Does size‐selective harvesting erode adaptive potential to thermal stress?

Abstract

Overharvesting is a serious threat to many fish populations. High mortality and directional selection on body size can cause evolutionary change in exploited populations via selection for a specific phenotype and a potential reduction in phenotypic diversity. Whether the loss of phenotypic diversity that accompanies directional selection impairs response to environmental stress is not known. To address this question, we exposed three zebrafish selection lines to thermal stress. Two lines had experienced directional selection for (1) large and (2) small body size, and one was (3) subject to random removal of individuals with respect to body size (i.e. line with no directional selection). Selection lines were exposed to three temperatures (elevated, 34°C; ambient, 28°C; low, 22°C) to determine the response to an environmental stressor (thermal stress). We assessed differences among selection lines in their life history (growth and reproduction), physiological traits (metabolic rate and critical thermal max) and behaviour (activity and feeding behaviour) when reared at different temperatures. Lines experiencing directional selection (i.e. size selected) showed reduced growth rate and a shift in average phenotype in response to lower or elevated thermal stress compared with fish from the random‐selected line. Our data indicate that populations exposed to directional selection can have a more limited capacity to respond to thermal stress compared with fish that experience a comparable reduction in population size (but without directional selection). Future studies should aim to understand the impacts of environmental stressors on natural fish stocks.

Directional selection, not the direction of selection, affects telomere length and copy number at ribosomal RNA loci

“…Short relative telomere length associating with directional selection on body size is intriguing as, after ten generations of recovery, both directionally selected lines had lower growth rate and reached smaller adult body size than sh which had not experienced directional selection (Sadler et al, 2024). As telomeres shorten with cell division, unless repaired by telomerase (Chan and Blackburn, 2004), fast growing, larger individuals (random-selected sh) are expected to have shorter telomeres.…”

“…After ve generations of harvesting, small-selected sh were smaller, had higher juvenile growth rate and higher reproductive investment than large-selected sh (Uusi-Heikkilä et al 2015). After ten generations of no-harvesting (a 'moratorium'), the random-selected sh had a 12% faster growth rate than both the large-and small-selected lines (which had similar growth rates) (Sadler et al, 2024). In our model there is a contrast between the large-and small-selected lines that both experienced directional selection on body size in contrast to the random-selected line that experienced the same population reduction (75% harvesting) but no directional selection on body size.…”

“…Directional selection may elicit a faster loss of genetic diversity than expected under a population reduction alone(Frankham, 2012). Although we do not measure inbreeding or genetic diversity, slower growth associated with directional selection may indicate that these lines experience some inbreeding depression whose effects extend to telomere maintenance(Sadler et al, 2024). However, the relationship between telomere length and inbreeding is controversial.…”