EFFECTS OF FRESH-WATER CANAL DISCHARGE ON FISH ASSEMBLAGES IN A SUBTROPICAL BAY - FIELD AND LABORATORY OBSERVATIONS

A 14 mo trawl survey was conducted at 8 study sites in Biscayne Bay, F lorida, USA, to compare the species composition and structure of juven ile fish assemblages found near the mouths of freshwater flood control canals with those in similar areas with relatively stable salinity re gimes. Water temperature, salinity, dissolved oxygen and depth measure ments were recorded during fish sampling and bottom vegetation was als o quantified. The survey yielded a total of 38134 individuals from 95 taxa. Fish species composition was similar among sites, but more speci es were collected from stable- versus variable-salinity areas. Mean fi sh abundance and the mean abundances of Eucinostomus gula, Lagodon rho mboides, Opsanus beta and Lutjanus griseus shared a general pattern of increase from north to south, with highest values occurring at one or more of the canal-influenced sites. In contrast, mean species richnes s and the mean abundances of Lucania parva, Haemulon sciurus, H. plumi eri, and H. parra were significantly greater at stable-salinity sites than at variable-salinity sites. Freshwater challenge experiments were then conducted on each of the fishes above, as well as on 2 relativel y uncommon species, Cynoscion nebulosus and Cyprinodon variegatus. The mortality of groups exposed to a single, rapid, freshwater pulse (i.e . salinity was changed from approximately 32 ppt to 0 to 32 ppt over 2 h) was compared with that of controls. Of the 8 fishes that dominated the nearshore habitats of Biscayne Bay, 5 exhibited no mortality and L. rhomboides, L. parva, and H. plumieri exhibited 12.5, 50 and 100% m ortality rates, respectively. Mortality was 100% for the relatively un common C. nebulosus and C. variegatus. Results suggest that the differ ential osmoregulatory abilities of the species tested may underlie som e, but not all, of the structural differences observed between fish as semblages from stable-salinity habitats versus those adjacent to fresh water canals.