A fish that we would consider to be resident in a tributary mouth would be expected to be present throughout most of the year. Unfortunately, we have no studies available in which fishes have been sampled over most of a year in this habitat. Anderson (1988) did a cluster analysis (single linkage, Euclidean distance) on Smith's (1985) field data for the Hudson Valley and characterized a cluster as a tidal stream mouth assemblage. This cluster included American eel, banded killifish (Fundu-lus diaphanus), pumpkinseed (Lepomis gibbosus), and bluegill (L. macrochirus). We did a summer and early fall study on Quassaic Creek (RKM 96.5) and categorized a set of nine "resident" species that we consistently saw in the stream mouth (Lake and Schmidt, 1997). In addition to the fishes listed by Anderson (1988), we included fathead minnow (Pimephales promelas), fallfish (Semotilus corpo-ralis), redfin pickerel (Esox americanus), redbreast sunfish (Lepomis auritus), and tessellated darter (Etheostoma olmstedi). Neither study had a long enough time span to categorize these fishes, other than hypothetically.
With the exception of fallfish, redfin pickerel, and fathead minnow, all the species listed as residents are widespread organisms in the tidal Hudson River and were listed as long term components of the freshwater tidal marsh community (Schmidt, 1986). Fallfish has been encountered in the mouths of several tributaries, notably RES observed (literally tripped over) a fallfish nest in the tidal Saw Kill (RKM 157.5). However, we doubt that they are widespread in Hudson River tributary mouths and same can be said for redfin pickerel. Fathead minnow is an introduced species that appears to be increasing in abundance and distribution in Hudson River tributaries. In recent years, we collected larvae and saw adults in several tributary mouths (Stockport Creek-RKM 194.5, Moordener Kill - RKM 221, Coxsackie Creek- RKM 204.5, and Lattintown Creek - RKM 111).
Therefore, there doesn't seem to be a community of fishes that is unique to the tidal mouth habitat in the Hudson River, but rather an assemblage consisting of American eel, widespread estuarine species, and, in some tributaries, species usually associated with upland habitats. From our observations of the resident fish fauna, we could best characterize the tidal tributary mouths as transitional zones between the tidal Hudson River and true riverine habitats. We would expect to see large variation in the species composition of the fish fauna of such an area when viewed over seasons and over years. We have no such observations to consider, however.
Two of the species that we commonly observe in the tidal tributary mouths are worth discussing briefly. Not surprisingly, seining or gill netting rarely produced American eels. However, in those instances where we have shocked in tributary mouths, we have seen many, often very large, eels. We suspect that the biomass of American eel in tributary mouths is very high and exceeds all other resident species combined. The freshwater subadult eels may reside in tributary mouths for up to perhaps fifteen years (Helfman et al., 1987). They are nocturnal scavengers and predators of fishes and invertebrates (Moriarty, 1978) but we do not know if they feed exclusively in the tributary mouths (in which case they would probably be a highly significant predator in the habitat) or if they venture out into the tidal Hudson River. TRL has observed American eel feeding on alewife eggs and has seen them aggressively interacting with ripe females (inducing them to release eggs?).
Tessellated darter (Etheostoma olmstedi) is common in Hudson River tributary mouths, as it is elsewhere in the Hudson River (Schmidt, 1986). Our seining efforts in Quassaic Creek (RKM 96.5; Lake and Schmidt, 1997) took them regularly and we frequently collected yolk-sac andpost-yolk-sac larvae in the drift from most tributary streams (Schmidt and Lake, 2000). It is unclear whether tessellated darter have a pelagic larval stage or whether the lar-vaeare simply washed down with the current; however, tessellated darter larvae are commonly taken in ichthyoplankton surveys in the main Hudson River (J. Young, ASA Analysis and Communication, personal communication).
Havinglarvae in the drift is reasonable for species whose presence in tributaries is likely to be brief (anadromous and potamodromous species) and where most of the ontogenetic development occurs in the tidal Hudson River. For a small, benthic, relatively sedentary species resident in tributary mouths (which we presume characterizes tessellated darter), drifting larvae could be considered a loss to the population. We do not know how far a given individual larva may drift, or, in fact, how far a captured individual may have drifted at the point of capture, so we have no evidence of whether the drifting darter larvae are leaving a given tributary or not. If these observed larvae are a significant loss to the population, one might expect that there is a mechanism is place that recruits individuals back into the population, one of which maybe upstream migration of juveniles or adults. We have no evidence that such migrations may occur in this species.
Summary. We were able to identify few species of fishes resident in tidal tributary mouths. Those that we did consider as residents are also widespread in freshwater tidal marshes in the Hudson River. The tidal mouths of tributaries may be a very challenging habitat for fishes and we might expect the abiotic conditions to fluctuate dramatically, daily and tidally. Other that the American eel, little is known about the biology or population dynamics of these species in the Hudson River.
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