The size, population structure, relative fish abundance, diet and heavy metal concentrations of fish in Hamilton Lake (Lake Rotoroa) were examined. A combination of gill nets, fyke nets and minnow traps were used at each of 11 sampling sites located primarily at stormwater inflows around the lake. Sampling was undertaken between 9 December 1993 and 2 March 1994, with one overnight sampling occurring at each site. A total of 1073 fish were captured, comprising nine different species: catfish (Ictalurus nebulosus), common bully (Gobiomorphus cotidianus), goldfish (Carassius auratus), longfinned eel (Anguilla dieffenbachii), perch (Perca fluviatilis), mosquitofish (Gambusia affinis), rudd (Scardinius erythropthalmus), shortfinned eel (Anguilla australis), and tench (Tinca tinca). Abundance of rudd has declined since 1990, with a decline in catch rates of 67%. Conversely, perch abundance has increased since 1990, with catch rates increasing by 300%. During this period perch have surpassed rudd as the most abundant fish species. The declines in rudd abundance are probably associated with the disappearance of submerged aquatic macrophytes, largely Egeria densa, from Hamilton Lake during 1990. Ages of rudd were determined using scales, while perch were aged using opercular bones. Back-calculation of fish length involved the use of both scale proportional (SPH), and body proportional hypotheses (BPH). Results from the SPH appeared more accurate, due to better compatibility with actual observed fish lengths. Sizes of one and two year old rudd appear to have declined over the past six years in Hamilton Lake. In 1987, the mean lengths of one and two year old rudd were 87 mm and 207 mm respectively, while in 1991 the same aged fish were 69 mm and 127 mm in length. Perch growth rates, however, do not appear to have changed. Poorly formed growth rings on both scales and operculum prevented tench from being aged. Despite the total collapse of submerged aquatic macrophytes, which formed the bulk of the rudd diet (84% by volume), aquatic vegetation still remained the most important food item of adult rudd in the present study, although this now comprised entirely of emergent marginal vegetation. This suggests that rudd >200 mm in length are obligate herbivores. Perch fed largely on invertebrates, before a transition to a largely piscivorous diet at approximately 200 mm in length. Tench appeared to be specialist feeders on molluscs. Concentrations of four heavy metals (arsenic, lead, copper, and zinc) in the white muscle of fish were examined. Concentrations of the majority of heavy metals analysed were well below maximum permitted levels. Arsenic concentrations were highest in bottom feeding fish, particularly catfish (mean=0.778 mg kg⁻¹ wet weight, compared to the maximum permitted level of 2 mg kg⁻¹ wet weight) due to high arsenic concentrations in the lake sediment resulting from sodium arsenate application for weed control in 1959. Arsenic in the present study was analysed using a dry ashing technique. Arsenic concentrations were higher than those of previous studies in the lake undertaken using a wet ashing technique, which fails to liberate all arsenic from the fish flesh.