No-one seems to dispute the fact that fish are significantly smaller today than they
were a couple of decades ago. It is evident in virtually any stock where size data exists.
Nobody catches any of the “really big” ones anymore - they’re all “fish stories” from
years ago. We know that fish are a lot smaller today than they were in the past, but the
question is "WHY?"...There are a couple of theories, that may or may not explain the
actual causes of slow growth in fish.
One theory that seems to be fairly well accepted today is that slower growing fish
are the result of “size selective mortality” associated with fishing. Here is an
explanation: “First, it is important to remember that not every individual in a population
matures at the same time; some will mature earlier, and some will mature later. Now,
imagine that you fish only the larger individuals (presumably the older fish). Thus the
fish that survive to breed are mostly the smaller (presumably younger) individuals (note
that some large fish will also be able to breed, unless you catch all the large fish). These
are the genes that will be passed on to the next generation. Repeat this over several
generations. The fish that will be able to pass on their genes are those that are not
removed by fishing. Thus fishing may, in the long run, artificially favor (or select) fish
that are smaller and breed earlier. Of course, this will only occur if fishing mortality is
size selective.” Size selective fisheries are those with rules about mesh sizes and
releasing smaller fish (like lobster). (Another explanation for the prevalence of small fish
is “high fishing mortality” - just catching too many, and obviously using a method that
catches the bigger ones. The result is that the population consists mostly of (smaller)
youngsters.)
Besides being smaller in average size, it has been consistently noted in a wide
range of stocks that fish are “maturing at younger ages.” Eggs are found in much
smaller fish now than what was noted a few decades ago. The first thing that I want to
question is the age of these fish. It is believed that they are relatively young but since the
population is exploited they are trying to “reproduce quickly before they get caught.” I
cannot prove or disprove the exact reasoning that might be used by the fish...and I doubt
that anyone else can. I do, however, have some doubts about the accuracy of aging,
especially the aging of malnourished fish (which are known to not lay down as many
otolith rings, therefore potentially looking younger), because there is a point where aging
becomes a subjective call since a fish must be "2" or "3" never "2 1/2" years old. It has
been noted during the 1990s in at least one Atlantic Canadian fish stock (capelin) :
“...changes in physical structure of the otoliths causing problems in age determinations.”
I cannot categorically disprove the idea that exploitation of fish stocks causes them to
mature at younger ages...maybe it does, but I seriously doubt it. (Of course underfed
animals are smaller when they mature and "why" is not difficult to comprehend.)
The second thing that I want to question is the proof for the hypothesis that small
size in fish is a result of fishing methods. It’s not a bad theory, but how could it be tested
to see whether or not it is valid? Where are the “control” populations?
One question pertains to the size selectivity of different fishing methods. Which
methods are most likely to be very size selective? The species subjected to these methods
should show the “cropping” effect, and species not exploited in like manner should be
spared from the effect. Mesh sizes and practices such as releasing small lobsters, will
obviously result in selectively catching the bigger ones. But why would longline fisheries,
for example, catch all the bigger fish first? Larger specimens of swordfish and shark were
commonly hooked before, but now only much smaller ones are being caught. These
stocks show the declining size trend just like the netted fish, stocks are looking like they
were “cropped off.” How did the hooks select the big fish? Littler ones will take the same
hooks, because they do now. One might reply “overfishing” but I still wonder why there
seems to be NO survivors from the “really big” group left to catch.
If certain species in an area are subjected to fishing exploitation, the “controls”
could presumably be other species in the same area that are not similarly exploited. The
fished things should get smaller and the not-fished things should stay the same size. So if
“not fished” animals show the same declining size pattern as their “fished” neighbours,
maybe the effect is not caused by fishing in the manner that is presumed. One problem
with looking at this of course is the lack of attention paid (and therefore data) to the
“not-fished” things. But sometimes it can be found.
One example of a “not-fished” species that is disappearing is the barndoor skate
(only caught as a bycatch, but formerly known to exist in a farily large range, now
practically gone). This is one of the larger skates, and the fact that it needs to grow to a
larger size would make it more vulnerable in a situation of food shortage...regardless,
little is known about it except that for some reason (apparently not “fishing”) the
population is in a severe decline, resembling similar species in the same area that have
been targetted by fisheries.
A very lightly exploited species would presumably look more like a “not fished”
one than a “fished” one. I’ll use capelin in Atlantic Canada as one example. Recognized
as “the most important forage species,” in the late 1970s, scientists recommended that no
more than 10% of the projected mature biomass be removed annually in a commercial
fishery. And it has never been a major fishery. Stock status reports on capelin make
estimates of how much is eaten by natural predators. Here are the numbers from 1999
(although it is stated that these are a bit old and some predators are known to have
declined since the estimates were made) ...puffins 12,000 tons, seabirds 250,000t, minke
and fin whales 400,000t harp seals 700,000t (1998), cod 0.7-2.25 million tons, greenland
halibut 200,000t (cod and halibut estimates from 1980s). Anything remotely near these
numbers when compared to the 1998 TAC for Atlantic Canada (47,545t) is convincing
evidence that the fishery removes only a very small portion of the capelin stock, and
therefore has, as the researchers claim, “no detectable effects within the population.”
Another stock that appears to me to have been lightly exploited is Northern
Labrador Arctic charr. There is a commercial fishery and a “food fishery” by natives.
Landings seem relatively low, often being reported in kg instead of tons - over the last
decade annual landings have been well under 50t for this fish - and the low landings are
attributed to low effort.
Yet, the sizes of the capelin and the arctic charr are decreasing in tandem with the
sizes of the “overfished” groundfish. It cannot be that the natural predators of the capelin
are now “cropping off” the big ones. The charr shows a major declining trend in
weight-at-age and average fish size, in recent years there have been none of the bigger,
older fish in the catch. The diminishing size of these “not heavily exploited” fish must be
caused by an “environmental” factor, something negatively related to feeding, something
that is affecting the system overall. The environmental factor most commonly pointed to
(at least in the Atlantic Canadian literature) is water temperature. Cooler water
temperatures in the early 1990s were correlated with a decline in fish stocks. Some,
maybe all fish, apparently eat less food when conditions are colder - and I don’t dispute
this. But to what extent was the temperature drop the cause of the drop in the stocks, and
the size of the individual fish? Later in the 1990s the water temperature increased but the
expected increased growth of the groundfish has yet to occur.
I have another concern about “cropping off” as a side effect of overfishing. Has it
ever happened before? Has it ever been demonstrated to be a reversible effect? If so, the
theory would be more credible. In Atlantic Canada there was a marked decrease in the
abundance of the groundfish in the 1960s which was obviously the result of too intense
fishing pressure. The abundance of fish went down because they were heavily exploited
by “size-selective” fisheries - but did the weight-at-age and size-at-maturity also go down
at that time? Were there reports of “slinky” fish in the 1960s? My impression is “no” but
I don’t really know - that sort of data is very sketchy from the 1960s. Reducing fishing
effort at that time however resulted in a rebounding of the stocks. The same thing is not
happening now, at least not in Atlantic Canada. (There was a little temperature dip in the
60s as well, and the return to normal temperaturess coincided with the rebuilding of the
stocks, which could easily have been a coincidence. What we have now is a very
different looking picture.)
So - I think the small fish today are the result of an environmental factor moreso
than cropping off by size-selective fishing gear. I’m not convinced that it is water
temperature or changing currents. The most likely environmental cause of slow growth in
fish is a shortage of
nutrients, which has resulted from the drop in the overall biomass.
Hunger is evident in populations of marine mammals and sea birds as well - it is clearly
affecting the whole ecosystem.
