The nonrandom size distribution and size-selective transport of age-0 Atlantic 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 tomcod, was sorted, by estuarine hydrodynamic forces, into a distinct longitudinal size gradient throughout a 93-km section of the lower estuary. This distribution was evident as a well-ordered progression of mean lengths, 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 during March and April, and the active upriver migrational movement of juveniles greater than 20 mm in length beginning in late April, were deduced from systematic changes in the longitudinal distribution of tomcod mean lengths and associated variances. Abrupt increases in tomcod mean length, variation in length, and population density often occurred just seaward of the 1.0 parts per thousand salinity-intrusion boundary. During March-May, vulnerability of tomcod to power-plant entrainment at km 60 - km 69 increased whenever freshwater flow decreased and the 1.0 parts per thousand intrusion boundary moved upriver. The late April - early May appearance each year of age-0 tomcod in impingement samples at km 69 is consistent with the hypothesis of an upriver migration of juvenile tomcod beginning in late April.