How Do You Tell The Age Of A Crab Or Lobster.

[Dreamtime]

Summary:

It's very difficult ot tell the age of a crab or lobster. But if we can't tell their age when they're caught, how do we manage the fisheries?

Transcript:

Back to creatures, to crabs in fact. Giant ones. Caleb Gardner studies crabs in Tasmania, and they’re huge.

Caleb Gardner: They get up to at least 14 kilograms. The very biggest ones which we get are pretty much around the size of the neck of a trap so it’s possible that there’s one bigger out there which simply can’t fit in and of course there’s some fantastic fishermen stories about the big one that was hanging on the outside but we couldn’t just quite catch it.

Robyn Williams: When you’re looking at the legs how big do they span?

Caleb Gardner: Oh I guess, probably a metre I guess would be the longest legs that they have.

Robyn Williams: That’s huge and no doubt quite welcome in the kitchen.

Caleb Gardner: They are huge animals and yes, certainly welcomed in the kitchen, delicious to eat and highly valued.

Robyn Williams: To get to that size the question is how long it takes them and therefore how old they are.

Caleb Gardner: We have very little idea of how long it takes them to get from a tiny baby I guess. The size which they settle from the plankton is only a couple of millimetres and so from that small size up to the size when we begin to get them in our pots we really have no idea. But we have some idea of the growth rates of the animals around the size when the fishermen start to catch them before they become legal size and the growth rates of them at that stage is outrageously slow compared to any other crustacean that I’m aware of so certainly it’s going to be I would say surprising.

Robyn Williams: Give me a clue of the span that you can track, what does that come to?

Caleb Gardner: OK well, the way the crustacean grows is by moulting so they shed their shell and then expand and then have a period of time at least until they moult again. Now most crustaceans which are fished commercially and which we look at, say rock lobsters, that period is normally between each one of those moults is say six months or a year. In giant crabs the ones that are round the legal size it looks like at the moment it’s maybe up to 5 years for a male and possibly even double that for a female, maybe 8 years or so for a female crab between each one of those moults.

Robyn Williams: Yes I know, I get terribly cheery about eating a very old man or a very old woman. I remember when somebody tried to serve me an orange roughie and had a tantrum because the orange roughies exist from a very, very deep level, these of course are fish, and they can be up to 100, 120, 130 years old can’t they?

Caleb Gardner: That’s right, eating an animal that old, is emotional almost isn’t it, it certainly does make you worry. In terms of commercial species though where you do fish them and harvest them, fishing an animal, which lives for such a long period, isn’t so much a problem provided you know about it. And if you don’t know that they live for a long period, and they do, that’s when you really get into strife and so that’s what I guess my research is trying to avoid.

Robyn Williams: So if you’re cropping all of these very big animals because obviously there is more flesh you might find that they reproduce slowly and you simply could not replace the population?

Caleb Gardner: That’s right and it really comes down to what proportion of the population, which we take at any one time. In the end in fishery management you just try to balance the amount which you are taking against the rate at which it can be replaced.

Robyn Williams: Where do you find them around Tasmania at the moment, everywhere?

Caleb Gardner: Yeah, Tasmania is the most important region for the fishery of giant crabs. They’re found in deeper waters right around the edge of the continental shelf on the west and on the east, not very many of them on the south and there’s only the occasional one in Bass Strait, but very few.

Robyn Williams: What do they eat?

Caleb Gardner: A whole range of animals are eaten. Sea urchins, star fish, hermit crabs other giant crabs and I’ve also found some other odd bits and pieces in there things like penguin feathers and seal fur as well, so quite a broad range and clearly a scavenging animal.

Robyn Williams: You’re not telling me that they are catching penguins?

Caleb Gardner: I don’t think they’re catching penguins but I guess the odd sort of dead beast drifts down from the surface down to the depths where they live.

Robyn Williams: How are you managing to investigate the possible age of the giant crab, what techniques are you using?

Caleb Gardner: OK, because they are so long lived compared to well any other crustacean that I’m aware of, looking at how fast they grow moves onto a while new level of difficulty. You can imagine what with lobsters we measure age simply by tagging animals and you go back again a certain period of time later, catch them again and you measure the change in growth. Now if an animal’s only moulting every five, eight years whatever the period of your research then has to be extraordinarily long or else you have to tag a large number of animals and that’s unfortunately not possible with giant crabs. You typically catch very few animals and so there has been tagging work done with them that’s yielded some results in Victoria and South Australia but we’ve sort of struck difficulties in Tasmania so we’re trying some novel approaches here. First of all looking at the accumulation of a pigment in their brain, the idea being that this pigment accumulates in a linear fashion with the age of the animals and so that gives you a scale then to compare their age.

The other thing which we’re looking at is the composition of the shell. As I said earlier the animals grow by moulting and so a critical piece of information in determining how old they are to gain an estimate of the duration it’s been since the last time they moulted. Now when they moult they absorb elements out of the sea water around them, calcium mainly, but also other small amounts of some other materials and some of those are naturally radio active occurring compounds and they degrade in the shell and looking at that rate of decay gives us an idea of change. So specifically I’m looking at the thorium radium decay.

Robyn Williams: When do you think you’ll know the answer?

Caleb Gardner: Well we have a lot of results coming in already actually and some of these estimates for the moment are giving us estimates of say round five years or so of duration since moulting so it’s good in that it’s giving us similar results to that which we had from tagging and it’s certainly I guess supporting this previous work which is giving us this idea that they are just outrageously well, extremely long lived.

Robyn Williams: The biggest one that you mentioned right in the beginning. If it were going to be really old according to your latest estimates, what sort of figure would you come up with?

Caleb Gardner: Well the biggest ones are males, the oldest ones are likely to be the female crabs who are a bit slower growing and also a bit smaller. The biggest one of those who knows, I guess in the ballpark of orange roughie potentially. So perhaps maybe 100 years,

Robyn Williams: Well would you eat a 100 year old crab? Caleb Gardner might. He’s at the University of Tasmania where his colleague Stewart Frusher is the rock lobster man. And the odd thing about lobsters is their size is different according to geography.

Stewart Frusher: It really depends where you are. In Southern Tasmania we’ve had lobsters which were tagged 20 plus years ago and we still get recaptures from some of those. In the north of the state though, the growth rates are a lot faster and they reached the fishery in maybe 5 to 6 years. Down south those animals which we caught 20 years ago were still undersize so those lobsters could be well in excess of that well and truly in their 40 plus years old, we just simply don’t know. The problem you have with lobsters is you can’t age them.

Robyn Williams: And of course one of the problems is if they are that old then growing up, maintaining population of reasonable sized adults will take a long time.

Stewart Frusher: Only certain size lobsters can be taken and therefore you’ve got a new set of recruits coming into the fishery each year.

Robyn Williams: You said there’s a variation between north and south what’s the scale?

Stewart Frusher: It’s quite substantial. We believe that there’s more variation between north and southern Tasmania than there is in any other lobster fishery in the world. We see lobsters in southern Tasmania, the females in particular into slightly deeper water those animals actually die before they reach the legal size because they are growing so slow. In the north of the state those animals are growing up to 4 to 5 times as fast and indeed they are immature by the time they get to the legal size. We estimate that the males for instance are growing six times as fast in the north of the state, they’re averaging on a year at about the legal size limit, about 20 to 25mms of growth each year. Down south it’s less than 5mms.

Robyn Williams: Why is so low down south?

Stewart Frusher: We’re not 100% sure, certainly there’s a lot cooler water down south and the other aspect is that they’re very dense populations down south and it may be that there’s a density dependent affect. We can set a pot down south and it’s not uncommon to get between 50 and 80 lobsters in that pot. Virtually all of those lobsters are undersized lobsters. In the north of the state we may see an average between two and three lobsters per pot.

Robyn Williams: And as for their sex lives it’s difficult but then look what the youngsters go and do.

Stewart Frusher: After the female moults the male then comes and courts the female and then deposits a spermatophore. The female then releases the eggs quite shortly afterwards and as those eggs are extruded they are fertilised and then they’re placed on the tail where they’re incubated for up to three to four months before they hatch.

Robyn Williams: Then what happens?

Stewart Frusher: Then we have quite an extraordinary part of the life cycle of the lobster because they then hatch out as lava, they are a little bit more like miniature spiders than lobsters, and they then go via ocean currents out to sea and they’ve been found between 100 and a 1000 kms from the shore and it’s thought that for the southern rock lobster that they are out at sea anywhere between 12 to 24 months.

Robyn Williams: Doing what?

Stewart Frusher: Well a lot of people think they are just drifting around the ocean but we suspect that they have some sort of mobility and they manage to get themselves in the big oceanic currents which means that they do come back to our part of the world which is rather fortunate for us.

[tuffy]

Well, this thread certainly elicited a lot of responses, Carl.

Methinks you have baffled them all with science, so to speak.

Russ.

[adjustedpete]

Thanx mate, I was just pondering that very question about the age of lobsters. Seeing those giant ones lately has got me thinkin'. I didn't know that there wasn't much research done on them. I dont eat them so i am not racked with guilt for eating an old man either. Since learning about growth rates in fish, I have been alot more diligent in returning fish back to the sea and not taking more than a feed. That is, if I can even get a feed.

[Sir Reel]

Does anyone know of any more infomation about lobsters.Like when is the best time to put a lobster pot out in NSW South Coast and at what depth. What months are best etc? Is there anything I should look out for?

Edited July 28, 2008 by bbayjohn