QUESTIONS AND COMMENTS RE: The Effects on Fish stocks of Fishing/Overfishing and other human activities -- how do opposing theories compare to the "Starvation" theory?

There is a fair amount of speculation and disagreement among different parties as to what are the actual effects on fish stocks of fish removal/fishing (if any). As well there is concern and speculation about the effects on fish stocks of other human activities (mainly pollution, altering normal runoff patterns and disturbing critical habitat such as estuaries, as well as the physical alteration of bottom habitat by bottom trawling).

The first issue that I will address here is the question of how fishing methods have affected the size of the fish in the sea. It is generally known that individual fish on average are smaller than they used to be, not only are they younger but they appear to be growing more slowly that in the past. (This refers to ocean stocks, apparently is it not necessarily the case in enclosed waters like the Mediterranean. The characteristics of the Mediterranean sea probably make it a “different kettle of fish” from the ocean at large.)

The idea that fish are smaller and slower-growing now because size-selective fishing methods have exerted a “culling” or “cropping off” effect, has been frequently described to me.

“...depending upon the gear used, fishing generally results in the cropping off of the faster growing animals. Given the variation that we have observed for size-at-age in populations we could be changing the populations such that those animals which survive to reproduce at older ages will tend to reflect the slower growing component only -- whether this causes a trend in the genetic pool over time is still an open question.”

“What about selection pressures? If there are size limits on the fish that can be taken, those who are smaller will have a better chance of escaping the nets or being returned. Thus giving a selection pressure for smaller size at age.”

This is an interesting idea that I had not considered. It does make logical sense that such a thing could happen, with the result being like “selectively breeding” the slower growing fish. But it seems to me that it would take quite a long time for such an effect to be measurable. A very significant change in size-at-age has been seen in the last twenty years - it seems to have happened rather quickly if “cropping off” is what caused it. Also, for such an effect to be marked, it seems that fishing would have to remove the majority of the faster growing individuals to inhibit their reproduction to such an extent. To my knowledge fishing quotas are not usually set high enough to take out the majority of fish of a given age. So a lot of the faster ones must also normally escape the traps along with the slower ones. The tendency for greater reproductive success of the slower-growing component might be a factor, but again to what extent does it explain the observed trends? As opposed to other factors, such as food shortage, that might be causing the same change? This question has yet to be answered, along with the question of whether or not size-selective culling actually does have the effect of causing slower growth in fish. (This theory is also discussed on my “small fish” page. It is doubtful to me that size-selective culling is a major cause of smaller fish because the trend is seen across so many species, from heavily exploited ones to lightly exploited ones and across all fishing methods - trawl, gillnet, longline and trap fisheries.)

Other reasons are proposed for the smaller size of fish:

“...smaller fish at age is a symptom of thermally induced increased metabolism, more energy is consumed on respiration, and less is available for somatic growth.”

That comment referred to how warmer water might slow growth. But it is also proposed that colder water can inhibit growth because fish don’t feed as well if it is too cold for them. Both of these are probably true, as fish apparently have an optimal temperature range in which they live. Exceeding their upper or lower temperature tolerance points is very stressful for them. But the exact temperature tolerance range must be species specific. A small temperature change in an ecosystem might be expected to cause a significant problem for a few species, but when essentially all fish species are growing more slowly after a change in temperature of less than 2 degrees, there are most likely other factors involved.

When a fish stock is noted to be at high abundance these days, it always turns out to be a fish species that is relatively small. Sardines for example are apparently doing well off California. There is a relatively high abundance of capelin on the Scotian Shelf. Salmon stocks are doing well in Alaska, most notably the pink salmon, which is the smallest of the bunch. The tuna species that are most common these days are the smaller ones (skipjack and yellowfin vs bigeye and bluefin). The last few “thriving” groundfish are small flatfish. So the smaller species seem to be doing better than the larger ones, and within stocks the smaller fish outnumber the bigger ones. Virtually no “record big ones” are being found any more. It seems that all the large size records for fish are from decades ago.

One reason that I suspect for the relative success of the smaller fish these days is that it is easier for a smaller fish to get enough to eat. Is that likely to be true or false? Smaller (as in "younger"), animals are normally growing faster so their metabolism and therefore need for food would be relatively higher than older ones, but they do not need to meet the energy demands of reproduction at that stage...and there are two other simple advantages to being smaller. One is that a smaller fish can generally eat “smaller stuff,” and the other advantage can be illustrated by the example of an imaginary filter feeder that I used in my book:

“The bigger an animal gets, the harder it can be to get enough to eat. They have to somehow become more efficient feeders as they get bigger in order to support their increased bulk. The problem can be explained by looking at a basic mathematical concept. As a 3-dimensional object of a constant shape gets larger, the ratio of surface area to volume gets smaller. This can be illustrated by considering an imaginary filter feeder that is shaped like a perfect cube, with one side of the “die” representing the mouth, or feeding apparatus of the animal. If one of these creatures stands one inch high, the one square inch of area that is the mouth needs to take in enough nutritients to support one cubic inch of flesh. As the creature grows bigger, the surface area and the volume both increase. When the creature stands 10 inches high, the side that represents the mouth will take in an area of 100 square inches. But the total volume of flesh will have increased to 1000 cubic inches. The feeding efficiency at this point will have to have increased ten-fold since each square inch of mouth now needs to take in enough nutrients to support 10 cubic inches of flesh. This is very likely a factor contributing to the observed phenomenon in so many fish species that “all of the big ones are gone.” It is simply harder for the bigger creatures to get enough to eat.”

Another idea that has been explained to me several times is the notion that the physical damage done to the bottom habitat by draggers has altered the environment to such an extent that the normal species mix can no longer flourish there. That could be true. But the explanation generally goes on to say that “especially the little fish” would suffer from the loss of all the little nooks and crannies that they formerly took shelter in. That is probably a factor, but currently on the heavily dragged bottom of the Grank Banks there are little cod surviving and bigger cod dying off for no explained reason. (The size of cod has decreased significantly since the closure of the fishery - so it’s hard to blame size-selective fishing methods for the most recent decline. It seems that cod can only live a few years there now, there are practically none over five years old...maybe food is so hard to find that a fish that big can’t get enough?)

An inverse relationship between abundance of fish and their condition (“plumpness”) has apparently been often noted until fairly recently.

“For the Scotian Shelf, groundfish condition factors generally rose during the years of the great foreign fishery (roughly 1960s), as simple theory said they should. (Fewer fish were expected to mean more food for each one and so better condition.) If I remember correctly, average condition then began to fall as biomass started to recover in the later 1970s (as was expected) but continued to fall once biomass tended down from the early or mid-1980s onwards. That was remarked on and documented at the time. The guys at BIO have now been struggling with the issue for more than a decade and I am not in the least surprized that they agreed with you that it is important.”

The change in the last decade to declining abundance along with declining size-at-age of fish has also been noted in Pacific salmon. It’s a relatively new trend, which does not fit with the old “simple theory” which seemed to explain thinner fish when their numbers were unusually high since more had to compete for the normal amount of food. If thinner fish were the result of “less food” then, and smaller populations of fish are appearing thinner now (when in the past the smaller population appeared more plump), an overall food shortage in the system is one theory that could reasonably explain the picture. It might be “simplistic,” but it also might be true.

One writer pointed out an apparent inconsistency in my theory this way:

“...the paradigm that even nearshore/coastal fisheries have shown the same kinds of reductions -- even in the face of what most ecologists would agree to be a situation of substantial marine eutrophication.”

Declining coastal stocks are certainly a reality. My point was only that the declines appear to be more severe in the offshore stocks. (The best example - but not the only one - that I know of is the various cod stocks in Atlantic Canada. The cod that are growing the best are situated in the northern Gulf of St. Lawrence, an inshore area which extends into the estuary of a major river. The worst growing stock (Grand Banks) is the one situated farthest from shore and land-source nutrient run-off. In between the two are a couple of other cod stocks - whose indicators are between the other two. Even the “good” inshore cod stock, however, appears to be in fragile condition and unable to withstand any amount of fishing pressure.)

So why would coastal fish stocks not take full advantage of the “oversupply” of nutrients in the area? I think it is because the nutrients are largely unavailable to fish, and it takes a long time for them to pass through the lower levels of the web in significant quantities to actually become fish food. To some extent this does occur, so inshore fish are slightly better fed than offshore ones...however adding “sewage and fertilizer” is simply not an effective way to feed the ecosystem. The result of this method of “feeding” is sickness for some areas and partial benefit for a few creatures nearby, but nothing useful at all for the fish that are offshore. In a few places “cleaning up” the waterways has actually been associated with a decline in “fish productivity,” although whether or not this represents cause and effect is still an open question. An interesting article that explores this idea in Puget Sound called “Managing the Waterways -- Too Clean for the Fish?” can be read on the internet.

"...mate, where did you get the idea that all fishing is the largest source of predation for fish stocks?"

There is a very significant difference between us and the other predators of fish. All of the others do no damage, nor do they deplete the system overall because everything that they take they also give back. Carnivorous fish, seabirds, seals, all return the nutrients to the system in an appropriate manner because they also live and die there. We may not be the largest source of "predation" but we most definitely are the source of "depletion."


Pollution is frequently cited as one of the major factors causing the decline of the fish stocks. And without a doubt toxic run-off causes a lot of harm to marine life. But the major disappearance of fish at the present time is not primarily due to the effects of pollution. Pollution may sicken or kill individuals but it is unlikely that the main effect would be weight loss, seen across a wide spectrum of species. Looking again at our cod stocks, it becomes evident that those that are growing the best (at the mouth of the St. Lawrence River) are probably receiving the highest dose of industrial pollution because of their location, while the stock that is doing the most poorly (Grand Banks) is in an offshore location very remote from where toxic chemicals are likely to enter the ocean. So the fish that are experiencing the most severe decline are exposed to the lowest level of pollutants...and the lowest level of nutrients.

“Where did you get the idea that fishing has a greater impact on fish stocks than habitat change, sea temp, current changes, human induced and natural?”

This writer seemed to be trying to get at the question of whether or not fishing can actually be shown to be the factor that “regulates” fish stocks -- meaning in the relatively short term. (In a way the fishermen are pointing out that their specific fishing practices, and they themselves, should not be blamed for the changes in the system.) It’s a good question and it is asked by fishermen because the trends in fish stocks don’t seem to be closely related to the annual harvests. Some fish seem to become unusually abundant despite being fished, while others show declines that seem out of proportion (and not as “predicted”) based on the fishing harvest. So, other than cases such as removing a major fraction of the biomass in one year, it cannot be clearly shown that “fishing regulates fish stocks.” That is true, and it is because, as the fishermen suspect, the fish stocks react more to conditions in the system overall than they do to a given year’s fish harvesting. But the unrecognized factor in the system overall that is affecting the balance of all marine a steady decline in “everything” (total marine biomass). Incidentally, ask an older fisherman what he has seen over his lifetime, the answer I always get is...“everything going down.”

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