Abundance of fish and crustacean postlarvae on portable artificial seagrass units: daily sampling provides quantitative estimates of the settlement of new recruits

Artificial collectors and seagrass units have mainly provided qualitative s amples of epifaunal abundance or have been difficult and time-consuming to sample. Consequently, they are useful for distinguishing temporal or spatia l trends in abundance or they are deployed for several weeks and, as a resu lt, the quantitative samples are cumulative. We developed a portable artifi cial seagrass unit (ASU) with buoyant plastic artificial seagrass (47 cm lo ng by 15 mm wide strips) that can be retrieved, harvested and re-deployed w ith 98% catch efficiency in about 5 min by two people from a small boat. Th ey can quickly and easily quantify settlement:of crustacean and fish postla rvae over tidal or deil periods. When set for 24 h, postlarvae settled from the plankton during the night and their abundance is the result of a disti nct settlement event. When set for longer periods, the numbers of postlarva e may, represent several settlement events and post-settlement activities. Crustacean and fish postlarvae and juveniles used ASUs deployed within seag rass in a similar way to natural seagrass. Estimates of juvenile tiger praw n abundance from beam-trawl catches showed similar densities in natural sea grass (2.93 Penaeus semisulcatus de Haan m(-2) day) to those in the ASUs (2 .40 P. semisulcatus m(-2) day(-1)); their density was significantly lower o n bare trays (0.48 P. semisulcatus m(-2) day(-1)). When deployed on bare ar eas, more epifaunal crustacean postlarvae were collected from the ASUs (e.g . Portunus pelagicus Linnaeus, 1.21 m(-2) day(-1); caridean shrimp 4.03 m(- 2) day(-1)) than from the bare trays (e.g. P. pelagicus, 0.46 m(-2) day(-1) ; caridean shrimp 0.78 m(-2) day(-1)). However, greater abundances of the p ostlarvae of other crustacean taxa were collected from the bare trays than the ASUs (e.,a. Sergestes spp. 1.21 and 0.31 m-2 day(-1); tiger prawn postl arvae 0.15 and 0.06 m(-2) day(-1), respectively). Sampling with portable AS Us allows settlement to be assessed temporally (e.g,, daily or tidally) or spatially (e.g., distinct areas affected by different current regimes). The strength of settlement can be used to evaluate the productivity of nursery habitat for fishery populations. (C) 1999 Elsevier Science BN. All rights reserved.