Abstract
This study quantified size-dependent cannibalism in barramundi Lates calcarifer through coupling a range of prey-predator pairs in a different range of fish sizes. Predictive models were developed using morphological traits with the alterative assumption of cannibalistic polyphenism. Predictive models were validated with the data from trials where cannibals were challenged with progressing increments of prey sizes. The experimental observations showed that cannibals of 25–131 mm total length could ingest the conspecific prey of 78–72% cannibal length. In the validation test, all predictive models underestimate the maximum ingestible prey size for cannibals of a similar size range. However, the model based on the maximal mouth width at opening closely matched the empirical observations, suggesting a certain degree of phenotypic plasticity of mouth size among cannibalistic individuals. Mouth size showed allometric growth comparing with body depth, resulting in a decreasing trend on the maximum size of ingestible prey as cannibals grow larger, which in parts explains why cannibalism in barramundi is frequently observed in the early developmental stage. Any barramundi has the potential to become a cannibal when the initial prey size was <50% of the cannibal body length, but fish could never become a cannibal when prey were >58% of their size, suggesting that 50% of size difference can be the threshold to initiate intracohort cannibalism in a barramundi population. Cannibalistic polyphenism was likely to occur in barramundi that had a cannibalistic history. An experienced cannibal would have a greater ability to stretch its mouth size to capture a much larger prey than the models predict. The awareness of cannibalistic polyphenism has important application in fish farming management to reduce cannibalism.
Highlights
Polymorphism, the occurrence of discrete intraspecific morphs, is triggered by genetic differences, phenotypic plasticity, or a combination of both [1,2]
Mouth width at close (MWc) or open (MWo) presented slightly negative allometric growth as fish grew larger, decreasing from 13% to 9% total length (TL) (15–135 mm TL) and from 17% to 15% TL (35– 135 mm TL), respectively (Figure 1)
The empirical results in the present study showed that barramundi cannibals (25–131 mm TL) could ingest conspecific prey of 78–72% of cannibal TL, respectively
Summary
Polymorphism, the occurrence of discrete intraspecific morphs, is triggered by genetic differences, phenotypic plasticity, or a combination of both [1,2]. Some individuals of Arctic charr exhibit a broader or larger mouth, faster growth rates and more aggressive behaviour than others [10] In aquaculture, these traits are selected for, thereby leading to inadvertent selection of cannibalism in a farmed fish population [11], and causing frequent occurrence of intracohort cannibalism in piscivorous species. Aquaculture conditions enhance the propensity of some individuals to become cannibals due to restriction of fish dispersing, overcrowding, and uneven food distribution, leading to size heterogeneity and cannibalism [11,12,13] As a result, such conditions promote development of cannibalistic polyphenism
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