Developmental changes in anti-predator defences of marine larval fish

Williams, P. James (1994) Developmental changes in anti-predator defences of marine larval fish. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis describes behavioural experiments that investigate anti-predator defences employed by larval fish. The first anti-predator defence investigated was cessation of movement, or freezing. Smaller size classes (mean length 6-10 mm) of larval lumpfish (Cyclopterus luitipus) used the freezing response in the presence of a predator, despite the fact that this behaviour precluded foraging. Interestingly, fifteen week old larvae (mean length 15 mm) no longer used this defence, presumably because the predator no longer posed a threat. -- The second anti-predator defence investigated was the escape response, defined as a period of high acceleration followed by burst swimming. Escape response performance was measured in larval winter flounder (Pleuronectes americanus), ranging in size from newly hatched (3.5 mm TL) to metamorphosed juveniles (10 mm TL) . All escape response performance measurements (mean and maximum speed, distance travelled during the first 100 ms of the response, and total distance travelled) increased with larval length. There was no obvious decrease in performance during metamorphosis, nor was any increased rate of improvement noted after metamorphosis. -- The escape response performance of length ranges of an additional four species of larval fish, including cod (Gadus morhua), capelin (Mailotus villosus), herring fciupea harengus), and radiated shanny (Ulyaria subbifurcata) were measured. These results were combined with the winter flounder data to produce general models for the following performance measurements: mean and maximum speed, distance travelled during the first 17 and 100 ms of the response, and total distance travelled during the response. In all models, except that relating distance during 100 ms to larval length, the logarithm of the performance measurement was significantly linearly related to larval length. Distance travelled during the first 100 ms was linearly related to larval length. -- The potential for increased drag, and subsequent reduced performance during escape responses occurring very near the surface was investigated, in an attempt to partition some of the within-length variation observed in the general models. No overall reduction in performance was detected in responses near the surface; in fact, some performance measurements actually showed significant improvement.

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