Seahorses are one of the most iconic examples of a monogamous species in the animal kingdom. Documenting monogamous behaviors of seahorses have proven to be quite complicated to study in the wild because of their low population densities and cryptic habits. Another challenge involves interpreting these behaviors in captivity because recreating realistic densities of wild populations in the laboratory can be difficult due to their patchy distributions. This study investigates the relationship between stocking density and mating and competitive behavior from the context of the field biology of the dwarf seahorse, Hippocampus zosterae (Jordan & Gilbert). Animals were housed in 38 liter tanks at a range of densities and sex ratios (from 2-8 animals per tank), and their reproductive and other social behaviors were monitored from tank introduction through copulation. At low tank densities and even sex ratios but comparatively high field densities, activity level in trials was low. A higher level of males in tanks across all densities increased competition, activity levels, and aggression leading to partial egg transfers and failed pregnancies, resulting in lower reproductive success. Across seahorse species, mean and maximum wild densities were consistently lower than those used in captive breeding, with adult sex ratios that were significantly female biased. However, significant variation exists in wild seahorse densities across species, with higher densities detected in focal/mark recapture studies and on artificial habitat structures than reported with belt transect sampling techniques. Interchange of knowledge gained in both captive and wild contexts will allow us to better understand the biology of this genus, and improve reproduction in captivity. Interpreting captive reproductive behaviors of seahorses within various densities reported from natural populations will help us predict the impact of conservation efforts and increase the likelihood of long-term persistence of populations for this threatened genus.