Why do a fish contain higher pesticides

[RayRay]

When someone sprayed pesticides in a field next to a pond. Why do the fish have higher pesticides than any other pond organism.

[canalon]

Depends on the fish... But maybe if you think about how how pesticides are accumulated, and how a fish can ingest them you will find your answers (hint: trophic levels)

[AstusAleator]

The process is called bioaccumulation. Here's a link I found in a quick google search for you.
http://www.marietta.edu/~biol/102/2bioma95.html
Essentially the process is as follows:
microorganisms eat pesticide infected things. invertebrates eat hundreds of the microorganisms and plants infected with the pesticides, accumulating a pretty large amount of the toxin in them. The invertebrates are generally then eaten by larger invertebrates or amphibians, which ingest hundreds of similar organisms, accumulating an even higher concentration of toxins in their bodies. Then the fish eat hundreds of these organisms, accumulating the highest concentration of toxins in themselves.
Or, in detritous/vegetative feeding fish, the toxins accumulate because the fish are essentially eating the sludge (poop mixed with various phyto- and zoo- plankton) that has a high concentration of the toxin.

The toxins are then stored in the fishes liver, kidney, and pyloric cecea(sp?) if they have them.

The accumulation can be taken one step further though:
Like in the case of DDT, the toxins can accumulate in the eagles or other raptors that eat the fish. DDT was shown to soften eagle eggshells, increasing the chance of infant mortality.

So there you have it. Hope this helped.
PS: if this was a homework assignment, try not to submit what I've said verbatum. Actually try to understand it.