The Downturn of the Atlantic Cod (Gadus
morhua) in Eastern Canada
What is happening to these fish, and why? by Debbie MacKenzie, November, 2002
(**Note: In 2004,
DFO has finally agreed with my diagnosis:
cod are starving, because they cannot find enough food,
their habitat is "energy poor" (starved) due to the cumulative effects of
fishing.)
classic adult cod (NOAA), fish predator,
consistent with today's "healthy cod".
20" cod, arched back, flat stomach
(click thumbnails for larger views)
19.5" cod, emaciated bottom-feeder?
Atlantic cod caught recently in Nova Scotia, besides appearing very
lean, are showing a strange body shape, with the back arched and the
head turned down, in contrast to the classic appearance of the species.
Cod now looks more decidedly like a bottom-feeding fish than it did in the
past. Fish stock assessment scientists see a recent dramatic downturn in
the numbers of Atlantic cod…but surprisingly, a ‘downturn’ can also
literally be seen in the faces of individual codfish...as these fish struggle to adapt to the current poverty of their normal
prey fish in the ocean(?)
Perspective:
A decade ago, cod stocks “crashed” in Atlantic Canada, an event
generally agreed to have been the
result of “overfishing." Despite a ten year fishing moratorium,
however, these stocks are showing no convincing signs of recovery. Most perplexing,
perhaps, is the continued, and increasingly acute, absence of older,
mature fish from these stocks. With 10 years to grow without fishing
pressure, there should now be a significant stock component of larger
adult fish. But there is not…and in fact the oldest age now attained by
cod on the Grand Bank of Newfoundland, is less that it was ten years ago.
Stock assessment indicators for cod in general continue to fall. Why? ...Might they
really be simply 'starving?'
A look at cod on the Scotian Shelf (Atlantic coast of Nova
Scotia):
An informal look at two lots of Atlantic cod caught by a commercial
fisherman on the Scotian Shelf during late summer of 2002, may reveal an
important clue, since the profile of cod seems to be literally changing.
The cod fishery on the “Eastern Scotian Shelf” (4VsW) remains closed (under
moratorium since 1993) but a greatly scaled back cod fishery continues in
bordering area “4X” which extends from Halifax southwestward, and includes the
naturally more productive Southwest Nova/Bay of Fundy area. (click on map
at right to enlarge) The fish that I examined were caught near the
northeastern limit of “4X” in waters bordering on the closed area, “4VsW". The Eastern Scotian
Shelf has become known to fishermen and scientists in recent years for the
“baffling” production of “slinky” cod, fish in "very poor
condition." (quotes from DFO) It seems likely that these poor-looking cod
caught in eastern 4X are similar to their “slinky” relatives in 4VsW.
(click to enlarge)
August 8, 2002 (fish from 44:04N, 64:02W, in photo at right above) - I wanted to photograph profiles of cod for gross
comparison to DFO’s photo (left) in which well-fed and starved cod are
contrasted. It was a surprise to find that the fresh-caught cod (on baited longline) resembled neither the well-fed nor the lab-starved examples from DFO, neither did these fish appear as intermediate forms between the two
extremes. Besides appearing lean, the cod caught in early August appears
rather to have a grossly altered body shape ~ a flattened stomach profile
and almost a ‘humpback’ in comparison to the standard codfish shape. The
orientation of the head is more downward-looking, giving the overall
impression that this fish is a bottom-feeder. (However, this particular fish was 20
inches long, a size at which cod are known to normally feed primarily on
fish, in contrast to the small invertebrates that prevail in the diet of
younger cod. (Liem and Scott, 1966) For what it’s worth, the
stomach of this fish contained only one
very small crab and one small shrimp.)
September 27, 2002
(photo at right) - The same fishing vessel caught a load of cod at a
location further east in “4X” (44:12N, 63:30W), very close to the edge of
"4VsW". Again, the cod were typically small and
very lean. The example in the photo was 19.5 inches long, in the same size
range as the cod photographed in August, but appearing in much poorer
condition. Again, the ‘downturned’ appearance of the head is pronounced,
and in this case the belly looked positively sunken in. (There was no
recognizable food in the stomach of this fish - just a bit of
clear mucus.)
Another common finding, the redness of the mouth, that was visible in the
August cod, is also more pronounced in this one. Both of these examples
were representative of the whole catch of cod at the time (meaning: I did
not pick out the couple of sorry-looking fish from two boatloads of
healthy cod. It is well known that when members of a fish species of a
certain size are caught together that the condition of individuals is
usually very similar.)
What are cod eating in Atlantic Canada today?
This is surprisingly difficult to ascertain from the literature, as
recent publications from DFO seem to reveal more interest in what seals
are eating than in what cod are eating. Routine fish stock assessment
reports do not include dietary assessments, although growth rates and
"condition factors" are now regularly included, which is helpful. The
growth rate of 4VsW cod has declined substantially, with fish in the 1970s
growing to 68-72cm in 7 years, but recently reaching only 54-59 cm at that
age. (DFO, 1998)
Recent scientific investigations into the failure to thrive of Atlantic
cod have focused on two likely causes of increasing natural mortality: (1)
excessive seal predation and (2) adverse environmental conditions, which
lead directly and indirectly to poor survival of fish (often ultimately
starvation). (FRCC, 2002)
These two suspected causes of high natural mortality in cod are unrelated, and should
theoretically result in OPPOSITE physical effects on individual fish. This
is an important insight, which should help to distinguish between the two
scenarios.
1. Regarding seal predation: According to conventional thinking,
and past observations, if predation (natural or human) lowers fish
population numbers below the level that the system can naturally support,
the surviving fish are left in
better-than-average physical condition. This is because each fish experiences a relatively greater
availability of food, once the competition has been lessened. Prior to
recent years, a pattern had long been noted in fishery science that when
fish populations had been over-harvested and were at low abundance for this
reason,
that the individual fish remaining were unusually plump. Whether the
excessive predation is from human fishing or seals feeding, this effect on
the condition of individual fish should be the same. Extreme degrees of seal
predation, if this is what is occurring, should be associated with
exceptionally good condition of the remaining cod.
Recently and unexpectedly, however, cod and many other fish have been
found to be simultaneously at low abundance and in poor condition.
2. Regarding adverse environmental conditions: If the growth of cod is being restrained by high natural mortality due
to “adverse environmental conditions,” this implies that individual fish
will likely be highly stressed, and predisposed to conditions such as post-spawning mortality (no energy
reserves) as well as greater mortality from factors such as parasites and
infections, and also predation (weaker specimens are less able to escape
predators). A fish population so affected wherein individuals are
undernourished will actually benefit - 'condition-wise' - by having its
numbers reduced, since food availability to individuals will be increased.
So seal predation, rather than hurting, may actually benefit a fish
population that is suffering from a shortage of food. (A lower number of
spawning fish in better condition may be better equipped to maintain the
species than a higher number that are dying of starvation.) Therefore,
making the wrong decision and removing seals in a situation of severe
"adverse environmental conditions" might actually work to aggravate
the poor physical condition of cod...
Recent biological data on Atlantic cod indicates generally slowed
growth, and poor "condition" (or fat reserves) of the few remaining fish.
(See
publications of the Canadian Department of Fisheries and Oceans (DFO) and
the Fisheries Resource Conservation Council (FRCC).) This finding does not
support, but rather contradicts, the currently favored hypothesis that
seal predation is now preventing the growth and "rebuilding" of these fish
stocks. Regardless, it appears that the announcement of a seal cull to
"aid" the groundfish recovery in Atlantic Canada is imminent:
"The council is still of the opinion
that the large numbers of seals are impeding groundfish recovery."
FRCC.2000.R.5 "Reductions in seal populations are required in areas where groundfish
spawn and on juvenile groundfish nursery areas. Unlimited, unhampered and
unrestricted seal predation on highly vulnerable groundfish cannot be
allowed to continue if groundfish stocks are to be allowed to rebuild."
FRCC press release, April, 2002.
However, judging by the appearance and
condition of the fish, "adverse environmental conditions," now seem
certain to be the
cause of the poor growth of cod stocks. It is most commonly suggested
that what is "adverse" is water temperature (if too low, fish feeding is
suppressed). But the "adversity" could also be a decline in overall system
productivity, which could result from various factors including climate
change or altered patterns of water circulation. (These two factors have
not changed dramatically in Atlantic Canada. My hunch is rather that
system productivity has been relentlessly lowered by centuries of fishing,
which has induced a general decline in marine nutrient cycling.
Most scientists will
disagree with my suggested 'cause' but that is not the main point...the
first order of business is determining whether or not a significant
lowering of marine productivity has occurred. Because if it has, it
is very important that this be acknowledged.)
Do these observations on the changed physical shape of Scotian Shelf
cod offer a useful insight into why the stocks are failing to “rebuild?”
The altered appearance of today's Atlantic cod should help to tease
apart the possible causes of their failure to thrive. A direct
"environmental" affect on cod feeding, such as cold water or lowered
oxygen, might induce the fish to eat less of its normal prey even though
adequate prey existed (appetite suppression?)...but a more indirect
"environmental effect" on cod would result from lowered system
productivity; in which case fish would simply be faced with a lower
abundance of their normal prey (normal appetite, just cannot find enough
to eat?). This seems most likely to be the problem, based on
physical assessment of the fish. In the case of lowered system
productivity (a much more worrisome "environmental" situation), changes would
be expected to also occur across all other species, and
this does appear to be the case.
Although my observations may be dismissed by some as “anecdotal” and my
conclusions as “mere speculation”…here is my interpretation of what is
most likely happening now to Atlantic cod off eastern Canada:
- A declining availability of their normal prey in the water column
(essentially all small fish, cod are not discriminatory) is forcing cod to
continue bottom-feeding as they enter adulthood. This repetitive behavior
(looking downward, rooting for food in the bottom sediment) has actually
caused the fish to grow now into an unusual shape with a markedly down-turned
head, as compared to the classic codfish look (normally adult cod have the
mouth oriented forward as befits one who pursues prey fish and snatches
them from the water.) Cod are not nearly as well suited physically to the
intensive
bottom-feeding style as are fish such as haddock, for instance, which may
help to explain why cod stocks are faring worse
than haddock stocks today.
- Rooting in the bottom for food (not the normal manner of acquiring
prey by adult cod) has possibly also caused unusual abrasion or inflammation
around the mouth, therefore the reddened appearance of the mouths of the
cod in the recent photos. (Note the contrast below between the facial structures
of normally-fish-eating cod (left) and normally-bottom-feeding haddock
(right): haddock has a much bigger eye and almost a 'beaked' mouth for
digging in the mud, hence it seems that it will feed more successfully than cod
under today's conditions: 'mandatory bottom feeding?')
- Laboratory studies are unlikely to reveal the whole picture in nature,
as starving a cod in a tank merely resulted in a very lean fish, but one
with the normal forward orientation of its head.
Starving cod in the
ocean, on the other hand, changes the profile of cod, as pelagic-feeding
adults lower their heads in a struggle to survive by feeding at a lower trophic
level (bottom invertebrates). Besides offering a contradiction to current thinking that
cod are feeding poorly
due to a direct effect of water temperature, the observation of this
particular physical change seems to discredit another current hypothesis:
that fish growth has slowed because a genetic shift towards slower-growing
individuals has been induced by the "size-selective culling" effect of fishing gear.
- A gradual lowering of overall marine productivity would be expected
to induce broad changes in the cod population that are consistent with
those that have already occurred. For example: a long, downward trajectory
including contraction of the range of the species, with the remainder of
the population concentrated in those areas that are relatively mid-range
and are naturally richest. The northernmost extreme of the range is where
cod disappeared first: the commercial cod fishery collapsed in Labrador a
decade before the major Newfoundland "crash" in 1992. (FRCC,
2000) The
offshore component of Newfoundland cod is now
in desperate condition, while inshore components are growing reasonably
well only in a few large bays (areas where production is enhanced by
terrestrial runoff). In Nova Scotia the best growing cod are concentrated
in the Bay of Fundy where the topography naturally enhances marine
production. This pattern would be predicted under a scenario of declining
ecosystem productivity, as would the prey-switching to organisms living at
'lower' points in the food web, which seems evident in Scotian Shelf cod
today.
- What about the other natural predators of small marine fish? All of
these would be expected to be similarly stressed, and possibly also
switching to bottom-prey. One visible example of this in Nova Scotia is
the common cormorant, a fish-eating bird that is now seen fishing inside
all of the shallow inlets along the Atlantic coast, in contrast to decades
ago when these birds were mainly seen near coastal islands. But if the
only food can now be obtained by diving to the bottom, the birds will save
energy by fishing in the very shallowest coastal areas. Most often cormorants
will now surface with small crabs, rather than fish, in their beaks, and
the crab seems to be quite an awkward meal for them to consume
compared to the ease with which these birds can gulp down small
fish...they usually rip one leg off and then lose the thing....(Aside:
strange behavior in marine wildlife includes this
cormorant who tried unsuccessfully to
eat hot dog pieces offered by the author.) (Further aside: how did this
codfish paper stray into a discussion of cormorants?...only because it
must; the entire ecosystem needs to be considered to make sense of
anything.)
- The lack of small prey fish seems most likely to have resulted from a
decline in the abundance of their major prey, zooplankton, which
implies a decline in productivity of the whole ecosystem. The interpretive
stumbling block which remains, to those following classical reasoning, is
“why are zooplankton declining when their food, phytoplankton, appears to
be at an unusually high abundance?” Is this where the food web has become
unhinged, at the point between phytoplankton and zooplankton? How can this
be? Well…it’s complicated, but the strength and nature of the
interdependence of zooplankton and phytoplankton is just one aspect of
marine ecology that perhaps needs to be reassessed now…and, especially in
Atlantic Canada, it is impossible to convincingly explain the recent
dramatic shift in the marine food web by either “global warming” or
“pollution.” It seems more likely to simply have been the ultimate result of
centuries of heavy fishing…a result which should have been predictable had
we correctly grasped how the ecosystem works. (We have largely failed to
appreciate the ecological role of fish in supporting zooplankton, and
indeed in enhancing basic nutrient cycling/ecosystem productivity…because
we thought that mature fish were “surplus,” i. e. removing them would not
weaken the system, or negatively affect organisms smaller than themselves.
It looks like we were wrong about that. Today’s trends are showing us what
can be produced by a fishless sea…and it’s NOT a great excess of
the lower
forms that would normally be their prey…the declining trends in formerly
dominant marine species can even be detected in
barnacles and seaweeds (check out the
seaweed photo galleries).
- The scientists responsible for assessing and protecting the health of marine
life urgently need to address the question of significantly declining marine
productivity that may have already occurred. This theory challenges the usual view, but unfortunately it
seems that overwhelming evidence supports it.
Regardless of how unpleasant this issue might be, we need to honestly
look at all of the facts and ask the questions: "What if we
have been wrong about what 'drives' marine productivity?" "What if
the current failure of the Atlantic cod is essentially due to simple starvation?"
FRCC.2000.R.5 "Uncharted Waters" Annual Report of the
Fisheries Resource Conservation Council and Conservation Requirements for
Atlantic Groundfish Stocks for 2000.
FRCC.2002.R.1 "2002/2003 Conservation Requirements for
Groundfish Stocks on the Scotian Shelf and in the Bay of Fundy (4VWX), in
Sub-Areas 0, 2 + 3 and Redfish Stocks" (online at:
http://www.dfo-mpo.gc.ca/frcc/2002/jan2002e.pdf )
Liem, A. H. and W. B. Scott. "Fishes of the
Atlantic Coast of Canada." Fisheries Research Board of Canada, Bulletin
No. 155. Ottawa, 1966.
