Atlantic cod (Gadus morhua) from the eastern Scotian Shelf, 2002, shows physical signs of starvation

Atlantic Cod on the Eastern Scotian Shelf - Does the changing profile of the “slinky” cod reflect simple starvation?

Brief submitted to the FRCC at Halifax, N. S., November 20, 2002

by Debbie MacKenzie


In 1999, I made a presentation to this council and submitted the report “Wake Up and Feed the Fish! A New insight into the Causes of the Collapsing Fisheries.” My major suggestion was that simple food shortage is a root cause of the ongoing failure to thrive of many fish stocks. At that time, the conclusion was largely deduced from the widespread pattern of slowing growth rates and declining abundance of multiple commercially fished species, as reported in the stock status reports from DFO.

The pattern evident in 1999 appears to be continuing, even worsening, today, and although other explanations may be offered for the slowed fish growth (like “cold water” or “size-selective culling” by fishing gear), a general lowering of system productivity (or “nutrient cycling”) would also obviously be expected to cause a slowing of growth, since less food would thereby be produced for fish.

A general lowering of ecosystem productivity, however, would be expected to be reflected in all marine species, commercially exploited or not. A search for other evidence of declining marine production has revealed some surprising and disturbing results. For instance, I discovered that over the last half century there has been a dramatic decline in barnacle growth on exposed rocky shores. (This is described and illustrated in my web article, “The Barnacle Zone” posted at: As an index of plankton production, barnacles seem to clearly indicate that marine production has been steadily dropping.) Similarly, changing trends in local seaweeds over recent decades are consistent with what would be expected if overall nutrient availability in seawater should drop. This includes the decline in production of Irish moss in areas that were once profitable for harvesting, and is also illustrated on my website: . And a recent study by U.S. government scientists (NASA/NOAA) used twenty years of satellite data to show that marine phytoplankton production in northern oceans has been declining over that time. (1) In short, there is ample reason to suspect that a systemic decline in production may be occurring, and an important question is “is this production decline severe enough to be limiting the growth of groundfish?”

I knew that cod had been growing poorly on the Scotian Shelf, and that those in the eastern area (4VsW), especially, were in very poor condition. In recent years, those cod have often been described as “slinky” in reports by fishermen and scientists both. Still, I was surprised when I actually saw some intact specimens of Scotian Shelf cod this past summer. (See attached photographs.) Besides appearing lean, the body shape of the cod appears to have changed remarkably. In contrast to the classic appearance of the species, these codfish (adults approximately 20 inches long) showed a flattened belly profile, an arched back and a downturned head. According to dietary data collected in the past, cod at this size feed primarily on other fish. (2) However, these fish gave the strong impression that they were feeding on bottom invertebrates instead - behaving more like haddock than adult cod. The flattened belly profile, arched back and downturned head just seem to suggest “bottom feeder,” whereas the usual build of adult cod is that of a fish-predator with eyes and mouth directed straight ahead. These changes were more pronounced in cod that were caught farther east in 4X. The leanest one in the photographs was caught very near the edge of 4VsW…which suggests to me that this may also be the appearance of the “slinky” cod that live in the closed area.

It seems most likely to me that the problem facing these cod is a simple shortage of small prey fish, a situation which has necessitated continued bottom-feeding instead, even by these young adults. The repeated behavior of looking downward, and possibly digging into the sediment for invertebrates, seems to have caused these fish to actually bend and grow into this unusual shape. If production of the ecosystem overall is lowered, it is predictable that animals that must feed at the higher trophic levels will be eliminated first. It should also be predictable, I would presume, that those animals would try to continue to survive by feeding at a lower trophic level than that for which they are naturally suited. And that this attempted adaptation would be quite stressful, and ultimately unsuccessful. I suggest to you that this explains the altered appearance of the Scotian Shelf cod in my photographs, the unexplained increased natural mortality of mature cod, and also gives a reason for the relatively better growth in haddock stocks as compared to cod stocks. However, there may be another and better explanation for the altered physical appearance of the cod, but I cannot guess what it might be.

If the mature Scotian Shelf cod is now simply unable to find enough of its normal prey (as I believe is suggested by its altered physical appearance), this contradicts the “feeding inhibited by cold water” hypothesis, and also the “genetically slowed growth” hypothesis. This finding would also seem to contradict the “cod recovery is inhibited by excessive seal predation” hypothesis.

The Role of Seals

Concern among fishermen, and also the FRCC, is clearly increasing that seals are eating too many cod and other groundfish, and are thereby preventing the recovery of the stocks. Although most seem to agree that human fishing is what caused the stocks to “crash,” many people believe that seals are now preventing their “rebuilding.” And the intention of the FRCC to recommend killing greater numbers of seals in the near future is very clear. I think that this is ill-advised for the following reasons:

1. An appropriate level of predation is actually vital to the health of fish, since they produce such large numbers of excess young. In the absence of any predation, year classes would have astronomically high numbers of individuals which would starve together before reaching maturity. Importantly, predators reduce the number of individual fish to a level that can be supported by the available food resources in the environment. If predation is overly intense, and a fish population is lowered by more than it needs to be, then the surviving fish will have an unusually fast growth rate (3) (which will then help them to escape natural predators…this appears to be part of the natural balance between species that was worked out millions of years ago. However, accelerated growth obviously does not help fish to escape human predation.) This positive effect of predation on fish growth is well known, and the same effect should result from either over-predation by humans or over-predation by seals. Examined in this light, the exceptionally poorly nourished Scotian Shelf cod would appear not to be experiencing too intense predation pressure, but rather a predation pressure that is not intense enough…given the current impaired ability of the ecosystem to produce food for adult cod. Although it may seem counterintuitive, reducing natural predation on these cod could actually result in a further worsening of their physical condition, and accelerated cod mortality due to starvation. This is how a seal cull could ‘backfire.’ Has the council considered this possibility?

2. The culling effect that natural predators have on fish populations is a much healthier one than the effects of human predation. This is because natural predators remove the weaker individuals and allow the bigger and stronger ones to survive. Fish that are strong enough to survive the early years of more intense predation will eventually grow big enough to be immune to this threat (given an adequate food supply). And these will be the best spawners. Human fishing, on the other hand, tends to selectively remove the biggest and most successful fish. Unlike the situation with seals, there is no “size refuge” where cod can outgrow the threat of human predation. Therefore, if the fish population cannot support both human and seal predation due to low numbers, the safest strategy will be to discontinue human predation and not to interfere with seal predation.

3. Natural checks will prevent seals from destroying their prey. Seals will not drive their prey fish to extinction, because as their prey availability/food supply is lowered the seal population will inevitably be restrained by this fact (unlike the human predators). It appears that this dynamic may be occurring now, as harp seals in Atlantic Canada have been shown to be in declining physical condition (5), pregnancy rates have dropped and the age at maturity has risen. Immunity will also eventually be lowered in food-stressed seals, and they can become vulnerable to very high mortality due to epidemics of infectious disease. Approximately half of the European seal population died this year due to a distemper epidemic. Seal populations can therefore be rapidly lowered by natural causes. This will surely occur before the extinction of their prey.

4. One observation regarding the recently increased numbers of seals, and their increased consumption of fish, is this: The fact that there are more seals living in our coastal waters now than there were a few decades ago, does not in itself lead to the conclusion that total natural predation pressure on immature cod has increased. This is because other predators that also consumed young cod are no longer in the picture, including large numbers of mature cod, halibut, and other fish that once grew large enough to catch the same small cod that seals eat. To conclude that natural predation on small cod has increased overall, it would first be necessary to quantify the extent to which small cod predation has decreased due to the generalized relative absence of large fish in the ocean today. Does the recent increase in fish consumption by seals outweigh the recent decrease in fish consumption by other fish? Who knows? My guess is “No.” (From the point of view of the seals, the competition for small fish has decreased. And from the point of view of the cod stock, the fish now increasingly relies on the seals, rather than larger fish, for thinning their numbers by natural predation. We may choose to perceive this change as an “ecosystem imbalance” but it is more likely just a natural way that the ancient multispecies network of marine life works to re-establish equilibrium.)

5. Number comparisons can be deceptive. For example, regarding 4VsW cod, the FRCC has quoted DFO (in FRCC.2002.R.1):

“The models of cod consumption by grey seals imply a range from 5,400t to 22,000t of cod being removed by seals. These are relative to estimated biomass of 32,000t to 37,000t respectively.”

A person with no understanding of population dynamics might get the impression on reading this that the grey seals are eating a dangerously high fraction of the total cod population, and in fact that they might easily consume all of the fish within the next few years. But a direct comparison of these numbers is meaningless because the consumption by seals is an annual total, while the biomass estimate is a simple snapshot of the standing stock of cod at a given time. When considering the impact of a particular biomass removal route on an annual basis (such as seal consumption), at the very least this needs to be compared to the total biomass that is annually added to the stock (through production of juveniles and growth of individuals). The addition and subtraction of “cod biomass” at the younger ages normally occurs at a much greater rate than at the older ages. And the tonnage that flows in and out of the youngest portion of the cod stock on an annual basis is undoubtedly far greater than any single, static “biomass estimate.” Young cod, especially the very youngest, are normally fast growing but short lived, transient members of the stock. Biomass gains and losses happen quickly, and this is normal. The removal of a given tonnage of juveniles simply does not have the same implication for the population as the removal of a similar tonnage of mature adults. This is another unfair bias in the common comparisons of seal consumption to fishery removals.


Atlantic cod off Nova Scotia are visibly in very poor physical condition. Adult cod mortality due to poor condition/starvation has recently been described in the Newfoundland and Gulf of St. Lawrence cod stocks. Those people responsible for safeguarding the health of the Scotian Shelf cod need to carefully assess starvation as a cause of the unexplained rising natural mortality in adult cod. (5) Sampling of cod stomach contents should be done to compare with data from earlier years. It is hypothesized that prey-switching to lower trophic levels, as well as a generally inadequate feeding success, will be evident if new dietary studies are done on mature cod, especially those on the Eastern Scotian Shelf. A focus on the dietary habits of cod has the potential to shed more light on today’s problems than a continued focus on what is being eaten by seals. And a seal cull can be expected only to aggravate the problem of starvation faced by individual codfish.

Until we gain a better understanding of the interrelationships between marine species, can explain the dynamics of the marine system as it is changing today, and can soundly justify any further interference with marine life that we may feel inclined to make…the most prudent decision is to leave it all alone. Leave the fish alone, and leave the seals alone; we have already committed too many blunders and have caused too much damage to marine life. If, as increasingly appears to be the case, starvation really is becoming a severe problem for cod and other fish today, there may be an intelligent way in which humans can help to correct this problem. Or maybe there is not. Maybe all that we can do is to leave the ocean to try and heal itself.

(For more discussion of these points, see “The Downturn of the Atlantic Cod (Gadus morhua) in Eastern Canada” posted on my website at: ) - Debbie MacKenzie



(1) Phytoplankton in northern oceans have declined from 1980s levels. NASA press release, online at

(2) A. H. Liem and W. B. Scott. 1966. Fishes of the Atlantic Coast of Canada. Fisheries Research Board of Canada, Bulletin No. 155.

(3) W. Nigel Bonner. 1990. The Natural History of Seals. Oxford: Facts on File. (196pp)

(4) M. O. Hammill, M. C. S. Kingsley, G. G. Beck, and T. G. Smith. 1995. Growth and condition in the Northwest Atlantic harp seal. Can. J. Fish. Aquat. Sci. 52: 478-488.

(5) Caihong Fu, Robert Mohn, and L. Paul Fanning. 2001. Why the Atlantic cod (Gadus morhua) stock off eastern Nova Scotia has not recovered. Can. J. Fish. Aquat. Sci. 58: 1613-1623.

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