Crabs Love Warmer Water!

Source: World Climate Report

Who would ever guess that in 2011, one of the most popular television shows in the world is about fishing for crabs in “the vast Bering Sea.” Deadliest Catch premiered on the Discovery Channel on April 12, 2005 and currently airs in over 150 countries. If you don’t know, the show portrays the real life events aboard fishing vessels in the Bering Sea during the fall Alaskan king crab and the winter Opilio crab fishing seasons. With so much interest in the show and so much concern about climate change in the high latitudes of the Northern Hemisphere, it was just a matter of time before we explored the world of crabs and climate change.

Our interest in this subject actually came about given a recent article in the Journal of Experimental Marine Biology and Ecology. The article was produced by three scientists from Oregon; Stoner et al. acknowledge that “This study was conducted as part of the AKCRRAB Program (Alaska King Crab Research, Rehabilitation, and Biology) funded by the NOAA Aquaculture Program and the Alaska Sea Grant College Program.” They note in their introduction that “Red king crab (Paralithodes camtschaticus) (RKC) was the most economically valuable crustacean fishery in Alaska from the late 1960s, until the population collapse in the early 1980s. Both over-harvest and unfavorable environmental conditions probably contributed to low fishery recruitment. Various fishing closures have been imposed in the Gulf of Alaska and Bering Sea for more than two decades, but the stocks have not recovered substantially.”

Regarding any link to climate change, Stoner et al. state “Temperature is a dominant environmental factor that mediates the behavior, physiology, growth, survival, distribution, and recruitment of ectothermic animals living in temperate and high latitudes. Consequently, climate-driven changes in ocean conditions can cause significant fluctuations in the distribution and abundance of marine populations. In the Gulf of Alaska and Bering Sea, oceanographic regimes linked to climate conditions occur on a multi-decadal scale, and these climate cycles have been linked to major temporal shifts in the composition of marine fish and invertebrate communities. Longer-term trends in sea surface warming and loss of sea ice have already been observed in the Gulf of Alaska and Bering Sea, and the potential impacts on economically important species are large.”

Fair enough! To test how the red king crabs respond to various water temperatures, Stoner et al. reared red king crabs for 60 days with water temperatures ranging from 1.5ºC to 12ºC. Some of the crabs were cultured in populations while others were grown in isolated cells. The key figure below tells much of the story – the red king crabs grow best in warm water … the warmer the better from their perspective! Not only are the growth rates in terms of size impressive at high temperatures, but the growth rates in terms of weight are just as impressive. Weights for red king crabs at 12ºC averaged nearly seven times more than the weights for the crabs reared at 1.5ºC.


Figure 1. Growth rates of red king crabs cultured in populations and individually (in cells) for 60 days at four experimental temperatures. Values are mean ± standard deviation for all of the surviving crab. Curves were fit to the data with two-parameter exponential functions (from Stoner et al., 2010).

In discussing their findings, Stoner et al. note “There is little doubt that variation in growth rate associated with temperature is related to differential rates of metabolism and food intake, as observed in other crustaceans. We have observed that the general activity of age-1 RKC doubles between 2° and 10 °C and that feeding rate increases 2.8 times.” The warmer they get the more they eat!

Surely something must come along and spoil this story? Consider the following concluding comments from the Stoner et al. team. They state “The significance of temperature-related growth and metabolism for RKC aquaculture is obvious. Exponential increase in growth rate with temperature means that crabs can be reared to release stage in a shorter time, reducing costs for facilities and labor. In this experiment there was no apparent disadvantage of high temperature in terms of crab survival; however, at some point above 12°C survival will decline as the crabs reach their upper range of physiological tolerance. Also, we predicted that cannibalism would increase with increasing temperature as indicated in at least one field study with Callinectes sapidus Rathbun 1896. Such a result was not observed in this study”. By the way, you know “Callinectes sapidus Rathbun 1896” as blue crabs.

More directly to the climate change issue, Stoner et al. conclude “Increases in water temperature associated with climate change have already been observed in Alaska, and it is clear that these changes will have direct effects on the growth and age-at-recruitment for RKC as well as other taxa. No apparent adverse effects of temperatures as high as 12 °C were observed on the growth and condition of juvenile RKC in this study, and accelerated growth might have a positive, indirect effect on survival. Depending upon temperature, the size of RKC juveniles could easily span <4 mm to >10 mm one year post-settlement. Larger size associated with high temperature could provide for earlier refuge in size from the typical fish and invertebrate predators on RKC.”

Once again, we find a situation where most would believe that warming would have a devastating effect on the red king crabs in “the vast Bering Sea.” But when a scientific experiment is conducted, we find evidence that the crabs love the warmer water. Having seen fishermen on Deadliest Catch nearly freezing to death show after show, a bit warmer water would probably go over pretty well with them as well!

Reference:

Stoner, A.W., M.L. Ottmar, and L.A. Copeman. 2010. Temperature effects on the molting, growth, and lipid composition of newly-settled red king crab. Journal of Experimental Marine Biology and Ecology, 393, 138–147.

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