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 Micropogonias undulatus is commonly caught as bycatch in shrimp trawls and has also 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 industrial fishery, have primarily harvested juvenile fish. In contrast, the most intensive fisheries in the Atlantic have targeted adult fish. We hypothesized that population-level effects of exploitation differ qualitatively between fisheries that take primarily juveniles and fisheries that target adults, even in the face of bycatch of juveniles. We compiled data on Atlantic croaker fisheries, life history parameters, and population abundance in the Gulf and the Atlantic and followed changes in these variables over time. In the Gulf, long-term intensive fishing of Atlantic croaker juveniles has caused severe declines in abundance since the 1930s but little change in adult life history parameters: large fish remain common, the size distribution has been relatively stable, and the size at maturity has not changed. In the Atlantic, however, targeted fishing of adults has caused both declines in abundance and major changes in adult life history parameters: size at maturity, 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 changes in Atlantic adult life history parameters by reducing the number of fish that survive to adulthood, making the population more sensitive to adult fishing. We explore several hypotheses to explain why juvenile fisheries (Gulf) would have smaller effects on life history than targeted adult fisheries (Atlantic). If our observations are correct, reductions in bycatch legislated by the U.S. Magnuson-Stevens Act should allow Atlantic croaker to increase in abundance in the Gulf (but have little effect on size distributions) and to increase both in abundance and in adult size in the Atlantic. [ABSTRACT FROM AUTHOR]