DOI: 10.25338/b8h06d
title: Effects of incubation temperature on the upper thermal tolerance of the imperiled longfin smelt (Spirinchus thaleichthys)

description: Upper thermal limits in many fish species are limited, in part, by the
 heart’s ability to meet increased oxygen demand during high temperatures.
 Cardiac plasticity induced by developmental temperatures can therefore
 influence thermal tolerance. Here, we determined how incubation
 temperatures during the embryonic stage influence cardiac performance
 across temperatures during the sensitive larval stage of the imperiled
 longfin smelt. We transposed a cardiac assay for larger fish to newly
 hatched larvae that were incubated at 9, 12, or 15 ℃. We measured heart
 rate over increases in temperature to identify the Arrhenius breakpoint
 temperature (TAB), a proxy for thermal optimum, and two upper thermal
 limit metrics: temperature when heart rate is maximized (Tpeak) and when
 cardiac arrhythmia occurs (TArr­). Higher incubation temperatures
 increased TAB, Tpeak, and TArr­, but high
 interindividual variation in all three metrics resulted in great overlap
 of individuals at TAB, Tpeak, and TArr­ across temperatures. We found that
 the temperature where 10% of individuals reached Tpeak or TArr­, and
 temperature where number of individuals at TAB relative
 to Tpeak (ΔN(TAB, Tpeak)) is maximized correlated more closely
 with upper thermal limits and thermal optima inferred from previous
 studies compared to mean values of the three cardiac metrics,
 respectively. Larvae of embryos incubated at 15 ℃ did not have a higher
 temperature where ΔN(TAB, Tpeak) is maximized compared to larvae incubated
 at 9 or 12 ℃, suggesting that incubation temperatures beyond 12–15 ℃ are
 unlikely to shift larval thermal optima to match warming waters. Overall,
 by measuring cardiac performance across temperatures, we defined upper
 thermal limits (10% thresholds, Tpeak: 14.4–17.5 ℃; TArr­: 16.9–20.2 ℃)
 and optima (ΔN(TAB, Tpeak): 12.4–14.4 ℃) that can guide
 conservation strategies for longfin smelt, and demonstrated the potential
 of this cardiac assay for informing conservation plans for the early life
 stages of fish. 
description: # Effects of incubation temperature on the upper thermal tolerance of the
 imperiled longfin smelt (Spirinchus thaleichthys)
 [https://doi.org/10.25338/B8H06D](https://doi.org/10.25338/B8H06D)
 Contact: Yuzo Yanagitsuru, yyanagit@ucdavis.edu,
 yuzo.yanagitsuru@gmail.com All analyses were conducted in R version 3.6.3
 File: LFS\_cardio\_breakpoint.R includes script that calculated all
 breakpoint temperatures File: lfs\_cardio\_stats.R includes script for
 statistical analyses. Description of column headers File:
 Heart\_rates\_compiled.xlsx 'Analyzed dataset' includes data
 that were used for analyses. 'with missing measurements'
 includes data that were incorrectly assayed (temperature steps were
 missing during assay) * Date - Date assay conducted * ID - Individual fish
 identification number * Clutch - Clutch/cross identification (i.e. ID for
 different pairs of male/female broodfish) * Temperature - Temperature in
 degrees Celsius that heart rates were measured * Acclimation - Incubation
 temperatures of individuals that were measured * BPM - Heart rate in beats
 per minute measured * Arrhythmia - Identifier for reached arrhythmia or
 didn't reach arrhythmia. 0 = not arrhythmic, 1 = arrhythmic * Peaked
 - Identifier for reached peak heart rate or didn't reach peak heart
 rate. 0 = not peaked, 1 = peaked * Notes - Visually measured indicates
 that heart rates were measured by human observation as opposed to
 Danioscope software. File: Cardiology\_Analysis\_Datasheet.xlsx * ID -
 individual fish identification number * Acclimation - incubation
 temperatures of individuals that were measured * Clutch - clutch/cross
 identification (i.e. id for different pairs of male/female broodfish) *
 Batch - identification number for grouping of individuals measured
 together (i.e. fish that have the same batch number were measured at the
 same time) * Tarr - temperature at which arrhythmia was first observed *
 Breakpoint - Arrhenius breakpoint temperature, the temperature at which
 the rate of exponential increase in heart rate changes * Tpeak -
 temperature at which heart rate was at its maximum * maxhr - heart rate in
 beats per minute when heart rate was at its maximum * minhr - heart rate
 in beats per minute when heart rate was at its minimum * Slopes - rate of
 increase in heart rate prior to reaching Arrhenius breakpoint temperature.
 Slopes were calculated from plots of Arrhenius temperatures
 (1/1000\*temperature) vs ln(heart rate) Some values are not available for
 Tarr and Tab because some individuals did not become arrhythmic and others
 had a continual increase in heart rate from which Tab could not be
 determined