Biophysical Simulations Support Schooling Behavior of Fish Larvae Throughout Ontogeny

Abstract

Schooling is very common in adult and juvenile fish, but has been rarely studied during the larval stage. Recent otolith micro-chemistry studies of coral reef fish have demonstrated that cohorts of larvae can move through similar paths and settle within a few meters one from another. However, little is known about the processes involved in the formation and maintenance of these cohorts. Here we use a biophysical modeling approach to examine whether local hydrodynamics, various individual behaviors, or larval schooling can explain cohesive patterns observed for Neopomacentrus miryae in the Gulf of Aqaba/Eilat (Red Sea), and whether schooling is feasible in terms of initial encounter probability and cohesiveness maintenance. We then examine the consequences of schooling behavior on larval settlement success and connectivity. Our results indicate that: (1) Schooling behavior is necessary for generating cohesive dispersal patterns, (2) Initial larval encounter of newly-hatched larvae is plausible, depending mainly on initial larval densities and patchiness, and (3) schooling behavior increases the rate of larval settlement while decreasing the percentage of realized connections. Together with mounting evidence of cohesive dispersal, this numerical study demonstrates that larval schooling throughout the pelagic phase is realistic, and has a significant effect on settlement success and connectivity patterns. Future research is needed to understand the mechanisms of fission-fusion dynamics of larval cohorts and their effect on dispersal. Our findings should be considered in future efforts of larval dispersal models, specifically in the context of marine connectivity and the planning of marine protected area networks.

Physical connectivity simulations reveal dynamic linkages between coral reefs in the southern Red Sea and the Indian Ocean

“…We acknowledge that to explore the observed genetic break for various species in the Red Sea, we investigated the circulation-driven physical connectivity where the simulation of larval behaviours were omitted and particles were defined as passive. Previous studies using biophysical models have revealed that the simulation of larval behaviours may alter the model results 65,66,70 . On the other hand, Raitsos et al .…”

Marine fish larvae consistently use external cues for orientation

“…In our analyses, every larva is gauged against its own potential of autocorrelated directional precision, and can be readily compared against other larvae from either the same or different species ( Figure 2). CRW simulations assume a von Mises distribution of Δθ, which is widely used to simulate CRW 7,10,[20][21][22] . Yet, this distribution may not perfectly represent the real distribution of Δθ.…”

“…Larval directional movement using external cues 21,[24][25][26] and autocorrelated swimming 21,22,27 were previously implemented in biophysical models of larval dispersal, largely demonstrating in both cases that these behaviors substantially affect dispersal, increase settlement success and alter connectivity patterns 21,22,[24][25][26] . The mathematical methodology behind such an implementation was generally described in 10 , and is largely based on a sampling of swimming directions from a von Mises distribution centered around the direction of the cue source for orienting larvae, or around the swimming direction in the previous time step for autocorrelated movement 21,22 . In both cases, kappa, the von Mises distribution concentration parameter, governs the simulated precision of directional movement.…”

Perspectives on Larval Behaviour in Biophysical Modelling of Larval Dispersal in Marine, Demersal Fishes

“…Settlement-stage fish larvae of several species are known to school, shoal or otherwise aggregate in the pelagic environment prior to settlement (see summary in [60]), but most of these species also aggregate to a greater or lesser extent following settlement. So, in such species, this behaviour may be more related to what the larvae do after, rather than before, they settle [31].…”

“…Furthermore, genetic or otolith microchemistry evidence implies group dispersal over the full PLD in some situations in two species (a Red Sea pomacentrid, genus Neopomacentrus, that schools post settlement [62], and a New Zealand cryptobenthic tripterygiid [63]), indicating that a wider variety of species might have aggregating larvae than previously thought. In the case of the Red Sea pomacentrid, modelling indicated that schooling behaviour was necessary to achieve the patterns observed [60]. Although the model assumed an ontogenetic downward vertical migration to >25 m, the settlement-stage larvae of another species of Neopomacentrus are known to have modal depths of <5 m during the day [64].…”