Fall 2020

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)


Moss Landing Marine Laboratories


In marine systems, fishes excrete dissolved nutrients rich in nitrogen, which is a biolimiting nutrient essential for regulating primary production and macroalgal growth in the ocean. Often overlooked in attempts to explain variability in kelp forest productivity, relatively little is known about the magnitude of nutrients excreted by fishes and what factors may influence nutrient excretion rates. I investigated the supply of nutrients excreted by the dominant fishes (30 species representing ~85% of total fish biomass) on nearshore rocky reefs in central and southern California. Using rapid field incubations, I measured the amount of dissolved ammonium (NH4+) released per individual (n = 460) as a function of body size and developed predictive models relating mass to excretion rates at the family-level. Mass-specific excretion rates ranged from 0.08 – 3.45 μmol · g-1 · hr-1, and per capita ammonium excretion ranged from 5.9 – 2765 μmol · individual · hr-1. Ammonium excretion scaled with fish size; mass-specific excretion rates were higher in smaller fishes, but larger fishes contributed more ammonium per individual. When controlling for body size, ammonium excretion rates differed significantly among fish families with the highest excretion by surfperches (Embiotocidae) and the lowest excretion by scorpionfish (Scorpaenidae). By trophic guild, planktivores and micro-carnivores excreted more ammonium per gram of body mass than macro-carnivores, piscivores, and herbivores. However, when body size differences were considered, the highest mean excretion rates per individual were observed in larger-bodied fishes such as California sheephead (Semicossyphus pulcher), Kelp bass (Paralabrax clathratus), and Lingcod (Ophiodon elongatus). The excretion rates reported here are consistent in magnitude with those for fishes inhabiting freshwater and tropical marine reefs, but are among the first measured in temperate kelp systems, enabling researchers to determine further quantify the role of consumers in nutrients cycling.