Mortality Rates of Chum Salmon during Their Early Marine Residency
Alex C. Wertheimer and Frank P. Thrower
Abstract.—Interannual variability in chum salmon Oncorhynchus keta mortality during early marine life is thought to have a major influence on recruitment. However, few estimates of daily mortality are available for chum salmon during this period, and average values reported in the literature are unrealistically high when used in a simple life-history model. We analyzed survival to adult of seven groups of chum salmon, marked as juveniles, and released at different times and sizes at Little Port Walter, Alaska to estimate average daily mortality during early marine residency for an early emigration group and a late emigration group. We assumed that differences in proportions of groups surviving to adult between the initial releases of unfed fry and subsequent releases of fed fry for each group were due to natural mortality during the time interval between releases. For both groups, mortality was highest during the period immediately after release, declining rapidly thereafter. Average daily mortality was 8.1% for the early release during their first 21d in the ocean and 3.9% for the late release during the first 32 d in the ocean. After May 4 (54 d and 33 d postrelease, respectively, for the early and late groups), average daily mortality was less than 0.6% for both groups. These results support the paradigm that most of the mortality of chum salmon in the ocean occurs early in their marine residency, and the results provide realistic rates for demographic modeling of the abundance of chum salmon in marine habitats.