NEW: Annual Report - George Mason University

110
F. Adult and Juvenile Fish Trends
Trawls
The mean annual catches per trawl at Stations 7 and 10 combined for the 20 years we have
sampled are gathered in Table 13. Annual means are shown for all species combined and
separately for eight typically abundant species. The mean catch of all species combined in 2003
was almost as low as the previous low of 1996. This is shown graphically in Fig. 156a. The
LOWESS diagram (Fig. 156b.) indicates a continued slow increase since 1997, but that trend
appears to be leveling off in 2003.
Most of the decline is apparently caused by the low numbers of YOY and juvenile white perch,
whose average catch per trawl was lower than the previous low of 1989. Since white perch are
known to spawn in Gunston Cove and also in the Potomac River from Alexandria to Aquia
Creek (Lippson et al. 1979), the primary spawning area may just be shifted further upriver or
further downriver. Since 2003 produced higher river discharge events in the spring than is typical
(Fig. 2), the shift is more likely to have been downriver. However, the down trend has persisted
through 4 or 5 years, and the pattern of river discharge has not been regular. The population of 2
year old and older white perch seems to be robust, and I think that we will see a continued
dominance of the white perch population in Gunston Cove in future years. Nevertheless, the
offshore bottom in the upper part of Pohick Bay and Accotink Bay and the shorelines of these
bays are increasingly occupied by submersed aquatic vegetation (mostly Hydrilla), and this may
be less suitable for white perch spawning or use by juveniles. It certainly interferes with
sampling the fishes with either a trawl or a seine, and the presence of thick beds of submersed
aquatic vegetation was noted at Station 10 as early as May 15. Indeed, reduced ability to catch
the white perch that are there may explain part of the decline.
No juvenile or adult gizzard shad were collected in 2003 at all, though their numbers have been
low since 1989. Fewer pumpkinseed were collected, but more bluegill were taken.
On the other hand, the mean catch per trawl of blueback herring was high in 2003, and the mean
catch of alewife, bay anchovy, spottail shiner, and brown bullhead were comparable to most
previous years.
The mean annual catch per trawl at Station 9 in the river channel for all species combined and for
eight typically abundant species are presented in Table 14. Of the species considered at Station 9,
only the bay anchovy catch and perhaps the spottail shiner catch might be judged low, and the
abundance of bay anchovy is usually low in the trawls at Station 9. Larger numbers of white
perch, brown bullhead, channel catfish, tessellated darter, and hogchoker have been caught since
2000. All are known to feed on benthic animals or on other fishes. This might be explained by an
increase in the abundance of benthic animal prey associated with the placement of dredged
sediment in the deep basin in the Potomac channel off Fort Belvoir.