Summary
In this study, the effects of abrupt temperature change on the hatching success and larval survival of eggs, yolk‐sac larvae (YSL) and larvae above nest (LAN), for both largemouth bass Micropterus salmoides and smallmouth bass Micropterus dolomieu were quantified. Temperature had a significant effect on hatching success and time to 50% mortality, with large heat shocks causing accelerated mortality. The temperature changes shown to influence survival of all life stages, however, were beyond what is typically experienced in the wild. Micropterus salmoides had greater egg hatching success rates and increased survival rates at YSL and LAN stages, relative to M. dolomieu. Additionally, egg hatching success and survival of LAN varied across nests within the study. These findings suggest that temperature alone may not account for variations in year‐class strength and may emphasize the need for protection of the nest‐guarding male Micropterus spp. to ensure recruitment.
Methodology
Littoral zone snorkel surveys were performed to locate nests with male M. salmoides and M. dolomieu guarding unhatched eggs (450 mm total length, LT) or small (1 km away from each other, reducing the likelihood that a single female may have deposited eggs into multiple nests in this study. Once a suitably sized nesting male of either species was located, it was marked with an individually numbered nest tag and the location was drawn on a map of the site. Nest depth was estimated visually and ranged from 0.5 to 3.0 m, consistent with nest depths of these species in other systems (Beeman, 1924; Kramer & Smith, 1962; Bozek et al., 2002; Suski et al., 2003) and in Lake Opinicon (Philipp et al., 1997; D. P. Philipp, unpubl. data). Following discovery of a nest, the snorkeller captured the nesting male with conventional fishing gear and brought the fish to a boat where LT was measured to the nearest mm, and the fish was then held in a closed cooler of lake water.
While the male was absent, the snorkeller placed a clear 1 m2 acetate sheet, divided into 2 cm2 cells, on top of the nest and outlined the egg mass with a grease pencil. The number of eggs was counted in three 2 cm2 cells (at the centre of the egg mass, midway to the edge, and at the edge) and these values were averaged. This average egg count (per 2 cm2 cell) was scaled up to the area of the egg mass to estimate the total number of eggs for each nest (Raffetto et al., 1990). Approximately 100 eggs were transferred to a glass jar filled with lake water using a kitchen turkey baster and placed in an insulated cooler. A weather monitoring station at QUBS, which recorded lake temperature every 60 min (Campbell Scientific thermocouple probes, Model 105T; www.campbellsci.com) indicated that, on the days of egg collections at the time of day when eggs were collected, mean ± s.e. water temperature at 0.2 and 3.3 m depth was 17.3 ± 0.1 and 16.4 ± 0.1◦ C, respectively. For the entire 2008 nesting season (6 May to 17 June), temperatures averaged 18.3 ± 0.1◦ C at 0.2 m and 17.6 ± 0.1◦ C at 3.3 m depth. Examination of three successive years (2006, 2007 and 2008) of temperature data during the nesting period (i.e. 6 May to 17 June) showed that the largest hourly increases and decreases in water temperature were 1.34 and 1.02◦ C at 0.2 m, and 2.48 and 2.75◦ C at 3.3 m depth, respectively. Once egg collection was complete, males were released on top of their nests and brood-guarding resumed within 2 min. The snorkeller remained at the nest while the male was absent, to deter brood predation. Eggs were transported by boat to the aquatic facility at QUBS. No more than 2 h elapsed from the collection of eggs to their arrival at the aquatic facility.