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

Larval and age-0 sablefish Anoplopoma fimbria reside in neustonic waters of the North Pacific during spring and summer. We estimated the potential imp acts of elevated surface temperatures on ecological processes of growth, co nversion efficiency, and behavior in early juvenile sablefish. Growth exper iments tested a wide range of temperatures from 6 to 24(degrees)C, with fis h receiving ad libitum or low (3 % body weight d(-1)) rations. With unlimit ed food, growth increased rapidly as temperature increased to 14(degrees)C, then displayed a more gradual rise to 22 degreesC. Growth rates at the war mer temperatures were among the highest recorded for teleosts, attaining a maximum of 3.3 nun 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 degreesC, there was a severe decline i n growth for both ration levels, and few fish survived the 3 wk experiments . Gross growth efficiency, measured at temperatures of 6 to 22 degreesC, di splayed an interactive effect of temperature with ration level consistent w ith bioenergetic relationship, Conversion peaked at 16 to 20 degreesC for f ish receiving ad libitum rations, and at 10 degreesC 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 rate s are driven by high consumption rather than unusually efficient energy tra nsfer. Experiments analyzing sablefish behavior in thermally stratified wat er columns demonstrated increasing movement into colder water as ration lev el decreased, in agreement with an energy conserving strategy. Average mont hly temperatures within the major nursery areas of neustonic juveniles (nor th of 40 degreesN) did not exceed 19 degreesC during the last 19 yr. These results suggest that juvenile sablefish are capable of tolerating and thriv ing at increased temperatures, with the critical caveat that sufficient foo d resources must be available. Thus, impacts on early life stages exerted b y El Nino conditions, oceanographic regime shifts, or climate changes induc ed by current global warming scenarios are likely to be a consequence of in direct effects on circulation and productivity patterns rather than direct effects of warmer temperatures. However, because the upper thermal limit fo r growth nearly coincides with the upper limit for survival, exhibiting a s harp demarcation between favorable growth conditions and intolerable temper atures, 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 warmi ng events increases with climate change.