THE NONRANDOM SIZE DISTRIBUTION AND SIZE-SELECTIVE TRANSPORT OF AGE-0ATLANTIC TOMCOD (MICROGADUS-TOMCOD) IN THE LOWER HUDSON-RIVER ESTUARY

From early March until about mid-May 1975 and 1976, the entire Hudson River population of recently hatched Atlantic tomcod, Microgadus tomco d, was sorted, by estuarine hydrodynamic forces, into a distinct longi tudinal size gradient throughout a 93-km section of the lower estuary. This distribution was evident as a well-ordered progression of mean l engths, beginning upriver with the smallest larvae and increasing in a downriver direction, with the largest larvae being nearer the estuary mouth.;Aspects of the passive estuarine transport of larval tomcod du ring March and April, and the active upriver migrational movement of j uveniles greater than 20 mm in length beginning in late April, were de duced from systematic changes in the longitudinal distribution of tomc od mean lengths and associated variances. Abrupt increases in tomcod m ean length, variation in length, and population density often occurred just seaward of the 1.0 parts per thousand salinity-intrusion boundar y. During March-May, vulnerability of tomcod to power-plant entrainmen t at km 60 - km 69 increased whenever freshwater flow decreased and th e 1.0 parts per thousand intrusion boundary moved upriver. The late Ap ril - early May appearance each year of age-0 tomcod in impingement sa mples at km 69 is consistent with the hypothesis of an upriver migrati on of juvenile tomcod beginning in late April.