Effects of freshwater 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, Florida, USA, to compare the species composition and structure of juvenile fish assemblages found near the mouths of freshwater flood control canals with those in similar areas with relatively stable salinity regimes. Water temperature, salinity, dissolved oxygen and depth measurements were recorded during fish sampling and bottom vegetation was also quantified. The survey yielded a total of 38134 individuals from 95 taxa. Fish species composition was similar among sites, but more species were collected from stable-versus variable-salinity areas. Mean fish abundance and the mean abundances of Eucinostomus gula, Lagodon rhomboides, 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 richness and the mean abundances of Lucania parva, Haemulon sciurus, H. plumieri, 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 relatively 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% mortality rates, respectively. Mortality was 100% for the relatively uncommon C. nebulosus and C. variegatus. Results suggest that the differential osmoregulatory abilities of the species tested may underlie some, but not all, of the structural differences observed between fish assemblages from stable-salinity habitats versus those adjacent to freshwater canals.