Interpreting Maturation Data for American Shad in the Presence of Fishing Mortality: A Look at Historical Data from the York River, Virginia

Abstract

To enable better monitoring, forecasting, and establishment of restoration targets, we undertook a study of maturation by American shad Alosa sapidissima in the York River, Virginia. Throughout the study, the need for historical information about the status of the stock and how it relates to the current depressed state was evident. We identified an early study (covering the years 1957–1959) that might allow reconstruction of maturation schedules for historical time periods and enable comparison with contemporary maturation schedules. When a commercial fishery is operating, mature survival can differ from immature survival for a given age because the immature fish remain offshore and do not experience the fishing mortality imposed on adults in the spawning river. Thus, a maturity model that can account for the relationship between mature and immature survival is required and was developed here. A process for reconstructing data matrices from published data summaries was developed that led to the discovery of errors in the historical source. For the year in which the errors could be corrected (1959), maturation parameters were estimated using the method developed here for various levels of the ratio of mature to immature survival (including that inferred from the exploitation rate reported in the historical study). Additionally, microfiche data were located that provided the information necessary to estimate maturation parameters for 1955 and 1956. The estimated maturation schedule for 1959 was earlier than the present estimated schedule, while those for 1955 and 1956 were later. Because a greater range of ages was sampled in 1955 and 1956 and the maturity estimates for those years were in agreement with each other and in line with ecological expectations, we accept those estimates over the ones obtained from the 1959 data and conclude that at present the maturation of American shad occurs earlier than it did during the historical period.