Examining the early life history of the coho salmon (Oncorhynchus kisutch) from a study of otolith microstructure

Abstract

Birth date can be an important correlate to evolutionary fitness because birth timing can profoundly influence growth and survival. As part of an ongoing study, we report here on birth date and growth patterns for two populations of coho salmon (Oncoryhnchus kisutch) exposed to different stream environments, one from Washington (where salmon are native) and the other from Wisconsin (where this species was introduced in the 1960s). Fish ear bones (otoliths) were used to measure fish age (in days) and from microscopic examination of 400 fish, we have thus far generated results concerning birth date and growth patterns for each system. Both populations show long hatching seasons (>6 wk), but fish from Wisconsin hatched apprximately 3 weeks later, emerging from nests at least one month following the spring flood season. Wisconsin fish also grew at a much slower rate (nearly 40% slower) and had a longer nest residency than Washington fish, findings that probably reflect the existence of warmer, more productive stream conditions out west. To date, our examinations have shown that birth date is a poorer predicator of growth than birth size or metablism (both of which can be inferred from otolith morphology.