SKINNER: INTERRELATION OF WATER QUALITY. GILL PARASITES, AND GILL PATHOLOGY 



that "sublethal effects of each poison can sum 

 within the individual fish and kill it." Minimal 

 risk, hazard and lethal levels, and median lethal 

 concentrations (LC50, the concentration that kills 

 50% of the test organisms in 96 h) of certain pollu- 

 tants in the marine environment are published 

 by the National Academy of Sciences Environ- 

 mental Studies Board (1972). Although both the 

 National Academy of Sciences and conservation 

 organizations have emphasized the need for eco- 

 logical information on long-term effects of pesti- 

 cides on wildlife at sublethal doses, most field 

 studies are done after the animals have been 

 found dead. Mitrovic (1972) asked for studies of 

 subtle damage resulting from long-term expo- 

 sure at subacute levels. Local studies are needed, 

 since environmental conditions vary with loca- 

 tion, and temperature, salinity, and pH play a 

 part in the toxicity of poisons (Trussel 1972). 

 Subtle indications of damage, according to John- 

 son (1968), may be a change in behavior caused 

 by lowered efficiency of the organism. He sug- 

 gested that the illustration of physiological and 

 ecological effects of sublethal quantities of envi- 

 ronmental pollutants will lead to a more realistic 

 view in establishing tolerance levels for all toxic 

 pollutants. This requires year-round monitoring 

 to take into consideration seasonal variation, 

 variation in drainage as a result of precipitation, 

 runoff, and irrigation, as well as fluctuating 

 physical or chemical factors. 



MATERIALS AND METHODS 



The study period extended from May 1975 to 

 August 1976. The three host species were yellow- 

 fin mojarra, Gerres cinereus (Walbaum), a bot- 

 tom feeder; gray snapper, Lutjanus griseus (Lin- 

 naeus), a predator; and timucu (needlefish), 

 Strongylura timucu (Walbaum), a surface feeder, 

 since they were available on both sides of south 

 Biscayne Bay and remained in one locality for 

 extended periods (Cervigon 1966; Randall 1968; 

 Cressey and Collette 1971; Starck 1971). 



Collection stations for the fish were the mouths 

 of Black Creek, Moody, and Mowry Canals; the 

 Arsenicker Keys; Elliott Key; and a canal in 

 Sands Key (Fig. 1). South Florida Water Man- 

 agement District maintains salinity control 

 structures a short distance inland from the south- 

 western shoreline of Biscayne Bay at Black 

 Creek, Moody, and Mowry Canals. Directly up- 

 stream from the gates the water is brackish. The 

 flood gates open automatically according to the 



difference in the water level on both sides when 

 the water exceeds a certain height on the upland 

 side. For many months during the study period 

 the gates of the salinity structures remained 

 closed because of the low freshwater table inland 

 and the danger of saltwater intrusion into inland 

 wells. Fish were collected downstream from the 

 salinity control structures near the entrances of 

 Moody and Mowry Canals into the bay, at the 

 confluence of Black Creek and Goulds Canals 

 where they enter the bay, in mangrove creeks 

 and close to shore at Arsenicker and Elliott Keys, 

 and in a manmade canal and lagoon in the inte- 

 rior of Sands Key. 



Collections were made between April 1975 and 

 August 1976 during three to five trips per week, 

 depending upon weather conditions. The total 

 number of fish collected was 356, of which 186 

 were from the southwest locations and 170 from 

 southeast Biscayne Bay (Table 1). Only one spe- 

 cies was collected on a given day from one area to 

 prevent exchange of parasites from one host spe- 

 cies to the other. The yellowfin mojarra were 

 caught by gill net, 75 mm mesh size (stretch), and 

 occasionally on hook and line; the gray snapper 

 on hook and line; and the timucu (needlefish) on 

 hook and line, and by beach seine. Collection 

 trips to the stations were alternated regularly, 

 depending on weather conditions and need. Be- 

 cause of the gear used, size ranges of fish were 

 the same in both localities. Sex ratios were simi- 

 lar, with an average of 52% males and 48% fe- 

 males. 



Fish were collected at depths between 0.5 m 

 and 2.5 m. Water samples for the salinity read- 

 ings were obtained from depths of 0.3 m, 1.0 m, 

 and 3.0 m. An average of 2.5 salinity measure- 

 ments per month were made at each station and 

 each depth. To avoid contamination a closed, 

 weighted plastic bottle was lowered to the 

 desired depth where it opened and filled with 

 water. Additional salinity data for the years 1975 

 and 1976 from Black Creek, Mowry, and Moody 

 Canals downstream from salinity structures 

 were made available by the U.S. Geological Sur- 

 vey (USGS) and South Florida Water Manage- 



Table 1.— Numbers of fish hosts collected in the southeast and 

 southwest locations in Biscayne Bay, Fla., between May 1975 

 and August 1976. 



S E Biscayne Bay 



S W Biscayne Bay 



Gerres cinereus 

 Lut/anus griseus 

 Strongylura timucu 



Total 



69 



57 

 44 



170 



52 

 80 



54 



186 



271 



