We used combined field and laboratory studies to investigate the relationships among foraging, food selection, and macronutrient intake and retention by the New Zealand marine reef fish, Girella tricuspidata (F. Girellidae). Gut analyses of field‐caught fish revealed a diet comprising significant proportions both of algal and non‐algal (copepods, hydroids, and organic detritus) foods, with the relative proportions of algal and non‐algal food items varying from 65% of non‐algal foods in the winter to 34% in autumn. Observations of the diurnal feeding pattern of free‐ranging wild fish showed an increase in bite rate from sunrise to late afternoon, followed by a decrease toward sunset. In laboratory studies we recorded (a) gut throughput rates, (b) macronutrient intake, and (c) macronutrient assimilation by captive G. tricuspidata fed one of three algal species. The algae were Enteromorpha intestinalis and Ulva lactuca, both of which are preferred foods of G. tricuspidata, and Gracilaria chilensis, which has not been recorded in the stomach contents of the fish despite its co‐occurrence in some habitats. The two species of preferred algae were found to have a higher starch and lower protein content than the non‐dietary G. chilensis. The fish regulated their intake and utilization of the algae such that similar amounts of protein were ingested and assimilated from all three species, but less starch was ingested and assimilated from the non‐dietary G. chilensis than from the preferred species. Gut throughput times correlated positively with starch availability in the algae but were lower for all treatments than previous studies would suggest are associated with fermentative digestion. The data suggest that omnivory in G. tricuspidata is likely based on complementarity, rather than substitutability of algal and animal foods.

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