A Review of Biophysical Models of Marine Larval Dispersal

Abstract

Larval dispersal is arguably the most important but least understood demographic process in the sea. The likelihood of a larva dispersing from its birthplace to successfully recruit in another location is the culmination of many intrinsic and extrinsic factors that operate in early life. Empirically estimating the resulting population connectivity has been immensely difficult because of the challenges of studying and quantifying dispersal in the sea. Consequently, most estimates are based on predictions from biophysical models. Although there is a long history of dispersal modelling, there has been no comprehensive review of this literature. We conducted a systematic quantitative review to address the following questions: (1) Is there any bias in the distribution of research effort based on geographical or taxonomic coverage? (2) Are hydrodynamic models resolving ocean circulation at spatial scales (resolution and extent) relevant to the dispersal process under study? (3) Where, when and how many particles are being tracked, and is this effort sufficient to capture the spatiotemporal variability in dispersal? (4) How is biological and/or behavioural complexity incorporated into Lagrangian particle tracking models. (i.e. are key attributes of the dispersal process well captured.)? Our review confirms strong taxonomic and geographic biases in published work to date. We found that computational 'effort' (i.e. model resolution and particle number) has not kept pace with dramatic increases in computer processor speed. We also identified a number of shortcomings in the incorporation of biology, and behaviour specifically into models. Collectively, these findings highlight some important gaps and key areas for improvement of biophysical models that aspire to inform larval dispersal processes. In particular, we suggest the need for greater emphasis on validation of model assumptions, as well as testing of dispersal predictions with empirically derived data.

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

“…Calls for "a deep integration of oceanography and behavioural ecology" (e.g., [12]) in dispersal modelling are not uncommon. In spite of this, what Swearer et al [8] call the "exemplary manual of recommended practices for biophysical modelling by " which includes explicit advice about incorporating behaviour of larvae has been cited only 76 times between 2009 and 2017, whereas the number of publications over the same period that used biophysical dispersal models was more than 700 [8]. Citations per year of North et al [13] increased to a high of 16 in 2015, but averaged only 8.5 per year since then.…”

“…To this should be added settlement competency, although strictly speaking, it is not a behaviour, but an ontogenetic developmental milestone. Only 6.5% of larval-fish dispersal models (and 5% of models overall) published between 1980 and 2017 reviewed by [8] included horizontal swimming by larvae. The review of [8] did not explicitly address the use of swimming orientation in dispersal models except in the context of detecting and orientating toward settlement habitat, and only 5% of all models included this behaviour.…”

“…It can no longer be a justifiable simplifying assumption to ignore such abilities when attempting to model dispersal. However, a recent review of biophysical larval dispersal modelling [8] (across all marine taxa, not just fish) showed that a majority of models (56%) assumed that larvae were passive throughout their PLD, with 41% assuming some form of active behaviour for all or part of the PLD. Surprisingly, the likelihood of published marine larval dispersal models assuming passive larvae increased marginally over the period 1980-2017 [8].…”

“…However, a recent review of biophysical larval dispersal modelling [8] (across all marine taxa, not just fish) showed that a majority of models (56%) assumed that larvae were passive throughout their PLD, with 41% assuming some form of active behaviour for all or part of the PLD. Surprisingly, the likelihood of published marine larval dispersal models assuming passive larvae increased marginally over the period 1980-2017 [8]. Many published models continue to make the simplifying assumption of passive dispersal without justification or try to justify doing so by pleading the lack of data on the behaviour of the species being modelled (e.g., [9], p. 3).…”

Distribution, growth, and transport of larval fishes and implications for population dynamics

“…Individual-based biological-physical models have allowed us to move beyond the early concept of larvae as passive particles to understand the importance of larval traits, such as vertical distribution behaviors (Paris et al, 2013;Vaz et al, 2016) and swimming abilities (Rypina et al, 2014). Despite the increased availability of sophisticated models that can incorporate larval behavior, the vast majority of recent modeling studies of larval dispersal continue to treat larvae as passive tracers (Swearer et al, 2019). Furthermore, those studies that do incorporate larval behavior tend to focus on a single species, or comparisons between very different species (Faillettaz et al, 2018;Kough and Paris, 2015;Petrik et al, 2016).…”