Effects of temperature and parental background on the embryonic survival and metabolic rate of newly hatched Arctic charr Hannu Huuskonen 1, Olli-Pekka Penttinen 2 and Jorma Piironen 3 1
Karelian Institute, Department of Ecology University of Joensuu, P. O. Box 111, FIN-80101 Joensuu, Finland E-mail:
[email protected]
2
Department of Biology, University of Joensuu P. O. Box 111, FIN-80101 Joensuu, Finland E-mail:
[email protected]
3
Finnish Game and Fisheries Research Institute Joensuu Game and Fisheries Research Kauppakatu 18–20 FIN-80100 Joensuu, Finland E-mail:
[email protected]
Key words: embryonic survival, standard metabolic rate, parental effects, temperature effects, Arctic charr
Abstract Arctic charr (Salvelinus alpinus (L.)) is an endangered species in Lake Saimaa, south-eastern Finland. As a part of a stock enhancement programme, several year classes of cultivated brood stocks have been founded. Genetic diversity of Lake Saimaa Arctic charr has recently been studied and it has proved to be low, probably due to population decrease and subsequent inbreeding. To study possible interactions between the genotypes and environment during early ontogenetic development, we created family groups by crossing randomly selected single parents from cultivated brood fish. The eggs of these Arctic charr families were incubated at 2 °C and 7 °C, and embryonic survival was determined at the eyed stage. After hatching, oxygen consumption of alevins was measured at the incubation temperatures. Temperature was found to have effects on both embryonic survival and standard metabolic rate of Arctic charr while parental background affected only survival. Variability in the embryonic survival could be attributed to the female at both temperatures whereas the male only had an effect at the lower temperature. Low incubation temperature was advantageous for Arctic charr due to higher utilization of yolk reserves and lower mortality which indicates adaptation of Arctic charr to cold water temperatures. The Big Fish Bang. Proceedings of the 26th Annual Larval Fish Conference. 2003. Edited by Howard I. Browman and Anne Berit Skiftesvik Published by the Institute of Marine Research, Postboks 1870 Nordnes, N-5817, Bergen, Norway. ISBN 82-7461-059-8
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Introduction Arctic charr (Salvelinus alpinus (L.)) is an endangered species in Lake Saimaa, south-eastern Finland. Genetic diversity of Lake Saimaa Arctic charr has proved to be low (Primmer et al. 1999), probably due to population decrease and subsequent inbreeding. As a part of a stock enhancement programme, several year classes of cultivated brood stocks have been founded, but they have suffered from increased embryo and alevin mortality as well as disease susceptibility. Poor survival of Arctic charr is not restricted to the Lake Saimaa population alone, since de March (1995) reported high mortality during egg incubation also in Labrador and Norwegian strains. The distribution of Arctic charr covers alpine and Arctic regions, and it can be considered the most cold adapted of all the salmonids (Johnson 1980). Lake Saimaa Arctic charr spawn during October-November, at the time when lakes begin to freeze in this area so that the egg incubation takes place under the ice where temperatures are well below 5 °C. Under experimental conditions, Lake Saimaa Arctic charr preferred cobbles to finer material as spawning substrate (J. Piironen, pers. obs.). Females dug a redd before laying their eggs and covered it by tail beats after fertilization. According to Pavlov et al. (1994), Arctic charr females in the Lakes Onega and Ladoga, Russia, did not bury their eggs after spawning but the eggs were freely spread among the rocks and gravel. Organisms tend to be more stenothermal during early embryogenesis than in later developmental stages (Cossins and Bowler 1987). According to Gruber and Wieser (1983), embryos and fry of Arctic charr survive well at temperatures between 4 and 8 °C but obviously embryos are viable at lower and higher temperatures as well (e.g. de March 1995, Bebak et al. 2000). Increase in temperature results in accelerated development but it may also elevate the mortality and the percentage of abnormal embryos which are known to increase towards both thermal limits of a species (Cossins and Bowler 1987). De March (1995) observed greater hatching success of Arctic charr eggs at 3 °C than at 6 °C. The aim of this study was to find out whether embryonic survival and metabolic rate of newly hatched Arctic charr are affected by parental background or temperature. The role of parentage in embryonic and larval survival has been studied in numerous fish species (reviewed by Kamler 1992), but information about parental effects on fish metabolism is scarce. Since metabolic rate represents a measure of an animal’s physiological processes, combining information on survival and metabolism could give new insight into the factors behind poor survival of Lake Saimaa Arctic charr. If mortality is parentally affected, it may have serious consequences for stock conservation, since some families (genotypes) can be selected against thus affecting the genetic diversity of the brood fish. To study this, family groups of Arctic charr were created by crossing randomly selected parents from cultivated brood fish following the North Carolina Design II (Lynch and Walsh 1998).
Materials and Methods Parental fish (mean length 68.3 cm; sd ± 5.2 cm and mean weight 3.6 kg; sd ± 1.0 kg) originated from 1991-1994 year-classes of cultivated brood fish (first hatchery generation) founded from wild spawners. A total of 15 blocks of randomly selected two female x two male factorial fertilizations, i.e. a total of 60 families, were carried out at the Saimaa Fisheries Research and Aquaculture
S U R V I VA L
A N D M E TA B O L I S M I N
ARCTIC
CHARR
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in south-eastern Finland during 15-26 November 2001. After the fertilization, the eggs were left under the flowing water overnight before removing the dead, unfertilized eggs. Then the eggs from each family were divided into eight groups of 80-90 eggs. Four groups of eggs (four replicates of 80-90 eggs) from each family were incubated at 2±0.2 °C and 7±0.1 °C. The experimental design is known as a North Carolina II design and it allows the total phenotypic variance in the offspring traits to be partitioned into female and male effects and, in addition, observable variance components can be expressed in terms of hypothetical underlying causal factors i.e. female effects can be divided into maternal and genetic effects (Lynch and Walsh 1998). Due to practical limitations, however, only 12 families were used in oxygen consumption measurements and hence we did not include quantitative genetics in the statistical analysis. As a result, we were able to separate the effects of female and male and their interaction. Three four-family blocks of Arctic charr randomly selected from the experimental blocks were fertilized on the 15th (block 1: females and males 1 and 2), 20 th (block 2: 3 and 4) and 26 th (block 3: 5 and 6) of November, 2001. The incubation took place in floating plastic cylinders with a net bottom (mesh size ca. 2 mm, bottom area ca. 78.5 cm2) kept in 1.1 m2 circular rearing tanks (water depth ca. 30 cm). Water flow into the tanks during incubation was about 18 l min-1. Dead eggs (i.e. those turning totally or partly opaque) were counted and removed from the incubators for the first time about one week after fertilization, and at about two week intervals thereafter. At the eyed stage of embryos (2-15 January, about 343-355 degree days at 7 °C, 6-20 March, about 222-224 degree days at 2 °C), the final embryonic survival was determined after ‘chocking’ the rest embryos by pouring them at least twice from incubators into a plastic cup. By this treatment the unfertilized eggs turned opaque and they could be removed before counting the remaining live embryos. The embryonic survival was then expressed as a percentage of live, eyed embryos from the total amount of eggs at the beginning of incubation. After hatching (in February at 7 °C, in May at 2 °C), oxygen consumption of the alevins was measured at 2±0.1 °C and 7±0.1 °C by two automated intermittent-flow respirometers equipped with YSI 5750 polarographic oxygen sensors (Forstner 1983). The term alevin applies here to hatched, endogenously feeding fish (e.g. Kamler 1992). The number of fish in each family varied from six to ten (Table 1) and the measurements of the family blocks were made in the same order as the fertilizations. At 7 °C block 1 was measured on 83-92 days post fertilization (dpf), block 2 on 92-100 dpf, and block 3 on 99-107 dpf. Measurement periods at 2 °C were 176-182 dpf in block 1, 177-186 dpf in block 2, and 182-187 dpf in block 3. Both respirometer systems included three parallel acrylic chambers (volumes 148-167 ml) and the flow rate was about 200 ml min-1. The oxygen consumption in each chamber was recorded for 30 minutes every second hour and the average rate during this period was extrapolated to an hourly value. The signals from the polarographic oxygen sensor were fed on-line into the computer and integrated each minute. Microbial oxygen consumption in the respirometer was measured at the beginning and end of the experiments and it was subtracted from the total decline of oxygen. Each experiment lasted for 18-22 hours (overnight) and they took place in the dark. After the experiments the fish were anaesthetized with tricaine, weighed (fresh mass, FM) and measured for total length (TL). Length (l) and height (h) of yolk were also measured and yolk volume (V) was calculated by the equation for a prolate spheroid: V = 0.5236 * l * h2.
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Table 1. Size (FM = fresh mass, TL = total length), yolk volume (V) and number (n) of Arctic charr alevins used in oxygen consumption measurements. Values indicate mean ± se. Within-family statistical differences between the sizes of newly-hatched alevins incubated at different temperatures are denoted as follows: ns p>0.05, * p
