Thursday, September 27, 2007

Removing claws is harmful to crabs

Is this a surprise? Even though the claw can grow back, it's not good for the crab to break off a claw.

Some crab fisheries are called sustainable because they only take a claw and throw back the rest of the crab. Well, now researchers have discovered that breaking off a claw can be harmful to the crabs.

I guess there's a reason crabs have claws, and the crabs would prefer not to have pieces broken off. This doesn't completely overturn the sustainability argument, crabs have a better chance of surviving loss of a claw than being cooked whole. But it's a bit too easy to assume everything's fine if the crab gets thrown back.

I wonder if an ecological study would show that clawless crabs have trouble even if they survive the shock? This study didn't look at survival value of claws, it only looked at death from the direct injury of claw removal.


Kevin Z said...

LOL, duh. Crab claws are their primary defense against predators beyond their exoskeleton. And also their offense against their own prey and in interactions with other crabs.

Did they take only one claw or both claws? Crabs can autonomize appendages in nature if they are injured and regrow them. But they clearly are at a severe disadvantage.

I wasn't aware that fishermen were only removing claws and calling that sustainable. What a load of *crab*.l

Mark Powell said...

In general, they take one claw but sometimes both are taken. Here's some info on the risk to crabs:

"Survival depends partly upon the skill of the fisherman, who must separate the claw
from the crab’s body at the proper joint. If the joint is pierced or snapped quickly enough, the
crabs own defense reflex contracts its muscles and sheds the claw cleanly. If a crab is de-clawed
incorrectly (if, for example, part of the body is taken with the claw), the crab may bleed
excessively or may be unable to regenerate a new claw, which significantly increases the
likelihood that the crab will die..."

Anonymous said...

Clearly there is a misunderstanding about the meaning of sustainability. Sustainability has little to do with claw removal and lot to do with the rate of exploitation that will achieve the optimum yield. The question should be, does mortality from claw removal exceed the mortality rate that would allow the optimum yield? I think the historical evidence suggests that this has been a sustainable fishery for some time.

Mark Powell said...

Some seem to believe that the fishery is sustainable because they don't kill crabs, they only take claws. That is an incomplete picture.

Anonymous has a good point, mortality rates matter. Also, how is the ecology changed by producing clawless crabs? Do they have lower reproductive success?

My main point is that taking claws instead of crabs does not ENSURE that the fishery is sustainable, important questions remain.

Kevin Z said...

I don't have the time to work it out right now, but one could easily use game theory to determine the effect that claw removal has on a population of crabs. Which can be extrapolated to understand the relationship between claw removal and harvest quota.

Regardless, this is NOT sustainable fishing because there is a mortality factor that is ignored. Until one determines that mortality from the practice is minimal, this industry cannot label itself as sustainable.

Anonymous said...

Kevin - you still don't get it. Sustainability is directly related to the fishing mortality rate. It does not matter if they die from having a claw broken off or from being harvested in the whole. As long as the mortality rate is low enough to achieve or sustain the optimium yield then the fishery is sustainable. Again, this fishery has been going on a long time relative to the life span of stone crabs and would appear sustainable.

chels said...

An unknown mortality rate does not null sustainablity. In practice sustinability is a spectrum. It is important to define where the line of certifiable sustainability is, but in this case it is clear that the demands on the fishery taking into consideration the mortality rate (even though we don't know what it is..) is not causing a shortage in crab. Frankly the fisherys aren't taking claws only to appease those worried about the crabs wellbeing, they take the claws because the amount meat in the body is not worth the trouble of cooking it. Sustainability is a perk of farming these particular kinds of crab.