species for more than15 years. Over that time, we’ve learned a lot about how more acidic waters may affect these species and the fisheries that depend on them.
In this study, 48 female adult southern Tanner crabs were exposed to different pH levels (8.1, 7.8, or 7.5) for 2 years. Researchers examined the effects of OA on the exoskeleton (thickness, structural integrity, elemental content, phase of calcium carbonate) of mature female southern Tanner crabs (post-molt). Only 7 crabs survived in the lowest pH conditions. These crabs experienced a reduction in the hardness of their shells by an average of 60%. Lower seawater pH resulted in softer claws, thinner carapaces, less calcite in their shells (alteration in mineral content), and internal dissolution of the carapace.
In this study, females carrying eggs were collected from the Aleutian Islands and fertilized in the lab. Crab larvae were raised in a seawater lab, and 90 individuals were chosen to be exposed to different levels of pH for 127 days. Researchers observed how OA altered juvenile golden king crab growth and survival. Only 14 crabs successfully molted in the lowest pH conditions. At the end of the experiment, 12 crabs survived in pH 8.1 (the "ambient" treatment), 8 crabs survived in pH 7.8, and 8 crabs survived in pH 7.5. Crabs reared at 7.5 pH were significantly smaller and took longer to molt than those reared at ambient pH, while crabs in pH 8.1 grew fastest and had the best survival. Juvenile golden king crabs exposed to pH levels below 8.1 had significantly lower growth and survival than those exposed to surface ambient water.
In this 2-year experiment, females carrying eggs were reared in one of three pH conditions (8.1, 7.8 and 7.5). At the end of the experiment, there was no direct effect on either embryo or larval pH treatment. The results from this study suggest that snow crabs are well-adapted to projected ocean pH levels within the next two centuries, although other life-history stages need to be examined for sensitivity and potential interactive effects with increasing temperatures.
Different crab species show different levels of resiliency. Red king and snow crab stocks have crashed recently in Alaska. We cannot tie this directly to OA at this time, but OA is going to be one of multiple stressors affecting crab in the long term. We need more monitoring to know how crab species are currently being affected. The life stages that are most sensitive occur geographically in places we know little about as far as ocean chemistry. Bringing together the crab industry and researchers has been key and will continue to be important going forward.
To learn more about the computational models that are being developed to explore the economic impacts on the Alaska crab fishery from ocean acidification, visit the Alaska OA Network website.
When we look at how ocean acidification is likely to affect fisheries, the more sensitive species are predicted to have declining populations over time. This means fewer crab will be available for fisheries, resulting in declining harvest over the next 50 years in Alaskan waters. It is important to note though, that these crab species may be able to evolve over time to be better adapted to lower pH (more acidic) water. However, it is very difficult to estimate how much that will help in the face of rapidly-changing ocean conditions.
For Alaskans dependent on these species, understanding how they may fare in a higher-acidity environment is critical. Formally evaluating the risks of ocean acidification to crab fisheries - and assessing the benefits of pre-emptive human responses - will be important for future decision making and adaptation.
As you've learned, many species that are important to Alaskans have negative responses to ocean acidification in the lab. In order to make informed descisions for ecosystem health and effective fisheries management, we need to keep our finger on the pulse of OA in Alaska. Head to the next module to learn how we're doing just that.