“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 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 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.”


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