FISHING - IS IT A CAUSATIVE FACTOR IN MASS CORAL BLEACHING?
“One of the most obvious and widespread losses to reef biota is the reduction in fish
populations from intense overfishing in most reef areas of the world. Coasts without
adequately managed reefs have suffered intense overfishing for both local and export
purposes, to the point where the positive effects of fish on those reefs have been
compromised.” (Sebens, 1994)
Two questions to be considered:
1. Is fishing an independent risk factor in coral bleaching? (by virtue of
biomass/nutrient extraction from the system as a whole, rather than just the more limited
“trophic interaction” type of effect) Another way to phrase this question could be: “Does
vulnerability of coral reefs to bleaching episodes increase as standing stocks of reef fishes
are depleted?” Or...“Which came first, the weakened fish stocks or the weakened coral
stocks?”...well, one must admit that the answer to that one is rather obvious, since major
amounts of reef fishing clearly preceded the onset of mass coral bleaching. (Decreased
amounts of coral are expected to “produce” or “support” lower amounts of reef fish, but
might not also decreased amounts of fish be similarly expected to “produce” or “support”
decreased growth of corals?)
2. What ecosystem changes have been possibly (or definitely) induced by fishing in other
marine areas? ..and how closely are these changes paralleled in the tropical systems? What
are the important lessons?
Considering the first question: “Is fishing an independent risk factor in coral
bleaching?”
The mass coral bleaching events of the last two decades, especially those in the last few
years, have triggered a surge in concern for the welfare of the reefs. Analysis of the pattern
of reef destruction (a term that includes all forms of degradation, not just mass bleaching
events) clearly shows that the risk entailed by reef communities is in proportion to the
extent of human contact. This pattern appears to be stronger than the pattern of increasing
water temperature (global climate change?) alone, not just for general “degradation,” but
for mass coral bleaching as well.
If, as argued in this paper, mass coral bleaching events are caused primarily by nutrient
starvation, (with temperature highs being the final stressor), a causative connection to
fishing is plausible, once fishing is viewed as the removal of “nutrients” from the
ecosystem.
The stressors of increased nutrient-rich runoff, sedimentation, physical damage, higher
water temperatures and fishing removals, will obviously be overlapped in many or most of
the damaged reefs that are situated near to human population centers. Determining the
extent of impact of each individual stressor therefore becomes difficult; teasing them apart
may be impossible. But it would make an interesting study to try to find examples that
illustrate different combinations of the stressors, especially to isolate fishing from the
others, and also to compare the bleaching susceptibility of reefs so remote that they have
never been fished to any degree. This research did not include an exhaustive inventory of the status of
the world’s coral reefs and their fishing histories, but from the sample of reports read, there may be an underlying trend of more heavily fished reef areas being more
susceptible to mass coral bleaching. This is certainly not a claim to have proven the connection.
It’s just a hunch, a question worth investigating.
A useful approach might be to classify reefs according to the combinations of human-stress
that they have experienced, and look for a pattern in degree of bleaching susceptibility. In
large geographic areas the effects of climate change should be felt fairly uniformly by all
reefs - when the pattern of bleaching varies in an area like the Caribbean, for instance,
looking for the key differences in the non-climate variables may be important. Possibly:
Case 1 - Reefs subjected to high levels of multiple human impacts, eutrophication, heavy
fishing and physical destruction. These seem to be quite susceptible to bleaching events and
coral death...and the negative effects of fishing alone may well be obscured by the other
stressors.
Case 2 - Reefs that are situated in areas that spare them the effects of nutrient rich run-off
but which have been accessible enough that they have experienced heavy fishing
exploitation. If very fish-depleted, these may be also fairly vulnerable to bleaching events(?)
Regardless, these reefs might offer the best information on the effects of fishing alone.
Case 3 - Reefs that are in very remote locations (e.g. mid-Pacific atolls?), so as to have
always been relatively inaccessible to fishermen. They may experience “global warming”
but be healthier and more resistant to bleaching due to the fact that not only have they been
spared the “runoff,” but they are co-existing with relatively robust fish populations(?)
These reefs would be the best examples to serve as the unfished “controls” in the
investigation of the effects of fishing on reefs.
Case 4 - Similar to case 3, reefs nearer to human populations but that have been effectively
protected from fishing removals, and therefore still enjoying relatively high standing stocks
of reef fish. These may also be relatively less susceptible to coral bleaching.
Case 5 - Reefs with a long history of intense reef fishing, with fish stocks now depleted to
low levels, but maintaining a strong healthy coral community that is relatively
bleaching-resistant. (This finding would contradict the hypothesis.)
Case 6 - Very remote, virtually untouched reefs suffering heavy coral bleaching, illness and
death, in the presence of large, healthy fish stocks. (This finding would also contradict the
hypothesis.)
Case 7 - The unlikely scenario of a reef that has been spared from fishing, yet subjected to
significant amounts of human-related runoff. (This would help to isolate the precise effects
of “eutrophication” without witnessing the effect added to the effects of heavy fishing. It is
suspected that this reef would show a higher level of resistance to “disease/algal
overgrowth/bleaching” types of degradation than the usual “eutrophied AND fished”
pictures.)
There may be a continuum of bleaching vulnerability that parallels the continuum of fishing
intensity. (A similar pattern, of increasing vulnerability of corals to infectious diseases with
increased reef fishing, would also be anticipated if the underlying pathology is simple
malnutrition.) How well does the observed pattern of mass coral bleaching fit with these
predicted scenarios? One cannot be certain, but it appears that there “just might be” a rough fit
between the hypothesis and the observations.
Here are a few observations from the coral bleaching reports, just a few “snapshots”:
- In Indonesia, the eastern and north-eastern areas, the relatively inaccessible reefs are in
“excellent shape.” (“...underexploited fisheries are found in areas of low human population
density such as parts of eastern Indonesia.”) (case 3?)
- In 97-98, Japanese reefs suffered severe bleaching over large areas with significant
mortality, although it was less extensive on offshore islands. (case 1, case 2?)
- Reefs off Brunei are rich in coral and fish species as fishing pressure is low. Healthy coral
coexisting with healthy fish. (case 3?)
- There has been significant coral bleaching in the Bahamas, noted to be much worse in the
central parts, on reefs near New Providence (the center of the human population, the city
of Nassau is located there...hence, probably also the greatest intensity and history of
fishing.) Elsewhere in the Bahamas, “The Andros Reef Complex is one of the longest reef
systems in the Western Atlantic with few anthropogenic impacts because of its remoteness
and low population.” According to scientists who surveyed the Andros reefs in 1997-1998,
“The surveys revealed low to moderate partial coral mortality with patchy occurrences of
recent mortality caused by coral disease outbreaks and bleaching during 1998. Of particular
interest are the extensive thickets of the elkhorn coral, Acropora palmata, found to be in
good condition and localized areas of luxuriant fore reef carpets with high coral cover.
Macroalgal cover was low to moderate and the abundance of herbivorous fish and
commercially significant fish (e.g., grouper) was high. Overall, this assessment revealed the
Andros Reef Complex is in good condition and has few signs of degradation or significant
overfishing.” Elkhorn coral (possibly the top casualty from mass coral bleaching events) is
becoming a real rarity, as are unfished reefs. Coincidence? It may be reasonable to assume
that the effects of global warming would be felt approximately uniformly in the Bahamas.
So why such a contrast in the condition of reefs near New Providence vs. Andros? (case 1,
case 3?...maybe a bit of case 2 in there as well, if there are non-eutrophied yet damaged
reefs easily accessible from Nassau?)
- “The majority of Pacific coral reefs remain in good to excellent condition, with only those
reefs near large urban areas being chronically degraded. But it is these reefs that are often
most important for subsistence fishing, recreation and tourism, shoreline protection and
other benefits.” (AIMS) (case 3, case 1?)
- Reefs off the Florida Keys have experienced “fairly serious” coral bleaching. There is
very little evidence to suggest that high terrestrial nutrient input is an issue as these reefs are
well flushed by “clean” open ocean water. However, regarding Florida, “recreational
fishing is the area’s primary tourist-related boating activity, and commercial fishing its
fourth largest industry overall.” Significant fish removal/significant coral bleaching. (case
2?)
- In the Gulf of Mexico, located over a hundred miles from the Texas/Louisiana coastline,
is “The Flower Gardens,” a well developed coral reef community that was designated as a
marine sanctuary in 1992. This reef experienced a small degree of bleaching in 1995, but
appears to be uncommonly resilient. “In contrast to many other coral reef sites, this reef
community has shown no significant declines during an ongoing 25 year monitoring period.
The remote location of the Flower Gardens helps to protect the reefs from most fishing
and diving pressures.” (NOAA) (case 4?)
- In March, 2001, National Geographic Magazine printed an article about Palmyra, a very
remote central Pacific atoll. Never inhabited by humans and very rarely visited by
fishermen, the coral reefs there are reported to be in exceptionally good condition. The
author quotes:
“Jim Maragos, a coral biologist with the U.S. Fish and Wildlife Service, told me that in 30
years of research he’s dived thousands of Pacific reefs. ‘These are the most spectacular
that I have ever seen,’ he said. ‘There are just magnificent schools of sharks, humphead
wrasses, bumphead parrotfish, large groupers--fish that are basically being wiped out
elsewhere in the world, especially in the Pacific.’”
(I’ve searched for, but been unable to find, any report of mass coral bleaching on Palmyra.
Since the article was printed in 2001, might one infer that major coral damage did not
occur there in the huge bleaching event of ‘97-’98? -- possibly case 3?)
This discussion is very far from a conclusive study. But possibly it is suggestive of a
trend - one where the intensity of fishing relates directly and independently to the degree of
vulnerability to coral bleaching. Examples of “cases 5, 6 or 7” have not been found yet, which
certainly does not mean that they do not exist. It is merely suggested that this is an avenue of
research worth pursuing.
---
A second question regarding the risks to coral reef ecosystems imposed by fishing is this
one:
What ecosystem changes have been possibly (or definitely) induced by fishing in
other marine areas? ..and how closely are these changes paralleled in the tropical
systems?
Temperate zone marine systems appear to have been the focus of more, and longer-term,
scientific studies than have the tropics. Many trends that have been revealed in those
systems may also exist in the exploited tropics. Similarities are worth looking for, and
lessons learned in one area could well be applied in the other. There are many similarities
between fisheries stories from differing latitudes.
A few common themes:
- Long-term declines in marine life have occurred over centuries of human exploitation.
The difference in the gross abundance of sea life at first European contact, compared to
what exists today, is huge -- and suggestive of a significant decline in overall marine
biomass.
- Marked declines in the abundance of organisms targeted by human fisheries. This trend
is most noted in species that live at the higher trophic levels. Transient “blooms” of their
prey species are sometimes noted following the decline of top predators...in some cases,
the stocks of top predators seem exceedingly fragile (unable to “rebuild”), while their usual
prey seems very robust (e.g. NW Atlantic groundfish vs. their crustacean prey, lobster and
crab). This seems to suggest “species replacement” and occurs in only the most (formerly)
highly productive areas. But it falls short of real “replacement,” and in many cases does not
happen at all, deep sea fisheries being a prime example...fish removed from those areas are
apparently replaced by “nothing”. (Merrit and Haedrich)
- Simultaneous declines in targeted and non-targeted species, with those at the higher
trophic levels most vulnerable. This has been documented in the northern areas - but the author has
not seen it as such in the tropics, although it’s possible that the decline in sharks and larger
carnivores (i.e. big, possibly ciguatera-loaded fish) may be out of proportion to their level
of exploitation. And, of course, the decline in corals is occurring in “non-targetted” species.
- Significant declines in zooplankton abundance. This has been recorded in the North
Atlantic and North Pacific - “cause unknown.” Might this trend be occurring in the tropics
as well? Is there any old data for comparison? Decreasing numbers of these primary and
secondary consumers in the temperate seas, makes an interesting comparison to decreasing
numbers of coral polyps, which sit in the same ecological niche, in the tropics. Could these
be two expressions of another common theme?
- **Slower growth rates** This is a widespread phenomenon in marine organisms in
temperate systems, and it has been occurring over decades. The great majority of species
with data sets show slower rates of growth today than in their past records. Generally
stunted growth, with lower weights-at-age, lower condition factor and smaller
sizes/younger ages at maturity, are well known to be occurring in wild fish in general,
tropical fish included. The most widely accepted theory to explain this is that the
“size-selective culling effect” of fishing gear has “cropped off” the faster growing
individuals in the fish populations, resulting in a predominance of smaller fish, that are
genetically programmed to grow more slowly. The same argument is offered for the
widespread trend of declining age and size at maturity in fish.
This “genetic shift” has never
been proven, however, and several observations seem to argue against it. One observation
is that both heavily exploited and lightly exploited species show the same change (e.g. in
the NW Atlantic, herring are showing declining size at age and size at maturity, yet the vast
majority of herring continue to be harvested by their natural predators, whales, birds and
fish, and not fishermen. So the “culling” effect of human fishing should be negligible in
this case...but, there it is.) Elsewhere, scientific studies have demonstrated that size at
maturity in tropical fish increases with increased food supply, strongly suggesting that it
similarly decreases with decreased food supply (Hart and Russ, 1996). The “phenotypic
plasticity in reproductive life-history traits” of Caribbean fish has also been demonstrated to
be considerable, with the expression of these traits, including size at maturity, being
observed to vary considerably with changing environmental conditions - including food
availability (Abney and Clemons, 2000). The study demonstrated that size at maturity, in
the fish that were studied, shifted up and down in response to environmental factors, not necessarily
genetic changes.
Another observation on the fish stocks with declining growth rates is this one: the dropping
weight-at-age data for individual species is steeper for the older age groups. Virtually all of
the growth lines slide downward (the very youngest cohorts may be exempt), but the older
ages drop more quickly (and then they disappear from the graph - “cropped off” - there are
several temperate species with very good time-data sets to illustrate this, e.g. cod, charr). The
weight loss becomes increasingly pronounced as the fish grows and must move up to the
higher trophic levels - so it’s not really looking like a pre-programmed, lifelong tendency to
grow more slowly. The oft-described “genetic shift to smaller adult sizes and smaller size at
maturity” deserves another look - stunted growth can also clearly result from a decreased
availability of food.
**Water Temperature** --
Not a characteristic of the fish stocks as much as the thinking of fish researchers, another
common theme between the literature on temperate and tropical fisheries is a very high
degree of interest in investigating the effects on marine life, of recent changing
temperatures. Water temperatures are very gradually increasing, but the lesson for the
tropics that was learned in the NW Atlantic is that “water temperature changes do not
adequately explain all downward trends.” In fact, the notion that changing water
temperatures have been driving the changes in the fish stocks has been all but discarded
in the North. While the temperature changes are certainly worth monitoring and
investigating, there is a danger that the mantra “coral bleaching is caused by rising seawater
temperatures” will be accepted as the whole story, and that there will be no index of
suspicion regarding other possible causes. With corals, by their easily observed and sessile
nature, investigation of the “food deprivation” theory should be fairly simply done.
It is predicted that, with greater nutritional reserves (lipid and protein), corals will exhibit a greater
tolerance for rising water temperature. It should be a simple undertaking to provide an
experimental group with supplementary feeding (use whatever they are fed in aquariums)
to build up their fat and protein reserves before subjecting corals to the heated water
tolerance test. In many ways corals should be the ideal test subjects to help prove or
disprove the theory of the “starving marine ecosystem.”
