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Fishery Bulletin 97(1), 1999 



sible explanation for perceived differences in trawl- 

 induced mortality between small and large size-class 

 fish may be attributed to larger fish having a greater 

 proportion of muscle tissue and larger energy stores 

 than smaller fish; the larger fish are thus able to 

 avoid net contact during prolonged trawl tows, (Fritz 

 and Johnson, 1987). This may cause small fish to be 

 more susceptible than large fish to stress and mor- 

 tality due to capture. Consequently, high mortality 

 of small individuals, directly or indirectly due to 

 trawling, could potentially reduce the local stock of 

 juvenile fish. This reduction in juvenile fish in turn 

 could reduce the reproductive potential of a species 

 and alter species diversity and composition within 

 affected areas (Wassenberg and Hill, 1989). 



Greater survival during November was most evi- 

 dent for silver perch, gulf pipefish, and gray snap- 

 per. However, we were not able to assess whether 

 observed increased survival was the result of changes 

 in handling procedures that we instituted (culling 

 versus no culling), or to seasonal factors, such as dif- 

 ferences in water temperature as noted by Jean 

 (1963), between our survey months. The 2-min cull- 

 ing time and high August air temperature may have 

 substantially stressed the fish in our study. Also, the 



higher water temperature in August may have re- 

 duced the ability offish to recuperate from handling 

 because of increased respiratory demand ( Alderdice, 

 1963; Bond, 1979) and lower ambient dissolved oxy- 

 gen levels (Raymont, 1980). However, even with the 

 changes in handling instituted in November to re- 

 duce stress, net injury alone appeared to cause sub- 

 stantial mortality in mojarra, spotted seatrout, sil- 

 ver perch, and goldspotted killifish, Floridichthys 

 carpio. Mean size of fish caught was larger in No- 

 vember than in August; this too may have contrib- 

 uted to the higher survival observed. 



Fish abundance per holding pen was not recorded 

 until the end of the 36-h mortality observation pe- 

 riod. Because of this, and the fact that predation of 

 live and dead individuals within the pens was not 

 measured, the mortality for smaller species or 

 smaller individuals of a species may be conservative. 

 However, the initial confinement may have also en- 

 hanced the survival of injured or disoriented fish by 

 reducing predation that might have been experienced 

 in an open system. Even if bycatch organisms sur- 

 vive trawling and culling they may be quite suscep- 

 tible to predation. High postrelease predation on 

 bycatch has been observed by prawn trawlers (Hill 

 and Wassenberg, 1990), and this fate probably awaits 

 much of the bycatch overboarded during bait-shrimp 

 trawling. Many bait-shrimp boat captains have re- 

 ported schools of hardhead catfish following boats 

 and feeding on the bycatch as it is thrown overboard. 

 We similarly observed numerous hardhead catfish 

 and other predatory fish following our boats and feed- 

 ing on bycatch, and numerous such predatory indi- 

 viduals were collected during our sampling. 



Trawl-induced mortality may occur past our 36-h 

 observation period. Numerous fish still alive after 

 the 36-h observation period had missing caudal fins 

 or ulcers, or both. Such fish are were susceptible to 

 subsequent mortality through predation or infection, 

 or both. Moreover, individuals recaptured and culled 

 again in heavily exploited areas could suffer high 

 mortality rates because of cumulative stress. To un- 

 derstand better the effects of this fishery on the resi- 

 dent fauna, we need additional evaluation of the ef- 

 fects of temperature on mortality; the consequences 

 of postrelease predation, delayed mortality due to 

 initial sublethal damage, and subsequent infection; 

 and finally an evaluation of the potential for recap- 

 ture of bycatch fish. 



Acknowledgments 



We thank D. Colby, M. Hall, M. Durako, and S. 

 Kennedy for advice and guidance on the study de- 



