QUESTIONS AND COMMENTS RE: "Species Replacement Theory"
It has been pointed out to me repeatedly that if one marine species suffers a decline that
another will increase to replace it in such a way that the total biomass is basically
unchanged. It is tempting to believe, especially when a prey species is observed to
flourish once its predators go into decline. Regarding the disappearance of major
quantities of our usual fishing targets, we are told that the “production” has gone into
other species that are of no particular interest to us, creatures that we might not even
notice (non-commercial species of fish, and the likes of worms, jellyfish, and
micro-organisms). It is fairly easy to “prove” the declining biomass of our traditional
fishing targets, but it is quite another thing to “prove” that they have not been replaced
with an equivalent amount of some other organisms elsewhere in the system (largely
because the data has never been collected on the “uninteresting” creatures). But I guess
that is the thing that I must try to prove...
“To support your argument you need to show that nothing else is replacing them.”
(OK, I will try.)
For some reason we are told that this is the starting point:
“The simplest starting point must be that biomass is stable. (After all, the oceans and
their biota have been there a very long time. Since the waters have neither filled up with
fish nor become complete deserts, there must be some stability.)”
The marine ecosystem appears to have existed in a condition of “some stability” for a
very long time - but nutrients were always recycled internally before fishing by humans
became well established. Our removal of large quantities of fish (and our haphazard
replacement of fish flesh with sewage and soot) appears to have effected a real change in
the system...the “stability” is no longer apparent, nor is it at all obvious that “biomass is
Yet the theme is constantly repeated...
“Current dogma predicts (other) species should increase in biomass to replace depleted,
“Many fishermen in our province could have told you that the weight of evidence
suggests that heavy fishing leads to species replacement (such as skates replacing the
dogfish that replaced the cod of George’s Bank) but not to any major change in total
A quick check on the stock status of skate reveals that it is in decline as well, so
replacement must have happened again, with yet “another” species...
“The species replacement idea first emerged in the Great Lakes, where the salmonids
were fished down and the fishery then shifted to whitefish, which proceeded to support
catches roughly equal to those that the salmonids had once provided. Then the whitefish
were fished down but the industry moved to, I think, lake herring --and still maintained
much the same total catch.”
I can see how this sequence of events might have suggested the “species replacement
theory,” but not how it offers any proof, nor why this idea should or could be generalized
to the ocean.
The “biomass is constant” idea leads to fishery management guidelines like this one:
“As long as the amount taken is not above the stock recruitment levels (not always the
case unfortunately), there is plenty of scope for stock replenishment. (i.e. you can
sustainably remove biomass every year and the total standing crop will remain the same
year on year.)”
One problem with that approach is that the calculations and predictions are based on one
species only and take no account of the interactions of that particular fish with the rest of
the system. This approach has been used repeatedly and found repeatedly to be flawed.
Mathematical “models” used in fisheries management are notorious these days for their high failure
rates. It’s because all the models are based on a false assumption “total production
always remains the same” or “total biomass is basically stable.” Another way to state it
is the old adage “There are lots of other fish in the sea.” It’s just not so.
I also have never been told by a fisherman that there seems to be no major change in total
biomass. Most of those that I know are clearly concerned about the future of fishing as a
viable occupation. One of our First Nations people was asked to speak at the closing
ceremonies of the conference of the “International Ocean Institute” in Halifax in 1998.
“I’m thinking about the kids and our grandkids, are they going to have to walk Mother
Earth with a face mask on? I hope not...What you are telling me is you don’t know how to
fish...You use the word ‘technology,’ but in my time there has been a decrease in
everything...If something isn’t done soon, there will be no more time for any of us. There
has to be something better than technology. It was sad a few years ago when our seals
got the blame for taking the cod. It wasn’t their fault...those who harvest the earth’s
resources must begin putting as much back into it as they remove...”
-- Charlie Labrador, Mi’kmaq Elder of the Ocean