In addition to overfishing, other factors play a part in the diminishing stocks of commercial fish species. Some are beyond human control. Most are traceable to human activity. For example, developing human populations along the world’s coasts have added to the pollution of inland rivers and streams; estuaries and lagoons that previously sheltered and fed juvenile fish have been filled in and developed. Almost three-quarters of the species in the US fisheries must live in estuaries at some stage in their growth.
By-catch, the netting and killing of unwanted fish, is another factor in the shrinkage of fish stocks. The dolphin and porpoise kill in tuna fishing became well known to canned-tunafish buyers in the 1980s, and the methods in which purse seine nets are used in the tuna fisheries were changed as a result. However, in the US shrimp fishery alone, an estimated 172,000 US tons of juvenile fish are thrown out each year, contributing to a noticeable decreases in the populations of snappers and groupers in the Gulf of Mexico. The estimated by-catch in Alaskan fisheries amounts to over one-half million US tons a year. Worldwide, as much as 30 percent of the fish caught may be wasted as by-catch.