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QUESTIONS AND COMMENTS RE: the effects of natural climate variability (i.e. long-term cyclical changes) on rates of marine primary production. Also the role of plankton and the possible effects of “fishing” on plankton/primary production levels. Long term changes in environmental conditions (climate) are frequently related to observed changes in “production” in marine systems. It is widely believed that the changes in productivity are “driven” by the environmental changes. This is another theory that is undoubtedly true to some extent, but it is rather difficult to clearly relate historical changes in fish stocks to past climate swings...although it apparently has been done. I am left wondering if what we have today is not one picture superimposed on another - environmental changes affecting the production cycles in the ocean (via temperature and current alterations) but also a general declining trend overall underlying the picture because the system is being steadily drained of the “fish biomass.” Rather than a “cycle” the picture might be better described as a “spiral” that is following a general downward direction... I posted a challenge to the validity of the “temperature changes cause fish declines” explanation - trying to stimulate debate with the scientists on this point: “Your message to Fish-Sci about temperature change is interesting, but for the most part, as naive as the earlier question dealing with fish weight/biomass/production. Fish exhibit maximal growth at an optimal temperature, and that temperature depends on the species and sometimes even the population within the species. Therefore, you can easily see an increase in growth when you move from cold (e.g. 2 - 4 deg C) to optimal (e.g. 10 deg C) followed by a decrease in growth when moving from the optimal (e.g. 10 deg C) to the stressful (e.g. 15 deg C). This is precisely the scenario that herring within the Atlantic are facing. In addition, although a 1 to 1.5 deg C change doesn’t seem much to you it can and does have a significant effect on fish - behavior, physiology, and all. What is really important is where the termperature change is. so if yo move from 10 to 11.5 for a North Atlantic species you are going to see little effect, but if you move from 6 down to 4.5 you may will be crossing a physiological border that will have a great effect, especially on appetite. Likewise, a temperature change from 15 deg C to 16.5 deg C could very well be lethal. Whether temperature or food is more important, I would really say that temperature is a significant factor because it determines the ability to search for and capture food, it determines the rate of digestion, and temperature through its influence on metabolic rate, determines the proportion of ingested energy that can be channelled into growth.” Yes, temperature appears to have a real affect on growth rates, but probably a more basic factor in the equation is the availability of food, without which there is no growth, digestion or even survival. Food scarcity is undeniably capable of causing slower growth rates...and slower growth rates in fish is a widespread phenomenon. “...marine biomass is ‘never’ stable. A general stability, subject to cyclical and irregular environmental change, remains a useful abstraction against which the effects of other driving forces can be discussed -- though not as a standard against which anything can be measured.” “Stronger and colder Labrador current driven by atmospheric events...where water temperatures started out close to freezing and then got colder (as off northeast Newfoundland), it’s not too hard to accept that the loss of cod productivity cannot be matched by an increase in something else since there isn’t very much that can be highly productive as such low temperatures. (Cooling trends in warmer areas, however, should see a shift in species composition more than an overall decline.)...You appear to have set this sort of change aside and instead have sought a different mechanism for your supposed decline in productivity. (Which you have blamed on the removal of marine protein by fisheries.) You have claimed to find it in a lack of nitrogen available to support primary productivity. That mechanism can, I think, be shown to be grossly improbable...” Of course I cannot deny that cooling temperatures might negatively affect the productivity of cod. But a serious inconsistency appears when it is noted that the cooling trend off Newfoundland reversed itself several years ago (in the later 1990s the water temperature actually rose to a point slightly above the long term mean) and the warming was most definitely NOT accompanied by an increase in cod productivity - quite the opposite has occurred, the cod stocks in the area continue to decline. Regarding my theory about the “lack of nitrogen”... “...even if those declines could be proven, they would not provide support for your hypothesis. To get that, you would need to show that nitrate levels in the photic zone of the world ocean have fallen, roughly at the rate that protein has been removed from the sea. You have not done so. Granted, nobody has yet posted figures that would show that no such fall has occurred (though I suspect that the necessary data does exist).” Nitrate levels and actual food that is edible by fish are two different things (and I do suspect that both are diminished). This is discussed in detail on the page dealing with nutrient cycling. “If El Nino means a drop in nutrients available to algae and plankton this will limit their production, fish which feed on these will in turn be limited, as will fish which feed on these fish, etc, thus the whole system’s biomass will fall.” “Warm water increases basal metabolic rate so metabolic costs are higher and less weight is gained.” “On the west coast, there was a somewhat strong correlation between long-term changes in the catch data of Pacific salmon and indices of climate change.” (I believe this refers to the PDO - Pacific Decadal Oscillation) Apparenty this is also true for sardines. The size of the Pacific salmon however, has apparently been steadily decreasing since 1967 -- the fluctuation that follows the climate change is the size of the catch and not the size of the individual fish. The decline in the average size of the fish appears to nicely follow the steady progressive depletion of the system overall. Many other fish species have also shown a steady decline in growth rates over recent decades. As I said, I think we have one picture superimposed over another. There are opposing views as to what effect fishing removals might have on the level of plankton in the ecosystem, and there is real uncertainty about trends and historical data on plankton. “Are plankton levels severely depleted from historic levels? How good is our monitoring? I can’t really answer this one but...I suspect there are huge unknowns involved in trying to construct estimates of past and recent plankton population levels.” “Plankton are a big sink for carbon-fixing from the atmosphere - if greenhouse gases have been increasing then more atmospheric carbon has been entering the system. ...Does this carbon end up as more fish flesh?” It does seem to make sense that rising atmospheric carbon dioxide should stimulate an increase in plant growth (phytoplankton), especially since that exact effect (accelerated growth) has been noted in some land plants. If phytoplankton has not taken advantage of the added CO2 that is available and increased it’s own growth rate, why not? It’s because it’s growth is limited by another factor - fixed nitrogen/fertilizer. Plankton records, such as they are, do not show increases...on the contrary decreases have been noted in some places (I am referring now to decreases in zooplankton - which of course could be connected/limited by the availability of phytoplankton for grazing - and also could be, and surely are, affected by numerous other factors as well.) So it’s hard to draw conclusions about trends in plankton levels. I have pointed to the decline in the population of the largest plankton feeders (right whales) as evidence that plankton levels are not what they used to be. This, of course, has been disputed. “Are the right whales starving? The short answer is no.... ...but I am becoming convinced that food limitation is a serious concern at present ...During the 1990s the most dramatic observed change in the right whale population has been an increase in the average interval between calves. In 1980-1992 the average was 3.7 years (the modal interval is three years - one each for pregnancy, lactation, and replenishment of energy stores). The average for 1993-1998 is over 5 years, ...A decline in food availaility for right whales might be expected to manifest itself first in reproductive effects, including an increase in the time needed to accumulate the surplus energy needed for pregnancy and lactation.” “There is no evidence for any long-term decline in productivity which might impact right whale feeding. Their preferred prey includes the older copepodites and adults of ‘Calanus finmarchicus.’ which is the dominant zooplankter in much of the North Atlantic. (Incidentally, I always make a point of specifying zooplankton rather than simply plankton when talking about right whale food, because I think too many people get misled into thinking the whales eat phytoplankton). For a right whale, however, the abundance of ‘Calanus’ over broad areas of ocean is unimportant. What a right whales ‘cares’ about is the concentration of copepods in patches the size of its mouth opening. That concentration must be extremely high in order to pay back the high costs of metabolism, locomotion, migration, foraging, and reproduction. The most important factors in determining the location and value of appropriate feeding grounds for right whales are physical-oceanograpic concentrating mechanisms rather than biological productivity. Changes in circulation patterns are therefore more likely to impact right whale foraging success and nutritional status than alterations in nutrient supply.” This provides a very interesting insight into factors involved in the feeding success of right whales. No doubt it is complicated. Whether or not it is significant to the discussion, a decline in the population of “Calanus finmarchicus” has been recorded off Atlantic Canada, with levels now being significantly lower that those recorded in the mid 1980s. (DFO Atlantic Fisheries Researach Document 96/05) The size of the concentrated patches of plankton that the whales need, may possibly have changed as well...there seem to be many uncertainties. Regardless, it appears that the right whales are having some feeding difficulties at present, and whether or not it is temporary or is fact something worse than they have experienced at other “low” times in their past...remains an open question. What effect does the removal of fish from the sea have on the level of phytoplankton in the sea? This question definitely elicits differing opinions... “The trophic cascade theory argues that removing (or adding) species at the top of a food web has cascading effects on lower levels of the food chain. One example is where fish removal is used as a management tool to control water clarity in some small lakes in England.” (Removing fish translates into lower levels of algae.) But then another writer mentions a practice of adding fish to drinking water reservoirs in the Mediterranean, so that they will maintain water clarity (presumably by eating the algae). “Unfortunately, the bottom of the food chain doesn’t sit there in balance, so there is a ‘bottom-up’ influence as well. This is why marine scientists can’t agree whether it was the removal of fish or the addition of nutrients to coastal waters that was responsible for what you see (in New England, referring to lots of algae and few fish) - or a complex combination of the two.” “Algae have potential for greater production if you remove the fish that feed on them.” “How can removing fish cause a drop in plankton? Should it not be the reverse?” The way that I imagine fish removal resulting in decreasing amounts of algae is this: Algae nourish fish and fish nourish algae. The living fish excrete nitrates constantly in their bodily wastes (including ammonia from their gills) which serves to fertilize the plant component of the ecosystem. Removing a large fraction of the fish population takes away an important source of fertilizer as well as eliminating plant consumers. It seems to me by this logic that more fish in the ocean should support the growth of more algae and vice versa (realizing, of course, that it is slightly more complicated than this). The idea that fewer fish would result in an increase in algae seems to be based on the “species replacement theory ” |