Date of Award

2003

Degree Type

Thesis

Degree Name

Doctor of Philosophy

Faculty

Faculty of Computing, Health and Science

First Advisor

Paul Lavery

Second Advisor

Glenn Hyndes

Abstract

Recreational line fishing is highly targeted at predatory fishes, making them vulnerable to overfishing. These same fishes play a role in trophic structure by regulating prey species. Despite increasing numbers of fishers, few studies have investigated the potential effects of recreational fishing on fish populations and subsequent trophic effects. This project investigated whether there were differences in fishes and benthos between unfished and recreationally fished areas, and whether the removal of targeted fishes influenced trophic structure. The study was conducted at the Ningaloo Marine Park, Western Australia, which had Sanctuary (no-take) and Recreation {recreationally fished) Zones. Data were collected from three regions (Mandu, Osprey and Maud) and replicated over time. Fish assemblages, benthos and trophic interactions were compared between zones at each region. At Ningaloo the lethrinids (emperors) are a top-order predatory fish and the preferred target of recreational anglers. The algal-grazing urchin Echinometra mathaei comprised 51% of macro invertebrate abundances and was heavily preyed upon by lethrinids, being recorded in 50% of the guts of sampled fish. In nil regions, Sanctuary Zones had a greater biomass of lethrinids than Recreation Zones, but there were no differences in non-targeted fishes between zones. Despite the consistent effect on lethrinids, there were inconsistencies among regions in the predator-prey relationships. At Mandu, Echinometra mathaei abundances were inversely related to lethrinid biomass, suggesting a strong predator-prey interaction. In the Recreation Zone, the abundances of E. mathaei were four times greater, and macro-algal cover was half, that of the Sanctuary Zone. Furthermore, algal composition differed between zones, and this was driven by fucoid brown algae, which dominated the diets of E. mathaei. This was interpreted as evidence of a trophic cascade resulting from the removal of lethrinids at the Recreation Zone. At Maud, different results were recorded. Abundances of Echinometro mathaei and lethrinids were both higher in the Sanctuary Zone, than the adjacent Recreation Zone. E. mathaei reside in the crevices of rock, dead coral or Echinapora coral, which provided refuge from predation and this habitat was more available in the Sanctuary Zone. It is suggested that the availability of this habitat confounded the effects of predation. Macro- algal cover was lower in the Sanctuary Zone indicating a grazing effect from E. mathaei. At Osprey there was higher cover of E. mathaei habitat in the Sanctuary than the Recreation Zone. However, there were no differences in macro-algal cover, which was consistent with a lack of difference in E. mathaei abundances. The effect of E. mathaei grazing was unlikely to have been confounded by fishes that graze macro-algae, as they did not differ between zones at any region. These results indicate that recreational fishing reduced fish populations below that of adjacent protected areas at Ningaloo Marine Park, and in one region this resulted in a trophic cascade. This may be the first study that has recorded evidence of a trophic cascade where recreational line fishing is the only means of extraction. However, the results also show that this is not a consistent response to reduced fishing pressure; in other regions, changes in predatory fish abundance did not result in differences in the abundances of their prey, suggesting no trophic cascade. The studies have contributed towards an understanding of fish-habitat interactions and provide a baseline for future monitoring of the Ningaloo Marine Park. They also have important implications for marine park managers in terms of defining their expectations when implementing Sanctuary Zones. The results also show that Sanctuary Zones have the potential to be effective tools for fisheries management.

Included in

Agriculture Commons

Share

 
COinS