Growth and behavioral responses to elevated temperatures by juvenile sablefish Anoplopoma fimbria and the interactive role of food availability

Abstract

Larval and age-0 sablefish Anoplopoma fimbria reside in neustonic waters of the North Pacific during spring and summer. We estimated the potential impacts of elevated surface temperatures on ecological processes of growth, conversion efficiency, and behavior in early juvenile sablefish. Growth experiments tested a wide range of temperatures from 6 to 24°C, with fish receiving ad libitum or low (3 % body weight d-1) rations. With unlimited food, growth increased rapidly as temperature increased to 14°C, then displayed a more gradual rise to 22°C. Growth rates at the warmer temperatures were among the highest recorded for teleosts, attaining a maximum of 3.3 mm d-1 in length and a specific growth in weight of 11.8 %. A similar response to temperature was observed at low rations, although at lower overall growth rates. At 24°C, there was a severe decline in growth for both ration levels, and few fish survived the 3 wk experiments. Gross growth efficiency, measured at temperatures of 6 to 22°C, displayed an interactive effect of temperature with ration level consistent with bioenergetic relationship. Conversion peaked at 16 to 20°C for fish receiving ad libitum rations, and at 10°C for fish on restricted rations. Conversion rates of sablefish were comparable to those calculated for a diverse array of fish species, suggesting that the rapid growth rates are driven by high consumption rather than unusually efficient energy transfer. Experiments analyzing sablefish behavior in thermally stratified water columns demonstrated increasing movement into colder water as ration level decreased, in agreement with an energy conserving strategy. Average monthly temperatures within the major nursery areas of neustonic juveniles (north of 40°N) did not exceed 19°C during the last 19 yr. These results suggest that juvenile sablefish are capable of tolerating and thriving at increased temperatures, with the critical caveat that sufficient food resources must be available. Thus, impacts on early life stages exerted by El Niño conditions, oceanographic regime shifts, or climate changes induced by current global warming scenarios are likely to be a consequence of indirect effects on circulation and productivity patterns rather than direct effects of warmer temperatures. However, because the upper thermal limit for growth nearly coincides with the upper limit for survival, exhibiting a sharp demarcation between favorable growth conditions and intolerable temperatures, juvenile sablefish at the southern limit of their distribution may suffer the direct effects of elevated temperature; recruitment of juveniles to southern populations may become more sporadic if the frequency of warming events increases with climate change.