Temperature-dependent temporal variation in the size and growth of Thames estuary smelt Osmerus eperlanus

Abstract

Long-term monitoring data collected in the Thames estuary, UK, were used to investigate inter-annual trends in year-end size and growth rate of smelt Osmerus eperlanus (L.). Patterns were related to variations in the thermal opportunity for growth, defined in terms of temperature-dependent cumulative degree-days at estuarine temperatures ≥5°C. Year-end size was positively correlated to the thermal opportunity for growth, whereas growth rate was negatively correlated. Cohort-specific comparisons of estuarine growth patterns indicated significant differences in starting sizes, possibly related to the influences of temperature on early larval development. Interannual differences in within-estuary growth rate as a function of cumulative degree-days were found and suggested thermally dependent growth. Smelt in normal and warm years showed similar rates of growth that differed significantly from rates observed in cold years. Reductions in the proportions of older fish in warm years were also found and interpreted in the context of spatial segregation patterns found in freshwater environments. Overall results suggest that use of mid-estuarine environments during larval development and migration to colder outer-estuarine areas are growth/maturation optimization strategies that enhance individual fitness. Thus, smelt migration phenologies in the Thames estuary appear to be driven by short-term variations in the suitability of thermal habitat. © Inter-Research 2007.