Winter flounder settlement dynamics and the modification of settlement patterns by post-settlement processes in a NW Atlantic estuary

Abstract

For fishes with bipartite life cycles, locations of high quality nursery grounds are deter- mined by processes controlling larval supply as well as those affecting early juvenile mortality and emigration. From April through June 2000, distributions of settling and early juvenile winter flounder Pseudopleuronectes americanus were measured to examine how pre- and post-settlement processes determine the location of the primary nursery ground in the Navesink River/Sandy Hook Bay estuar- ine system (NSBES), New Jersey. The settlement pattern, measured with fine mesh (3 mm) traps that captured flounder ≤ 8 d into the post-metamorphic age but excluded predators and prevented emi- gration, was spatially dynamic. Fish settled on organically rich substrata (organic content = 5 to 12% by weight) 2 wk earlier in the Navesink River (mid-April through mid-May) than on similar substrata just 15 km downstream in Sandy Hook Bay (May through mid-June). Local retention mechanisms combined with spatial variation in spring warming, which probably affected larval-stage durations, appeared to be responsible for the dynamic settlement pattern. To determine whether spatial pat- terns of flounder settlement were dramatically altered by post-settlement processes, we compared settler supply (measured using traps) with juvenile distributions (measured using beam trawls, which do not prevent post-settlement mortality and emigration). The index of settler supply explained 95% of the variation in juvenile abundance patterns in the Navesink River (p 20 mm standard length were commonly trawled. However, larger juveniles were nearly absent in Sandy Hook Bay, where juvenile distributions were not related to settlement (r 2 = 0.15, p = 0.31). Thus, the upstream distribution of juvenile winter flounder in the NSBES, which is similar to that observed in other estuarine nurseries, appeared to be produced by the rapid modification of settlement patterns by post-settlement processes. However, pre-settlement processes that produce spatial variation in the timing of settlement could affect the ways in which settlement patterns are modified by age, time and/or size dependent post-settlement processes.