Anderson et al : Current velocity and catch efficiency in sampling settlement-stage larvae of coral-reef fishes 



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Figure 1 



Location of South Bock Cay (So. Bock) and southeast Lee Stocking Island (S.E. LSI) study sites 

 near the Caribbean Marine Research Center (CMRC), Lee Stocking Island, Bahamas. 



Light traps and channel nets may not only differ in cap- 

 ture success for any particular species, but in the collection 

 of larvae under varying environmental conditions such as 

 current velocity and turbidity. Species (and the ontogenetic 

 stages of these species) can differ in their sensitivity to 

 light cues (Doherty, 1987; Choat et al., 1993), in swimming 

 ability (Stobutzki and Bellwood, 1994; Leis et al., 1996; 

 Stobutzki and Bellwood, 1997; Wolanski et al., 1997), and 

 in the ability of larvae to interact with current velocity. For 

 example, at relatively high current velocities, settlement- 

 stage larvae that are photopositive but that have limited 

 swimming speeds may not have the ability to respond to a 

 light cue and swim into a light trap as they are carried past 

 the sampling device. Conversely, larvae with strong swim- 

 ming abilities may be able to avoid capture by channel nets 

 at lower current velocities. Thus the relationship between 

 the sensitivity to light cues and the ability to respond to 

 such cues by larvae (determined by developmental stage, 

 swimming ability, and current velocity) should determine 

 the relative catch efficiency of these sampling devices. 



Here we compare and contrast the relative number, taxo- 

 nomic composition, and size of settlement-stage fish larvae 

 caught in light traps and in channel nets deployed at two 

 reefs that differ substantially in current velocity. Because 

 these devices collect larvae either actively or passively, we 

 examined 1) whether light traps are more selective and 

 catch fewer taxa (families) than channel nets, 2) whether 

 light traps catch larger settlement-stage larvae than chan- 



nel nets, and 3) with higher current velocity, whether the 

 relative effectiveness of light traps to channel nets de- 

 creases, resulting in fewer taxa and a lower abundance of 

 settlement-stage larvae in light traps (with lower current 

 velocity the opposite is true). The relative abundance and 

 taxonomic composition of larvae caught by these two sam- 

 pling devices, and how they may be modified by current ve- 

 locity, might result in different interpretations concerning 

 both magnitude and variation in larval supply. 



Methods 



Study sites 



We conducted this study using light traps and channel 

 nets deployed each day, from 30 July through 11 August 

 1997 at Lee Stocking Island (LSI), Bahamas, bracketing 

 the new moon phase in the lunar cycle when settlement- 

 stage fish larvae are more abundant (Thorrold et al., 

 1994b). Two fore-reef sites were selected a priori for study 

 based upon our impression of marked differences in cur- 

 rent velocity: South Bock Cay (So. Bock) northwest of LSI 

 and southeast LSI (S.E. LSI), approximately 7 km south of 

 So. Bock. So. Bock is near a channel between two cays and 

 experiences moderate to strong tidally driven currents, 

 whereas S.E. LSI is far from a channel where there is 

 relatively low current velocity (Fig. 1). 



