Northern cod discovered
‘belly up’ in Newfoundland…were they frozen, starved or suffocated?
(see also update on this story)
Debbie MacKenzie, April 7/2003
Another mysterious development
in the ongoing crisis in Atlantic Canadian fisheries has been reported by
the CBC. From the CBC news website:
dead cod likely froze to death
06 Apr 2003 5:35:04
SMITH SOUND, NFLD. - Thousands of dead cod turned up off Smith
Sound Trinity Bay, Newfoundland this weekend. Fishermen noticed small
numbers of dead cod earlier in the week and by Saturday the bay was
full. Fishermen estimate that about a quarter-million pounds of cod
Tom Mills, a Smith Sound resident, described the cod as flawless,
"with no signs that they were killed by seals or pollutants." Mills
estimates the cod surfaced over a 25-kilometre area and predicts the
final loss to reach thousands of tonnes, which will likely have a
disastrous impact on the vulnerable east coast fishery.
Each year, small numbers of cod surface due to 'super chilling', an
atmospheric affect that draws cod to a body of water that's too cold
for them to survive, and they eventually freeze to death. Provincial
authorities are unsure what caused this latest tragedy and have
cautioned people not to eat the cod until test results come in.
But residents of Smith Sound are taking to the waters and scooping
up as much cod as they can store, despite the do-not-eat warning.
(story online at:
How sad. Has the saga of the
troubled northern cod now come full circle? Five hundred years ago it was
claimed that one could harvest cod from Newfoundland waters simply by
lowering a basket into the sea. In this latest pathetic twist of the story,
it seems that harvesting the northern cod might again be that easy…because
thousands of tons of them are floating dead on the surface of an inshore
bay. The scientific opinion is that these cod have died from the cold. But
that explanation at first sounds just a bit unlikely. Have not
Newfoundland waters always been very cold?
The cod population involved in
this incident represents the last hold out of a once-phenomenal biomass that
dominated the entire Grand Bank. And it now seems certain that cod will be
unable to survive much longer in this precarious position. Why not?
Frozen cod at Lower
Lance Cove, Newfoundland, April 9/03.
Photo courtesy Patricia Pike.
Why did the cod die?
The media report was based on
an interview with a spokesperson from the Department of Fisheries and
Oceans, as well as the observations of local residents. And the explanation
is that the cod found themselves in water that had been “super chilled” and
was therefore too cold for their survival. It was also noted that this
phenomenon has been seen before in this area in recent years, but that on
the other occasions only “small numbers” of fish had been affected. The
important question posed today is “Why have such a large number of cod
suddenly ‘frozen to death’ this year?”
Stresses have been mounting on
the northern cod that indicate that the marine environment has become
increasingly hostile to its continued survival. But this does not mean
simply that the ocean has become colder. A more plausible explanation is
that the many shifting trends have been forced by an ever-dwindling food
supply. Surviving northern cod are now increasingly concentrated in a few
inshore bays (FRCC, DFO). This is in marked contrast to the earlier,
long-established pattern where the bulk of the cod population migrated
offshore to spend the winter in warmer, deeper water and moved inshore
during the spring and summer months to feed. Cod that live inshore
year-round enjoy a relatively greater availability of food, but they run the
added risk of becoming weakened or killed by exposure to extremely cold
(sub-zero) surface water. The water has not gotten colder, but it appears
that a greater proportion of the cod are trying to survive in this
precarious position. What is essentially a food shortage issue for cod could
therefore be ultimately manifested as a ‘freezing to death’ issue.
There are many other
indications that food shortage is now a major problem facing northern cod.
- Growth rates, and more
significantly the condition (essentially the fat content), of cod has
declined in recent years, and this decline has been sharpest in the areas
with lower natural levels of food production (i.e. offshore Newfoundland
waters as compared to inshore, and in Nova Scotia a marked drop in condition
has occurred on the Eastern Scotian Shelf as
compared to the richer Bay of Fundy).
- Age-at-maturity in northern
cod has declined significantly (DFO, 2003). The tendency of fish to become
mature at smaller sizes and younger ages is known to be a possible
consequence of a declining food availability (Stearns and Crandall, 1984).
- In recent years, scientists
and fishermen in Newfoundland (and elsewhere in Atlantic Canada) have
repeatedly expressed concern at the decline in capelin, traditionally the
major, oil-rich prey of the northern cod. Scientific surveys on the Grand
Bank suggest that a large decline in capelin abundance has occurred there.
Capelin are also smaller now, on average, than they were in the past.
- Scientific monitoring has
revealed a decades-long trend of a declining abundance of zooplankton* in
Atlantic Canada. And zooplankton (tiny animal-plankton) are the major
food of young cod, as well as the food for capelin. (* This development has
received far less scientific attention than it deserves, which represents a
major weakness in DFO’s Science program.)
- Accelerated natural mortality
due to poor physical condition (post-spawning mortality) was believed to
have been a factor in the sudden crash of the cod stock that occurred in the
early 1990s, and which coincided with a short term dip in water temperature.
- The age of cod being consumed
by harp seals has been inexplicably rising. From DFO (2003):
“From 1986 to 1996 cod age 0
and 1 were the predominant age groups found in harp seal stomachs. In 1997
and 1998 older fish (ages 3-5) were the dominant age groups and fish as old
as age 7 were found more frequently than in previous years.”
Since harp seals are not
growing bigger or stronger, this pattern suggests that their cod-prey are
becoming physically weaker and increasingly unable to escape predation. In
the earlier years, it seems most likely that the seals refrained from eating
the larger cod simply because they could not catch them. The role of natural
predators is to enhance the health of its prey species by selectively
removing the weakest individuals. Adult cod slowed (or killed) by factors
such as food starvation, extreme cold, or oxygen deprivation, would become
easy prey for seals that they could have avoided had the fish been in better
Why are the cod concentrated
in Smith Sound?
A relatively dense aggregation
of Atlantic cod has persisted in Smith Sound, in an inner region of Trinity
Bay, following the demise of the bulk of the larger offshore stock
component. Why have cod persisted in this particular location?
Faced with an environment that
is producing ever-lower quantities of their food, a fish such as the
Atlantic cod will make any and all possible adjustments that will enhance
its survival odds. Besides slowing the individual growth rate, reducing body
fat stores and reproducing at smaller sizes, the fish can be expected to
contract its geographic range to the area that naturally produces the most
edible material. Due to the nutrient input from terrestrial run-off, inshore
locations such as Smith Sound produce somewhat more food, and will
ultimately become the only remaining viable habitat for cod. But, as general
food availability declines, susceptibility to death by ‘freezing’ in these
relatively shallow, inshore areas can be expected to increase. This is
because poorly nourished animals have lowered resistance to all physical
What explains the timing of
this particular fish-kill?
If the immediate cause of the
recently reported cod deaths was cold water, why did this occur in early
April, and not in January or February? It seems unlikely that much heat
remained in the surface water of Smith Sound during those winter months.
This is supposedly explained by some vague “atmospheric effect,” but the
factors previously discussed may be more important.
First of all, fish with
borderline energy reserves to withstand the intense cold would be expected
to run out of energy and perish at the end of winter.
However, the suddenness of this
event and the fact that cod of many different sizes died simultaneously,
suggests that some other environmental variable may have abruptly changed
for the worse and contributed directly to their deaths. The following is
speculation on my part, but it will be interesting to see what the final
verdict of science is on the cause of this fish kill.
A lack of oxygen in the
bottom water of Smith Sound may have contributed to fatally ‘drawing’ the
cod to the surface. It is now early spring. This is the time of year
when daylight lengthens, seawater becomes less turbulent, and the ‘spring
bloom’ of phytoplankton occurs in the North Atlantic Ocean. In this area,
these blooms have been noted to be becoming more intense, although somewhat
shorter lived, in recent years as compared to decades ago (Gregg and
Conkright, 2002, DFO, 2000). The increased intensity of the ‘greeness’
relates to the fact that tiny algae are more numerous in the surface water.
Since zooplankton normally eat
the phytoplankton, a decline in the numbers of zooplankton can readily be
seen to be related to the observed increase in ‘greeness.’ One problem,
however, is that uneaten algae will sink to the bottom and undergo bacterial
decomposition, which can reduce the oxygen content of the water to levels
below that needed for fish survival. This scenario may have developed in
Smith Sound, and in such a semi-enclosed inshore area the whole effect may
easily have been exacerbated by nutrient input from terrestrial sources,
such as spring run-off. (The loss of oxygen from the bottom water in Smith
Sound may have been severe enough to kill bottom life there, such as crabs,
and if this is discovered to have occurred it adds considerable credibility
to this hypothesis.)
The loss of zooplankton can
therefore be seen to contribute in multiple ways to disabling the ability of
the ocean to sustain fish such as cod. Fishermen cannot be blamed for
failing to see the zooplankton decline, but for DFO Science this seems to be
above copied from DFO SSR G3-03(2000), page 6. Click to enlarge. I
have added the red and green lines, just my 'eyeball' linear
regressions, to show the rising tendency of the three plant
(phytoplankton) indicators and the declining tendency of the three
animal (zooplankton) indicators. This interpretation agrees with DFO's
text comments at top left of the page.
My final observation is that
the direct effect of seals on zooplankton is a positive one. Seals excrete
copious amounts of live invertebrate (worm) eggs directly into the water.
These little creatures ‘are’ zooplankton, and they are also eaten by larger
forms of zooplankton. The entire assemblage of tiny planktonic animals is
critical to zooplankton health and to the control and moderation of
phytoplankton growth. Therefore, these, and the seals, are the last
creatures that should be ‘excluded’ from Smith Sound. Even in a hypoxic bay,
seals can survive, eat the dying fish and convert at least a portion of that
food into live zooplankton, while avoiding some degree of the bacterial
decomposition which would necessarily occur should many dead fish sink to
the bottom. Thus an air breathing, zooplankton excreting predator, such as
the seal, is in a unique position to help correct this type of unhealthy
“stewards” must quickly realize these things, and learn to value the seals
as agents that actively promote the health and stability of marine
ecosystems. The FRCC has recommended that “a designated Seal Exclusion
Zone Control Team should be established immediately to keep seals out of
Smith Sound year round.” (FRCC, 2003). This plan is sheer folly,
and will undoubtedly hasten the imminent total disappearance of the northern
cod. The “stewards” should now give careful consideration to a truly
original approach; they should consider trying to have an active, positive
impact on the health of cod; I think they should try feeding the fish.
- Debbie MacKenzie
update on this story, April 11.
For more information see
Seals and Cod.
For a look at starving cod in Nova Scotia see The
Downturn of the Atlantic Cod.
For a discussion on the role of spawn in the health of marine ecosystems see
Fish Eggs: the Perfect Food?
For a lengthy scientific argument regarding nutrient dynamics in marine
ecosystems, including the reasoning that leads to the prediction of fish
kills such as this one, see: CHALLENGING BASIC
DFO, 2000. State of
phytoplankton, zooplankton and krill on the Scotian Shelf in 1998. DFO
Science Stock Status Report G3-02(2000).
DFO, 2003. Northern (2J+3KL)
cod Stock Status Update. DFO Can. Sci. Advis. Sec. Status Report 2003/018.
FRCC, 2003. 2003/2004
Conservation Requirements for 2J3KL Cod Stocks. Report to the Minister of
Fisheries and Oceans. FRCC.2003.R.2.
Gregg, Watson, W. and Margarita
E. Conkright. 2002. Decadal changes in global ocean chlorophyll. Geophysical
Research Letters 29 (15): 10.1029/2002GL014689 (section 20, pages 1-4)
Stearns, S.C. and Crandall, R.E.
1984. Plasticity for age and size at sexual maturity: a life-history
response to unavoidable stress. p. 13-33. In Potts, G,W, & Watton, R.J.
(eds.). Fish reproduction: strategy and tactics. Academic Press. London.