Higher Trophic Levels

Juvenile Fish Populations in Florida Bay

Florida Bay Project Profile - 55

---

Question: How do today's populations of juvenile fish, including seatrout, compare with those found prior to the seagrass die-offs which began in western Florida Bay in the early 1990's?

Background/Project: Widespread losses of seagrasses which began in the late 1980's in Florida Bay were accompanied by microalgal blooms and increased turbidity from suspended bottom sediments. These events may change the way the bay functions as a nursery area for fishes, particularly spotted seatrout, Cynoscion nebu-losus. Spotted seatrout, whose prime spawning habitat is located in western Florida Bay where seagrass losses have been significant, spends its entire life in estuarine waters. To determine whether these disturbances affected seatrout populations and other resident fishes and invertebrates, data on the abundance and distribution of juvenile and planktonic larval fishes (ichthyoplank-ton) collected in 1984-85 prior to seagrass die-offs were compared with similar data from 1994 through 1996.

Findings: There were only minor changes in the total abundance of juvenile and small adult fishes collected by trawls. However, the species assemblage —the kinds of fish inhabiting the regions of the bay changed during the past decade. Similar observations were made by other investigators sampling in Johnson Key basin and several carbonate mudbanks in the bay. In the 1980's, the fish community was dominated by resident benthic (bottom-dwelling) and epibenthic (just above the bottom) fish such as rainwater killifish and mojarra. In contrast, in the 1990's, the bay anchovy, a pelagic (open-water) planktivore, dominated the fish community. This shift from benthic species to pelagic species may be related to the loss of seagrass cover, changes in the food web that have resulted in an increased supply of zooplankton to feed anchovies or other planktivorous fish, or to modifications in some water quality parameter. Results from ichthyoplankton sampling also demonstrate an increase in anchovy larvae. Collection of trout larvae from 1994-1996, indicated that spotted seatrout, which typically spawn in the western portion of Florida Bay near Cape Sable and in nearby estuarine waters where salinity levels reach 20 ppt., have expanded their spawning and nursery range into the central subdivision of Florida Bay, especially in Whipray Basin. Juvenile surveys by the FMRI and NOAA researchers confirmed this range expansion. Data obtained from trawls conducted in 1997 show species composition of juvenile/small fishes similar to observations prior to 1990. That is, there has been a shift back toward more killifish and mojarra with a corresponding decrease in bay anchovy. However, the total abundance values for all fishes is low relative to the 1984-1985 values.

Status: This comparative decadal project is complete, but monitoring of the distribution and abundance of juvenile and small resident fishes continues. Studies have been initiated that examine the growth of snappers and barracudas, and the growth and metabolism of spotted seatrout relative to environmental factors, particularly salinity.

Restoration Impacts: Population levels of recreational fish and other species that use the bay as a nursery area may be influenced by the restoration of freshwater surface flows into the Everglades and eventually Florida Bay.

Funding Sources: NOAA Beaufort Laboratory; NOAA Center for Sponsored Coastal Research; NOAA National Marine Fisheries Service Southeast Fisheries Science Center; United States Geological Survey; National Park Service; Florida Marine Research Institute

Related Project Profiles:  

• FBPP 32      • FBPP 52