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Subject: Re: depensatory exploitation
From: "Trevor J. Kenchington" <[log in to unmask]>
Reply-To:Scientific forum on fish and fisheries <[log in to unmask]>
Date:Wed, 2 Aug 2006 14:30:17 -0300

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I have been away on business and hence am a few days late in  
responding to this thread.

Rom Lipcius wrote:

> Thanks for the comment and suggestion, Trevor.
> As I'm sure you know, the normal assumption in most fisheries is  
> that exploitation rate or fishing mortality rate is constant as a  
> function of abundance.

I do not know whose normal assumption that is but it strikes me as a  
foolish one, so obviously contradicted by everyday observation. There  
is a common suggestion that the management target should be a fishing  
mortality rate independent of resource abundance (across some range  
of abundances) but management targets and actual events in fisheries  
rarely converge very closely.

The "across some range of abundances" is important also: Either as  
part of an existing plan or as a response to declining resources,  
management generally aims to cut mortality rates when abundance (or  
more often biomass) falls very low -- a "compensatory" response, if  
you wish. When biomass is very high, managers generally leave  
fisheries alone but that is a mistake and the best strategy is  
usually to hold mortality down, thus preventing investment in the  
infrastructure needed to handle huge catches (such as result from  
applying F[MSY] to virgin biomass), which investment will then drive  
demands to maintain high catches as biomass falls. I guess that would  
be a "depensatory" response of a kind.

> Hence, in fisheries management we attempt to find that rate (F or  
> u) which produces a sustainably fished stock, for example by  
> maximizing yield or protecting a fraction of the spawning stock  
> using target or limit biological reference points. In this case the  
> form of the function relating exploitation rate to abundance is  
> linear (= constant), and is analogous to a predator's Type I  
> functional response.

The message is getting distinctly old but I will repeat it yet again:  
Examining the behaviour of fisheries using ecological concepts and  
models applicable to predator/prey relationships will mislead and  
confuse you. Ecologists have been trying to do that for decades and I  
don't think that it has ever been helpful.

Human behaviour, particularly the behaviour of humans who are part of  
a globalized, technologically-advanced society, is just too complex  
to be modeled under the simplifying assumptions used for non-human  
predation. There are many reasons why. A few of the more obvious: The  
food value to a natural predator of a unit of fish (an individual  
herring, 200 mm in length, for example) is effectively constant,  
whereas the dollar value of that unit to a fisherman depends on a  
highly-variable market price, some of the variations in which are  
themselves driven by resource biomass. then, of course, teher is teh  
variability in the cost of exerting a unit of fishing effort.  Next,  
the hunting ability of a predator of a given type and size is, if not  
constant, then at least stationary. The fishing efficiency of a boat  
of particular type and size usually increases rapidly over time, with  
the rate of that increase being highly unstable. Another, and perhaps  
less obvious, issue is that fishermen are less predators than the  
mouthparts of meta-predators, whose bellies are the consumers of the  
seafood products. hence, the heavy exploitation of northwest Atlantic  
cod in the 16th and 17th centuries, when there were few cod-eating  
humans in North America but a hungry market in Europe.

Something as apparently-obvious as a fishery's responses to changing  
resource abundance can be massively affected by such factors,  
potentially reversing any predictions made from predator/prey models.

> Constant exploitation rates as a function of abundance may not be  
> that uncommon, as exemplified by the following recent paper:
> David B. Eggleston, Eric G. Johnson, G. Todd Kellison and David A.  
> Nadeau. 2003. Intense removal and non-saturating functional  
> responses by recreational divers on spiny lobster Panulirus argus.  
> Mar. Ecol. Prog. Ser. 257: 197207.

If the only example that can be offered is of recreational hand- 
collecting by depth- and endurance-limited divers, I would say that  
"uncommon" is a fair summary.

> When the response is non-linear, it is not necessarily depensatory,  
> particularly at low to moderate population levels. The actual form  
> depends on a combination of fisher behavior and the behavior and  
> response of the species, similar to what we see in predator-prey  
> dynamics. The two major non-linear forms of the response are a Type  
> II functional response (= depensatory) in which exploitation rate  
> or proportional mortality increases as abundance declines. The  
> other function is a Type III functional response (= compensatory)  
> whereby proportional mortality/exploitation rate decreases as  
> abundance declines from moderate to low abundance levels.

The far more likely response, if the consequences of changing biomass  
could be separated from those of other factors, would be one that  
varied between "depensatory" and "compensatory", depending on just  
how low biomass went -- fishermen at some point increasing effort to  
try to maintain catches in the face of falling catch rates but then  
cutting effort (ultimately to zero) if CPUE fell below break-even.  
The aggregation behaviour of the resource certainly plays a part. So  
does its changing spatial distribution, as abundance declines, which  
may result in a withdrawal from the coast (increasing harvesting  
costs) or a concentration there, making the fish more vulnerable.  
Then there are important temporal factors: An immediate drop in  
biomass can cause harder fishing as fishermen seek to utilize their  
investments in boats and their own lives to maintain their incomes,  
even if the dollar returns from their effort barely exceed the short- 
run costs of going out again. However, if the low biomass (and hence  
low CPUE) is then maintained over an extended period, boats are laid  
up, old fishermen retire while their sons take jobs ashore, and  
effort falls.

Again: The system is far too complex to be summarized in three types  
of "functional response" and needs to be addressed from socio- 
economic, not predator-prey, angles.

> You are correct that there is likely to be a fair amount of  
> information in stock assessments on fishing mortality rates and  
> exploitation rates along with stock abundance/biomass, but I am not  
> aware of many attempts to characterize the form of the function  
> quantitatively (other than in theoretical treatments), particularly  
> with direct population estimates of exploitation rate and  
> abundance, as opposed to modeled estimates of exploitation or  
> fishing mortality rates. I am now thinking that it would be useful  
> to conduct a comprehensive review of this issue across various  
> fisheries, and will be requesting suggestions as to where I can  
> most readily acquire this information.

I think a comprehensive review could be very valuable. So would a  
review of attempts to model fisheries using ecological models.  
However, I would suggest that the lack of attempts to characterize  
the responses of fishing mortality rates to changes in resource  
abundance in terms of the abstractions of predator/prey theory should  
be seen as a warning that such characterization is not helpful.

> I should also note that Lobo Orensanz, Gordon Kruse and Tito de  
> Morais kindly provided some initial leads on this issue, in  
> particular the following publication that I am currently using to  
> extract some of the published information:
> Orensanz, J.M., Armstrong, J., Armstrong, D.A. and Hilborn, R.  
> 1998. Crustacean resources are vulnerable to serial depletion- The  
> multifaceted decline of shrimp and crab fisheries in the greater  
> Gulf of Alaska. Reviews in Fish Biology and Fisheries 8: 117-176.

Serial depletion is a major problem, especially with sedentary  
shellfish but also with other resources that have highly-subdivided  
spatial structures. That is an extra complication added on top of my  
above generalizations, in as much as it allows a behaviour of  
expansion onto new resources (or those only lightly exploited in the  
past), besides any increase or reduction in effort imposed on  
previously-exploited components.

Trevor Kenchington

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