“rotting fish as food for thought”

03 June 2011

this thread on the fish kills, courtesy of marine science professor flor lacanilao, is quite an eye-opener.  more and more it looks like the fish kills are the result of too many fishpens, bad governance (what else is new), and ignorance, as well as disregard, of the limited carrying capacity of our lakes and coastal waters.

the first email starts out with a link to philippine star that has DENR scientists  saying that the fish drowned!  sec. paje?  hello?

From: Raul Suarez <raul.suarez@lifesci.ucsb.edu>
To: PhilScience@yahoogroups.com
Sent: Tuesday, May 31, 2011 6:38 AM
Subject: [PhilScience] Re: rotting fish as food for thought

Recently, there were a couple of fish kill incidents reported in the Philippine Star:



According to the first report, the fish “drowned”. The second article states “Scientists said the onset of the rainy season led to a sharp drop in water temperatures that depleted oxygen levels in the lake.”

I thought it might be useful to ask students to read these articles and to answer the following questions:

1. What is wrong with the statement that the fish (mostly bangus) “drowned”? (What is the actual phenomenon and what term better describes it?)
2. Can any species of fish drown? If so, in what ways are these species different from bangus?
3. How would a drop in water temperature affect the solubility of oxygen and the oxygen consumption rates of aquatic organisms? (Is the statement above by “scientists”, by itself, a satisfactory explanation for the observed fish kill?)

I will try the above 3 on my own students here. These questions come from my ecological physiology perspective. Of course, there are other relevant questions that could be asked from the perspectives of limnologists and freshwater ecologists.

My quick internet search indicates that there have been a number of journal articles authored by Filipinos concerning the effects of aquaculture on Philippine lakes. In the interest of educating the public and government officials, I hope these findings make their way into the Phil Star.

The scientific literature seems deficient in terms of characterizing the effects of exotic species on lake biodiversity in the Philippines. I wonder, for example, whether the introduction of Tilapia has caused the collapse of populations of local species or even their extinction? What are the community-level effects? In what sense can this kind of aquaculture be considered “sustainable” or “environmentally friendly”?

Below are links to 2 journal articles concerning these issues:

the next email is a response to prof. suarez’s question re the connect between water temperature and oxygen depletion.

On Tue, May 31, 2011 at 9:02 AM, Jose Christopher E. Mendoza <protojcem@yahoo.com> wrote:

Hello Prof Suarez,

…When I read the “drowning” statement in yesterday’s report, I had this sudden cartoonish mental image of myself with my eyes popping out of their sockets in reaction. I think this also suggests that the English vocabulary of our news writers (and gov’t. officials?) is also on the decline… an “asphyxiatingly” worrisome state of affairs (?).

Regarding your third question/point on water temperature and DO, I re-read the scientist’s statement you alluded to, which actually said “It is bad for the bangus when sudden heavy rains cause the temperature to cool at the surface, in contrast with the hot temperature of the water at the bottom”. Just trying to recall my undergrad ecology/limnology now: in a lentic environment with sufficient depth, the cooler surface water would then tend to sink and the warmer water near the bottom would tend to rise, and the resulting convection currents would promote mixis. If the bottom has become hypoxic/anoxic due to microbial decomposition of organic matter (from fish feed, fish waste), then the hypoxic/anoxic conditions may be brought to surface & sub-surface levels where the fish are kept, negating the higher oxygen solubility brought about by cooler water temperature. So in a way po, mukhang tama din naman yung sinabi nung scientist.

Then again, the writer of the article could have done more research (kahit Google man lang) and put more science in the article to explain the deleterious effects of excessive fish farming to the natural environment. I remember a few experiences where I or a colleague would spout out a lot of “science stuff” (already conscientiously couched in layman’s language) to a reporter only to have the intent or meaning still distorted in the final report. Which is why I think media may have to share the blame when our local science workers come out with low credibility & gravitas.

P.S. I’d also like to share a blog post on a similar incident in Singapore last year, for comparison.

Jose Christopher E. Mendoza, M.Sc., Ph.D. :: Systematics & Ecology Laboratory :: Department of Biological Sciences :: National University of Singapore :: 14 Science Drive 4, 117543 Republic of Singapore

and finally, from prof. lacanilao, a letter to the editor he wrote in 1997 that was published by the inquirer, philippine star, and manila chronicle.  good as new.

Re: [PhilScience] Re: rotting fish as food for thought
To: PhilScience@yahoogroups.com,philmarsci@yahoogroups.com, taxo2006@yahoogroups.com

Pollution and fish kill
The NBI report on the Manila Bay fish kill (PDI, 1/21/97) again confirms the industry’s continued disregard of the environment and the government’s failure to enforce pollution laws. Unless these two are given serious attention, fish kill from various causes will recur. And to find out the cause each time this happens is to continue evading prevalent pollution problems.

Organisms have a capacity for tolerance to environmental changes, like pollution. The tolerance, however, is limited and varies among organisms and with the kind of change or pollution. Some species are affected more than others by a given pollution. The organism’s tolerance is widest for survival, narrower for growth, and least for reproduction.

A given level of water pollution, for instance, may prevent an organism to reproduce, but it will continue to grow. A higher level may arrest growth but allow the organism to survive. At the limit of tolerance for survival, any factor of environment, man-made (e.g. pollution) or natural (e.g. temperature), can trigger fish kill, affecting all species with similar tolerance properties.

Hence, at some pollution level, certain species are inhibited to reproduce. Others are prevented to grow or to survive. The overall effect is lower fish catch. The slow and gradual damage to fish stock causes more losses in fish harvest than the occasional fish kill, which is just an indicator of the pollution condition. The gradual but widespread losses caused by pollution are partly responsible for the country’s decreasing fish catch in inland and coastal waters recorded since 1991. These water bodies provide subsistence livelihood to 90 percent of our fisherfolk.

The NBI suggestion to prosecute violators is long overdue. Perhaps the Senate committee on environment and natural resources can look into the possibility of dealing with suspected polluters in the same way suspected common criminals are treated – arrest (or stop operation) first, then investigate. – FLOR LACANILAO, professor of marine science, UP Diliman, Quezon City

read too
Unregulated Fish Pen and Cage Operations Mess Up Coastal Ecosystems by ALECKS P. PABICO

2 Responses to “rotting fish as food for thought”

  1. June 3, 2011 at 3:39 am

    you are a member of a very interesting email group. :)

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