Predation is considered one of the main costs to reproduction but is rarely examined from a physiological perspective. In particular, little is known about the influence of brood predation pressure on the physiology of parents engaged in care. Brood defense, even when there is no direct threat to the parent, can be costly as it requires constant vigilance and chasing predators to protect the developing brood and maintain parental investment (i.e., fitness). Our goal was to examine the influence of natural variation in nest predation pressure on the physiology of the teleost smallmouth bass Micropterus dolomieu, an animal that provides sole-paternal care for developing offspring. More specifically, we used indicators of anaerobic (lactate dehydrogenase [LDH]) and aerobic capacity (cytochrome c oxidase [CCO] and citrate synthase [CS]) in axial white muscle and pectoral red muscle to test for differences in antipredator performance of nest guarding males across six lakes with natural variation in nest predation pressure. Pectoral red muscle enzyme activities and protein concentrations were highly conserved among populations, while axial white muscle showed differences in LDH activities, CCO activities and protein concentrations. However, there was no evidence for higher metabolic capacities in fish from lakes with increased brood predation pressure. Clearly, factors other than predation pressure have a greater influence on white muscle metabolic capacities. Additional research is needed to clarify the extent to which biotic and abiotic factors influence the enzyme activity and organismal performance in wild animals, particularly at the level of the individual and population.
Fish were sampled from six lakes within a single ecoregion in southeastern Ontario, Canada: Big Rideau Lake, Charleston Lake, Indian Lake, Newboro Lake, Opinicon Lake and Sand Lake. Study lakes were chosen due to the inherent variation in nest predation pressure as documented and described in Gravel and Cooke (2009) with a series of metrics such as number of predators in proximity to nests when male is present (perceived predation pressure) and when male is absent (actual predation pressure), time to egg consumption in the absence of males and proportion of nests predated. By using non-parametric ranking tests, lakes were ordered from lowest to highest in nest predation pressure: Big Rideau Lake < Newboro Lake ≤ Charleston Lake < Indian Lake < Sand Lake < Opinicon Lake. Lake surface area, mean depth and predation pressure metrics are summarized in Table 1 (taken from Gravel and Cooke (2009) ). These predation pressure metrics were measured on the same individuals which were sampled for muscle enzyme activities. Predation pressure metrics were again measured in 2008 and 2009 and lake ranking has been very similar, lakes with the lowest and highest predation pressure rank identically over the years with some variation in the medium predation pressure lakes (Gravel, M.-A. unpublished data).
Within this ecoregion, differences in lake depth and turbidity cause lakes to warm differentially, allow for temporal variation in peak spawning dates (Kubacki et al., 2002) and enable data collection within one spawning year. At the onset of spring, the six lakes were visited daily by snorkelers. Portions of the littoral zone were swum (approx. 1 to 3 km) and when present, parental males on fresh eggs were identified (n ≤ 30) and nests were labeled with a numbered tile. All data collection occurred during May and June of 2007. Fish were sampled on fresh eggs and were collected by rod and reel (using heavy angling gear — all angling durations < 20 s) within 3 days of egg deposition for physiological analysis of adult males (n = 10 nesting adult males per lake).
Parental male fish were removed from their nest and placed in a foam-lined trough filled with fresh lake water for hook removal. Fish were then euthanized by cerebral percussion within 2 min of being on board the boat. Pectoral red muscle and axial white muscle samples were taken with a disposable scalpel, wrapped in foil, and immediately placed in liquid nitrogen until later transfer to a − 80 ºC freezer. Pectoral red muscle was taken anterior and ventral to the pectoral fin, when it laid flat against the fish, while the axial white muscle sample was taken mid-way down the body, 1 cm above the lateral line.