JavaScript: Weighted Random Generator
I'd suggest you select a random number that's from 0 to the total weight of your fish.  Then, each fish can have a piece of that random range according to it's weight which gives it the weighting you want.  This ends up giving each fish a number of buckets that corresponds to it's weighting value and then you pick a random number from 0 to the total number of buckets and find out who's bucket the random number landed in.  

It's the same idea as turning each weighting number into a percentage change and then picking a random number from 0 to 99.

This code should do that:

var FISH = {
    "level1": [
          //["name",      xp, weight] 
            ["Shrimp",    10, 95],
            ["Sardine",   20, 85],
            ["Herring",   30, 75],
            ["Anchovies", 40, 65],
            ["Mackerel",  40, 55]
        ]
};


function getRandomFish(level) {
    var fishForLevel = FISH[level];
    var fishTotalWeight = 0, fishCumWeight = 0, i;
    // sum up the weights
    for (i = 0; i &lt; fishForLevel.length; i++) {
        fishTotalWeight += fishForLevel[i][2];
    }
    var random = Math.floor(Math.random() * fishTotalWeight);
    // now find which bucket out random value is in

    for (i = 0; i &lt; fishForLevel.length; i++) {
        fishCumWeight += fishForLevel[i][2];
        if (random &lt; fishCumWeight) {
            return(fishForLevel[i]);
        }
    }
}