Trawling Surveys at ANERR

Historically, Apalachicola Bay has been a nursery habitat for several commercially and recreationally important fish and invertebrate species. Recently, concerns over the reallocation of upstream waters from the Apalachicola River have led scientists to evaluate the biomass and abundance of various plant, invertebrate and vertebrate species from within the river, bay and surrounding lands. This study, begun in July 2000, attempts to recreate a trawling program during the 1970s. The goals are to:

1. Describe the major trends in the spatial and temporal distribution of major fish and invertebrate species within the Apalachicola Bay System.
2. Relate the occurrence, abundance and seasonality of fish and invertebrate species to natural environmental variation such as temperature and salinity regimes and periodic events such as storms or changes in river flow.

Monthly trawling is performed at 12 sites within the boundaries of ANERR, including St. George Sound, East Bay and St. Vincent Sound. Typically, trawling is performed over a two-day period during the middle of the month. Five 2-minute tows are performed at each site with a 16ft (4.8m) otter trawl. Fish and benthic macroinvertebrates are identified to species, the first 20 are measured and all are counted. Physical and chemical water parameters are measured using a handheld YSI 85 at each station and an adjacent datalogger provides continuous (30-minute) data on temperature, specific conductivity, dissolved oxygen, pH, turbidity and depth. Rainfall data are taken from a weather station located adjacent to East Bay.

Monthly Trawling Survey
Two areas of the Apalachicola Estuary System are subject to different salinity regimes due to freshwater and saltwater inputs. East Bay is predominantly influenced by the distributaries of the Apalachicola River and terrestrial runoff. The Mid Bay area, while under the influence of the river, is equally subject to cyclical tides and movements of higher salinity water from St. George Sound. Temperature and salinity are two major driving forces in the distribution of most estuarine species because they are the most dynamic.

In an estuary such as Apalachicola Bay with shallow waters and riverine input, salinity is more variable than temperature on a spatial scale. Even though salinity may be the most important factor, it is certainly not the only factor affecting fish distribution. This study shows that two areas of the estuary have similar water temperature regimes but very different salinity regimes. Higher catch per unit effort of fish corresponded with lower salinity within East Bay. Although this suggests that salinity may be a driving force in fish distribution, it is important to consider other factors such as habitat. These areas, in fact, have very similar characteristics, including soft bottom sediments and lack of submerged aquatic vegetation; however, the East Bay stations are located closer to brackish marshes.

Little is known about the availability of food, the pressures of predation and other physical constraints such as current. All of these factors have an effect on the abundance and distribution of these species. The success of an individual is dependent on the totality of the environment in which it resides. It is clear that river flow has an immense effect on the salinity of East Bay; however, it is not clear how events such as high river flow and low river flow due to drought conditions may affect the distribution of the fish species.