Irregular Growth Patterns in the Otoliths of a Short-Lived Forage Fish Do Not Reliably Indicate Reproductive History

Irregular growth patterns in fish otoliths are considered to reflect the energetic demands associated with reproduction and hence have been referred to as spawning zones or spawning checks. Here we evaluated the reliability of these checks for identifying the past reproductive history of capelin Mallotus villosus, a small pelagic fish found throughout the Northwest Atlantic that typically experiences high mortality associated with spawning. Ovarian histology was used to definitively identify females that had previously spawned via the presence of residual oocytes. Only half of the individuals confirmed to be recovering from a previous spawning episode were identified as having spawning checks in their otoliths, indicating that not all spawning episodes result in the formation of spawning checks. In addition, some individuals that appeared to be immature also had spawning checks, suggesting that processes other than spawning may be implicated in the creation of these otolith growth patterns. Analyses of otolith growth increments for fish of known reproductive status indicated that higher growth during the first primary growth season may allow capelin to reach sexual maturity at a younger age. These results strongly suggest that otolith checks are unreliable indicators of repeat spawning in capelin and may be of questionable utility for identifying past reproductive history of fishes in general. It is recommended that future research to quantify iteroparity in capelin be based on reproductive histology rather than otolith-based methods.