Effects of the Southern brown shrimp, Penaeus subtilis, predation and artificial feeding on the population dynamics of benthic polychaetes in tropical pond enclosures

Abstract

In less intensive shrimp culture systems, polychaetes are known to be a major food item of several penaeid species. Under these conditions, very little is known about their population dynamics and the possible interactions with penaeid predation and artificial feeding. This work examined the effects of Penaeus subtilis, predation, stocking density and supplemental feeding on the population dynamics of polychaetes in pond enclosures. The work was conducted in a shrimp pond, where forty-five 9-m2 open-bottom enclosures were built. The study consisted of four treatments: enclosures with feed and shrimp (i.e., FS); enclosures without feed and shrimp (NFNS, control); enclosures with shrimp and no feed (NFS); and, enclosures with feed, but no shrimp (FNS). For enclosures with shrimp, four initial shrimp stocking densities were used (i.e., 5, 10, 15 and 20 shrimp/m2). Benthic polychaetes were collected 2 days prior to shrimp stocking and were sampled every 10 days over the complete rearing cycle. A total of 1,631 substrate samples of 20.43 cm2 containing 20,283 polychaetes were collected for analysis. Overall, polychaete density and dry biomass over the production cycle varied from 956 to 11,921 polychaetes/m2 and 1.17 to 2.58 g/m2, respectively. Six polychaete families were identified and ranked relative to their numerical occurrence and frequency (Cn) as follows: (1) Spionidae (Cn=52.3%), (2) Capitellidae (Cn=37.9%), (3) Eunicidae (Cn=6.8%), and (4) Nereidae (Cn=2.7%), Pilargidae (Cn=0.3%) and Sabellidae (Cn<0.1%). P. subtilis final body weight varied from a minimum of 4.2 g to a maximum of 9.3 g, with final survival ranging from 42 to 69%. Shrimp predatory pressure produced a reduced polychaete growth in NFS and FS enclosures when compared to other treatments. Polychaete numerical abundance and biomass were greatly affected by higher shrimp stocking densities, while their population patterns appeared to be governed by other environmental and endogenous cues. Artificial feeding promoted higher polychaete levels even when shrimp were present, but it was not effective in alleviating P. subtilis grazing pressure at increased stocking densities (i.e., 15 and 20 shrimp/m2). Results showed that P. subtilis predation, shrimp stocking density and external food supply are major factors to be considered when establishing sustainable limits for polychaete use as a naturally occurring food source in aquaculture systems.