Selective mortality on early life-history traits of a temperate reef fish

<p>Variability in recruitment and early life-history traits is widespread in many marine organisms. Phenotypic variation is particularly prevalent in the early life-history stages (e.g., larvae and juveniles) of reef fish, and provides the basis for selective mortality on growth and size-related traits, with important ecological and evolutionary consequences. Recruitment variability can alter the effective densities experienced by these early life stages, raising additional questions about the interplay between selection and density-dependent processes. While many examples of growth- and size-selective mortality have been documented for young reef fish (typically caused by predators), few studies consider how the strength and/or direction of selective mortality changes with ontogeny, or how these patterns may be mediated by density. I explore spatio-temporal variability in early life-history traits of the common triplefin, Forsterygion lapillum, using metrics derived from otoliths (a re-analysis of two previously collected data sets). I evaluate patterns of variation in traits with respect to early life-history stage (either larvae or post-settlement juveniles) and document shifts in the distributions of traits that are consistent with selective mortality favouring slower growing individuals. I conclude that a cohort of juveniles (sampled after settlement) was comprised of individuals that were smaller at hatch and grew slowly throughout the pelagic larval period relative to a cohort of larvae (sampled prior to settlement). I then conducted an experiment using a set of mesocosms to evaluate whether selective mortality on early life-history traits in common triplefin could be caused by a natural predator, the variable triplefin, Forsterygion varium. Specifically, I exposed groups of fish of each stage to a pair of predators and I used otoliths to reconstruct the traits of fish that survived versus fish that were consumed (i.e., I recovered otoliths from the guts of predators). Selection trials were conducted across realistic density gradients for each developmental stage. Fish size was negatively correlated with relative fitness for larvae (indicating larger fish were consumed preferentially by predators) but not for juveniles (where no size-selective mortality was observed). These patterns were consistent across the range of densities evaluated. Both larvae and juveniles experienced significant selection against fast larval growth (estimated from growth increments in otoliths), and the strength of selection was inversely related to density (i.e., strongest at lower densities, weakest at higher densities). However, juveniles also experienced selective predation for fast growth at the larval-juvenile transition. As with larval growth, selection was strongest at lower densities and weakest at higher densities. Collectively, these results suggest that predators may preferentially target larger larvae, and faster growing individuals regardless of developmental stage. However, this effect may be mediated by density, such that the strongest selection occurs during low recruitment. Density-dependent selection could explain how faster growing individuals can survive this vulnerable stage. These results provide evidence for carry-over effects of larval growth on juvenile survival, and suggest conspecific density should be considered when evaluating patterns of selective mortality.</p>

Selective mortality on early life-history traits of a temperate reef fish

<p>Variability in recruitment and early life-history traits is widespread in many marine organisms. Phenotypic variation is particularly prevalent in the early life-history stages (e.g., larvae and juveniles) of reef fish, and provides the basis for selective mortality on growth and size-related traits, with important ecological and evolutionary consequences. Recruitment variability can alter the effective densities experienced by these early life stages, raising additional questions about the interplay between selection and density-dependent processes. While many examples of growth- and size-selective mortality have been documented for young reef fish (typically caused by predators), few studies consider how the strength and/or direction of selective mortality changes with ontogeny, or how these patterns may be mediated by density. I explore spatio-temporal variability in early life-history traits of the common triplefin, Forsterygion lapillum, using metrics derived from otoliths (a re-analysis of two previously collected data sets). I evaluate patterns of variation in traits with respect to early life-history stage (either larvae or post-settlement juveniles) and document shifts in the distributions of traits that are consistent with selective mortality favouring slower growing individuals. I conclude that a cohort of juveniles (sampled after settlement) was comprised of individuals that were smaller at hatch and grew slowly throughout the pelagic larval period relative to a cohort of larvae (sampled prior to settlement). I then conducted an experiment using a set of mesocosms to evaluate whether selective mortality on early life-history traits in common triplefin could be caused by a natural predator, the variable triplefin, Forsterygion varium. Specifically, I exposed groups of fish of each stage to a pair of predators and I used otoliths to reconstruct the traits of fish that survived versus fish that were consumed (i.e., I recovered otoliths from the guts of predators). Selection trials were conducted across realistic density gradients for each developmental stage. Fish size was negatively correlated with relative fitness for larvae (indicating larger fish were consumed preferentially by predators) but not for juveniles (where no size-selective mortality was observed). These patterns were consistent across the range of densities evaluated. Both larvae and juveniles experienced significant selection against fast larval growth (estimated from growth increments in otoliths), and the strength of selection was inversely related to density (i.e., strongest at lower densities, weakest at higher densities). However, juveniles also experienced selective predation for fast growth at the larval-juvenile transition. As with larval growth, selection was strongest at lower densities and weakest at higher densities. Collectively, these results suggest that predators may preferentially target larger larvae, and faster growing individuals regardless of developmental stage. However, this effect may be mediated by density, such that the strongest selection occurs during low recruitment. Density-dependent selection could explain how faster growing individuals can survive this vulnerable stage. These results provide evidence for carry-over effects of larval growth on juvenile survival, and suggest conspecific density should be considered when evaluating patterns of selective mortality.</p>

Recruitment patterns of freshwater amphidromous fishes (Pisces: Gobiidae, Eleotridae) to the Cimaja estuary, Palabuhanratu Bay

The sustainability of amphidromous fishes is determined by the success of the larval recruitment process from marine to freshwaters habitats. This study aimed to determine the recruitment pattern of freshwater amphidromous fish to the Cimaja River estuary, Palabuhanratu Bay in terms of season and daily. Amphidromous fish were caught monthly on the 25th of Hijri or waning crescent from December 2020 to August 2021. Sampling in one whole day with observation intervals every 4 hours was carried out in June 2021 to reveal the time of larvae and juveniles of the amphidromous fish recruit to the Cimaja River estuary. A total of 13 species of amphidromous fishes representing five genera and two families was captured. The recruitment of amphidromous fishes into the Cimaja River occurs between the peak of the rainy season to dry season. Generally, amphidromous recruit into the estuary waters of the Cimaja River is at the juvenile stage (11.9-21.8 mm body length). However, the recruitment of some amphidromous fish species appears at the postflexion stage (6.9-11.9 mm BL). Based on diel observations, larvae and juveniles of amphidromous fish are generally preferred to recruit to the Cimaja River estuary in the early morning (03:00 am) and afternoon (03:00 pm). Recruitment of amphidromous fish larvae to the Cimaja River follows a semi-diurnal tidal cycle which is a tidal type in Palabuhanratu Bay. Amphidromous fishes use tidal flux to recruit and migrate upstream of the Cimaja River through estuaries.