Summary
Predation risk has the ability to greatly influence the behaviour of reproducing individuals. In large long‐lived species with low risk of predation for parents, reproductive behaviours often involve caring for offspring (i.e. defending broods from predators) and these behaviours are essential for offspring survival. Our objectives were to test for the presence of natural variation in nest predation pressure in an aquatic environment for a species that provides sole‐paternal care, smallmouth bass (Micropterus dolomieu), and to determine if natural variation in predation pressure influences parental care behaviour. We used snorkeler observations and a series of metrics to assess predation pressure and parental care behaviour in six lakes within a narrow geographical range. Lakes differed in all predation pressure metrics: number of predators in proximity to nest when males were present, time to predator arrival and number of predators that consumed eggs when males were absent and total number of nests that was preyed upon. Similarly, parental behaviour varied between lakes. Parental smallmouth bass spent more time engaged in anti‐predator defences in lakes with high predation pressure, while males from low predator pressure lakes remained close to their nest. Conversely, males from lakes with low and high predation pressure showed a similar willingness to defend their nests during simulated nest predation events. Our results show that natural variation in aquatic nest predation pressure across multiple lakes can be significant and has the ability to influence baseline parental care behaviour. Such variation provides opportunities to study the costs and consequences of parental care and to evaluate how this could influence demography and community interactions in aquatic systems.
Methodology
Study Area
Smallmouth bass were studied in six lakes in the spring of 2007 (<50 km between most distant lakes) within the same ecoregion in southeastern Ontario (Big Rideau Lake, Charleston Lake, Indian Lake, Newboro Lake, Opinicon Lake and Sand Lake). The last smallmouth bass stocked in Ontario was in 2000 and hatchery production (at the provincial level) has been negligible since the 1930s, hence there should be minimal influence from supplementation (Kerr 2006).
In the spring when temperatures reach approx. 15°C, male bass move into the littoral zone where they sweep out a nest in the substrate with their caudal fin, court females, spawn, and then provide parental care to the brood until the offspring become independent. Because of the ecological differences between lakes such as depth and turbidity, lakes warm differentially and allow for temporal variation during the reproductive season. Peak spawning dates, even within a small geographic region such as southeastern Ontario, can vary by approx. 10 d (Kubacki et al. 2002) enabling research to take place in multiple lakes within a small geographic area in a single season. Lakes were chosen because of their proximity to each other (less than 50 km) and the indication that they showed inherent variation in nest predation pressure (based on interviews of biologists with the Ontario Ministry of Natural Resources and local sunfish researchers, Frank Phelan and David Philipp). As the reproductive season began, snorkelers swam a subset of the littoral zone of each lake to identify the location of approx. 30 nesting males on eggs (≤ 4 d). Distances swam in each lake ranged approx. between 1 and 3 km. Snorkelers estimated the egg score (ES) in each nest [a categorical metric from a low of 1 to a high of 5 (Kubacki et al. 2002)] and age of eggs (fresh eggs are golden with a visible oil droplet and gradually whiten within a few days). Factors such as male size, number and age of eggs are known to affect the behaviour of nest guarding males (Ridgway 1988, 19889; Suski et al. 2003) and were considered in analyses. Individual nests were identified with a numbered marker. Study sites within a lake were selected based upon previous research by our team or colleagues and focused on areas with appropriate spawning substrate (i.e. coble and gravel). All sampling occurred from May to June 2007. All observations were collected at the egg stage when predation pressure can be quite high because fresh eggs are energetically valuable to predators and can be easily captured because they are immobile.
Lake Predation Pressure
We used several metrics to establish the level of nest predation pressure in the six lakes. All observations were made by a snorkeler and were recorded on dive‐slates. The first metric directly quantified the predation pressure of each smallmouth bass nest. This study was performed in conjunction with others (M.‐A.Gravel, unpubl. data) which required relocating unmanipulated individuals at a later date. For this reason, only a proportion of individuals were used in this study. To select individuals, one in every three nests were chosen along transects swam by snorkelers. Overall, 10 smallmouth bass nests were chosen from the nests previously marked by the snorkeler and were observed for 15 min with the snorkeler positioned 3 m from the nest. There was a short 1–2 min acclimation period but typically this distance was sufficient in preventing the disturbance of normal parental care behaviour. Only five of 59 fish reacted to the snorkeler and were removed from the baseline behaviour analysis. At 30 s intervals, the snorkeler recorded the number and species of nest predators that were within 2 m of the nest. The maximum number of predators within the 15‐min period was determined for each nest because individual predators could not be identified and reporting means would be ambiguous. We considered fish to be potential nest predators if they had been previously reported as being such for bass nests in the literature or if we had observed them doing so. For the purpose of this study that list included bluegill (Lepomis macrochirus ), pumpkinseed (Lepomis gibbosus ), rock bass (Ambloplites rupestris ), yellow perch (Perca flavescens ), black crappie (Pomoxis nigromaculatus ), largemouth bass (<15 cm) (Micropterus salmoides ) and conspecifics (<15 cm). The 2 m distance was chosen to ensure the same amount of visibility across all lakes. Moreover, previous studies of smallmouth bass have revealed that bass actively defend against predators within 2 m of the nest (e.g. Cooke et al. 2008). After the conclusion of the 15‐min observation period, the fish was removed from the nest by rod and reel. The snorkeler observed the nest area for an additional 15‐min period and noted the time elapsed between the removal of the parent and the arrival of the first nest predator. At each 1 min interval, the snorkeler would also note the number and species of nest predators present and engaged in consumption of eggs at the abandoned nest.