Summary
Ecological light pollution occurs when artificial lights disrupt the natural regimes of individual organisms or their ecosystems. Increasing development of shoreline habitats leads to increased light pollution (e.g., from cottages, docks, automobile traffic), which could impact the ecology of littoral zones of lakes and rivers. Smallmouth bass (Micropterus dolomieu) engage in sole paternal care, guarding their nest continually, day and night, to protect their developing offspring. Any alterations to their behaviour—either directly because of the response to light or indirectly due to changes in nest predator activity and associated response of the bass—could lead to increased energetic demands for fish that have a fixed energy budget and ultimately reduce reproductive success. To examine this issue, tri-axial accelerometer biologgers were externally attached to nesting smallmouth bass during the egg stage to determine whether light pollution (i.e., dock lights with low levels of continuous light and spotlights with high intensity irregular light simulating automobile traffic) altered behaviour of nesting males relative to control fish. Our study revealed that both types of light pollution increased overall bass activity level compared with the control group. The intermittent light treatment group had the highest activity and exhibited large fluctuations between night and day activity levels. Fish in the continual light treatment group displayed statistically higher activity than the control fish but showed limited fluctuations between day and night activity levels. Our results suggest that continuous or intermittent light sources, common in shoreline habitats that have been developed, have the potential to alter the behaviour and thus energy use of nest-guarding fish. This study contributes to the growing body of literature on the ecological consequences of light pollution in aquatic ecosystems.
Methodology
Fish Selection
Nest-guarding male smallmouth bass were located through snorkel surveys. Fish size, brood size and offspring developmental stage were visually assessed by a diver. Brood size was qualitatively scored using a scale of 1 to 5 (1 = low to 5 = high as outlined by Philipp et al. (1997)) with suitable scores considered as 3 and above. Nest-tending behaviours vary among egg development stages; so, only fish guarding eggs were used (Cooke et al. 2002). All fish sampled were between 380 and 510 mm in total length. Fish below 380 mm were deemed too small for accelerometer attachment, as the accelerometer tag would exceed 2 % of the fish’s total body weight if we used smaller fish, which could impede swimming behaviour (Brown et al., 1999). Though mass was not directly measured, fish in this size range are expected to exceed 1000 g, as calculated from known length-weight relationship of smallmouth bass in Lake Opinicon (Dey et al. 2010). The equation was log10 mass = −7.1004 × 3.884(log10 TL) with mass reported in grams and total length reported in millimetres.
Accelerometer Attachment
Once a suitable nest-guarding male was located (spaced at least 50 m from nearest nest that was part of experiment or in an area that was out of the direct influence of a treatment such as on the opposite side of a treed island), the nest was marked with a numbered PVC tile. All nests used in the experiment were in ∼0.5 m of water. The guarding male was angled off its nest using a variety of lures and bait. Fight time was minimized to less than 20 s to reduce stress associated with anaerobic exercise and capture (Cooke et al. 2003). While the fish was away from its nest, a snorkeler remained in the water and protected the nest from brood predators using a blunt pole. Following capture, each fish was measured (total length) and held in place in a foam lined trough filled with fresh lake water. Accelerometers were secured using tape to plastic and foam frontal and backing plates with 22.7 kg strength braided line threaded through the musculature on the back of the fish near the anterior aspect of the soft dorsal fins (Brownscombe et al. 2014). Accelerometers had an average weight of 28 g in air (∼18 g in water) including the backing plates, tape and braided line. Prior to release, the fish was rotated along its horizontal and vertical axes, and the time was noted to calibrate the device at the start of accelerometer logging (as per Brownscombe et al. 2014). The snorkeler left the nest area after the bass returned and resumed parental care duties (generally in <2 min).