Complexity affects habitat preference and predation mortality in postlarval Penaeus plebejus: implications for stock enhancement

Abstract

Global attempts to offset declines in fishery populations through stock enhancement have had varied levels of success due to the absence of preliminary studies to determine which habitats best support release species and the mechanisms controlling their distribution. Habitat preference was examined as a possible mechanism driving distribution of postlarval Penaeus plebejus, a current candidate prawn for stock enhancement in Australia. Occupancy of complex (artificial macrophyte) and simple (bare sand and mud) habitats by postlarvae was compared in the presence and absence of a choice between the habitats. Predation mortality was also compared amongst these habitats. P. plebejus settled into the different habitats randomly during the night, but actively selected macrophyte over the simple habitats during the day. Mortality caused by the predatory fishes Centropogan australis and Acanthopagrus australis was higher in simple habitats than in complex habitats, but was similar across habitats when large penaeid prawns, Metapenaeus macleayi (which are tactile rather than visual feeders), were used as predators. Postlarvae may select macrophyte habitats during the day to lower predation risk, but because nighttime foraging efficiency is reduced in their predators, which are primarily visual hunters, this may preclude the need of postlarvae to obtain shelter in macrophyte habitats at night. Predation mortality of stocked P. plebejus may be minimized by releasing postlarvae directly into macrophyte habitats. Studies such as these must precede all stock enhancement attempts because they identify optimal release strategies and allow ecological and financial costs of enhancement to be weighed against projected benefits, and thereby assess the practicality of enhancement as a management option.

Spatial and temporal variability in the condition of postlarval and juvenile Penaeus plebejus sampled from a population subjected to pilot releases

“…Although there is no definitive information on the maximum distance that postlarval and small juvenile Penaeus plebejus can traverse within nursery grounds over short periods of time, a previous 16 mo study reported that only 23.1% of postlarval P. plebejus were caught beyond ~1 km from the sea within an estuary (Coles & Greenwood 1983) and previous work has shown that once this species' postlarvae have settled within a preferred habitat during the day, they will rarely move out of this habitat to adjacent alternative habitats over a 12 h period (Ochwada et al 2009). Bare and macrophytic habitats in the present study were separated by ~1 km.…”

“…Together with the observed significant difference in condition between habitats, the spatial separation of the habitats suggests that the different habitats examined within Wallagoot Lake were sufficiently independent. However, the work by Ochwada et al (2009) did show that postlarval P. plebejus forage between different adjacent habitats during the night and thus may be able to traverse large distances during the night time. Future studies in estuaries with larger (> 5 km) spatial separation between habitats or pilot releases into mesocosms that can eliminate the possibility of inter-habitat movement are therefore necessary to facilitate a more thorough comparison of the condition of P. plebejus released directly into macrophytic habitat with that of individuals released into bare habitats.…”

Competition between wild and captive-bred Penaeus plebejus and implications for stock enhancement

“…Since epiphytes contribute significantly to the juvenile penaeid diet (Kitting et al 1984, Dall et al 1990b, competition for the epiphytes present in our competition for food experiment may have led to higher mortality rates in both wild and captive-bred juveniles when densities were increased. The structural complexity of macrophytes can also limit predation mortality for penaeids by providing refuge or by reducing a predator's foraging efficiency (Primavera 1997, Ochwada et al 2009). Competition for shelter may have, therefore, similarly increased at higher densities, leading to predation mortality for ineffective competitors.…”

“…Macrophytes and their epiphytes have been shown to contribute to the juvenile penaeid diet (Kitting et al 1984. Experimental work has also shown that predation by fish on postlarval and juvenile P. plebejus in estuaries is significantly lower in macrophyte habitats compared to bare areas (Ochwada et al 2009, Ochwada-Doyle et al 2010. Once maturing juveniles reach a mean (± SE) CL of between 1.90 ± 0.01 cm and 2.84 ± 0.02 cm (total length, TL, ~7.6 to 11.68 cm), they begin to emigrate from estuarine waters and move northward along the coast towards the species' spawning grounds (Ruello 1975).…”

“…Taylor 2010). Like other penaeids (Minello & Zimmerman 1985, Skilleter et al 2005, this species has mainly been shown to exhibit a strong association with macrophytic habitat during the nursery phase of its life cycle (Young 1975, 1978, Ochwada et al 2009). This penaeid−macrophyte asso ciation has usually been attributed to the abundance of feeding resources and shelter from predators available within macrophytes (Kitting et al 1984, Kenyon et al 1995, Liu & Loneragan 1997, OchwadaDoyle et al 2010.…”