Cross-shelf gradient of larval fish: importance to platform productivity

Abstract

Between 1995 and 1997, and using plankton nets and light traps, ichthyoplankton were sampled around three offshore platforms off Louisiana. From the Results: “The early life stages of fish collected at the various cross shelf sites closely approximate the adult faunal assemblages of Gallaway (1981). Larval and juvenile stages of coastal pelagic (e. g., engraulids, clupeids, and planktivorous carangids) and demersal/estuarine species (e. g., sciaenids and flatfishes) dominate catches at inner shelf ST54G. Catch per unit effort (CPUE) of reefassociated taxa (e. g., blennies, gobies, and snappers), predatory ocean pelagics (e. g., jacks, blue runner, most tunas and mackerels) and demersal/pelagic fishes (e. g., lizardfish) are greater at G194B than either ST54G or GC 1 8A, with CPUE generally low in all ecological-categories at GC 1 8A. Relatively abundant reef-associated species at G194B include: vermilion snapper (Rhomboplites aurorubens) and snappers of the genus Lutjanus spp., several members of the family Blennidae, and various sea basses and groupers. CPUE differed among sites and gear. Overall, catches were higher in surface light traps fished within rig structure than in traps floated away from the rig, while light traps fished at 20-rn had the lowest CPUE. Occasionally, standardized catches in surface plankton nets were up to 5-10 times higher than in light traps. These high plankton net catches may be due to differences in gear selectivity and/or the differential mortality experienced by the larger specimens selectively captured by light traps. Differences in length frequencies of taxa caught with each gear support this observation. Plankton nets generally collect larvae <6 mm standard length (SL) whereas light traps generally collect fish >5 mm SL. Preliminary analyses suggest that CPUE at ST54G was greater on the full than new moon. At G194B, however, both density estimates from surface plankton nets and CPUE from light traps suggest higher catches during the first quarter moon. However, differences between sites and between moon phases may be taxa-dependent. Data also suggest that CPUE are generally higher when currents are toward the west and north than towards the south and east. Furthermore, CPUE is generally higher before than after midnight.”