fish in general--

Question
1. I went out on a boat with an ecology class and collected species via otter trawl from a deep and shallow site. we calculated abundance (number of each species collected) and then calculated biomass by putting all of those species together in a bucket and weighing in grams. i was wondering why the most abundant species doesn't have the highest biomass (in my case) and why the second most abundant species doesn't have the second most highest biomass..etc?
also, which measure--abundance, or biomass, more accurately describes the impact a species might have on a habitat?
2. Why are larger, older, fish found in deeper water trawls (sites) and smaller, younger fish found in shallow water sites?

Answer
Dear Erin,

I'm not really sure if you weighed the species separately and then calculated biomass or if you weighed them all together.  I'm guessing by your question that you weighed them separately.  In that case:

1.  Size matters.  You can have 100 fish that weigh 1/2 gram that will have a lot less biomass in a small sample than 50 fish that weigh 2 grams apiece.  What the size of the sample belies is the fact that the overall biomass (over the whole ecosystem) may be much greater, due to abundance, for the millions of 1/2 gram fish than for the thousands of 2 gram predators.  So, you have 10,000 pounds of anchovies for every 1,000 pounds of salmon.  The measure of both abundance and its resulting biomass over the whole ecosystem will determine both the positive and potentially negative impacts on the habitat.

2. I don't know.  Different species of fish behave differently at different stages of their lives.  Anadromous fish like salmon and steelheads will start in fresh water, spend some time in the sea and won't even reappear until they are staging up just outside the estuary for their runs into the rivers and streams to spawn.  Perhaps the little fish are rising to the surface to escape predation from the big fish down below.  Perhaps the little fish need more sunlight to grow, or they're feeding on something that blooms in the light.  

You're asking questions that may have specific answers for specific kinds of fish.  Or, they may be questions that you may yet answer in your studies someday.

Les Smith
Nevada Department of Wildlife