What's the problem? Aren't these animals re-released to go about their lives?
While yes, these other, "undesirable" animals are usually thrown back into the sea as discard, most are already dead or dying by the time they make it on board the shrimp trawler. So that pound of wild-caught shrimp on your table also caused the deaths of up to 20 pounds of other species. The discarded animals become food for marine scavengers...they don't go back to leading their normal lives, swimming or crawling along the bottom happily-ever-after. And obviously, because they are killed, they are removed from the breeding population of their species thus depleting that species numbers even further.
The amount of bycatch varies from region to region and the type of fish that is the intended catch. Shrimp trawling causes the largest ratio of bycatch. The UN's Food and Agriculture Organization has documented shrimp trawling bycatch rates of up to 20:1 (20 pounds of other species are caught and discarded for every 1 pound of shrimp). They calculate the world average at just about 6:1. Shrimp trawlers in the USA have bycatch ratios that average between 3:1 and 15:1. To put this into perspective, in the Gulf of Mexico alone, being caught in shrimp tral nets kills from 25 - 45 million red snapper per year...and that isn't the only species in the bycatch.
In some areas of the USA, progress is being made to reduce bycatch by shrimp trawlers. One example is in the Gulf of Maine and the introduction of a new type of trawling net that doesn't have a top (conveniently called "topless trawl"). This new type of trawl net can reduce bycatch from about 30% of the total catch ( or 3:7 bycatch to shrimp ratio), down to about 10% of the total catch (1:9 bycatch to shrimp ratio). While most of the bycatch decreases with this new type of net, the flounder bycatch actually increases.
Yes, this new net is a little ray of good news. Other good news comes from shrimp fisheries with tight regulations, such as (again) the Gulf of Maine. But don't reach for that second pound of shrimp quite yet - bottom trawling has other effects that I'll bring up in a future post.
So what's a responsible consumer to do?
The first thing I'd like you to do is think before you buy. Think of every dollar you spend at the grocery store or fish market as a vote. By purchasing something, you are giving a big YES vote to how it's captured, processed, packaged, shipped, and marketed. Know what you are voting for. Also, research what you eat - I will continue to post on ocean topics here, but I encourage you to do your own research. Search the library, the internet, talk to local fishermen if you can, inform and educate yourself. Don't believe everything you hear or read :) Use the brain you were given to discover the facts, then decide for yourself how you want to shop, based on your own values system.
My personal philosophy is now to severely limit my consumption of commercial, wild-caught sea fish, down to almost nothing. I will perhaps eat a pound of local, Maine pink shrimp when they are in season, or a Maine lobster now and again, but I won't purchase and eat anything my research has shown me to be unsustainable. Is that the right choice for you? I don't know...that's for you to decide.
For further information:
He, P., D. Goethel, and T. Smith. 2007. Design and test of a topless shrimp trawl to reduce pelagic fish bycatch in the Gulf of Maine pink shrimp fishery. J. Northw. Atl. Fish. Sci., 38: 13–21. doi:10.2960/J.v38.m591
^ "Gulf of Mexico Red Snapper: Assessment Summary Report.". Southeast Data, Assessment, and Review (SEDAR) Stock Assessment Report of SEDAR. 2005. http://www.sefsc.noaa.gov/sedar/download/S7SAR_FINALreduce.pdf?id=DOCUMENT.
Morgan, LE; Chuenpagdee, R (2003). Shifting Gears. Addressing the Collateral Impacts of Fishing Methods in U.S. Waters.