Abundance and age-specific growth rates in relation to population densities of Fundulus heteroclitus in Waquoit Bay estuaries subject to different nitrogen loads.
F. heteroclitus was chosen for this study for three reasons. First, it is the most abundant fish in Waquoit Bay (2). Second, food availability, and the related issue of population density, limit its growth rates (3). Third, it has a relatively small annual home range (4, 5) so that populations are likely to remain within a single estuary during their life spans.
Fish were sampled in four estuaries of Waquoit Bay with different nitrogen loading: Sage Lot Pond (9 kg N [ha.sup.-1] [y.sup.-1]), Hamblin Pond (30 kg N [ha.sup.-1] [y.sup.-1]), Quashnet River (300 kg N [ha.sup.-1] [y.sup.-1]), and Childs River (475 kg N [ha.sup.-1] [y.sup.-1]). We performed seine tows 10 m along the shore with a 5-m-wide seine net at five sites within each estuary, and measured standard lengths of the F. heteroclitus caught. To age the fish, we collected scales on the lateral line, behind the operculum (6).
Abundance of F heteroclitus increased as nitrogen loading increased (regression, F = 10.71, P [is less than] 0.01) (Fig. 1, numbers on top left of top four panels, and Fig. 1 bottom left). Although the population density of F. heteroclitus increased with nitrogen loading, the median size of the fish did not vary (regression, F = 0.61, P = 0.44) [data not shown].
We calculated growth rates of each of the four year classes of F. heteroclitus using the modal length of each age class found from length-frequency histograms (Fig. 1, top four graphs) and standard-deviate determination of age cohorts (7). For example, the growth rate for age 2 was determined by subtracting the modal length of age 1 from the modal length of age 2. Growth rates of the four age classes of F. heteroclitus were variable, but appeared to be somewhat lower -- or at least did not increase -- as nitrogen-loading rates increased (fig. 1, bottom right).
The data suggest that there are more F heteroclitus in the more nitrogen-loaded estuaries, even though surveys in Waquoit Bay show that abundance of potential prey in estuaries subject to high nitrogen loads is half that of unenriched estuaries (J. McClelland, Boston University Marine Program, pers. comm.). For F. heteroclitus, however, the lower prey densities must still be sufficient to sustain -- or only slightly reduce -- growth rates. Thus the populations of F. heteroclitus in Waquoit Bay do not seem to be food limited. We lack information about what determines abundance of this species in Waquoit Bay, but one possible explanation for its increased abundance in estuaries subject to higher nitrogen-loading rates may be that higher nitrogen loads somehow lead to increased recruitment or immigration. This possibility needs further examination.
This work was supported by funds from a grant from the NSF Research Experience for Undergraduates Program (E.L.) and the Woods Hole Marine Science Consortium (C.F., J.T.).
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