I was fishing over the weekend and landed 12 redbreast sunfish from my favorite stream. Only one was a female. From another lake in the area I recently netted 43 gambusia and there were only 8 females in the whole batch. This made me curious, does the environment have any influence over the sex of fry? If so, how?

You landed them as in you were fishing and the fish bit onto your hook? If so, then another question would be "Are males more likely to bite onto a suspicious food than females are?" I could understand if that would be the case. Males of many species often partake in riskier behavior than the females. If the risk doesn't work out and the male dies, then the population is still basically okay.

I mean, consider a population of 20 males and 1 female. The next generation is going to be pretty small in number, considering how energy intensive eggs are to make compared to sperm and how females can't reproduce as frequently as males. But if that same number unbalance was in the opposition direction, 20 females and 1 male, it's possible that that male could mate with a large fraction of those females and the next generation would still be decently sized. So from a population viewpoint, males can die more frequently than females and the number of offspring isn't too affected.

That's why males are often more colorful while females are drab. Colorful fish are more visible, so being colorful increases your chance of being eaten. But it also increases your chance of being mated with. So if the males are colorful and there are only 10 of them for every 20 females because they are more likely to be eaten, it works out and the population as a whole is not affected by the increased predation rate.

This might be what's happening with your hook. It doesn't look like normal, safe food, so the females stay away from it. But the males, who are occasionally rewarded for taking risks, try it. If they die, well, that's a risk they've got to take sometimes to get ahead of the other males.

Edited by EricaWieser, 01 March 2011 - 11:41 AM.

As an angler I don't believe males bite more. I catch lots of females [fish I mean ] but never did a statistical analysis. It's an interesting suggestion.

More likely is that outside of the breeding season, males and females may congregate in single sex groups which may not be in proximity to the opposite sex group. I don't know this to be fact about either species but I think it is not uncommon. Pre-spawn females may congregate near a high quality food source while developing eggs while males may begin to stage near prefered breeding sites.

Mikeb,
In your area, male sunfish are probably starting to guard nest areas. When they are guarding, they will bite at just about anything! Catching mostly male sunfish in the spring is fairly typical in my experience.

When they are guarding, they will bite at just about anything!

Oh yeah, I didn't think of that. That's got to be it.

I guess I should try to answer the original question about whether environment influences gender, although we can be fairly sure that there are approximately equal numbers of sunfish of both genders and that the males may just be biting more as they defend their nests.

Yes, the environment can influence gender. Here are two examples.

1. Quote from the Xiphophorus Genetic Stock Center, http://www.xiphophor...tockcenter.html : "A peculiarity of the Cd strain is the occurrence every few generations of almost all-male or all-female broods, a source of constant anxiety in its keepers. No cause for this deviation from normal sex ratios in this genetically uniform fish has yet been identified." That strain of swordtails often gets all one gender broods, but they're not sure why. It might be environmental or it might be genetic or it might be genetic triggered by something in the environment.

2. Toxoplasmosis gondii. It's a parasite that, if a pregnant woman is infected, makes it more likely for the baby to be a boy than a girl.
Link: http://news.sciencem...6/10/12-01.html
"Parasitologist Jaroslav Flegr and his colleagues at Charles University in Prague, who have studied the effects of Toxoplasma in humans, had found preliminary evidence that latent infections might correspond with longer pregnancies. But when they examined the medical records for 1803 births in three maternity clinics in the city that routinely test for Toxoplasma antibodies, they found a much more striking difference between infected and uninfected women: Infected women have more boys. The usual sex ratio at birth is .51, meaning 104 boys are born for every 100 girls. But in the 454 women who tested positive for antibodies to Toxoplasma the ratio was .60--which translates to about 150 boys per 100 girls. For women with the highest levels of antibodies against the parasite, the ratio was .72, equivalent to 260 boys for every 100 girls, the team reports online this week in the journal Naturwissenschaften."

Edited by EricaWieser, 01 March 2011 - 06:58 PM.

Is the question whether the environment can influence the sex of developing fry, or whether the environment favors the survival of one sex over the other (after sex is determined)?

To be fair, the Toxoplasmosis example is one where the survival of the fetus is affected, not the sex of the fetus itself.

Thanks for all the great replies,

I understand the idea of the males' risk taking and it makes sense. The fish I caught were likely starting to guard their nest areas which led to a higher rate of males vs females. I was curious though whether specific environmental factors influence the sex of developing fry. For example, will offspring in a higher pH environment or a more saline body of water tend toward one sex?

The gambusia I netted really got me thinking about it, I caught dozens and only a handful were female. Coincidence, or does something in that specific area favor the survival of males over females, or do females just avoid the shallower water during this time of year?

Interesting information though from everybody.

There are many tropical fish in which temperature and pH or some combination of those affects the sex of developing fry. Apistogramma and Pelvicachromis cichlids and certain killifish are notorious for this in captivity. I dont know whether it's been well documented in N.Amer fish, but its certainly possible. I doubt whether wild populations get severely skewed by this; seems to me it would be disadvantageous.

That said, your collections of mostly male sunfish and mosquitofish probably had nothing to do with that. It's more likely just behavioral differences between the sexes and/or artifact of collecting method & location.

Edited by gerald, 02 March 2011 - 10:24 AM.

Temperature determination of sex ratio has been documented in Gambusia at least. It came up when I was considering trying to breed selectively for the melanistic trait, and I was getting rather more than 80% males (total sample size of about a hundred fry from several females). You can probably still find some studies linked if you look up that old post.

Male sunfish are caught in greater numbers during spawning only if you target the nest colonies.
If you fish the deep drop offs where the spawned out and hungry females are congregating you can catch 'em all day and never see a male.

The only fish that bite are the ones you put your bait in front of.

Good point. I usually do any brim fishing from shore, which would target the nests. I like sitting in the vegetation and having an excuse to not think for a while.