|
Multi-million dollar questions
Brief prepared for the Canadian House of Commons Committee on Fisheries and
Oceans
As an observer located outside the commercial fishing and the marine science groups, although fairly well acquainted with both, and a lifelong coastal resident of Nova Scotia, I have grave concerns about the ongoing changing patterns in fisheries, marine science and the declining health of marine life in general. What I see is this: Things are just getting worse; the anxiety of those dependent on fisheries, as well as the uncertainty of those offering scientific interpretations and management advice, is steadily increasing. In recent years these two messages have become familiar and repetitive…but no new insights are emerging, no clear visions of which new directions should now sensibly be pursued. I would like to suggest two new lines of thinking that may lead to an improved understanding of today’s problems: 1. A major focus of marine science needs to be shifted to the investigation of changing trends in marine plankton production, and to the study of subtle positive feedback pathways that may exist between fish and their food supply. The problem is that not only are populations of larger marine animals that changing today, such as the targets of the commercial fisheries, but that changing trends are also apparent now throughout the entire marine ecosystem. All of the little things are changing too. It is very important to consider the animal fraction of the marine plankton (zooplankton), the portion that provides food directly to small fish, and to realize that production/abundance of these tiny, critical organisms is NOT now being sustained at former levels. The Department of Fisheries and Oceans (DFO) has documented a significant declining trend in the numbers of zooplankton in Atlantic Canadian waters(1, 2), yet the department has failed to identify the reason for this plankton decline as an important research focus. This is a serious omission, since zooplankton production has important implications for fish production as well as for the continued survival of other larger forms of marine life, such as whales, seals, and seabirds. The declining trend in zooplankton suggests that the entire marine ecosystem may be slowing its rate of organic production. And if that is true, then the ramifications of this development extend far beyond the interests of commercial fisheries. 2. We could benefit from some new thinking on seals, and a holistic reassessment of the roles played by seals in marine environments must precede any experiments or new initiatives that involve deliberately reducing their numbers. Seal research has been an earlier focus of this committee(3), and is currently at the forefront again, with the Minister’s recent announcement of $6 million for a study on the experimental use of “seal exclusion zones” in an effort to aid the recovery of depleted cod stocks. Seal research is fine, but any such proposed “experiments” must not be conducted without the use of the appropriate scientific “controls.” And I strongly suggest that this type of experimental seal research not be undertaken before completing a holistic reassessment of the roles played by seals in the larger marine ecosystem. Seals have long lived in marine environments and their overall contribution to these systems cannot have been negative (or else they would have been eliminated millions of years ago(4)). Therefore, although it is counterintuitive to many, the removal of more seals at this point may not be without added risk to the health of today’s declining fish stocks. Positive contributions to ocean health that can be seen to be made by seals include the production of zooplankton (via the excretion of vast numbers of live worm eggs(5)), and the scavenging consumption of dead or dying fish that might otherwise undergo bacterial decay on bottom, with a resulting dangerous depletion of oxygen from the water. In an oxygen stressed, low zooplankton aquatic situation, air-breathing/zooplankton-excreting marine mammals such as seals may therefore perform a unique system-stabilizing role by consuming dead or dying fish, while not removing oxygen from the water or succumbing to hypoxia themselves. These observations are intended to suggest some directions in which the holistic effect of seals (and other marine mammals) on ocean health might usefully be investigated. They also serve as a warning of the nature of the adverse impacts on the marine environment that may result from the removal of seals (less zooplankton, less oxygen). Seals are an integral part of life in a healthy ocean, and their actions today appear only to be part of what naturally occurs when such a living system tries to recover from damage inflicted on it. As fish eaters, the seals will actively work towards the stabilization of an ocean environment that supports fish…but the same cannot be said for the bacteria that will break down dead fish in the absence of larger animal consumers such as seals. The recent decision to allow fishermen to shoot “nuisance seals,” as well as the planned implementation of “seal exclusion zones” in Atlantic Canada should be carefully reconsidered in this light. What are the important problems facing Atlantic Canadian fisheries today?
What are the major signs and symptoms of trouble in Atlantic marine life today? There are some broad patterns of decline that are affecting all marine species. Easiest to see, perhaps, is the ever-shrinking size of fish. We have assumed that this increasingly acute lack of larger fish only reflected too-intense fisheries which were removing all of the big ones. But it is clearly not that simple, as is evidenced by the continued decline in the size of northern cod during the first decade of the moratorium. Looking at other fish, even relatively abundant species not thought to be “overfished” such as mackerel, reveals the same pattern (which in this case remains unexplained). The largest mackerel has been shrinking for decades (ask any mackerel fisherman or mackerel scientist), while this species has been assumed to be “underexploited,” and this shrinkage cannot be explained by an especially high head count in these fish. The maximum body weight reached by Atlantic mackerel has fallen in recent decades from about 4 pounds (6) to approximately 2 pounds (7). There must be a reason for this, yet this is the sort of drastic biological change that has remained unexplained by DFO. The “condition” of fish (essentially how plump or well-fed they are) has also declined across the board, although this has been far more acute in the older ages of fish and in ocean areas where food production is naturally relatively slower. For example, cod on the Eastern Scotian Shelf are in exceptionally poor condition, appearing positively emaciated and described as “slinky” by fishermen and scientists (8), while those living on the western end of the shelf and in the Bay of Fundy today are still reasonably healthy (9). Does this reflect poor fisheries management to the east and good fisheries management to the west? Not at all, what this pattern suggests is a broad, general lowering of the larger ecosystem’s ability to support (feed) cod. Since the Bay of Fundy always produced a remarkably quicker growing codfish, more leeway has naturally existed there to scale production back somewhat while still supporting cod in reasonable health. The Bay of Fundy appears to represent an optimum natural habitat for cod, while other parts of the traditional range have always been more marginal. Cod in those areas always grew more slowly due to the combination of natural food availability and environmental variables such as temperature (e.g. Newfoundland and Labrador). Less naturally advantageous areas of the habitat will predictably be abandoned first by such a wide-ranging species, should a gradual lowering of food production occur system-wide. This may help to explain the recent pattern of collapse among the various Atlantic cod stocks. Is there other direct evidence of a system-wide slowing of food production in the ocean? If this hypothesis is true, then the signal should be detectable in a very wide range of marine species. Today’s poor condition and extremely slow growth of commercially exploited fish stocks supports the hypothesis, but possibly more compelling evidence is offered by changing patterns in species that have never been exploited directly by humans. One telling example is the pattern of decline in barnacle growth that has occurred over the past half century on the exposed Atlantic coastline of Nova Scotia. Barnacles have certainly not disappeared, but they have definitely retreated from the less food-rich, more marginal parts of the range that they formerly occupied. What this means is that wave-exposed rocks on this coastline that once supported heavy barnacle growth high in the intertidal zone, higher than the highest seaweeds, are now lacking the once prominent white “barnacle belts,” that had been described as forming highly visible “landmarks” in this area (10). An example of this change can be seen on the rocks adjacent to the famous lighthouse at Peggy’s Cove. The barnacle belt has vanished from the scenery there. In areas such as this one, barnacles can be seen today to still survive only in the crevices, which are the relatively high water flow/high food areas. Barnacles are not extinct, and they do not appear to have been killed by pollution or global warming, they simply have had their options reduced by a decline in their food supply, and this is what seems most likely to be reflected in their changing distribution pattern today. The reason that I have stated that a significant decline in barnacles has occurred over a “half century” is because objective scientific records were made in 1948 of barnacle populations at specific Nova Scotian sites, including at Peggy‘s Cove (10). Revisiting (and repeatedly photographing) those same sites in recent years has revealed that a major decline has occurred in the range occupied by barnacles. (I have documented this bit of research in an illustrated article posted on my website at http://www.fisherycrisis.com/barnacles.html .) Barnacles eat plankton. Therefore, the strongest conclusion suggested by their contracting range pattern in the intertidal zone is that the rate of plankton production has declined for some reason, and that this is what has forced the barnacle shift. If so, this is a very important observation, since the food availability to fish and all other marine animals is also ultimately dependent on plankton production. As it turns out, long-term plankton data collected by DFO also supports the hypothesis that food production in the coastal ocean, at the level of the plankton, has been declining. The animal fraction of the plankton, called “zooplankton” (which is the relevant food link for small fish, for example) has shown a significant decline in numbers in recent decades as compared to its abundance in the early 1960s, when such data was first routinely collected (1, 2). At that time, virtually all fish in Atlantic Canada were growing much faster and in greater numbers than they are today. The plant plankton, or “greeness,” of the seawater was lower then, but the density of tiny animal life apparently feeding on it was higher. But as the fish populations have fallen in recent decades, the tiny animal life that fed them has unexpectedly diminished as well. This looks ominous. It is disturbing to me that for the past two years I have tried to discuss my observations and the declining plankton question with scientists at DFO, and I have had no success whatsoever. No-one will comment on changing trends in barnacles, nor on starving fish, nor will anyone comment on plankton. This non-response is unacceptable, but I suspect that this committee has been until now unaware of the issues at the root of my concerns. By comparison, this committee has specifically recommended in the past that “DFO scientists go into the field and make first-hand observations of anomalous behavior by seals and fish, when such behavior is brought to their attention” (3). I agree with this. But this type of directive must surely extend to other significant ecological changes which might be brought to the attention of DFO by concerned citizens. Beyond the disappearing barnacles, I can tell you that over the 40-plus years during which I have been personally acquainted with this Atlantic coastline that there has been a remarkable decline in the numbers of small coastal marine animal life in general, including mussels, snails, small fish, starfish, anemones, etc. Beyond that, I have seen eel grass beds disappear, salt marshes that seem to be melting away, and distinct changes in seaweeds which suggest a generally lower level of fertilization (11). All of this is significant, as it all agrees with the basic hypothesis that marine production in general has been steadily slowing. The essential thing that is changing in the ocean is affecting much more than the commercially exploited fish stocks. In recent years scientists have acknowledged the need to take an “ecosystem approach” to assessing the health of the marine environment, but real progress in that direction has been very slow in coming. Connecting the dots in a modern “ecosystem approach” by fisheries managers must incorporate much more than comparing numbers of seals, cod and capelin. It is essential to recognize broad common patterns, and one important one that I suggest to you is this: As lowered food availability has gradually forced the barnacles to abandon rocks which they once covered in solid sheets, so that their kind now persist only in the relatively food-rich crevices, so is the Atlantic cod abandoning the wider continental shelves off Atlantic Canada and persisting only in the relatively food-rich regions such as the Bay of Fundy. This is a disturbing picture of mounting starvation in the sea, but it is a possibility that we cannot afford to ignore. Besides the decline in zooplankton, a paradoxical increase has been recorded in the density of the plant-plankton (phytoplankton) in the ocean. Since phytoplankton has long been assumed to be the major food source for zooplankton, this development seems to further confuse the issues. How can zooplankton be declining while their food appears to be highly available? Several explanatory hypotheses warrant investigation here, but one of these must be that we have not yet correctly identified all of the important food pathways that work to sustain zooplankton. Today’s situation, with falling fish stocks being unexpectedly accompanied by falling zooplankton stocks, while the abundance of the plant “food” for zooplankton rises…is a bizarre and unanticipated development that has not been predicted by currently accepted marine ecosystem models. And, as with the redistribution of barnacles, this pattern cannot be convincingly explained by “climate change” or “pollution” arguments. While DFO Science has reported on these significant negative trends (1, 2), and a gradual note of worry about zooplankton shortages has recently crept into Atlantic fish stock assessments (12, 13), the declining state of zooplankton has yet to be clearly elucidated as an important problem that needs to be investigated directly. The impact on fish stocks, and indeed on the entire living marine web, of collapsing zooplankton populations, can be expected to be dramatic. Slowed growth of fish in general, poor physical condition of fish (especially the larger ones), and an increasing vulnerability of weakened fish to seal predation, would be some predictable consequences in this case. These changing trends that are clearly seen today, would be entirely predictable in an ecosystem experiencing increasingly severe basic food limitation. Yet this hypothesis has not been seriously investigated, and DFO press releases have failed to include the documented decline in basic fish food (zooplankton). So the public, the fishermen, politicians, the FRCC, and committees such as this one tend not to see the whole picture. Trying to understand the “complex” relationship between seals and cod A big question today, for which an
additional $6 million has been earmarked for research, is this one: Much has been written on this issue in recent years, but some facts are getting more prominent play than others, depending on the sources of information, and doubtless this is due to the various political/economic implications. However, I think that an objective assessment limited to the basic biology of the seal-cod picture might be the most pertinent. Here is what I think are some important facts to consider: 1. Seal numbers have increased. Over recent decades seal populations in the Northwest Atlantic ocean have grown from a historic low point (to which they were reduced by humans) of under 2 million to a number in the vicinity of 5 million today. Signs of nutritional stress have recently been noted in the harp seal population (14), suggesting to some that these animals have reached the “carrying capacity” of the ecosystem for their kind. While some have declared that seal populations today are at “historic highs,” this does not ring true since there are indications that a population of around 40 million seasls lived in this part of the world prior to the arrival of the Europeans (15). The contrast between 40 million seals then and 5 million seals today is unlikely to offer a useful indication of the intervening change in the ocean’s basic “carrying capacity” for predators such as seals, however, because the 40 million seals co-existed with untold numbers of large fish with whom they competed for the small-fish prey. The loss of the large fish competition today no doubt distorts the impression of marine fish production that is given by the number of “top predators” (seals) that are still supported. 2. Feeding behavior of seals has changed in recent years. The earliest studies of the stomach contents of seals revealed that seals consumed almost exclusively very small fish, including small cod. Scientific research in recent years, however, has indicated that the size of cod eaten by seals has been increasing: where they once ate only cod aged 1 - 2 years old, seals are now eating cod up to the age of seven years (16, 17). There have also been anecdotal reports from Newfoundland of unusual behavior such as seals driving lethargic cod into shallow freezing water and seals eating only the bellies of larger cod. These reports have been offered as observations of recently changed seal behavior, which is inconsistent with what people were familiar with in earlier years. What is happening differently now with seals and cod? A possible interpretation: The age of cod that has traditionally been vulnerable to seal predation has been approximately 1 - 2 years old. Beyond that, surviving cod would normally have attained a “size refuge” at which they were too big and too fast to be overtaken and caught by seals. In the past, when individual adult cod often survived for decades, they would grow large enough not only to avoid predation by seals but also to offer competition to seals in feeding on smaller fish, including young cod. The dynamics of the population collapse that has occurred in the cod stock will have worked to the advantage of the seals, possibly on two fronts: (1) The lack of competition by large fish has probably resulted in a greater proportion of the natural predation on young cod being carried out by seals. (And the poor condition of the older age groups does not suggest that these fish populations have been excessively culled or thinned out as 2 year olds - if so, the survivors would be expected to be unusually well fed.) (2) As the age structure of the cod stocks (and other ground fish) have collapsed and the physical condition of the now-younger senior adults has continuously declined, these older fish have become ever more vulnerable to seal predation because of physical weakness and inability to escape from seals. If this scenario is what has led to the increasing amount of codfish consumed by seals, the fundamental cause is a shortage of food for cod rather than an excessive number of seals. In fact, it is unexplained natural morality on mature cod, and not on young cod, which has risen the most sharply in the well documented case of the severely threatened cod stock on the Eastern Scotian Shelf (18). If starvation is the root cause, and seals are not available to consume dead or dying adults, then some other marine scavenger will eventually do the job. Seal predation on cod with low energy reserves has more in common with the scavenger role than with the old style of seal predation in which only small cod were slow enough to be chased down and eaten. The ecological roles of predator and scavenger may therefore have become somewhat blurred in the altered cod-seal interaction patterns that are occurring today. It may be easier for people to appreciate that removing “scavengers,” instead of “predators,” from natural ecosystems may be a dangerous move. An analytical approach that consists only of determining the gross tonnage of cod flesh that has been physically swallowed by seals may give the mistaken impression that seal “predators” are now limiting cod numbers, when a more accurate interpretation is that dead and dying cod in an increasingly starved fish population are being cleaned up by seal “scavengers.” An appreciation of the different dynamics involved in these two reasons why seals might eat cod is not evident in recent research on the subject, which has been largely concerned with basic numerical quantification of how much cod finds its way into seal stomachs. 3. Seals may be especially valuable predators/scavengers in the ocean today, since various aspects of their life history enables them to function efficiently even as the wider marine food web has shrunken (19), and they may work in ways that actively tend to stabilize the declining ecosystem. (1) Seals excrete into the water the eggs of the several species of parasitic invertebrates, including those of the sealworm or “codworm” (Pseudoterranova decipiens ) (5). Although people have tended to view this as a bad thing, this is equivalent to adding live zooplankton to the ocean…so, when considered in the light of a wider problem originating in falling oceanic zooplankton numbers, the dispersal of worm eggs can be seen as an ecological positive. The rate of worm infection in seal stomachs has increased in recent years (5). Individual seals commonly host thousands of mature worms which produce huge numbers of eggs while living in the seal’s warm belly. It is interesting that as zooplankton has fallen, sealworm has risen, and this may represent one of the subtle compensatory mechanisms that became part of the stable marine ecosystem when these creatures adapted to living together millions of years ago. (2) As air breathers, seals do not remove oxygen from the water column. This may be an advantage in a situation where low oxygen content is stressing or killing marine life. A lowering of the oxygen content of bottom water in the Gulf of St. Lawrence in recent years has been identified by DFO Science as a potential impediment to the recovery of fish stocks in that area (20). Should high numbers of dead fish undergo bacterial decomposition on bottom, instead of being consumed by scavengers such as seals, then the problem of oxygen loss at the bottom could accelerate, kill more marine life, and become a vicious cycle. (3) As mammals, the reproductive style of seals differs significantly from that of fish. It appears to be somewhat easier, therefore, for seals to reach the point of being a predator capable of catching small fish. High maternal investment of energy and resources into a single pup quickly produces a young animal that is big enough to function as a small-fish predator (or scavenger). Marine mammals avoid the early dependence on plankton-feeding that characterizes most fish species, and which may now or in the future be limiting to their success (12). A comparison of seals and cod on the Eastern Scotian Shelf, for instance, shows that cod are now rarely surviving long enough to grow to sizes where they can become effective small fish predators (8). So the ecosystem has perhaps become more reliant on the seal to perform this important role, as well as that of consuming the severely weakened, “slinky” older codfish. An ill-considered plan to simply remove the seals from this picture may well result in an acceleration of the negative trends rather than the hoped for “rebuilding” of the cod stocks. 4. Canadian seal populations may not be as secure as many have assumed them to be. Recent high population estimates for seals in the Northwest Atlantic have often prompted the opinion that seals are not “threatened” and that they can safely be “sustainably harvested.” However, if the trends that are evolving in the marine ecosystem continue in the direction in which they have been heading, seals may soon be in serious trouble too. Seals may now be enjoying a relatively high food availability as the cod and other ground fish stocks are being lowered by starvation. A new role has appeared for the seal that was not there before: consuming dead or dying cod aged 3, 4, 5, 6, 7 years. But if the downward trends continue, specifically the contracting sizes of the fish stocks (as they react to the lowering of the zooplankton), this recent food subsidy for the seals will eventually disappear. Faced with a general lowering and dilution of their food supply, seals will not be able to make many of the adaptations that have worked, albeit temporarily, for the fish. Fish have slowed their individual growth rates dramatically, now taking many more years to reach the larger sizes as compared to their faster growth rates of decades ago (e.g. 13). Mammals will be simply unable to accommodate themselves to a food shortage in this manner. Fish have also substantially lowered the ages and sizes at which they mature and start to produce offspring (e.g. 12, 13). Again, this is a compensatory strategy that will be impossible for marine mammals to use. Should the food base for seals be lowered significantly, they may experience a population “collapse” rather more quickly than that which has happened to the fish. Seal reproduction will probably fail first, and a malnourished mammal population will then become increasingly prone to rapid mortality from sources such as viral epidemics. We have watched the fish populations decline relentlessly over decades. And despite our best efforts, we still fail to clearly understand why this is happening. We now need to change our thinking. A steady slowing of plankton productivity easily explains many features of the overall picture, and the study of this question must now become a priority for the government department which has been charged with the responsibility of protecting the health of marine life. And the knee-jerk killing of seals because fish are disappearing is entirely inappropriate and unjustified, and worse, this tactic has the (seemingly unrecognized) potential to worsen the problems. Any “experiments” involving the removal (or “exclusion”) of seals from marine ecosystems must be delayed until after a holistic reassessment of today’s problems facing marine life, as well as an objective assessment of the full ecological role of marine mammals, has been completed. Six million dollars for new seal research…this may yet be reasonable. But plankton research needs to quickly become a more urgent priority in our approach to the “management of living marine resources.” Since fishery science specialists tend not to be plankton specialists, perhaps DFO Science today is ill-equipped to tackle this problem. The recommended (by the senate) Prime Ministerial Task Force on the Atlantic groundfisheries will need to find a way to assess which factors affect marine zooplankton production more importantly than continuing to pursue the current narrow focus on the fish-eating role of seals, if it is to achieve the stated goal of discovering “why the groundfish stocks have not recovered.” Funding an investigation into the reasons for the paradoxical zooplankton decline, should it reveal human interventions that may have the potential to stop or reverse the current downward trend, has the potential not only to rescue the disappearing fish stocks, but also to ensure that other priceless components of the marine ecosystem, such as seabirds, whales and seals, do not follow the fish into oblivion. What might this be worth? Based on these considerations, it is strongly recommended that a marine research initiative be publicly funded to discover the reasons why important zooplankton populations have declined as fisheries have collapsed in Atlantic Canada. The Atlantic Canadian fishing industry, marine scientists and politicians have borne the brunt of international flak for failing to prevent the demise of the northern cod…a disaster which is now recognized worldwide as being the ‘icon’ of collapsing fisheries. But all of the finger-pointing is useless…since it may well have been geography alone which dictated that the northern cod stock should fall first as marine productivity has been slipping on oceanic scales. It seems appropriate, then, that it should be in Atlantic Canada that new insights and truly creative approaches be initiated to deal with this major global problem. See also: Notes and references 1. DFO, 2000. State of Phytoplankton, Zooplankton and Krill on the Scotian Shelf in 1998. DFO Science Stock Status Report G3-02(2000). “Large changes have occurred since the start of the time series in 1961. The phytoplankton colour index was much higher in the 1990s than in the 1960s and early 1970s, reflecting large increases in both diatoms and dinoflagellates in the 1990s compared to the earlier period. In contrast, indices of both total copepods and early stages of C. finmarchicus have declined to low values since the mid 1990s. The CPR krill index has also been below the long-term mean throughout the 1990s.” 2. DFO, 2002. Chemical and Biological Oceanographic Conditions 2000 - Newfoundland Region. DFO Science SSR G2-02(2002). Regarding plankton collections made along the line from Iceland to St. John’s, which have been ongoing since 1959: “…during the period after 1991, the abundance of all stages of Calanus finmarchicus as well as that of total euphausiids, a shrimp-like animal, has been lower than during the earlier period whereas the colour index, a measure of phytoplankton abundance, has been substantially higher.” 3. Standing Committee on Fisheries and Oceans - The Seal Report, 1999. (online at: http://www.parl.gc.ca/InfocomDoc/36/1/FISH/Studies/Reports/fishrp13-e.htm#toc ) 4. Eugene P. Odum, 1969. The Strategy of Ecosystem Development - An Understanding of ecological succession provides a basis for resolving man’s conflict with nature. Science 164: 262-70 (1969) 5. W. D. Bowen (ed.) 1990. Population Biology of Sealworm (Pseudoterranova decipiens) in Relation to its Intermediate and Seal Hosts. Canadian Bulletin of Fisheries and Aquatic Sciences 222. Ottawa: DFO, 1990. 6. A. H. Leim and W. B. Scott. 1966. Fishes of the Atlantic Coast of Canada. Ottawa: Fisheries Research Board of Canada, Bulletin No. 155. 7. DFO, 2001. Atlantic Mackerel of the Northwest Atlantic. DFO - Science SSR B4-04(2001). 8. DFO, 2003. Eastern Scotian Shelf Cod. DFO Sci. SSR 2003/020. 9. DFO, 2002. Assessment of cod in Division 4X in 2002. CSAS Research Document 2002/105. 10. T. A. Stephenson and Anne Stephenson. 1972. Life Between Tidemarks on Rocky Shores. San Francisco: W. H. Freeman and Company. 11. Debbie MacKenzie, 2001.
Evolving Trends in Marine Algae Populations, seagrasses and other intertidal
organisms: Signs and Symptoms of a mounting Nitrogen Deficit in the Ocean?
(online at:
http://www.fisherycrisis.com/seaweed.html
) 12. DFO, 2003. Northern (2J+3KL) cod Stock Status Update. DFO Can. Sci. Advis. Sec. Status Report 2003/018. 13. DFO, 2002. Biological Considerations for the Re-opening of the Eastern Scotian Shelf (4TVW) Haddock Fishery. DFO Science Fisheries Status Report 2002/03E. Regarding the greatly slowed growth rates of haddock: “The best-supported hypothesis is that the productivity changes observed are compensatory responses to stresses of environmental/ecological origin.”… “A decline in condition, as shown by this stock since 1980, may indicate a worsening in the feeding conditions.” Regarding variable recruitment: “Recent research…suggests that physical oceanographic events, mediated through the plankton, might be important in determining year-class strength but his needs confirmatory study.” (While still searching for a cause in physical oceanography, it seems that this author is coming close to the conclusion that the extreme slowdown in haddock growth has resulted from a decline in plankton productivity.) 14. D.M. Lavigne, S. Fink, D.
Johnston, and P. Meisenheimer, 1999. Harp seals and Cod Questions and
Answers. IMMA Technical Briefing 99-02 15. Farley Mowat, 1984. Sea of Slaughter. Toronto: Bantam Books. 16. DFO, 2003. Northern (2J+3KL) cod Stock Status Update. DFO Can. Sci. Advis. Sec. Status Report 2003/018. “Numbers of cod at age consumed by harp seals from 1986 to 1998 were estimated from otoliths found in seal stomachs and total consumption estimates calculated from the seal consumption model. 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 7 were found more frequently than in previous years…this shift to older, larger cod…Belly-feeding is a mode of predation on fish which are usually too large to be consumed whole. The seal takes a bite from the belly of the fish, removing the liver and gut, but not consuming the muscle or hard parts…Observations of belly-feeding were more frequent during 1998-2000 than in recent years…” 17. From: http://www.isuma.net/v01n01/doubleda/doubleda_e.shtml “Surprisingly, despite the low abundance of cod in the 1990s, seal stomachs show more cod consumed, in total, during this period than in the 1980s. Also there is evidence of significant amounts of cod up to seven years old consumed, while in previous decades, few cod older than two years were found in harp seal stomachs.” 18. Caihong Fu, Robert Mohn, and L. Paul Fanning. Why the Atlantic cod (Gadus morhua) stock off eastern Nova Scotia has not recovered. Can. J. Fish. Aquat. Sci. 58:1613-1623 (2001) 19. Daniel Pauly and Jay MacLean, 2003. In a Perfect Ocean: the state of fisheries and ecosystems in the North Atlantic Ocean. Washington: Island Press. 20. J. D. Dutil, M. Castonguay, M.O. Hammill, P. Ouellet, Y. Lambert, D. Chabot, H. Browman, D. Gilbert, A. Fréchet, J.-A. Gagné, D. Gascon and L. Savard. 1998. Environmental influences on the productivity of cod stocks: some evidence for the northern Gulf of St. Lawrence, and required changes in management practices. Canadian Stock Assessment Secretariat, Research Document 98/18 © Debbie MacKenzie 2003
|