Catch and bycatch: The qualitative effects of fisheries on population vital rates of Atlantic croaker

Bycatch in shrimp trawl fisheries can make up 60-80% of the catch by weight . Finfish taken incidentally in shrimp trawls include juveniles of species that are valuable in directed fisheries as adults. Atlantic croaker Micropo gonias undulatus is commonly caught as bycatch in shrimp trawls and has als o been a target species in both the Gulf of Mexico (the Gulf) and the South and Middle Atlantic bights (the Atlantic). The catch of Atlantic croaker, including bycatch, has historically been at least three times higher in the Gulf than in the Atlantic. Gulf fisheries, including the directed industri al fishery, have primarily harvested juvenile fish. In contrast, the most i ntensive fisheries in the Atlantic have targeted adult fish. We hypothesize d that population-level effects of exploitation differ qualitatively betwee n fisheries that take primarily juveniles and fisheries that target adults, even in the face of bycatch of juveniles. We compiled data on Atlantic cro aker fisheries, life history parameters, and population abundance in the Gu lf and the Atlantic and followed changes in these variables over time. In t he Gulf, long-term intensive fishing of Atlantic croaker juveniles has caus ed severe declines in abundance since the 1930s but little change in adult life history parameters: large fish remain common, the size distribution ha s been relatively stable, and the size at maturity has not changed. In the Atlantic, however, targeted fishing of adults has caused both declines in a bundance and major changes in adult life history parameters: size at maturi ty, common sizes (the range in length of about 90% of the individuals in a length frequency distribution), maximum length, and modal lengths of adult fish have been reduced. Bycatch of juveniles may have contributed to the ch anges in Atlantic adult life history parameters by reducing the number of f ish that survive to adulthood, making the population more sensitive to adul t fishing. We explore several hypotheses to explain why juvenile fisheries (Gulf) would have smaller effects on life history than targeted adult fishe ries (Atlantic). If our observations are correct, reductions in bycatch leg islated by the U.S. Magnuson-Stevens Act should allow Atlantic croaker to i ncrease in abundance in the Gulf (but have little effect on size distributi ons) and to increase both in abundance and in adult size in the Atlantic.