Baum et a\ : Bycatch of Hippocampus erectus in a Gulf of Mexico sfirimp trawl fisfiery 



729 



Indirect effects of the trawl fishery on mortality 



Direct immediate mortality from trawling and culling was 

 rather low, probably in part because trawl sets were of very 

 short duration in this live-bait fishery More importantly, 

 most H. erectus caught in the Hernando Beach fishery were 

 returned to sea, rather than retained as in some other Flor- 

 ida trawl fisheries (Vincent, pers. obs.). Indirect impacts of 

 the fishery may, however, be considerable. 



Seahorses caught in trawls may experience high postre- 

 lease mortality. A study in the live-bait shrimp trawl fishery 

 in Tampa Bay Florida (Meyer et al, 1999), found that only 

 one of four of the congeneric dwarf seahorse (H. zosterae) 

 caught as bycatch were alive in the holding tank of seawater 

 36 hours after collection (Meyer^). Such tows lasted only 5 

 minutes (Meyer et al., 1999); therefore trawl-induced mor- 

 tality could be greater in the Hernando Beach fishery (with 

 trawls of 30-50 minutes), although H. erectus are larger and 

 perhaps more robust than H. zosterae. Like other discarded 

 bycatch, seahorses may also be subject to intense predation 

 upon release. Predation on discarded fish has been observed 

 on prawn trawlers in Australia (Hill and Wassenberg, 1990) 

 and within the bait shrimp fishery of Tampa Bay ( Meyer et 

 al., 1999). Captains of bait-shrimp boats concurred that this 

 is commonplace in the Hernando Beach fishery, and we fre- 

 quently observed bottlenosed dolphins (Tursiops truncatus) 

 and schools of ladyfish (Elops saurus) swimming alongside 

 the boats, feeding on discarded bycatch. 



Trawling may significantly disrupt seahorse populations, 

 particularly if they are spatially structured, as the present 

 study suggests. The disproportionate removal of females 

 could reduce mating opportunities, especially if//, erectus 

 is monogamous, as are most studied seahorse species (e.g. 

 Vincent, 1995; Vincent and Sadler, 1995; Kvarnemo et al., 

 2000; Perante et al., 2002). Trawling, on account of re- 

 peated intrusion onto breeding grounds, could also disrupt 

 courtship and negatively affect reproduction, In heavily 

 exploited areas of the fishery where fishermen repeatedly 

 trawl productive areas, seahorses may face cumulative 

 stress. For example, tail injuries are likely a serious wound 

 for seahorses, given that their tails are essential to grasp 

 holdfasts and may play a key role in mating competition, as 

 they do with Hippocampus fuscus (Vincent, 1994). 



Benthic habitat degradation is another potential indirect 

 effect of live-bait shrimp trawling on seahorses. Bottom-fish- 

 ing gear can reduce habitat complexity by removing emer- 

 gent epifauna, smoothing sedimentary bedforms and by re- 

 moving structure-forming species such as corals and sponges 

 (Hutchings, 1990; Auster et al., 1996; Auster and Langton, 

 1999; Thrush and Dayton, 2002), Roller beam trawls also 

 affect habitat complexity by redistributing macroalgae and 

 seagrass (Meyer et al., 1999). We estimated that seagrasses 

 comprised between 50% and 80% of the volume of the catch 

 for each trawling operation. Although roller beam trawls are 

 assumed to have low impact on seagrass habitat (Tabb and 

 Kenny, 1969; Meyer et al., 1999), the effects of long-term re- 



petitive trawling have not been tested, and it is possible that 

 species composition and abundance, including that ofH. erec- 

 tus, have been adversely affected (Watling and Norse, 1998). 



Summary 



Despite the relatively low direct mortality of seahorse per 

 boat, the live-bait trawl fishery has the potential to affect 

 seahorse populations, both directly and indirectly. The key 

 question is whether the level of exploitation, and subse- 

 quent impacts, represents a conservation concern. Our 

 evidence is inconclusive. Perhaps only the skewed sex ratio 

 and low proportion of reproductively active males suggest 

 a potential problem. However, fishermen have consistently 

 reported that seahorse catch per boat has declined greatly 

 over the past two decades in this area. Effects of trawling 

 are also almost certainly greater in food shrimp trawl fish- 

 eries, which trawl with much larger gear for longer peri- 

 ods, and obtain substantially more bycatch, with higher 

 mortality. Our analysis should thus be seen as a first step 

 in identifying areas for which more information is needed, 

 specifically estimating abundance and fishing mortality, 

 and understanding spatial structuring in H. erectus. 



This paper focuses attention on the need for research on 

 and monitoring of small fishes that may be affected by non- 

 selective fishing gear Management responses to minimize 

 bycatch have focused primarily on seabirds, sea turtles, 

 and commercially important finfishes, but trawl fisheries 

 may also have significant impacts on the many small ma- 

 rine organisms obtained as bycatch, even if they comprise 

 only a small proportion of the bycatch. Bycatch excluder 

 devices are unlikely to be effective in reducing incidental 

 catches of these species. Temporal variation in CPUE and 

 spatial population structuring, as observed in our present 

 study for H. erectus, suggest that time-area closures may be 

 a pragmatic solution for reducing incidental catch. 



Acknowledgments 



This paper is a contribution from Project Seahorse. We 

 thank Jana Schulz for her assistance with fieldwork, Daniel 

 and Patricia Mohr for their support during fieldwork, James 

 Boxall for preparing the map, A. DeBruyn, L. Crowder M. 

 Kaiser, and an anonymous reviewer for providing helpful 

 comments on an earlier draft of this manuscript. This study 

 would not have been possible without the cooperation and 

 support of many of the shrimp boat captains and crew in 

 Hernando Beach. This research was funded through an 

 NSERC summer undergraduate award to JKB, support 

 from the Community Fund (UK) and Guylian Chocolates 

 Belgium for JJM, and an NSERC operating grant to ACJV. 



Literature cited 



Meyer, D. 1999. Personal commun. NOAA Center for 

 Coastal Fisheries and Habitat Research, Beaufort Laboratory, 

 101 Fivers Island Road, Beaufort, NC 28516. 



Adams, C, S. Larkin, and D. Lee. 



2001. Volume and value of marine ornamentals collected in 

 Florida, 1990-98. Aquar Sci. Conserv. 3(l-3):25-36. 



