Atlantic cod die-off at Smith Sound, Newfoundland, April 11/2003
- the water is very cold, but evidence also suggests the
that an important trigger of the fish-kill may have been low oxygen.
See earlier report on this story, posted April
See observations that support the hypoxia (low oxygen)
The week of April 7-11, as a steady stream of frozen cod has continued to
“bubble to the surface” of Smith Sound, so has the frustration, anxiety and
anger of the local people.
“It’s imperative that
they get the answers and none of us are going to be settled and be satisfied
until we get the answers.” -- John Efford, MP
“It’s a crime for this to
happen here…Seals caused it, that’s all that’s wrong. That’s the bottom
line…if they don’t do something about the seals there won’t be a cod left in
here.” -- Gilbert Penney, fisherman
The first theory favored by fishermen, and considered by scientists, was
that the incident had been triggered by seals chasing the cod from a region
of safer, warmer bottom water into the too-cold surface water where they
came into contact with ice crystals and froze to death. However, by
Wednesday afternoon, the government fisheries research vessel Teleost
was on the scene and scientists had determined that the entire body of water
in Smith Sound was very cold throughout the water column. Water temperatures
were discovered to be as cold as -1.7 degrees Celsius. According to DFO’s
Dr. George Lilly, “That’s about as cold as sea water in our area can get;
it can get a tiny bit colder than that, but not much more…the water is cold
everywhere -- there’s no such thing as warm water in which the fish might
have been living, and then they ended up being chased by seals, or fled from
seals into the cold water.” So, the main working hypothesis at this
point seems to be that the cod have been killed by harsh environmental
conditions, specifically unusually cold water, which has likely resulted
from what has been a colder winter in this area than we have had for a
couple of decades.
While their survival at these very cold temperatures may be precarious,
and can end suddenly if cod contact ice crystals (they can freeze fast,
almost in a snap) they do produce a type of internal antifreeze and can
exist in a “super chilled” condition to some extent in sub-zero water
temperatures. And many cod are still managing to survive in the very cold
water. April 11, DFO scientist John Brattey described the latest on the cod
deaths in an interview reported by CBC:
“Brattey says the water remains the coldest that the department has
ever recorded in the area, and some fish will continue to die. He says tests
show some of the cod have an anti-freeze protein, while others don't.
Otherwise, the dead cod appears to have been healthy up until it was
instantly frozen by contact with ice crystals. Brattey says the organs of
the dead cod are frozen solid, even though the flesh of the fish is
pliable.” (CBC St. John’s news website)
The avoidance of contact with ice crystals could clearly best be
maintained by fish staying in the deeper waters layers, and this has been
the usual winter habit of Smith Sound cod according to previous scientific
observations. And Brattey has also reported that acoustic surveys this week
have revealed "one very large, dense aggregation of fish moving around in
the sound," and that sampling these fish has shown them to be in good
So…we had a hard winter, and the water in Newfoundland is unusually cold,
and the freezing death of what may amount to thousands of tons of cod at
this time has simply been caused by a ‘freak of nature,’ an unpredictable
and unavoidable turn of events?
Maybe…but it seems unlikely that the “cold water” explanation will
satisfy the local people. People have lived in this area for a long time and
many can remember colder winters, but no-one remembers seeing such a fish
kill here before. People involved in the cod fishery have been interviewed
on radio and quoted in the print media:
“I’ve been fishing ever
since 1959 until ‘92 when she closed -- never seen fish like it before.”
“We want to know the
reason, science is there to give us a reason…We want to know the truth of
it. It’s heartbreaking…there’s got to be answers. There’s a reason for
everything.” - fish plant worker
by…officials, of cold water columns don’t cut it. Codfish have been avoiding
cold water columns for hundreds of years.” - Earle McCurdy, FFAW
But…“why here, why now is not clear.” -
DFO, April 11
The cold water is undoubtedly significant, but a puzzling feature of this
incident is the suddenness with which it occurred. And why now? Something
seems to have changed abruptly which tipped the balance and caused Smith
Sound to become hostile to the cod. And it is crucial to determine exactly
which variable that was.
For how long has the entire water column at Smith Sound been super-cold?
Ongoing monitoring of water temperatures has not been done, so it is not
possible to answer this question with any certainty. However, barring some
very unusual weather pattern, it might be reasonable to assume that there
has not been a recent abrupt temperature drop. Water temperatures may have
been as low as the most recent measurements for months. Why is the cold
water killing fish now?
An illustration of the abruptness and the timing of
the increase in chlorophyll that occurs during the spring bloom in
Atlantic Canada can be seen in the solid lines in this figure, taken
from DFO SSR G3-02(2000) (click to enlarge image).
There is, however, one oceanographic variable that probably has recently
changed quite abruptly in Smith Sound, and that is the concentration of
chlorophyll in the water. This is known to rise sharply, as marine algae
numbers increase “explosively” in the "spring bloom" at this time of year in
this part of the world. As suggested in my earlier
article, this could hypothetically have caused the fish kill if large
amounts of algae sank, rotted and used up the oxygen in the bottom water.
Fish may escape a situation like this by swimming upward, or they may die at
the bottom. And the Smith Sound cod may have done both. Is the "one very
large, dense aggregation of fish moving around in the sound" treading a fine
line between exposure to ice crystals above and hypoxia below? Maybe the
margins of safety on both sides are narrowing for the cod. The hypoxia
explanation at least offers a reason for the timing of the fish kill.
Observations to date that appear to support the
hypoxia hypothesis for the Smith Sound cod deaths include:
- The sudden onset of the kill, and the simultaneous involvement of many
different sizes of fish. These characteristics are typical of hypoxic fish
- Dead fish coming up from deep water (thereby not giving the appearance
of having strayed or been chased into shallow, icy surface water, a
suspected cause of smaller frozen fish kills in Newfoundland in the past.)
- Dead fish found on the bottom, in “significant” amounts, and including
other species. This was observed in bottom trawls done by the crew of the
Teleost. At least two other species, redfish and witch flounder,
were also reportedly found dead. Hypoxic events typically kill many
species near the bottom, including fish and invertebrates.
- The physical characteristics of Smith Sound may render it prone to
hypoxic water events if there is an intense bloom of algae (a deep
reservoir in a semi-enclosed waterway…if an area like this is not well
flushed by tidal currents, a significant amount of sunken algae can
accumulate and decompose at the bottom). This may offer an answer to "why
- Oceanographic records from Atlantic Canada confirm that:
(1) The timing of the fish kill coincides with the normal time of the
spring algae bloom. The spring bloom in this area generally builds
quickly in late March, peaks in early April, and disappears by late
April-early May (as illustrated in image above, right). Dates are slightly
later as one moves north.
(2) The intensity of the spring bloom in the Northwest Atlantic has
been increasing in recent years. (Gregg and Conkright, 2002, DFO 2000,
DFO 2002a) Therefore any algae-induced oxygen stress on waterways such as
Smith Sound would also be expected to be increasing. This may have been the
immediate survival threshold that was passed for the cod.
Time series data from various sources confirm the rising trend in the
intensity of the spring bloom. The longest plankton monitoring data sets are
from DFO’s Continuous Plankton Recorder (CPR) collections. Information
(spanning 40 years) from the Iceland to St. John’s transect indicate that in
recent years measures of phytoplankton (algae) abundance have been
“substantially higher” than in the earlier years (DFO, SSR G2-02(2002)).
This agrees with the pattern evident on the nearby Scotian Shelf, and both
areas have recorded a coincident declining trend in the abundance of
zooplankton…and, of course, in fish.
The suggested scenario of oxygen depleted bottom water is the same as one
of the known adverse consequences of nutrient pollution of waterways, such
as can be caused by excessive sewage input. It may seem unlikely to us that
nature, alone, would spontaneously provoke such a picture, but evidence is
accumulating elsewhere that naturally forced pulses of marine algae growth
have recently been causing hypoxic fish kills.
For example, a “mysterious and sudden die-off” of substantial quantities
of fish and invertebrates occurred off the coast of Oregon during the summer
of 2002. This previously unknown phenomenon was determined to be “a natural
event,” when oxygen loss was triggered by the decomposition of algae that
had flourished due to (normal?) coastal upwelling currents. And a similar
scenario has been regularly occurring at an upwelling zone off South Africa
where “almost every year for the past 10 years they have seen rock lobsters
literally walk out of the water and onto the beach…desperately looking for
oxygen.” (Floyd, 2002) Marked declines in zooplankton populations have been
recorded in these other ocean areas as well, which seems quite plausibly
related to the onset of these new patterns. (It is also interesting to note
that an intense, bizarrely unusual algae bloom occurred off Florida in 2002,
as the result of a ‘natural upwelling of nutrients.’ This caused a highly
visible ‘blackening’ of a large area of seawater and was also associated
with the deaths of marine animal life.)
Long established patterns of physical forcing, on which multitudes of
marine plant and animal life once appeared to thrive, are now being seen to
induce unusually sharp algae growth spikes, which is occasionally now
killing, rather that supporting, many forms of marine animal life. If the
rising phytoplankton and declining zooplankton trends continue in the
Northwest Atlantic ocean, it seems inevitable that ‘natural physical
forcing’ will eventually begin to trigger hypoxic events there as well. If
and when this occurs, it will probably coincide with the spring bloom. And
that may well be the important message in the Smith Sound cod die-off. This
incident may be just one more symptom of a much wider malady now afflicting
- Scientists should be (and probably are) measuring both the
concentration of chlorophyll and the oxygen content of seawater at various
points in the water column at Smith Sound. (However, no mention of these
variables has been made in recent media reports of interviews with
scientists working on the problem.)
- It will be important to examine the dead fish dredged from the bottom
to determine if they might have died simply of hypoxia rather than by the
formation of ice crystals, as appears to have happened to the surfacing
fish. It seems unlikely that these dead fish from the bottom will have
“frozen solid” organs and “pliable flesh” like many of those that have
risen to the surface.
- Peak hypoxic conditions, if triggered by the spring bloom, might be
expected to persist for at least a couple of weeks, but maybe somewhat
longer. Resolution of hypoxic conditions might be hastened if increased
turbulence of the water occurs. And if the water temperature rises, cod
will be able to more easily avoid the lower oxygen depleted area by safely
moving closer to the surface.
- A large quantity of decomposing dead fish on the bottom is
unfortunately likely to intensify and prolong any oxygen depletion caused
by the algae bloom.
- There is a risk of a similar scenario developing in subsequent years,
especially if the trends of rising phytoplankton and falling zooplankton
should continue. And this risk may be significantly heightened by
colder-than-normal spring water temperatures.
- Discovering the reasons why zooplankton stocks have fallen along
with fish stocks must become an urgent fisheries research priority.
This unanticipated coincidence suggests that zooplankton health may be
positively coupled to fish abundance, and not only to the availability of
phytoplankton as has previously been generally assumed.
See also: Original report on this story, posted
April 7, 2003.
See declining zooplankton and rising phytoplankton
trends in Atlantic Canada.
DFO, 2000. State of phytoplankton, zooplankton and krill on the Scotian
Shelf in 1998. DFO Science Stock Status Report G3-02(2000).
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)
DFO, 2002a. Chemical and Biological Oceanographic Conditions 2000 -
Newfoundland Region. DFO Science Stock Status Report G2-02(2002).
DFO, 2002b. State of the Ocean: Physical Oceanographic Conditions in the
Newfoundland Region. DFO Science Stock Status Report. G2-01 (2002).
Floyd, Mark. 2002. Scientists Find Cause of Dead Crabs, Fish off coast.
ODFW News Release, online at