Indeed there is a very strong relationship. There are studies from Lake Victoria that show this relationship. You may want to consult studies by Ole Seehausen and others at - Seehausen O, van Alphen JJM & Witte F (1997) Cichlid fish diversity threatened by eutrophication that curbs sexual selection. Science 277: 1808-1811. DOI 10.1126/science.277.5333.1808. and a paper by Witte which clearly show the impact of turbidity on the visual mate selection - all of which contribute to hybridization.

I agree with Ismael. Simply put, Seehausen et al (1997) reported that the different but closely related species of cichlid fishes of Lake Victoria started hybridizing in Lake Victoria when the water became turbid due to pollution. This was because the females could not differentiate between males of different species because they could not see the special coloured spots on the males. Since they have evolved to use these spots to identify their nonspecific males, they could not manage to do this when visibility was very bad in the turbid waters.

Tanx Ismail and Cyprian, but are Cichlids the only examples? and are there any reported adaptations by any species to overcome this ecological disadvantage?

I forgot the title of Frans Witte's paper, but it seems the Victorian cichlids have adapted by developing retina cells such that they can still see in turbid waters

Tanx Ismail.

On second thought i dont want to believe that poor water quality causes mating problems for only fishes that are dependent on visual cues, what of fishes undergoing olfactory toxicity, won't mating accuracy based on chemical cues also be affected?

In the Kazkahstan water bodies we did not observe any relationship in the Aral sea watershed and the Balkhsh lake basin. Against, hybridization was observed in the rather clear period. Probably ability to hybridization depend on population number and habitat state.

Dear colleagues, Nadir has raised a very important point on this topic what do you guys have to say?

Well, hybridization in clear water is probably owing to other reasons rather than clear water. But it is well established that water quality does have strong relationship with hybridization. And obviously water quality is one of the most influential factors of hybridization but not the only...

Great contribution Jon! Great thought!

Hi colleagues, quite a while.

I recently came across a term called 'pre-mating behavioural isolation' it seems that the existence or break-down of this factor strongly determines the incidence or frequency of inter-specific hybridization.Can anyone please relate this term to this ongoing discussion?

According to all, in addition, some species or population requires at least population size to have a successful reproduction or mating.

I think it might be quite important to distinguish whether we are talking about hybridization between coevolved species within ranges and macrohabitats where they have historically co-occurred, or hybridization between indigenous and introduced or displaced species, where they are not naturally co-occurring.

And secondly but no less important, are we talking about waters with predictable and relatively less variable physico-chemical condition, or waters with a high degree of seasonal and smaller-scale variation in such conditions?

Coevolved species in less predictable/more time-variable habitats will likely have evolved a range of tactics to minimize hybridization, the full complement of which should be relatively robust to environmental "noise." Highly novel perturbations, such as toxic chemicals or sustained or unseasonable anthropogenic turbidity in naturally clear spring waters, would be more likely to be highly disruptive of mating systems. Species adapted to more sable, less variable habitats might have mating systems that are less robust to physico-chemical perturbations.

Where I do most of my work, physical habitat conditions during mating and spawning are quite variable (including frequent episodes of high turbidity in some cases), but coevolved congeneric species appear to successfully keep hybridization to low frequency. The perturbations that are most strongly associated with increased inidence of hybridization in western North American fish assemblages are 1) environmental alterations that impair natural spawning movements and habitat selection by which native species often sort themselves spatially, or sometimes that distort spatial and seasonal thermal templates by which species assortatively segregate for spawning, and 2) the introduction of closely related nonnative species that are not co-evolved. In the case of nonnative species, hybridization and introgression appear to be facilitated by chronic habitat alterations, perhaps particularly those that disrupt or impair behavior and reproduction of the related native species and open up opportunities for niche expansion by the invasive nonnative (e.g. dams, catchment disturbance and sedimentation) The incidence of hybridization then may increase simply as a secondary function of demographic and encounter probabilities.

Re-reading your initial question, I suppose if we are talking about fishes that are highly reliant on visual cues for mate selection, then it may be a given they are adapted to at least modest water clarity, and it's almost a tautology to say that yes they might be disrupted by alterations like elevated turbidity that impair visually-mediated behavior.

But does such impairment result in hybridization (that is, less mate selection fidelity), or simply truncation of spawning behavior altogether? If hybridization is a serious evolutionary risk, the parsimonious solution may be to simply suspend spawning behavior under conditions that impair mate selection. It's known, for example, that Pacific salmon species do this; to compensate they have evolved some plasticity to defer actual spawning times to more favorable periods on a time scale of hours, days, or weeks.

Of course the examples I listed above fall into the category of "pre-mating behavioural isolation," by way of spatial and temporal segregation. Since at higher abundances there remains considerable overlap between spawning congeneric salmon species, I'd use the term "segregation" rather than "isolation" in the strict sense.

@Christopher segregation leads to isolation of some sort, and you're unlikely to achieve isolation without segregation dont u think so?

Sure, but relatively recent zoogeographic remixing of taxa, or human species introductions, or substantial habitat alteration (the original question fits here) can all make for incomplete segregation and reproductive isolation.