196 



Fishery Bulletin 97(1), 1999 



month, an average increase of 30.5% species-specific 

 survival was observed in November compared with 

 August. Silver perch (63.6%), gulf pipefish, Syng- 

 nathus scovelli, (48.1%), and gray snapper (35.0%) 

 had the highest survival increases. 



Average air and water temperatures observed were 

 31 and 32°C, respectively, in August, 23 and 24°C, 

 respectively, in November. Salinity averaged 35 ppt 

 during both months. 



Discussion 



Because roller trawls were designed to reduce 

 seagrass fragment collection, it has been assumed 

 that they have minimal impact on seagrass habitat 



(Tabb and Kenny, 1969; Berkeley et al., 1985). We were 

 unable to detect significant trawl impacts on shoot den- 

 sity, structure, or biomass of turtlegrass by intensive 

 short-term ( 18 trawls within three hours ) trawling. This 

 finding supports conjectures by Woodbum et al. ( 1957) 

 and Tabb and Kenny (1969) that roller trawls have 

 minimal impact on turtlegrass habitat. 



However, we did not test for effects of repetitive 

 trawling over turtlegrass beds over a longer period 

 of time. Trawling may cause elevated localized tur- 

 bidity, which could chronically reduce the potential 

 for seagrass to perform photosynthesis (Kenworthy 

 and Haunert, 1991). In contrast, limited trawling in 

 areas with substantial epiphytic growth and numer- 

 ous senescent turtlegrass leaves may enhance light 

 availability by removing old plant parts (Woodbum 



1» 



90 

 80 

 70 



40 

 3& 



zy 



vy 



a 



=»= 



05 



2A 



ao IZA 24.0 36.0 



2.0 



8.0 



24i) 



36J) 



E 

 u 

 O 



November 



D 



aO MB »0 36.0 



0.5 



12.0 



Hours after trawling 



Figure 2 

 lA-D) Cumulative percent mortality in August and November of the abundant (at least 20 individuals per sample month I species 

 collected with shrimp trawls and held in floating pens for 36 h. As noted in the text, handling procedures differed between the two 

 collection periods. Species codes: moj = mojarra, sij = silver jenny, gol = goldspotted killifish, gut = gulf toadfish, gup = gulf 

 pipefish, sil = silver perch, spo = spotted seatrout, pin = pinfish. gra = gray snapper, and pig = pigfish. 



