• Xia, Jigang
  • Elvidge, Chris K.
  • Cooke, Steven J.


Behavioral responses to alarm cues in aquatic species are typically examined with emphasis on the potential survival benefits accrued by conspecific receivers. By contrast, heterospecific responses to alarm cues and changes in responses with ontogeny in fishes are relatively unexplored. Taking an ecological niche perspective, we hypothesized that the response patterns of fish to risky chemical cues should be closely related to their degree of niche differentiation, which increases with ontogeny. We tested this hypothesis using the responses of adults from sympatric bluegill (Lepomis macrochirus) and pumpkinseed (L. gibbosus) populations to the alarm cues of conspecific and heterospecific adults and juveniles, including water as a control treatment. Responses measured consisted of changes in body posture (time spent with the dorsal fin 60°) and behavior (times spent still, frozen, at the surface, or on the bottom of the tank). Both adult bluegill and pumpkinseed spent significantly more time with their fins held >60° in response to adult versus juvenile alarm cues, with these responses mediated by donor species such that adult conspecific cues resulted in greater responses than heterospecific cues. The same general pattern was observed in the behavioral measures. These results demonstrate that behavioral response patterns to chemical alarm cues in sunfishes are highly plastic and are likely related to niche separation in adults. Our findings open new lines of research into the role of ecological niches in shaping behavioral responses of fish to risky information.


Fish collection and housing

Adult bluegill and pumpkinseed were collected via angling using live bait (earthworms Lumbricus spp.) and size 8 J-hooks from a boat on Lake Opinicon at the Queen’s University Biological Station (Chaffey’s Lock, ON, Canada) during late May to early July 2016. Only fish that were shallowly hooked (through the lip) were retained and held in species-specific coolers filled with lakewater (mean temperature 20.1 °C), that was changed via bucket at 15 min intervals. The fish were transferred to one of two 500-L circular, flow-through holding tanks continuously supplied with lakewater within 2  h of capture. Fish were held for at least 24 h prior to testing to allow them to acclimate to conditions under captivity. None of the retained fish died during transport or acclimation. After the acclimation period, only visibly healthy bluegill (69.0 ± 2.02 g, 13.2 ± 0.11 cm standard length, N = 75) and pumpkinseed (86.3 ± 2.98 g, 13.7 ± 0.14 cm standard length, N  =  75) were used as experimental fish and any individuals demonstrating injury or debilitation were released back to the lake. Once a fish had been assayed, it was measured (standard length and weight) and released back into Lake Opinicon in an area away from where fish collections occurred.

Alarm cue preparation

Two adult bluegill (14.6  cm and 15.8  cm standard length) and 2 adult pumpkinseed (14.4 cm and 14.7 cm standard length) captured via angling, as well as 18 juvenile bluegill (5.97 ± 0.16 cm standard length) and 14 juvenile pumpkinseed (6.53  ±  0.10  cm standard length) captured using a beach seine were used as alarm cue donors. Donors were euthanized via cerebral percussion followed by decapitation and lateral skin filets were immediately removed to produce alarm cues. In total, we collected 194.8, 233.2, 204.3, and 223.5  cm2 of skin from adult bluegill, adult pumpkinseed, juvenile bluegill, and juvenile pumpkinseed, respectively. Skin tissue was mechanically homogenized in chilled lakewater and filtered through polyester floss to remove debris. The final concentrations were adjusted to 0.1  cm2/mL with lakewater. The 4 alarm cue solutions, as well as lakewater as a control treatment, were packaged in 20 mL aliquots and frozen at −20 °C until use.

Experimental protocol

Trials were conducted in 3 glass aquaria (60 cm length × 30 cm width × 30 cm height) with capacity of 45 L. Each tank was equipped with a single air stone attached to the side opposite the camera. An additional 2-m length of airline tubing was attached to the air stone through which chemical stimuli (20 mL) could be injected without disturbing the test fish. The tank was divided into 3 horizontal sections on the exterior of the tank to facilitate the recording of behavioral changes in time spent at the surface or on the bottom of the tank. Surface swimming and bottom-dwelling behavior were defined as those that located within 5 cm from the surface or bottom of the aquarium, respectively.