The New Zealand common bully (Gobiomorphus cotidianus) is a member of the family Eleotridae which comprises around 35 genera and some 150 species worldwide. G. cotidianus are abundant throughout New Zealand and thrive in streams, large rivers and lakes, with broad tolerances for temperature, pH, flow and lake trophic status. It is known that male bullies clean and guard a nest site and attract one or possibly more females to spawn within the nest which is then guarded until the eggs hatch. However, due to intraspecific competition, some males miss out. Male alternative reproductive tactics (MARTs) are commonly used to achieve a successful reproductive outcome when the usual reproductive strategy becomes energetically inefficient for the individual. MART’s are common within the closely related family Gobiidae but have not yet been observed in the Eleotridae. Differential investments in gonadal mass versus parental care are known in many animals from fish to humans. Greater parental investment typically correlates with smaller gonad size. I investigated the relative investment made by male common bully in gonads versus seminal vesicles on the assumption that if non-guarding (parasitic) males occur, they would invest significantly less effort in seminal vesicles (SVs) compared with “bourgeois” (nest guarding) males. I examined these traits monthly to determine possible seasonal changes in reproductive strategies. We found that male investment in SVs does vary significantly between individuals and that the greatest investment in this structure in males coincides with the peak summer breeding season. A small proportion of males (usually larger individuals) invest heavily in SVs but it is currently unknown whether these, and only these, defend nest sites and whether the remainder rely on MART strategies for reproductive success. Seminal vesicle size was positively correlated with fish size whereas testis size declined with increasing size of males. Sampling of common bully from Lake Karapiro began in December 2013 and finished in January 2015 providing fourteen months of data to predominantly investigate male reproductive investment using calculations of gonadosomatic index (GSI) and seminal vesicle somatic index (SVSI). I found that although investment in both gonads and seminal vesicles increase in preparation for the breeding season, the increase in SVSI is considerably greater than the comparatively slight increase in GSI. I also examined female GSI and found that it followed similar trends to that of the SVSI with sharp increases occurring throughout late winter and spring. Calculations of somatic condition indices were made for both males and females throughout the 14 month sampling period. This identified similar condition trends for both sexes with peaks throughout summer suggesting a slight increase in somatic condition post-breeding season. Male condition was slightly higher and more varied during this summer period. A behavioural experiment was carried out to investigate male nest preference and guarding behaviour. Obvious nest guarding was displayed in 14 of the 40 nest arenas and 11 of the guarded nests were large pots compared with only 3 guarded small pots. GSI and SVSI data from the males within the experiment suggest an increased investment in seminal vesicles by males displaying nest guarding behaviour. However, the sample size for both of these elements was too small to provide statistically significant results.