Contrary to popular opinion, minimum sizes lower fishery productivity. This is not just the long term effect of the selective pressure for runts and slow growing fish. Even in the short term, catching the smaller fish will increase the total catch on a weight basis. Basically, throwing them back, or not catching them in the first place, does not mean you catch them when they get bigger. Most of them do not survive to a larger size.

PJ argued that this is not the case because minimum sizes tend to coincide with the onset of sexual maturity and maximum biomass of a population cohort.

pjb05 wrote on Mar 15^th, 2009 at 7:01am:


However there are several problems with this argument. I have not been able to find any independent verification that maximum biomass tends to coincide with the onset of sexual maturity. This argument may have the causation backwards. Minimum sizes will create an artifical age of maximum biomass. For many species these were set arbitrarily at the age of sexual maturity, which is perhaps the worst age for creating selective pressure for slow growing fish. Furthermore the age of maximum biomass in a natural setting, to which you could argue fish would adapt their first breeding cycle, and which in turn would be would be caused by or reinforced by breeding effort, would have little correlation to the age of maximum biomass of a fished stock. That is, fishing would change it all anyway. Furthermore, if you were to try to target minimum sizes to maximise catch weight, you would not target the age of maximum biomass of a single cohort (generation). Rather, you would target the age that maximises total growth rate when you take things like intergenerational competition into account. You get the wrong answer when you only consider a single cohort in isolation. The age of maximum biomass becomes almost meaningless when you talk about harvesting based on it, as the harvest itself would have a strong impact on the age of maximum biomass.

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Marine parks make fisheries management robust. All of the failures of traditional management tools can be resolved by simply using the same rules or strategies but reducing the fishing pressure or total catch. Of course, you soon get to the stage where you are better off just recognising the benefit of marine parks as a fisheries management tool and taking home more fish, rather than tolerating an inferior management policy that gives you more freedom to fish wherever you want, but less fish to actually eat. Even the worst fisheries management tools can be made to work if you make them strict enough, but this is not an advantage. You are giving too much credit to traditional tools on the basis that they are what you feel comfortable with, rather than any inherent advantage. The whole point of marine parks is that they change the assumptions around maximum sustainable yield, resulting in an increase in what can be caught sustainably when you take a realistic appraisal of risks associated with unknowable or unmeasurable factors etc into account. That is, when it comes to practical measures, you end up with mroe fish.

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But it does prove the case for marine parks PJ. Your argument has withered away to little more than acknowlwedging that marine parks are better, but claiming that it doesn't matter anyway because other options are magically 'better' because you are useed to them.

Your post appears to recognise this, and accept that this particular defense of traditional management tools such as minimum sizes is fatally flawed. That is, "all they do" is point out the inferiority of traditional management techniques, and the bandaid solutions that are falsely claimed as benefits. Given that this is pretty much the extent of the role fisheries management tools, you seem highly dismissive of failures in this key role. It's like saying that the only thing that arguments against traditional drug policies do is show that tradition drug policies do not work.

I would genuinely appreciate any feedback on the original issues, ie whether minimum sizes lower fishery productivity (as apparently aknowledged by pj), and that this is due not only due to selective pressures for slow growing fish, but also due to short term impacts like the effect of the various natural causes of mortality on population cohort biomass.

Can you verify that maximum biomass tends to coincide with the onset of sexual maturity, and that this is not a result of anthropogenic influences such as the release of undersize fish? That is, would we increase maximum sustainable yields by making less use of minimum sizes and more use of better management techniques? The point of this thread is to counter an apparent technical flaw in the benefits of marine parks, so it is a bit disingenuous to revert to arguing that it doesn't matter anyway whether you can take home more fish. IF you don't understand the technical arguments, you should not make them, or copy and paste them from questionable sources, in the first place.

So why can't you back it up? It looks to me like Parrish just made it up as he went along. Why were you so quick to concede such an important point and go back to to your vague waffle and slogans?

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Can you back this up?

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Go ahead and apply it for us then. It would be interesting to see you try to reason it through.

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Is this a bad thing?

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Can you explain the logic behind your claim about more destructive fishing methods?

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Yes you have made the claim, but when it comes to backing it up, you always fall short.

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What, only moderately overfished?

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The extent to which they are used does not affect whether it is a good idea to use them. If you reduce the harm by not using minimum sizes exclusively, why not eliminate the harm or minimise it to the extent possible? The fact that other tools are also used does not mean thqat inferior tools magically become a good idea.

So why can't you back it up? It looks to me like Parrish just made it up as he went along. Why were you so quick to concede such an important point and go back to to your vague waffle and slogans?

'Looks to me like Parrish just made it up as he went along' - what a brilliant analysis! Why don't you try to get that published FD?

PS what have I conceded?

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Can you back this up?

Yes and I have already. I have put up several papers that make this case (Parrish, Shipton, Hilborn, Buxton). You then stuck your head in the sand by ignoring their arguments and/ or calling the authors names.

Even marine parks advocates in their more honest moments say that fishermen are unlikely to benifit. They justify them on preservationist grounds. Ie they don't like what we do and want a 'look but don't touch' world.

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Go ahead and apply it for us then. It would be interesting to see you try to reason it through.

I just did. I can expand on them if you like.

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Is this a bad thing?

The point is that they can upset the natural balance, eg if the predator is more mobile it will respond less to area closures  than less mobile species. Reduction in the prey nos means less biodiversity and could have commercial implications if the prey is a valuable species.

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Can you explain the logic behind your claim about more destructive fishing methods?

Some species don't lend themselves to being caught by methods other than trawling. To make up for the lost grounds you would have to step up trawling to maintain the yield. More trawling in a smaller area means more ecological damage (Parrish).

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Yes you have made the claim, but when it comes to backing it up, you always fall short.

You just ignore what is inconvenient to your mantra.

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What, only moderately overfished?

A few are considered growth overfished. This is not a biological problem. These few remaining problems of overfishing can and are being adressed my methods less proscriptive than marine parks.

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The extent to which they are used does not affect whether it is a good idea to use them. If you reduce the harm by not using minimum sizes exclusively, why not eliminate the harm or minimise it to the extent possible? The fact that other tools are also used does not mean thqat inferior tools magically become a good idea.

What 'harm' are you talking about? Any substantial fishery has an impact on fish stocks. What are you saying - that you want the World's only fishery that only harvests the tiddlers?

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I'm just trying to verify his claims, that's all. So far they seem logically flawed and misleading. I have looked, and can't find anything to support his claims. Can you?

There backed up by real world data from actual fisheries.

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The selective pressures created by minimum sizes that reduce the growth rate of fish. Plus the short term effects of minimum sizes that leads to a reduction in yields. That's what this thread is about remember? One the one hand, you post a claim that this reduction does not exist so there is no problem, but when asked to back up this claim, you switch to arguing that it doesn't really matter anyway because minimum sizes are only a small part of the current management regime. Can you explain why you constantly switch to this line of argument if you don't even recognise the problems associated with minimum sizes in the first place? Surely if you honestly believed that the problems with minimum sizes don't exist you would argue that they don't exist, instead of constantly making up excuses for them.

I'm just putting your theories in a real world context - if you don't like that, well too bad. Selective pressure for slow growth rates is a phenomenon with scant evidence in real fisheries. It is also only likely to ever be apparent if the fishing pressure is very high (ie few fish get much past legal size) - and I am not arguing for that. Furthermore this selective pressure argument does not prove the case for marine parks. All it does is make the case for a fishing effort not overly heavy or somewhat lighter than the maximum sustainable yield.    

I posted responses to some of your other points here:

http://www.ozpolitic.com/forum/YaBB.pl?num=1192441509/346#346

freediver wrote on May 26^th, 2009 at 9:44pm:


The EIS into NSW fisheries states that they do not regard growth overfishing as a biological problem. Also look a how quckly overfished stocks have recovered eg salmon and kingfish in NSW. This is hardly a sign that they have been genetically damaged.

freediver wrote on Jun 8^th, 2009 at 6:59pm:



Well if you can't find the evidence of selective pressures and detrimetal genetic changes in an overfished stock, when are you going to find it? Also you apply a burden of proof to the status quo but treat your own theories as a proven fact and not requiring the same standards of proof. If you look at examples like these it goes to show that a lot of our inshore fish stocks have a high natural resistance to fishing pressure (yes a lot of them are fast growing) and that overfishing is reversible.

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Why would it be limited to overfished stock? All you need is a selective advantage for the change to be inevitable.

Duh, the selective pressure is greater in an overfished stock. If there is no sign of permanent harm (ie overfished stocks recover quickly) then it can't be very significant in an sustainably fished stock.

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Are you suggesting that the theory of natural selection must be proven in every situation before it can be used to make a prediction? I am not demanding proof of anything. You are.

I'm not talking about predictions - I'm talking about observations from actual fisheries.

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Almost everything is reversible. That doesn't make it a good idea. The lost opportunities from past mismanagement however cannot be regained. All that lost income, food, and recreational value were simply lost to us because some stubborn soul kept demanding absolute proof of the bleeding obvious before we allowed common sense to take it's course.

Uneccessary marine parks mean a permanent loss of productivity, income, food and recreational value because they lock up productive ares for ever. Your faith based mantra is flawed. Just because there have been some fisheries failures in the past or overseas does not mean marine parks are the best way forward.

tallowood wrote on Jun 13^th, 2009 at 7:33pm:


They are probably farmed fish - for which legal sizes do not apply.

tallowood wrote on Jun 13^th, 2009 at 8:01pm:


Snapper are farmed. Also there are different species of flathead with different legal sizes, you might have misidentified the ones you saw.

Tallow, I think that outside state waters, in federal fisheries, they can catch them at a smaller size.

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Like I siad, that recovery is not evidence of zero harm. You appear to think that anything other than a total permanent collapse of a fishery is good management.

If there is no sign of permanent harm (ie overfished stocks recover quickly)

Like I siad, that recover is not evidence of zero harm. You appear to think that anything other than a total permanent collapse of a fishery is good management.

The Dieckman paper is a good starting point on the harm done by minimum sizes in wild fisheries. It has also been demonstrated in a lab countless times how easy it is to change growth rates with selective pressures.

http://www.ozpolitic.com/fish/marine-parks-fisheries-management-tool.html#links

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But it is PJ, for three reasons. One, there is normally at least a few years of significantly reduced catches. Two, even once the stocks are recovered, the catches must be restricted to more conservative levels to compensate for the problems inherent in the management approach used. But these first two are more a topic for the other thread. Third, like I said, this apparent recovery is not evidence that no harm has been done via the mechanism of selective pressure reducing growth rate. Nor is it evidence that the harm done is not significant. That's because it is not a useful measure of the extent of the harm. Because of the relatively rapid and interactive changes our fisheries are undergoing, and the highly variable nature of Australia's climate, we have no way of knowing what the catch rates would be without the harm done by minimum sizes. Recovery does not mean that catch rates are equal or close to what they would be without this harm. The examples Dieckman use are extreme because the harm done by minimum sizes actually prevents the recovery of a collapsed stock. But to suggest that anything less than a long term collapse is insignificant is completely absurd. The harm done by minimum sizes will not normally become evident as a collapse of a fishery, or the prevention of the recovery of a collapsed fishery. If anything they will make it appear more resilient, while reducing total catches.

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Please quote where I talk about wanting zero harm or impact. You just finished accusing me with "Don't have me saying things I haven't actually said", though I have no idea what you were actually referring to.

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No. What on earth makes you think that?

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No doubt they do, due to reduced competition etc. I hope you are not suggesting that this somehow contradicts the theory of natural selection. Are you? It is not fishing pressure that reduces a fisheries productivity via this mechanism, but a specific management tool. I am not criticising fishing pressure for reducing the growth rate of fish. I am criticising minimum sizes. Don't let the confounding factors confound you.

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That is not obvious PJ. This is a simple matter of logic. In order to judge whether something had significantly affected their growth rate, you would need something to compare it to. You don't have that. That is why your conclusion is totally unfounded.

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Well, that is how it appeared. Even now you still claim this is an example of good management, despite the fact that the stock collapsed. You claim it is evidence that no harm was done, even though you have no 'unharmed' state to compare it to. If this to you is good management, it is no wonder you have trouble understanding the benefits of marine parks over inferior management tools. It's like you are arguing that the exception proves the rule. "Look at these examples of traditional managment tools failing, isn't this great proof that they are better than marine parks".

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But PJ, that was in response to claims by you that no permanent harm was done. This was not an outof-the-blue statment that I wanted zero harm. Also, you appear to be confusing zero harm with zero impact.

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No it isn't PJ. It doesn't mean that natural selection is over-ridden, it means that you cannot measure the impact easily. Also, the impact could be massive and still be over-ridden by the increase in growth rate due to the decrease in competition.

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A cure-all for a single problem? Is that supposed to make sense?

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You obviously thought they do, and for some reason still think they do, but as I pointed out the first time, they don't. They do not even mention the negative impact of minimum sizes on fishery productivty. But then again, you do have an unusual habit of interpretting an ambiguity (or in this case even silence) as direct support for your postion. Perhaps you would like to quote one of those papers explaining how marine parks will make the negative impact of minimum sizes on fishery productivity even worse....

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No it doesn't PJ. It tells us nothing at all about it. Or perhaps you would like to explain how the theory of natural selection predicts a different outcome for the case? You don't need a parallel universe to make a comparison. Or is this your magical thinking argument in a different guise?

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Well then perhaps you should ask a more specific question about the harm/impact. Or better yet, just move on.

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Well done PJ. That makes a lot more sense than proclaiming that a recovery in X years is somehow proof that the harm is not significant. Though it would still not get you around all those other confounding factors. But it is a good start nonetheless. I can tell you have been thinking about this now.

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I never intended it to make the case for marine parks on it's own.

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You mean, underfishing? Because of the flaws in traditional managment tools? The flaws you insist are not significant? Are you trying to help me argue in favour of marine parks now?

BTW, can you explain how easing up on fishing pressure would eliminate this problem?

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I want to increase catches. Among other things of course.

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Yes I have heard the phrase before. Though I must admit I have never heard the phrase 'cure all' applied to a single problem. That's why it threw me a bit.

BTW, I often have great difficulty figuring out what you mean. That's why I ask you to reference/link/quote people so often. It is so much easier to go to the original claim than work from your interpretation of it.

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Actually no you haven't. You have made several claims about them, but this is the first time you have claimed that the papers show that marine parks will make the harm done by minimum sizes worse. In any case, if there is such a claim somewhere, I'm sure you'd like to bring it up right now. This thread would be the perfect place for it after all. I remember noting that prof. Buxton's paper ignored the issue, though I don't think I bothered mentioning it at the time, as there were more significant problems with your interpretation of his paper.

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No it wouldn't PJ. Why do you think it would?

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Again, no.

Like I asked already, please explain how the theory of natural selection predicts a different outcome for the case?

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Where? What did he actually say?

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Most often it is a case of you misinterpretting what they write.

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There is no contradiction PJ. You can improve on more than one measure at the same time with marine parks. That's what makes them such great fisheries management tools.

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Wouldn't it make more sense to use marine parks?

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Are you admitting they made no such claim about the impact of selective pressure on minimum sizes?

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True, but it does prove you wrong. It demonstrates yet again your tendency to interpret an ambiguous statement, or no statement at all, as a statement in support of your position. After all, no scientists is seriously going to deny the impact of natural selection.

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You keep missing the point PJ. It is a matter of logic, not flat denial. That is why my simple questions leave you so stumped.

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And to measure that extent, you would have to be able to say what the recovery rate would have been if the harm was present. You cannot. You pulled a number - 10 years - out of your arse. So please explain where you got that from, and why the recovery rate we saw indicates no harm done. You cannot do this, because your argument makes no sense.

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I am not trying to wiggle out of real world observations. I am trying to state, as clearly as possible, that your observations do not back up your argument. They are meaningless. You are misinterpretting them. You are slipping up on some very basic matters of logic. You see, but you don;t understand, which is why you can't explain yourself.

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I need evidence that there is no contradiction? Again PJ, you are not making any sense.

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Again PJ you are missing the point. You claimed they said something about the negative impact of minimum sizes on fishery productivty, but they did not. They avoided the issue altogether. All the real world data in the world won't help you if it is measuring the wrong thing. Evidence, references, etc are not an alternative to thinking for yourself. You still have to figure out what it means. You took an inappropriate generalisation about marine parks and took it to be a specific statement about the impact of minimum sizes - an issue the paper didn't even touch on.

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This is about minimum sizes remember. Whatever you think they observed or said about marine parks, it has nothing to do with minimum sizes and you were just plain wrong to claim they did.

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This is getting rediculous PJ. You are assuming that because they didn't even mention the issue, they took it into account. How naive is that?

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So you did look for it, and that is the best you could come up with - a statement that does not even mention the negative impact of selective pressures on growth rate. Do you honestly think this justifies your claim?

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Please quote him.

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I thought we had cleared this up PJ. Remember what I said about saying something specific, not waffling on about impact, harm, pressure etc?

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No they don't PJ, which is why you are unable to explain how. The only point your 'ten years' example illustrates is that you are unable to come up with the actual time frame. You can only complete your argument by making up numbers.

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So you haven't looked at the work of Dieckman and Heino yet? Would you like me to provide another link?

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I am saying I know better than you, because you can't comprehend the issue.

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No it isn't. I explained it to you. It is a matter of logic, not evidence. You have to be able to udnerstand what the evidence means. That is where your argument fails.

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You seem to have forgotten the topic PJ. Either that, or you never figured out what is in the beginning.

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In other words, you did not quote him. You substituted your own deeply flawed interpretation.

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Like I said, you need to think for yourself. References are meant to back up your argument, not act as a substitute for a logical argument.

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Because they're articles are not about this topic PJ. I have explained why you are wrong plenty of times already.

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No it isn't. Read the thread title if you have forgotten. It's about the negative impact of the selective pressure arising from minimum sizes on a fishery.

Also, you did bring up marine parks here:

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However, the papers do not say the opposite. They do not even touch on this particular problem. You seem to assume that because they are critical of one or two marine parks they can be used to back up any criticism you make of marine parks in general. That does not make sense PJ.

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And what? You want me to go looking for it? That is not how it works PJ.

Catch limits, maximum sizes and resilience

http://www.ozpolitic.com/forum/YaBB.pl?num=1244944049/0

The effect has been empirically measured by these scientists:

'Darwinian Debt' May Explain Why Fish Stocks Don't Recover

http://www.sciencedaily.com/releases/2006/01/060112040047.htm

ScienceDaily (Jan. 12, 2006) — Why does it take so long for fish stocks to recover from over-fishing? This problem has been worrying both scientists and fishery managers who expect stocks to quickly rebound when fishing stops.

Now a research team from Stony Brook University believes they have an answer: continually harvesting the largest and oldest fish (as fishing regulations typically require) alters not only size but also numerous other genetic characteristics that are harmful to the overall population.

As reported in Ecology Letters, the researchers experimentally harvested captive groups of a marine fish named the Atlantic silverside. Removing the largest fish over several generations gradually caused a "Darwinian debt": the fish that remained in these populations became progressively smaller but surprisingly many other traits also changed including fewer and smaller eggs with lower survival and growth.

Even behavioural traits like foraging and feeding rate declined. Collectively these changes hamper population recovery and because they are genetic, they don't immediately go away when fishing ceases. How long it would take to undo the debt is now being studied by the research team.

'Undesirable' Evolution Can Be Reversed In Fish, By Letting The Big Ones Go

http://www.sciencedaily.com/releases/2009/03/090303193950.htm

ScienceDaily (Mar. 4, 2009) —  "Undesirable" evolution in fish – which makes their bodies grow smaller and fishery catches dwindle -- can actually be reversed in a few decades' time by changing our "take-the-biggest-fish" approach to commercial fishing, according to groundbreaking new research published by Stony Brook University scientists.

Intensive harvesting of the largest fish over many decades, while leaving the small fish behind, may have unintentionally genetically reprogrammed many species to grow smaller, said lead author Dr. David O. Conover, Professor and Dean of the Stony Brook University School of Marine and Atmospheric Sciences in Long Island, New York.

Although Charles Darwin showed 150 years ago that evolution equips life forms to be better adapted to prosper in their environment, unnatural evolution caused by man's size-selective fishing is causing fish to be smaller, less fertile, and competitively disadvantaged. This has also been a loss for commercial fishers who seek big fish for their livelihoods, recreational anglers in pursuit of trophy fish, and seafood consumers who desire large portions on their plates.

This study demonstrates for the first time ever that detrimental evolution in fish can be reversed, and pokes a gaping hole in theoretical models suggesting that genetic changes are impossible to "undo." It is the result of 10 years of research largely supported by a generous grant from the Institute for Ocean Conservation Science at Stony Brook University.

"This is good news for fisheries, but it also shows that reversal is a slow process," Dr. Conover said. "Over time, fish can return back to their normal size but the reversal process occurs much more slowly than the changes caused by fishing. So the best strategy is still to avoid harmful evolutionary changes in the first place".

Current fishery management plans are generally based upon assessment methods which do not incorporate long-term evolutionary dynamics. It could take years before evolutionary change is incorporated into such plans, since the concept remains quite controversial among scientists.

"It took scientists a long time to reach a consensus on climate change, and acceptance of this phenomenon might require a long time, too," Dr. Conover said.

Dr. Ellen Pikitch, Executive Director of the Institute for Ocean Conservation Science, said, "We now have proof that the negative ramifications of common fishing practices can linger for decades, leaving future generations to grapple with a legacy of diminished ocean productivity. It is essential that fishing be transformed to minimize its evolutionary and broader ecological consequences." Evolutionary dynamics are a fundamental principle of ecosystem-based fishery management (EBFM), a holistic approach that considers the connectedness of different species and the links between species and environmental influences, rather than managing each species in isolation. Dr. Pikitch was among the earliest proponents of EBFM, and lead author of a seminal 2004 article in Science on the concept.

"We have interfered extensively with the natural course of things, and while it is very encouraging that the harmful effects of size-selective fishing may be reversible, the length of the recovery period is sobering," said Dr. Pikitch, who is also a Professor of Marine Science at Stony Brook University. "Restoration of ocean fisheries requires prompt and widespread adoption of an ecosystem-based approach to fisheries management."