THERMALLY-INDUCED CHRONIC DEVELOPMENTAL STRESS IN COHO SALMON - INTEGRATING MEASURES OF MORTALITY, EARLY GROWTH, AND DEVELOPMENTAL INSTABILITY

Developmental stability, or homeostasis, facilitates the production of consistent phenotypes by buffering against stress. Fluctuating asymme try is produced by developmental instability and is manifested as smal l random departures from bilateral symmetry. Increased fluctuating asy mmetry is thought to parallel compromised fitness, in part, because st ress promotes energy dissipation. Compensatory energy expenditures wit hin the organism are required to complete development, thus promoting instability through reductions in homeostasis. Increased heterozygosit y may enhance developmental stability by reducing energy dissipation f rom stress through increased metabolic efficiency, possibly by providi ng greater flexibility in metabolic pathways. Traditionally, fluctuati ng asymmetry has been used as a bioindicator of chronic stress, provid ed that selective mortality of less fit individuals did not reduce str ess-mediated increases in fluctuating asymmetry to background levels p roduced by natural developmental error, or create data inconsistencies such as higher asymmetry in groups exposed to lower stress. Unfortuna tely, absence of selective mortality and its effects, while often assu med, can be difficult to substantiate. We integrated measures of early growth, mortality, fluctuating asymmetry (mandibular pores, pectoral finrays, pelvic finrays, and gillrakers on the upper and lower arms of the first branchial arch) and directional asymmetry (branchiostegal r ays) to assess chronic thermal stress (fluctuating temperatures as opp osed to ambient temperatures) in developing eggs from two different co ho salmon (Oncorhynchus kisutch) stocks and their reciprocal hybrids. Hybridization provided insight on the capacity of heterozygosity to re duce stress during development. Although egg losses were consistently higher in crosses exposed to fluctuating temperatures, egg mortality w as predominantly a function of maternal stock of origin. Post-hatch lo sses were higher in crosses exposed to ambient temperatures than in cr osses exposed to fluctuating temperatures during embryogenesis. Observ ed patterns of early growth revealed no heterosis. but instead reflect ed maternal effects, with some crosses slowing growth and yolk utiliza tion when exposed to fluctuating temperatures. Analyses of fluctuating asymmetry also showed no effects from heterosis. While analyses of co mposite asymmetry scores and branchiostegal rays were inconclusive, an alyses of individual characters showed significantly higher fluctuatin g asymmetry in pelvic finray counts and a marginal change in the numbe rs of fish asymmetric for this character in crosses exposed to chronic thermal stress. In contrast, the fluctuating asymmetry. in lower gill raker counts was significantly higher in crosses exposed to ambient te mperatures and there were significantly more fish asymmetric for this character. Data on mortalities and fluctuating asymmetry indicate pelv ic finray development was thermally stressed, while the heightened flu ctuating asymmetry in lower gillraker counts under ambient temperature s was due to a greater frequency of less fit fish that had not been cu lled by thermal stress. Changes in early growth patterns in response t o developmental stress yielded no parallel responses in meristic chara cters. We conclude that chronic thermal stress produced both selective ly lethal and sublethal effects that directly shaped fluctuating asymm etry and fitness profiles in these crosses. Implicit in this conclusio n is that developmental instability analyses can detect more than just chronic sublethal stress, thus providing substantial credence for usi ng instability studies as proactive bioassessment methodologies.