One bothersome point in this debate is the fact that some species that are in severe
decline seem not to thrive even the presence of what appears to be abundant stocks of
their “normal” prey. Atlantic Canadian groundfish seem not to benefit at present from the
relatively abundant capelin and shrimp in the area. Why not?
“It’s a little hard to see how cod could be short of food when the shrimp resource seems
to be exploding -- likely (in part) because of the release of predatory pressure.”
___________
Here’s part of a debate (excerpted from email exchange) with a marine biologist that
includes this issue...
The (scientific data avilaible) deals only with exploited groundfish, ‘not’ with the species
that current dogma predicts should increase in biomass to replace depleted,
over-exploited types. Thus, their data cannot indicate whether the "SUM
TOTAL OF LIFE" (my phrase that means ‘total marine biomass’) is declining or staying
steady.
You are right of course. The data does not exist to calculate the "sum total of life" - but
it's still a valid concept and missing numbers don't convince me that a significant portion
of the marine biomass is not also missing. I am challenging "current dogma" because I
just do not see evidence of species replacement as predicted.
However, to support your argument, you will need to show that nothing else is replacing
them. I doubt that you can do that
You "doubt that I can do that?" Maybe not, but I can certainly ask the question. Can you
prove that they are in fact being replaced as predicted? It is not long since we were told
that "skates and dogfish are thriving where cod once reigned." In your note you
acknowlege the disappearance of the dogfish:
“...the weight of evidence suggests that heavy fishing leads to species replacement (such
as skates replacing the dogfish that replaced cod on Georges Bank) but not to any major
change in total biomass.”
...which leaves me looking for the skates. I only found the one SSR on skate, "4VWS
Winter Skate" on the groundfish-depleted and dogfish-depleted Scotian Shelf - no
reliable abundance estimate but "total mortalities have doubled since 1995." How many
skates are there on the Grand Banks? Are survey trawls full of them now that the fish are
missing? Or are they just theoretical skates? I know you'll say that it's not just "skate," it's
all sort of little things that we take no notice of, but I'm just not seeing "lots" of anything
much.
OK...there are populations of "feed" that are at high levels now, one example being the
abundant capelin currently on the Scotian Shelf. And most people seem to agree that it's a
rebound effect from the drop in predation. Which raises the question, however: why don't
the groundfish take advantage of this abundant food supply?
(like you said: “though it is a little hard to see how cod could be short of
food when the shrimp resource seems to be exploding -- likely (in part)
because of the release of predatory pressure.”)
I agree that this does present a conundrum. If their "prey" is now unusually abundant,
then what is their problem? Why don't they just eat it, grow fat and prosper? A biologist
at BIO asked me the same question. Here's what I wrote back but he's very busy and
hasn't yet replied to me - so, feel free to criticize...
“I'll try to answer the question. Anyone who does this is speculating...but this is what I
think...
First of all, describing the cod-capelin relationship as predator-prey might be a tad
simplistic. Is it not likely that capelin also eat cod larvae while munching on the
zooplankton? This partly reverses their relationship. With the balance of species tipped
the way it is today, large capelin stocks could eat enough larvae to have a negative
impact on the recovery of the groundfish. (Actually, you also mentioned this idea.)
But - the ones that survive the larval stage - why don't they grow fat on capelin? It's
because a lot more species than capelin and shrimp matter to the cod. The abundance
of cod doesn't reliably follow the abundance of capelin anyhow. (Here's a link to one
article that I read - does this look fairly accurate to you?
http://www.stemnet.nf.ca/cod/role.htm) A quote from that article:
‘For example, in the Barents Sea, the growth rate and condition of cod decline when
capelin are scarce, but here and in Iceland the evidence on this point is not clear. Here,
the abundance of capelin has been highly uncertain in recent years. However, we know
that in the 1970's, when capelin were definitely scarce, there was no obvious decline in
the condition of cod.’ (...suggesting that cod eat a variety of things. Also in the article
“here” means Newfoundland.)
In the 1970's the cod had a lot more "neighbours" around in the form of all the other groundfish stocks. No doubt at different stages they ate a lot of the young of these
neighbours as well as cannibalized their own. Now, especially in a spot like the Eastern Scotian Shelf, there are a lot less of all these fish. How much has the rate of
cannibalization increased since the decline in groundfish in general? There could have
been/probably has been a big change even in the last 10 years. How frequently is the diet
data collected?
I think the cod need the usual variety in their prey, and the usual concentration of their
traditional neighbours, at certain ages/sizes capelin might not be the easiest thing to
catch.
Cod cannibalism must be ‘way’ down on the eastern Scotian Shelf. It
takes a big cod to eat a small cod and there just aren't many big cod in
that area any more.
There could have been/probably
has been a big change even in the last 10 years. How frequently is the diet
data collected?
Very rarely. And such studies as have been done have not been able to
capture all of the subtleties: Cod have such variable diets that it
would take enormous numbers of samples to sort it all out.
So: You are acknowledging that prey is relatively plentiful, at least in
this one area. And you are
suggesting that the cod are starved, despite this abundance of prey,
because capelin is an unsuitable prey item, at least at certain times?
And that despite capelin being a known major food of cod in other times
and places?
I guess the idea just about holds together. However, before anyone could
take it very seriously, you would need some evidence that the cod 'are'
starving. If prey was scarce, that might be indication enough. However,
to postulate starvation in the presence of prey, you really need some
independent indicator of the starvation. (Skewed RNA/DNA ratios might do
it. Low fat content might also.) Simply low weight-at-age won't be enough:
The observed low temperatures on the eastern Scotian Shelf in recent
years will have slowed the cod down, including reducing their feeding
and so
slowing their growth.
So: Do you have any evidence that the fish are starving -- or even
food-deprived? Without it, I think we have to assume that fish do not
starve when prey are available.
RNA/DNA ratios I do not have. I do have a recent quote from DFO regarding the Eastern
Scotian Shelf cod stock (the directed fishery has been closed since 1993):
"Condition is the relative weight of the fish for their length, i.e. their plumpness. Up to
1989, there was a highly significant negative relationship between population biomass
(age 3+) and the condition of the fish in July. This suggested that in periods of high
abundance the condition of the fish was reduced. As the population biomass declined in
the early 1990s however, the condition index continued to fall and remained low until
1995. Although higher, there is greater uncertainty in the 1996 and 1997 estimates due to
relatively small sample sizes. These condition observations are consistent with the widely
reported loss of weight seen by the industry and the presence of "slinky" fish in the catch.
Investigations into the cause and significance of low condition in fish have suggested that
low temperatures can induce poor condition and that reduced survivorship and
reproductive success can result. This is also consistent with the appearance of colder
waters on the eastern scotian Shelf since 1986."
"Slinky" has been described to me by fishermen in terms that suggest "emaciated".
Regarding the colder water, has it not been recorded that this temperature trend has in
fact been reversed in recent years? You suggested that cod cannibalism must be very low
due to the absence of large ones. That makes sense. But despite warming conditions, no
fishing and "abundant prey" this stock is doing very badly. From the latest SSR: "Surveys
indicate that since the mid-1990s, there has been an increase in mortality of cod, other
than that attributable to fishing, and which has persisted even after the closure of the
fishery...The biomass is projected to decline 5% to 20%, even in the absence of any
fishery."
So, are the low condition "slinky" codfish evidence?...Enough to take the
"starvation" idea seriously?
You also pointed out another troublesome finding:
For the Scotian Shelf, groundfish condition factors generally rose
during the years of the great foreign fishery (roughly 1960s), as simple
theory said they should. (Fewer fish were expected to mean more food for
each one and so better condition.) If I remember correctly, average
condition then began to fall as biomass started to recover in the later
1970s (as was expected) but continued to fall once biomass tended down
from the early or mid-1980s onwards. That was remarked on and documented
at the time. The guys at BIO have now been struggling with the issue for more than a
decade and I am not in the least surprized that they agreed
with you that it is important. It is (for the particular area and
species).
So beyond the mid-1980s the "simple theory" failed? I am aware of this trend and wonder
if it has ever been seen before or observed to be reversible. When condition and
abundance decline at the same time, I think you are seeing something that might not have
occurred before (please correct me, I don't have all the facts), in any case it now looks to
me like a completely different ball of wax - not at all like the story of the "downturn" in
the 1960s.
Exactly (as to the ball of wax being different).
(In the 60s our favorite groundfish were overfished but their "neighbours" were virtually
untouched, and I believe that is why the survivors were able to maintain good
"condition.")
Belief is a risky basis for scientific conclusions. You seem to be
suggesting that cod were able to maintain condition because they had
unspecified and unexploited competitors (wolffish perhaps?). As
discussed above, you have accepted that they still have available prey.
I'm not clear why a lack of competitors should cause a cod to grow
slowly. That is certainly a novel hypothesis that need some support
before it can be accepted scientifically.
Yes, it’s a novel hypothesis, but you yourself said that “cod have such variable diets.”
Also regarding "low abundance + low condition" - I believe this is also being seen in the
declining Pacific salmon stocks. (I can't say exactly that it's "condition" but they have
seen a steady decline over at least the last couple of decades in the average size of
salmon (including dropping weight-at-age).
If you want to speak only
of the northwest Atlantic but to extrapolate from the known declines in
groundfish and to suggest that the region has seen a decline in total
biomass, I would be happy to take that idea on board as one hypothesis
(alongside the notion that species replacement is maintaining a constant
overall biomass).
[As Gary Sharp will no doubt point out, marine biomass is ‘never’
stable. A general stability, subject to cyclical and irregular
environmental change, remains a useful abstraction against which the
effects of other driving forces can be discussed -- though not as a
standard against which anything can be measured.]
...I'd agree that the northwest Atlantic ‘seems’ to be showing decreased total biomass.
Thanks.
_______________________________
I have been told as a criticism that “Anyone who looks for anything can usually find it.”
This is referring to scientific research, and of course it is true -- our bias always shows,
mine and everyone else’s. (Look for what you are hoping to find, ignore information that
appears to refute your pet theory, and juggle the statistics in the end if necessary...choose
the “level of significance” that suits you.) What is really needed is an objective look at the
data and a serious consideration of all possible explanations, including those that are not
the “favorites” of the researcher. Believe it or not, that is what I have tried to do.
“What is your affiliation? Where do you get your funding from? Why are you involved in
this issue?”
Some wonder what is my “real agenda,” what is it that I am really “promoting?” I have
been suspected of ulterior motives. Is it an end to fishing, and am I funded by an “animal
rights group?” “WHY” am I doing this...why am I so interested in this problem? Do I eat
fish?
I am wondering about your interest in this subject. What is your
background/affiliation? I ask this directly, because some of my colleagues
who are interested in your hypothesis are beating around the bush and
wondering. Please tell me "where you are coming from" with regard to your
affiliation and scientific background.
I eat fish and I go fishing. I grew up and still live in a fishing village, and count many
fishermen among my friends and family. My interest in this subject is because it is so
important. My "scientific background?" I have a nursing degree, am an engineering
school dropout, an inventor, a very interested student of many different things, and for a
long time the problems in the ocean have been on the top of my list. I read constantly,
lately including scads of marine biology research documents, I'm always "looking
around" at things, and I ask a lot of questions. (Re: “funding” and “affiliation” - I have
neither.)
Unfortunately, my lack of "formal education" in this field ruins my credibility with many.
Someone wrote to fishfolk yesterday "Most marine scientists do specialize in something
particular. They are not general practicioners"...maybe that's what I am trying to be, a
marine GP... Actually marine biologists resemble a lot of speciality doctors who are
intently studying the intricate details of the problems that have been identified. That’s
fine, but who is doing the “nursing” part? They can “doctor” the ocean but can they
“nurse” it? (They are two distinctly different jobs and it is generally the nurse who takes
care of routine details like “feeding” the patient...)
