Enhancement of Fish Stock by Habitat Manipulation in Artificial Waterways

Abstract

Two widely-recognised hypotheses propose that increases in fish abundance at artificial reefs are caused by: (a) the attraction and redistribution of existing individuals; and/or (b) the addition of new individuals by production. Inadequate experimental designs compromised by a lack of sufficient replication and/or spatial interspersion of reefs with controls have prevented researchers from distinguishing between attraction and production. Infrequent sampling has also led to insufficient temporal coverage of life history-driven variations in fish abundance and biomass at reefs. Detection and resolution of these trends, together with incorporation of fish age/length data, and the use of techniques such as stable isotope analysis to infer trophic link(s) between reef residents and potential sources of epibenthic nutrition at reefs should help demonstrate mechanisms underlying attraction and/or production.

Given that fish numbers tend to increase after artificial reef deployments, artificial reefs have been proposed as a means by which fish stocks could be enhanced by providing new habitat in areas where natural habitat vital for feeding, shelter and reproduction have been removed, such as residential canals. The purpose of my study was to investigate whether or not artificial reefs could be used to enhance fish production within residential canals.

Five reefs were deployed into Lake Rumrunner on the Gold Coast in southeast Queensland, Australia (28°02’59”S, 153°25’19”E). Reefs were interspersed among five soft sediment (control) sites of similar depth. Fish assemblages at reefs, controls and shoreline jetties were quantitatively sampled using a modified seine pop net. Very few fish were recorded at controls, but fish abundance and biomass increased dramatically at reefs shortly after deployment, stabilising at levels above that of shoreline jetties.

Reefs and jetties accommodated different fish assemblages and assemblage structure varied through time. Multivariate analyses indicated that the fish assemblage at jetties within Lake Rumrunner differed from assemblages at jetties in adjacent lakes in terms of biomass composition, accommodating numerous individuals of small-bodied species. The fish assemblage at reefs featured individuals of large-bodied species and occasional, itinerant individuals of species typical of offshore subtidal reefs. Among co-occurring (reef and jetty) fish species, strong differences in the distribution of Monodactylus argenteus (Monodactylidae) between reefs, jetties and controls suggested possible new production driven by reef deployment. Abundance and biomass of M. argenteus was significantly greater at reefs relative to controls (where no fish were caught) and jetties at all times after deployment. Individuals were consistently larger on reefs than on jetties. Analyses of length-frequency distributions through time indicated the size of individuals differed from one sampling time to the next for most sampling times at reefs, possibly representing cohort growth and movement through time...