There has been much debate about how male alternative reproductive tactics (ARTs) evolve. In particular, researchers question whether ARTs have evolved as a conditional, ‘best of a bad job’ strategy where one tactic has higher fitness than the other, or whether they have evolved as a result of a genetic polymorphism where both tactics have equal fitness. Despite the large number of species known to have ARTs, tests of equal fitness between tactics have only been conducted in a handful of species. We tested the prediction of equal fitness using the plainfin midshipman (
Porichthys notatus), a species with two well characterized male ARTs: guarding type I males and cuckolding type II males. We collected data across three years and three sampling locations to determine the proportion of each reproductive tactic, as well as the proportion of offspring sired by each male type using microsatellite markers. Our analysis suggests that males adopting the conventional guarding tactic likely have higher fitness compared to males adopting the cuckolder type II tactic. Also, we show that the guarding male tactic is able to gain paternity through cuckoldry, and that these males, who sometimes guard and sometimes cuckold, are responsible for the majority of paternity lost within nests. Indeed, the classic cuckolding type II males were responsible for only a small fraction of the paternity lost. These results highlight the degree of flexibility in male behaviour even among individuals adopting the same male tactic. Taken together, our results provide the first exploration of the evolution of male ARTs in plainfin midshipman and, given the tractability of midshipman system, a valuable next step will be to look for gene-by-environment interactions on tactic development and expression.