The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
xanthurus) red drum (Sciaenops ocellatus), sand seatrout (Cynoscion arenarius), snook
(Centropomus undecimalis) and mullet (Mugil cephalus). The numerically dominant bay
anchovy (Anchoa mitchilli) can be considered a transient in the lower river as adults are
more abundant in the bay, but all life stages are found within the lower river.
Various resident and transient species show distributional patterns and areas of peak
abundance in the tidal river. WAR/SDI reported that the numerically dominant Menidia
berrylina and a number of other species were most abundant in mid-river areas. As
previously described, freshwater species increased toward the dam, while a number of
estuarine species (Funduluds similes, Pogonias cromis, Sciaenops ocellatus, Mugil
cephalus and Cynoscion arenarius) increased toward the mouth of the river. PBSJ
(2003) calculated center of abundance (COA) values to represent the kilometer location
of peak density to describe distributional patterns for each species. Many species had
COA values in the upper part of the river. For example, of the seven most abundant
taxa collected, five had COA values above 9 kilometers. However, the COA for most
abundant species in the river (Anchoa mitchilli) was further downstream at kilometer 5.4
Studies from other tributaries to Tampa Bay that are less altered than the Hillsborough
River demonstrate that many estuarine transient fish species enter tidal rivers during
larval and juvenile stages and concentrate in low and mid-salinity waters (Peebles and
Flannery 1992, Peebles 2002). Freshwater inflows and the presence of low salinity
waters influence the presence and distribution of fish species within the lower
Hillsborough River. WAR/SDI pointed out that the Hillsborough River contained more
freshwater and estuarine fish species compared to the Palm River, where salinity values
were higher and the fauna was more marine.
It is the conclusion of this report, that by exerting a pronounced effect on salinity
distributions, flows from Sulphur Springs are likely a significant factor affecting fish
distributions in the lower river, particularly in the dry season. During a study period that
was unusually dry, PBSJ (2003, page 4-31) remarked that fish density was significantly
greater in the upper river, specifically near kilometer 14, just upstream of Sulphur
Springs. They suggested that the location of peak density might have been related to
the flow of water from the spring, as well as the availability of habitat that is suitable for
sampling.
Low dissolved oxygen concentrations also affect fish distribution and abundance in the
lower river. Based on an assessment of early life stages, WAR/SDI found that the Lower
Hillsborough had poor recruitment into the larval and juvenile age classes compared to
the Little Manatee River. These are stages during which many species become oriented
to bottom substrates and benthic feeding. They suggested that hypoxia in bottom
waters was a factor in this poor recruitment, either by its direct effects on the early life
stages or by reducing the density of fish food organisms in bottom waters. By affecting
density stratification in the river reaches near the spring, manipulation of flows from
Sulphur Springs could have a localized effect on fish populations in the lower river
through its interactions with dissolved oxygen.
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