Pilot whales

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Aleifr
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Pilot whales

Postby Aleifr » Tue Jul 06, 2010 1:05 pm UTC

I doubt there are any of you that haven't heard about the threat of extintion of pilot whales, or whales in general. Though, the actual facts are scarce.

I'm from a little country called the Faroe Islands, where we hunt pilot whales, and many are concerned that by doing so we contribute to the extinction of them. It's estimated that 850 are killed per year.

According to the IUCN (International Union for Conservation of Nature) the population of pilot whales in the area around the Faroes was 100 000 in 1996. (http://en.wikipedia.org/wiki/Whaling_in ... population)

According to this website: http://www.acsonline.org/factpack/PilotWhale.htm, each female pilot whale calves every 3-5 years, and does so from the age of 12-35 (their average lifespan is 45 years).

Assuming the females make up half of the population, their number would be 50 000, and since they are able to calve half of their lifetime, the females that can calve in a population of 100 000 should be aproximately 25 000. They calve in average every 4 years, that is 0.25 a year.
25 000 * 0.25 = 6 250

But I realize the problem with this is that I don't take other causes of death of the calves into account. I'm guessing a lot of the calves don't make it. And I also need to know mortality rate or the death rate. I'm no biologist, so I don't really know how to approach this. I'm pretty sure my guesswork above is very unorthodox, the females a probably very close to half of the population, but the ones that can calve are probably not half of the females. So what I want to know is how I account for the fact that not all calves make it and the mortality/death rate. If there's anything else that needs to be done differently, you're more than welcome to correct me.

Please, keep everything about the ethics concerning whale hunting out of the discussion, this is not about that.
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sgt york
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Re: Pilot whales

Postby sgt york » Tue Jul 06, 2010 3:03 pm UTC

Most of deaths in the population will take place in early life (I'd guess first 5 years). This will result in two things.

First, the age distribution will be heavily slanted towards younger animals; there will be more immature whales than whales of any other similarly sized age group. So, even though a whale is reproductively capable for half its lifetime, this does not mean that half the whales are reproductively capable.

Second, this means that although a mother may have a calf once every 4 years, she will not have a calf that reaches reproductive maturity once every 4 years. It may be 90% that survive or 50% that survive, but whatever the percentage is, you can be certain it's not 100%.

I don't know the specifics of pilot whale development but if a female only has a calf once every 4 years, it's probably because the calf takes 3 years or so to mature to the point where it can live on its own. So, many (likely most) female whales that are killed actually represent at least 2 whales that are killed; the calf (or calves) dependent on her will most likely die shortly after her.

If a mother calf or pod could take care of an adopted calf, she would be able to have more young of her own in the first place; and she probably would.

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Re: Pilot whales

Postby Coffee » Tue Jul 06, 2010 3:38 pm UTC

Pilot whales are social creatures; may it be that elderly females (past child-bearing years, assuming they live past that age) within a pod could adopt orphans? Maybe. Iirc the practice in the Faroe islands is to basically drive a whole pod onto the beach, so mother and calf would likely be caught together in many cases.

I do hope though that the practice is abandoned sooner rather than later; why carry it on at all? Culture?
Well a large aspect of my culture for a thousand years was inter-tribal warfare; some traditions are best abandoned.
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Aleifr
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Re: Pilot whales

Postby Aleifr » Tue Jul 06, 2010 4:09 pm UTC

Second, this means that although a mother may have a calf once every 4 years, she will not have a calf that reaches reproductive maturity once every 4 years. It may be 90% that survive or 50% that survive, but whatever the percentage is, you can be certain it's not 100%.


I agree. But the problem there is how many of them survive. Although most of this is guesswork, the guessing has to be realistic.

I don't know the specifics of pilot whale development but if a female only has a calf once every 4 years, it's probably because the calf takes 3 years or so to mature to the point where it can live on its own. So, many (likely most) female whales that are killed actually represent at least 2 whales that are killed; the calf (or calves) dependent on her will most likely die shortly after her.


According to this website: http://www.acsonline.org/factpack/PilotWhale.htm, the mother nurses the calf for 22 months. I'm guessing after that he can survive without his mother in the school. I think it is unnecessary to take the calves that are left alone after their mother is killed.

What I still need to know is what is the age-distribution of the females, or in other words how many reproductively capaple females are in a group of 100 000 whales. I also need to know the mortality/death rate, I'm pretty sure with those two parameters I could determine how much the population of whale grows per year. With that information I can whether the whale hunt in the Faroes is a threat to the population.

If anyone is wondering if this is homework or something, it's not. It's just that People talk about this a lot, but no one ever has any facts, not even the experts it seems.

Pilot whales are social creatures; may it be that elderly females (past child-bearing years, assuming they live past that age) within a pod could adopt orphans? Maybe. Iirc the practice in the Faroe islands is to basically drive a whole pod onto the beach, so mother and calf would likely be caught together in many cases.


Well, I agree and that busts sgt york's theory.


I do hope though that the practice is abandoned sooner rather than later; why carry it on at all? Culture?
Well a large aspect of my culture for a thousand years was inter-tribal warfare; some traditions are best abandoned.


I ask you once again politely not to make this into such a discussion. If it wasn't for the fact that I don't give a damn about whale hunting, I would probably be offended.
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Re: Pilot whales

Postby Coffee » Tue Jul 06, 2010 4:53 pm UTC

My apologies; I meant no offense. And you're right, this is the wrong forum for that kind of discussion.
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Re: Pilot whales

Postby sgt york » Tue Jul 06, 2010 5:11 pm UTC

Coffee wrote:Pilot whales are social creatures; may it be that elderly females (past child-bearing years, assuming they live past that age) within a pod could adopt orphans?


After the calf is weaned, that is possible. But before weaning, an older female couldn't do it. No milk; they are mammals.

Iirc the practice in the Faroe islands is to basically drive a whole pod onto the beach, so mother and calf would likely be caught together in many cases.


That would make the "one female = two whales" equation moot.

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Re: Pilot whales

Postby Omegaton » Tue Jul 06, 2010 6:00 pm UTC

I see no reason to expect that pilot whale mortality is high for the first five years. Perhaps for the first year, but it would be quite odd for an animal with parental care, large size, long time to maturity, and small number of young (you know, K-selected) to have a Type III survivorship curve (high mortality early). Rather, this type of life history suggests a Type I survivorship curve, similar to humans; although there may be a high chance of mortality early on, most of the mortality occurs late in life.

I did a fairly quick search and found a few things. The sex ratio is skewed towards females in many species of toothed whales. One study I found (off Japan) had a sex ratio of 1:1.72 (males to females) for pilot whales. Another study also found that the pilot whales near Faroe Islands are probably not restricted and intermingle with pilot whales of the Northern Atlantic, and they would have the ability to do so in the relatively short term (47 days); on the other hand, pilot whales are restricted to populations by water temperature.

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Re: Pilot whales

Postby gmalivuk » Wed Jul 07, 2010 1:21 am UTC

Aleifr wrote:I ask you once again politely not to make this into such a discussion. If it wasn't for the fact that I don't give a damn about whale hunting, I would probably be offended.
It's not your place to completely dictate the terms of the discussion. While this is the science forum, an occasional mention of someone's ethical views on whaling is acceptable in a discussion of whaling. As long as it doesn't turn into an outright moral debate, I see nothing wrong with a few people's opinions here and there.
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Re: Pilot whales

Postby meatyochre » Wed Jul 07, 2010 2:24 am UTC

I'm curious and just want to save a spot for my egosearch later. Mostly because I fall into the apparently small subset of people who've never heard of your country OR "pilot" whales in particular.

Carry on :)
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Re: Pilot whales

Postby Dream » Wed Jul 07, 2010 11:07 am UTC

Aleifr wrote:What I still need to know is what is the age-distribution of the females, or in other words how many reproductively capaple females are in a group of 100 000 whales. I also need to know the mortality/death rate, I'm pretty sure with those two parameters I could determine how much the population of whale grows per year. With that information I can whether the whale hunt in the Faroes is a threat to the population.

The raw numbers are not the only threat to a population of animals based on hunting. Selection of particular subsets of the population for hunting can have a large physical selective impact that also needs to be taken into account. The classic example is lobster potting, where government imposed guidelines on how large a lobster has to be before it can be caught lead to smaller lobsters being selected for artificially in reproduction, which leads to smaller lobsters overall. If I recall correctly, the same effect can be observed in trawler caught species like tuna and cod, where size based restrictions affect the average size of fish in the population. Before you could declare the whale hunt ethical or otherwise, you'd have to analyse its impact on behaviour and selection as well as overall numbers.
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Re: Pilot whales

Postby Omegaton » Wed Jul 07, 2010 7:46 pm UTC

Dream wrote:If I recall correctly, the same effect can be observed in trawler caught species like tuna and cod, where size based restrictions affect the average size of fish in the population.

This is a widely documented effect, and it it strengthened with overfishing.


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