Summary
The energetic costs of providing parental care are widely documented, but rarely do studies consider the role of environmental variation (e.g., predation pressure) in this context. Here, we tested if variation in nest predation pressure influenced the energetic costs of parental care in smallmouth bass (Micropterus dolomieu), a teleost fish species that provides lengthy paternal care. First, we documented that nest predation pressure varied among the six lakes studied and the relative predation pressure ranking was consistent across a three year period. We used a combination of traditional proximate body composition (PBC) analyses and electromyogram (EMG) telemetry to quantify activity costs of nesting fish across these populations. The traditional approach revealed declines in energy stores across the parental care period but showed no evidence of an increased energetic cost to parents from populations with higher nest predation pressure. Comparing the distribution of EMG data from the two extremes of predation pressure revealed that males from the site of highest predation spent more time at higher EMG levels relative to the parents from the lake of lowest predation pressure. Although not statistically significant, males from the site of highest predation pressure also spent 21–24 % of their time burst swimming when guarding young offspring compared to 10–11 % for males at the site of lowest predation pressure. These differences in overall activity, a large contributor to the energy use of fish, may translate into longer recovery times and decreased future reproductive opportunities.
Methodology
The study took place on six lakes in south-eastern Ontario within the Rideau and Gananoque River watersheds. We showed elsewhere (Gravel and Cooke 2009) that these lakes varied greatly in nest predation pressure in a given year. In order to establish whether variation in predation pressure across lakes remained consistent across years, variation in predation pressure was measured for three consecutive years (2007–2009). Each spring, when water temperatures reached approximately 15°C, snorkelers swam the littoral zones of the lakes to identify nesting smallmouth bass that were guarding fresh fertilized eggs (n≥40 per lake). Predation pressure was then measured on a random (n=10) subset of these fish when males were guarding fresh eggs. Predation pressure was only measured when males guarded fresh fertilized eggs to ensure consistency among the lakes and because this stage is known to be particularly vulnerable to predation because eggs are immobile and energetically valuable for predators (Cooke et al. 2008). Lakes warm differently due to variation in depth and size. Consequently, the sampling dates for the smaller shallower lakes were between May 8 and 16th, while the larger, deeper lakes were sampled between May 20th and 30th. Metrics of predation pressure are outlined in detail in Gravel and Cooke (2009) and consisted of measuring the maximum number of predators in close proximity to the nest (2 m radius) when the male was present (perceived predation pressure) and measuring the maximum number of predators that actively consumed fertilized eggs when the male was absent (actual predation pressure). The perceived predation pressure was assessed for 5 min. Preliminary analyses (Gravel and Cooke 2009) revealed that assessing the perceived predation pressure for 5 or 15 min yielded the same results. To measure actual predation pressure, males were removed from their nest by rod and reel and kept on board the boat in a cooler (40 L) of fresh lake water for 10 min. The proportion of nests depredated and the time to nest predator arrival in the absence of the male were also noted. The maximum values of nest predators were used in statistical analysis because individual predators could not be identified and reporting means would be misleading. Species composition of the nest predator community was similar among lakes and was composed of pumpkinseed sunfish (Lepomis gibossus), bluegill sunfish (L. macrochirus), and rock bass (Ambloplites rupestri), with the majority (i.e., >90 %) of predators observed being Lepomids.