The end of fishing for Alaska's prized Red King Crab?

 It's been a while since I wrote a science communication article. In September, I was lucky enough to visit Alaska and Canada on a final year university field trip. I became fascinated by the coastal ecosystems and fisheries there, and how climate change is impacting them, and have since decided to translate some of my own reading of papers into a more digestible article...

It sounds like a scene from a horror movie. Dozens of tanks lining a laboratory, each filled with seawater bubbling away with a lone crab inside. But this was no sci-fi experiment. This was the Alaska Fisheries Science Centre’s Kodiak Laboratory when a team of marine biologists decided to investigate how climate change might affect one particular species: the Red King Crab.

But why this species? Red King Crabs (RKCs) are central to the Alaskan fishing industry thanks to their high market value. For context, whilst shipping vessels earn around 5 USD/lb for King salmon (considered a delicacy), RKCs go for double that in Bristol Bay, one of the USA’s major fisheries (Long, 2022).

Last month, the Alaskan fishing community was rocked after authorities announced the cancellation of the Red King Crab season for the second year running due to insufficient stocks. However, unlike some commercial species, overharvesting is unlikely the main cause.

Growing up to five feet across, these crabs have amazingly complex life histories which encompass a variety of habitats across their different stages. After hatching from eggs, larvae spend three months feeding on plankton, before metamorphosing into juvenile crabs. These juveniles then spend another two years making their home among shells and stones on the sea floor. The constant movement of these environments deters most predators, allowing the crabs to mature before moving to live in adult groups in deeper waters (Swiney et al., 2017).

The rocky sanctuaries upon which juvenile crabs depend are threatened, however. Unlike the waters where adults thrive which are somewhat buffered from disturbance by their depth, these shallower, delicate ecosystems are particularly impacted by warming and acidification.

All species have a set of temperatures, their thermal tolerance, within which they can survive and operate. Crabs have an especially narrow thermal tolerance since they are ectothermic, meaning they can’t regulate their own body temperature (Long, 2022). When an organism finds itself outside of its thermal tolerance, it uses energy to combat this additional environmental stress. If too much energy is spent adjusting to new environments, vital cellular processes may lack sufficient energy and the organism will become continually weaker. Lower pH, as well as corroding crustacean shells, can alter biochemical reactions and exacerbate the stress of raised temperatures.

Although crustaceans are generally less sensitive than many marine species to these environmental stressors, it is still important we can predict their responses to manage their populations effectively. In 2017, the team of scientists led by Dr Katherine Swiney conducted the first study investigating how juvenile RKCs’ growth, morphology (physical form) and survival are affected by the long-term interaction of warming and acidification.

They first created six treatments with varied conditions by submerging heaters and bubbling carbon dioxide through seawater (temperatures: ambient, +2°C, and +4°C; pH: ambient pH ~7.99, and pH 7.8). They then randomly assigned a “young-of-the-year” (under twelve months) RKC to each treatment and reared them there. Whenever a crab died, they recorded their survival time and analysed their carapaces for any effects on growth rate or morphology.

Most strikingly, they found the highest temperature and lowest pH synergised to produce the highest mortality rate (97%). Interestingly, a +2°C temperature had the opposite effect with the lower pH. According to Dr Chris Long, part of the team, “The +2°C treatment actually pushed [the crabs] closer to their ideal temperature which made them better able to deal with the stress of reduced pH.”

The team did not find morphology to be affected by temperature or pH, whilst growth rate increased with temperature but not with a lower pH.

So, what do the results mean? Crucially, lowest survival was seen amongst crabs in the +4°C/pH 7.8 treatment, which mimics the conditions forecast in Alaskan waters in eighty years. This highlights the necessity for species to adapt if they are to survive changing oceanic conditions.

Another possible outcome is the crabs move to cooler waters. This could have potentially devastating consequences for polar ecosystems; RKCs have already invaded Norwegian waters and decimated marine biodiversity as non-native predators, a trend which could be replicated elsewhere (Oug et al., 2010).

There is hope though. Research is revealing some positive responses; Blue King Crabs for example have demonstrated the potential to adapt to changing pH (Long et al, 2016). More recent experiments suggest RKC larvae are relatively unaffected by acidification (Long, 2022).

Nevertheless, if RKCs can’t adapt quickly enough to keep up with the predicted warming and acidification of our seas, the question remains whether their stocks will ever recover enough for their commercial harvesting to resume once more.

Taken on an overnight ferry through Sitka Sound.

References

 

Long, Christopher, PhD, Research Ecologist/Research Fishery Biologist, National Oceanic and Atmospheric Administration. Personal communications, November 2022.

 

Long, W.C., Van Sant, S.B., Swiney, K.M. and Foy, R.J. (2016). Survival, growth, and morphology of blue king crabs: effect of ocean acidification decreases with exposure time. ICES Journal of Marine Science, 74(4), pp.1033–1041. doi:10.1093/icesjms/fsw197.

Oug, E., Cochrane, S.K.J., Sundet, J.H., Norling, K. and Nilsson, H.C. (2010). Effects of the invasive red king crab (Paralithodes camtschaticus) on soft-bottom fauna in Varangerfjorden, northern Norway. Marine Biodiversity, 41(3), pp.467–479. doi:10.1007/s12526-010-0068-6.

Swiney, K.M., Long, W.C. and Foy, R.J. (2017). Decreased pH and increased temperatures affect young-of-the-year red king crab (Paralithodes camtschaticus). ICES Journal of Marine Science, 74(4), pp.1191–1200. doi:10.1093/icesjms/fsw251.