I think one way pollution may indirectly affect a fish species is if it results in a depletion of the resources of its prey-food. Reduced availability of prey organisms may result in reduced foraging activity which will reflect in the condition of the fish species.

What kind of data are you dealing with?

I have compared the condition factor with the main trophic habits (divided into simple, general categories, as "carnivores, herbivores and omnivores) in a survey, on a northern Brazil estuary, and have found some seasonal differences concerning fish condition...

Yet I do not consider eutrophication along the estuary course - all sampling sites were established quite away from the city. However, it really seems there is a negative relationship concerning fish availability/diversity Vs. polluted areas, where omnivore fish are more likely to occur (namely, are more resistant to eutrophication). These usually present high condition factor values though....

Thanks Brenos for highlighting the possible exceptions, i didn't put omnivorous fishes into consideration .

You're quite welcome, Dr. Chukwuka!

You can find attached a few papers on this issue.

More here

The Visceral fat tend to be Exhausted when the fish Lays its egg or during its reproduction until then before they are going to breed most of the fishes will be fatty more near visceral. During the spawning they will migrate to the safer places where their fry will get adequate food and protection in this time they wont feed only this fat would be utilized by them during their spawning.This Spawning and Nursery Ground Depends on the habitat which they live. Every organism condition or its energetics depends on the food availability If the prey Availability is good the predator condition would be good here it is the fish condition !

Thanks Brenos for your publication on the Amazonian Coast, it addressed some aspects of this question particularly with regards to food availability in Estuaries.

What i would like to ask is does the high occurrence of diversity in fish catches during the dry season imply sufficient food for resident organisms in the estuary?

Thanks Seelam for your contribution. but i'd like to comment that spawning does not 'exhaust' the visceral fat , only a tangible portion of the total body fact is allocated to gonad production in fish of reproductive age.

Dear Dr. Chukwuka,

It may depend on several factors... A few are listed here: structure of the local ichthyofauna, current status of the ichthyofauna (regarding fisheries), artisanal fisheries Vs. industrial fisheries co-occurring in the area, etc...

In the Amazon region, the dry season is usually the fishing season, while the early rain season is the reproductive period for the most species, and fisheries are not allowed during this period....

Overexploitation of a given species will always incur in any level of due impacts to a given environment. If your question is related to overfishing during the dry season, I think there may be a negative correlation between catches and food availability in most cases.

Although the seasonal differences obviously depend on the latitude, these are generally well marked in the estuaries, which directly suffer the reduction of freshwater inflow during the dry season. Rain might enhances the productivity of the estuaries (a bloom can happen even in a very short period (hours-day) after the rain starts), so fish condition is expected to be better during that time of the year.

In general, estuaries are much more productive systems than the open sea, so the food availability is also likely to be higher in estuarine systems. However, the condition of fish might depend on a number of other factors, such as predation risk, competence, pollution, fishing pressure, etc.

And of course, as already mentioned above, the reproductive season determines the condition of the adults (that's why, in general, researchers tend now to use larval/juveniles stages to assess the seasonal variation in the condition of fish populations). This also depends on whether the fish behave as capital breeders or as income breeders (http://www.jstor.org/stable/3545800?__redirected).

Thanks David for your contribution. I'd like to ask if you could expantiate a bit more on the use of larval/ juvenile stages to assess seasonal variation in fish populations.

Secondly i'd wish to know if there are any draw backs, disadvantages or exceptions with respect to species or any other variable when using this method.

When assessing seasonal variability in the condition of fish populations, the spawning period must be taken into account, because the energy allocated for reproduction is not necessarily related to the actual nutritional condition of the individual, and this adds extra variability and complexity to the study of the condition. To avoid this issue, the nutritional condition of fish populations can be assessed by studying the larval/juvenile stages instead of the adults. Some methods of assessment of larval condition can be used as proxies for growth rate and even for recruitment, which gives information about the status of the population.

There are several methods to assess the condition of fish larvae. I think that the most straightforward technique is the Fulton index (see this thread: https://www.researchgate.net/post/What_are_the_limitations_of_the_Fultons_condition_index-K2), but probably the most accurate methods are the biochemical ones (http://www.sciencedirect.com/science/article/pii/S0065288108600644).

Among the biochemical methods, the ones I'm more familiarized with are the lipid contents (including the TAG/CHOL index) and the protein contents analyses and the RNA/DNA ratio. I can give you more references about these techniques, but the paper cited above of Ferron & Leggett (1994) gives a very good idea of the advantages and disadvantages of the different methods.

Thanks David.