Movements, Reproduction, and Mortality of the White Perch, Roccus americanus, in the Patuxent Estuary, Maryland

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Abstract
Over 8,000 white perch were studied in order to understand the population forces of movement, reproduction, and mortality in this representative tributary of Chesapeake Bay. Scales examined in 3,475 fish showed that the maximum ages attained in males and females were 9 and 10 years, respectively. Sizes attained at previous ages were calculated for both sexes, and the means of observed lengths and calculated lengths for corresponding ages compared well. The mean size and weight of males at corresponding ages was less than that of females. The effects of commercial fishing gear that selected out larger fish apparently produced smaller calculated lengths than observed lengths at corresponding early ages. Growth increments for both sexes were greatest during the early years of life. Most males and almost all females enter the commercial and sport fishery at the legal minimum size of 8 inches total length at around age group V. Dominant year-classes were not indicated by the data. First-year growth of white perch for the 13-year span, 1942–1954, was positively correlated with (a) growth span in the number of days in spring with water temperatures between 10° and 15°C, and (b) with solar radiation in light energy from February to June. It was negatively correlated with: (a) rainfall from February to May; and with (b) an index of population density in the form of commercial haul seine catch in the Patuxent River five years later than the origin of the year-class (r=−0.753; p=<0.01). The latter relationship suggests that when white perch populations were large in numbers, the individuals from which they arise were usually small-sized; when the populations were small in numbers, the individuals from which they arise were usually large. The white perch is a schooling, gregarious species, undertaking marked seasonal movements. Of 3,024 tagged and released largely in the river, 324 or 10.7% were recaptured almost entirely within the estuary. The results indicated that the population in the Patuxent is essentially self-contained, and that it can be managed on a local basis in the river. Extensive spring movements were observed from the lower and mid-estuary to upstream tidal fresh water for spawning, indicating the population is semi-anadromous. Summer movements were local and random. Fall movements were minor and usually downstream toward deeper water. In winter white perch remained in deep water and did not migrate until spring. None of the fish tagged outside the river mouth entered the patuxent; all recaptures came from upper Chesapeake Bay. A meristic study of white perch fin-rays from various zones in the river corroborated the tagging results, indicating that the population is homogeneous. Significantly different subpopulations, based on meristic characters, however, were found between other tributaries and areas in Chesapeake Bay. A study of gonad development and examination of fish on the spawning grounds showed that female white perch are oviparous and fertilization of eggs is external. They spawned from late March to May in non-tidal and tidal fresh and slightly brackish water areas when water temperatures were between 10° and 15°C. Males matured at a smaller size (50% at 100.3mm) than females (50% at 105.5 mm standard length). All males in age group II and older, and all females above age group IV, were sexually mature. The early sexual maturity of males partially accounted for the predominance of this sex on the spawning grounds throughout the season. Mortality rates were calculated from age frequency distributions in the form of catch curves. The annual mortality rate expressed as the annual probability of dying (50%) after age group IV for males was less than that for females (56%). Survival among exploited white perch populations in the Patuxent was relatively good when compared to values obtained from unexploited and exploited populations of other fish species. The apparent probability of dying for unexploited white perch under age group V which are subject to death largely by natural causes is slightly less than that for exploited fish above this age subject to death due largely to fishing mortality. A provisional life table was constructed for adult white perch beginning at age group IV, and the mean life expectancy for a cohort after this age was slightly more than 1 1/2 years, indicating that the chance of survival at any age is not very great. Liberalized management, and its assessment, of the white perch in the estuary is indicated by the population vital statistics. Reduction or removal of size limits on a local basis provide the only positive measure for biological improvement and effective use of the population.

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