438 



Effects of current speed and turbidity 

 on stationary light-trap catches of 

 larval and juvenile fishes 



David C. Lindquist 



Richard F. Shaw 



Coastal Fisheries Institute 



School of the Coast and Environment 



Louisiana State University 



Baton Rouge, Louisiana 70803 



E-mail address (for D C Lindquist): dlindqlig 1 lsu.edu 



Light traps are one of a number 

 of different gears used to sample 

 pelagic larval and juvenile fishes. In 

 contrast to conventional towed nets, 

 light traps primarily collect larger 

 size classes, including settlement-size 

 larvae (Choat et al., 1993; Hickford 

 and Schiel, 1999; Hernandez and 

 Shaw, 2003), and, therefore, have 

 become important tools for discern- 

 ing recruitment dynamics (Sponau- 

 gle and Cowen, 1996; Wilson, 2001). 

 The relative ease with which multiple 

 synoptic light trap samples can be 

 taken means that larval distribu- 

 tion patterns can be mapped with 

 greater spatial resolution (Doherty, 

 1987). Light traps are also useful 

 for sampling shallow or structurally 

 complex habitats where towed nets 

 are ineffective or prohibited (Gregory 

 and Powles, 1985; Brogan, 1994; Her- 

 nandez and Shaw, 2003). 



As with any sampling gear, there 

 are concerns about light trap sam- 

 pling biases and efficiency. Light 

 traps are taxon-selective because 

 they target fishes that are photoposi- 

 tive and able to swim to and enter 

 the trap (Thorrold, 1992; Choat et al. 

 1993; Hernandez and Shaw, 2003), 

 and size-selective because both pho- 

 totactic behavior and swimming abil- 

 ities change during ontogeny (Stea- 

 rns et al., 1994; Fisher et al., 2000). 

 LTnlike conventional towed nets, it is 

 difficult, if not impossible, to quan- 

 tify the volume of water sampled by 

 light traps. This is largely due to ex- 

 ternal, environmental factors such as 

 lunar phases, current speed or water 



clarity, which may have a large im- 

 pact on catch rates (Doherty, 1987; 

 Meekan et al, 2000). 



Few studies have attempted to ad- 

 dress the effects of environmental 

 factors on light trap performance. 

 Catches have been found to be lower 

 during full moons as compared to new 

 moons, either because of the greater 

 ambient illumination interfering with 

 light trap efficiency (Gregory and Pow- 

 les, 1985; Hickford and Schiel, 1999) 

 or because of higher abundances of 

 presettlement fish during the darker 

 lunar phases (Johannes, 1978; Rob- 

 ertson et al., 1988). Thorrold (1992) 

 showed that catches were greater for 

 light traps drifting with the current 

 as compared to traps anchored in the 

 current flow. Anderson et al. (2002) 

 found that anchored light traps were 

 less efficient at a high-current sam- 

 pling site as compared with a low- 

 current sampling site. The latter two 

 studies, however, did not provide any 

 information on catch rates with varia- 

 tion in current speed. The purpose of 

 this study was to assess the relation- 

 ships between catch rates from sta- 

 tionary (anchored or tethered) light 

 traps at offshore petroleum platforms 

 and concurrent measurements of cur- 

 rent speed and turbidity. 



Materials and methods 



Study sites 



Larval and juvenile fishes were col- 

 lected at five oil and gas platforms 



(platforms) in the north-central 

 Gulf of Mexico. These platforms 

 included: Mobil's Green Canyon 18 

 (27°56'37"N, 91°0'45"W; sampled from 

 July 1995- June 1996); Mobil's Grand 

 Isle 94B (28°30'57"N, 90°07'23"W; 

 April-August 1996); Exxon's 

 South Timbalier 54G (28°50'01"N, 

 90°25'00"W; April-September 1997); 

 Santa Fe-Snyder's Main Pass 259A 

 (29 C 19'32"N, 88°01'12"W; May- 

 September 1999); and Murphy 

 Oil's Viosca Knoll 203 (29°46'53"N, 

 88°19'59"W; May-October 2000). All 

 platforms had similar underwater 

 structural complexity, and had well- 

 developed biofouling communities 

 when sampled. 



Sampling procedures 



Sampling procedures have been 

 described in detail elsewhere (Her- 

 nandez and Shaw, 2003) and will be 

 briefly described here. Fish collec- 

 tions were made by using a modified 

 quatrefoil light trap with a Brinkman 

 Starfire II halogen light (250,000 can- 

 dlepower) powered through an umbili- 

 cal by a 12-volt marine battery. Light 

 traps were deployed in surface waters 

 within the platform structure along 

 a stainless-steel guidewire (within- 

 platform light trap), and tethered and 

 floated in surface waters to a distance 

 of 20 m from the down-current side of 

 the platform (off-platform light trap). 

 Light traps were deployed with their 

 lights off, fished with lights on for 

 10-15 min, and retrieved with lights 

 off. 



Sampling was undertaken general- 

 ly twice monthly coincident with new 

 and full moon phases. During each 

 trip, light traps were fished during 

 four to six sets per night, starting 

 at least one hour after sunset and 

 ending at least one hour before sun- 

 rise, over two to three consecutive 

 nights. Each sample set consisted of 

 a within-platform light trap collec- 



Manuscript submitted 4 February 2004 

 to the Scientific Editor's Office. 



Manuscript approved for publication 



1 December 2004 by the Scientific Editor. 



Fish. Bull. 103:438-444 (2005). 



