The metabolic cost of behavioural tradeoffs during the early life stages of three north-Atlantic marine fishes

Killen, Shaun S. (2007) The metabolic cost of behavioural tradeoffs during the early life stages of three north-Atlantic marine fishes. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis uses studies of physiology and behaviour to better understand the foraging and predator-avoidance tradeoffs experienced by young marine fishes. It has previously been speculated that, due to their small size and high growth rates, larval and juvenile fish may have a limited aerobic capacity compared to adult individuals. This could be an important constraint influencing the behavioural ecology of young fishes, but has been difficult to evaluate because there has been no single study to examine changes in metabolic rate or aerobic scope for any fish species over their entire life history. Thus, an important first step in this thesis was to examine the standard (minimal) metabolism, maximal metabolism, and aerobic scope for three species of north Atlantic marine fish (shorthorn sculpin Myoxocephalus scorpius, ocean pout Zoarces americanus, and lumpfish Cyclopterus lumpus) from the larval stage until adulthood. This research showed that young fish have a greatly increased mass-specific metabolic demand compared to adults, and that their aerobic scope is vastly diminished. The results also suggest that the relationship between aerobic scope and life history stage is greatly influenced by a species' developmental trajectory. -- A second group of projects then investigated the effects of increased mass-specific metabolic demand on anti-predator behaviour in young marine fishes. It is known that most animals will reduce foraging while in the presence of predatory threat, but that individuals are often more risky (i.e. they will continue to forage) during times when they are hungry or have increased energetic requirements. However, despite their high mass-specific energy demands, it was observed that young ocean pout and larval shorthorn sculpin both display large foraging reductions when exposed to predators. Due to the high energetic requirements of these fishes, these results suggest that these foraging interruptions could be very costly. To support this view, these studies also found that frequent, prolonged exposures to a predatory threat caused reduced growth, decreased lipid stores, and reduced ability to acquire certain essential dietary fatty acids that are crucial for normal development. -- A final study then examined how a limited aerobic scope may affect the choice of foraging strategy in young lumpfish - a species that can either actively forage by swimming, or use a "sit-and-wait" foraging strategy while clinging to the substrate using a ventral adhesive disc. Behavioural observations showed that lumpfish predominantly use the 'cling' foraging strategy when prey is abundant, but resort to the more costly 'swim' strategy to seek out food when prey is scarce. The metabolic cost of active foraging was also quantified using swim-tunnel respirometry, and a model was devised to predict the prey density at which lumpfish should switch between foraging modes to maximize energy intake. The results of this model do not agree with previous observations of lumpfish behaviour, and thus it appears that juvenile lumpfish do not try to maximize their net energetic gain. Instead, the data suggest that juvenile lumpfish forage in a manner that reduces activity and conserves space in their limited aerobic scope.

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