Responses of Largemouth Bass from Different Latitudes to Elevated Water Temperatures

Abstract

The effects of elevated temperatures on largemouth bass Micropterus s. salmoides, from Minnesota and Wisconsin (our northern stock) and from Tennessee (our southern stock), were compared at four first-year life stages. Our purpose was to determine if data gathered from one part of a speciesˈ range could be used to set thermal standards for that same species elsewhere within its normal range. Eggs and embryos, when first exposed early in development to an array of elevated temperatures, lost 50% of their maximum hatching success (TL50) at 29.5 C for the northern stock and at 29.1 C for the southern stock. When embryos from the same pooled egg lots were exposed late in development, their TL50 temperatures were 32.3 C and 30.9 C, respectively. Median lethal 24-hour temperatures (TL50) for 0–3-day-old hatched larvae acclimated to 20, 24, 27, and 30 C averaged 32.1 C for the northern stock and 32.6 C for the southern stock. Median survival times for juveniles depended on acclimation temperature, exposure temperature, and the difference between these. Expected 24-hour TL50 temperatures, derived from median survival times, are 31.0 C for the northern stock and 30.6 C for the southern, based on 22 C acclimation. When fish were reared at temperatures from 28 to 35 C for 6 weeks growth rates were not significantly different among temperatures or between stocks. At 35 and 36 C for the northern and at 36 C for the southern stock, mortality rates were significantly elevated and net biomass gains were significantly depressed, although neither variable differed between stocks (P > 0.05). The differences between stock tolerances exceeded 1 C in only one of the elevated temperature challenges performed (1.4 C for the TL50 of late-exposed embyros) and neither stock was consistently more or less thermally tolerant. We conclude that, in terms of responses to elevated thermal challenges, M. s. salmoides is a thermally consistent subspecies over the latitudinal range represented by our samples. We further conclude that thermal standards for these fish may be based on data obtained from samples taken from any usual population within this range, provided a 2 C safety factor is used where TL50 data are involved.