Odax pullus (Forster) is a herbivorous fish found in shallow reefs throughout New Zealand. Through grazing, O. pullus can exert significant top-down controls on macroalgae and has been shown to limit the distribution and abundance of dominant, habitat-forming species. Shallow reef ecosystems throughout southern New Zealand are threatened by the invasion of the macroalga Undaria pinnatifida (Harvey) Suringar, which can have harmful impacts to native macroalgal communities. This study aimed to identify the feeding preferences of O. pullus throughout the North Otago coast and explore the ability of O. pullus to provide biotic resistance against invasion by U. pinnatifida. Multiple-choice feeding experiments were used both in a controlled mesocosm environment and in situ to achieve this aim. During the mesocosm experiments, five species of macroalgae were offered to a single O. pullus individual for 22 hours, and preference was measured as wet weight consumed over this period. The protein and organic matter content of macroalgae used in the feeding experiments were also measured to determine if diet choices were related to diet quality. During these experiments, U. pinnatifida was preferentially consumed, with consumption of this invasive higher than on all native species combined. This preference appeared to be primarily due to the nutritional value of U. pinnatifida, which displayed the highest protein content in this study and a high mannitol content in the literature, both of which are important components within the diet of O. pullus. Similarly, feeding experiments in situ used multiple-choice feeding assays to measure dietary choices. The assays were monitored with an underwater camera, and preferences measured by the number of bites taken per minute on each species. The results were suggestive of a preference for U. pinnatifida by O. pullus but were constrained by limited replication. Preferences in the field were similar to those displayed in the laboratory experiments and appeared to be also driven by nutritional value rather than ecological variables such as macroalgal availability or diversity. Through the preferential feeding displayed in this study, as well as high mobility and consumption rates, O. pullus can clearly provide some degree of biotic resistance against invasion by U. pinnatifida, which indicates a prospect for biocontrol. Although strong modulative controls were observed at local scales, more research should be undertaken to identify the potential for biocontrol by O. pullus at broad scales. Widespread control may however be limited by the high densities of U. pinnatifida present throughout the study area and invasive traits that have facilitated global dispersal. Where feasible, biocontrol could be used in conjunction with the manual removal programme run by Te Rūnanga o Ngāi Tahu at high-value sites or sites that may enable dispersal to other areas. Even if biocontrol by O. pullus is impractical, this study has identified a positive ecological role of U. pinnatifida throughout the North Otago coast, as it provides O. pullus with readily available, high-quality diet.
Louis James Argent Alexander (Thu,) studied this question.