Size selective harvesting does not result in reproductive isolation among experimental lines of Zebrafish, danio rerio: Implications for managing harvest-induced evolution

Abstract

Mortality in fish populations is commonly based on body-size. In fisheries, larger fish are preferentially caught while in nature, predators eat smaller fish. Removal of fish of certain size from stocks generation after generation is a selective force which causes changes in behaviour, growth rate and adult body-size. Since behaviour and body-size are related, and influence mating preferences and determine reproductive ability, size-selective mortality may result in populations to not mate with each other. Our aim is to test this possibility using three experimental lines of zebrafish generated by removing large-sized, small-sized and random fish for five generations. We tested mating preferences among males and females, and whether they spawned together. We found that males and females across selection lines mated and reproduced with each other. Females generally preferred to be close to large sized males. Females of all lines readily spawned with males of other lines, and males of all lines fertilised eggs from females of other lines. Our study shows that size-selective mortality like in fisheries or due to natural predation does not result in mating and reproductive barriers. Thus, reproduction among fishery-exploited and unexploited populations would help exploited populations to recover. Size-selective mortality is common in fish stocks. Positive size-selection happens in fisheries where larger size classes are preferentially targeted while gape-limited natural predation may cause negative size-selection for smaller size classes. As body size and correlated behavioural traits are sexually selected, harvest-induced trait changes may promote prezygotic reproductive barriers among selection lines experiencing differential size-selective mortality. To investigate this, we used three experimental lines of zebrafish (Danio rerio) exposed to positive (large-harvested), negative (small-harvested) and random (control line) size-selective mortality for five generations. We tested prezygotic preferences through choice tests and spawning trials. In the preference tests without controlling for body size, we found that females of all lines preferred males of the generally larger small-harvested line. When the body size of stimulus fish was statistically controlled, this preference disappeared and a weak evidence of line-assortative preference emerged, but only among largeharvested line fish. In subsequent spawning trials, we did not find evidence for line-assortative reproductive allocation in any of the lines. Our study suggests that size-selection due to fisheries or natural predation does not result in reproductive isolation. Gene flow between wild-populations and populations adapted to size-selected mortality may happen during secondary contact which can speed up trait recovery.