Size-selective harvesting associated with commercial and recreational fishing practices has been shown to alter life history traits through a phenomenon known as fishing-induced evolution. This phenomenon may be a result of selection pathways targeting life-history traits directly or indirectly through correlations with behavioral traits. Here, we report on the relationship between individual differences in behavior and capture technique (beach seining versus angling) in wild-caught juvenile bluegill sunfish (Lepomis macrochirus). Both fish caught by using a seine net (seined) and fish caught by using a lure (angled) were individually tested under standardized laboratory conditions for their boldness, water-column use, and general activity. Observed inter-individual differences in boldness were strongly correlated with method of capture in the wild. Fish caught by angling were more timid and had fewer ectoparasites than fish caught using a seine net. However, this relationship did not carry over to an experiment in a large outdoor pool with seine-caught, individually tagged wild fish, where bolder individuals were more likely to be angled in open water away from refuges than more timid individuals, based on their previously assessed boldness scores. Our study is both novel and important, as it describes the relationship between capture technique and boldness in a natural population and underscores the potential risk of sampling biases associated with method of animal capture for behavioral, population, and conservation biologists.
Between 10 June and 20 July 2009, we used two different collection techniques to capture 230 juvenile (year 1+) bluegill sunfish (total length (TL), 98-148 mm; mass, 14.1-58.5 g) from the shallow littoral zone of several small bays located >600 m apart in Lake Opinicon, Ontario, Canada (44°34'N, 76°21'W). One of these collection techniques involved the use of a 20 m beach seine (5 mm mesh size) to capture a subset of the above individuals (N = 120, 98-148 mm TL, 14.9-51.8 g). Seining involved making numerous attempts of short duration (2-3 min for net movement, 5-7 min for fish collection from the net). Such brief seining attempts were used to minimize physical injury and stress among captured fish. Following each attempt, all captured fish of the appropriate size were immediately placed inside a cooler with fresh lake water (see below).
The second collection technique involved a standardized 10 s angling protocol by which we captured the remaining portion of individuals used in the study (N = 110, 99-148 mm TL, 14.1-58.5 g) over several collection trips. This short angling duration (i.e., 10 s) is common for sunfish (Cooke et al. 2003). These individuals were captured (angled) from the same bays as those sampled with the beach seine. Only small, barbless hooks baited with a 1 cm piece of earthworm (Lumbricus sp.) were used to capture fish. Moreover, individuals had to be caught, have the hook removed, and be placed inside a cooler within 10 s to be included in our study. Any individuals caught that did not follow this protocol, or individuals that were injured (e.g., any evidence of bleeding) as a result of angling practices, were excluded from further study. This protocol insured that any physical trauma and stress associated with capture would be minimized.
Experimental holding conditions and experimental apparatus
On arrival at the laboratory, fish captured either by angling or seining were placed in one of two large holding tanks (171 L, 61 cm x 67 cm x 42 cm, L xWx H). Each holding tank was aerated and continually provided with fresh water from the lake via a flow-through system. The tanks were exposed to ambient sunlight through several windows in the laboratory. All fish were held overnight prior to experimental testing the following day. Since angled and seined fish were, respectively, caught on consecutive days (not concurrently), the particular holding tank assigned to each group was randomly assigned each week to reduce the likelihood of any holding tank bias. Each holding tank was equipped with six plastic aquarium plants of various sizes (height range = 20-60 cm) to provide cover and minimize agonistic interactions between fish. The density of sunfish (<20 on average) held in each holding tank falls within the range of densities observed in free-ranging juvenile bluegill sunfish in the shallow littoral zone of Lake Opinicon and similar nearby lakes in eastern Ontario (A.D.M. Wilson, personal observation). Fish were fed according to a standardized protocol such that they were provided commercial flake food (TetraMin) ad libitum following Experiment 1 and prior to being moved to an outdoor pool for Experiment 2. Fish were not fed on the day of their capture to avoid confounding variables associated with any differences in feeding caused by stress during capture and experimental holding.
During behavioral testing, each fish was placed in one of two identical glass aquaria (82 L, 92 cm x 30 cm x 30 cm). Each aquarium contained lake water (23 ± 1 °C; replaced daily) and was exposed to overhead fluorescent lighting. Each aquarium was divided into thirds, both horizontally and vertically, with lines drawn on the front panel. In doing so, each aquarium was divided into nine distinct zones of equal size to facilitate the observation of fish water column use. A refuge area, consisting of three plastic aquarium plants for cover, was located on one side of the test aquarium and was separated from the remainder of the aquarium by an opaque white plastic partition equipped with a sliding door. This gated partition was located 25 cm from the left side of the aquarium (see Fig. 1 in Wilson and Godin 2009). Aquaria were covered externally with brown cardboard at both ends and the back wall to prevent external disturbance and interaction between subjects.