Temperate marine herbivorous fishes will likely do worse, not better, as waters warm up

Increased temperatures are associated with reduced body sizes, life spans, and reproductive outputs in shallow water marine fishes, reflecting the pervasive effects of temperature on metabolic rates in ectotherms. Herbivorous fishes have been seen as an exception to this trend, based on the hypothesis that physiological and demographic processes in these species are constrained by the inability to digest algae at low temperatures. It is thus argued that increased temperatures deliver a net benefit to herbivorous fishes. This study examines an alternative argument, that warming temperatures can have increasingly inimical effects on temperate piscine herbivores. We consider the hypothesis that herbivores experience greater oxidative stress at warmer temperatures, a consequence of temperature-related increases in metabolic rates. We use the age pigment lipofuscin to examine the rate of oxidative damage accumulation in populations of a temperate marine herbivorous fish, Odax pullus (Labridae), at different latitudes (temperatures) across New Zealand (175.3°E, 36.3°S–167.9°E, 47°S). We show a 55 % faster rate of oxidative damage accumulation in shorter-lived fish living at warmer latitudes. In these populations, it took 33–50 % fewer years to accumulate similar amounts of oxidative damage than in those living at colder latitudes, indicating greater oxidative stress in fish living at warmer temperatures. We conclude that at least some temperate piscine herbivores will be exposed to negative demographic impacts at their low-latitude range margins as temperatures increase.