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Update on
Atlantic cod die-off at Smith Sound, Newfoundland, April 11/2003 What’s new? 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 health. 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.” - fisherman “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 “Explanations 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?
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:
(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 ocean life. Implications
References DFO, 2000. State of phytoplankton, zooplankton and krill on the Scotian Shelf in 1998. DFO Science Stock Status Report G3-02(2000). http://www.dfo-mpo.gc.ca/csas/csas/status/2002/SSR2002_G3-02e.pdf 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). http://www.dfo-mpo.gc.ca/csas/csas/status/2002/SSR2002_G2-02e.pdf DFO, 2002b. State of the Ocean: Physical Oceanographic Conditions in the Newfoundland Region. DFO Science Stock Status Report. G2-01 (2002). http://www.dfo-mpo.gc.ca/csas/csas/status/2002/SSR2002_G2-01e.pdf Floyd, Mark. 2002. Scientists Find Cause of Dead Crabs, Fish off coast. ODFW News Release, online at http://www.dfw.state.or.us/public/NewsArc/2002News/July/080202bnews.htm
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