Particle-tracking Estimates of Larval Connectivity
Virtual particles representing larvae were released from 438 distinct
release sites distributed equidistantly along the Irish Sea coastline,
at 10 km intervals and 1 km offshore (Fig. 1B). By selecting release
sites 1 km offshore, all particles were released within the hydrodynamic
model grid cell adjacent to the coast, appropriate for simulating
spawning of near-shore species. Within a 100 m radius of each release
site, 625 release locations were randomly spaced to allow for some
stochasticity within the simulated currents and resulting particle
advection that represents unresolved sub-grid-scale and sub-time-scale
processes. Cohorts of 625 particles were released daily (at 12:00 noon)
from each of the 438 sites. To encompass releases during the full
spring-neap tidal cycle, particles were released for the first 16 days
of June. To capture dispersal variability over seasonal timescales, this
procedure was repeated for July, August and September, which covers the
spawning season of S. umbilicalis (Williams 1964; Underwood 1972;
Garwood & Kendall 1985). Adopting a similar experimental design to this
study, Robins et al. (2013) showed that release of 10,000 particles was
sufficient to represent dispersal and connectivity patterns within the
Irish Sea. Here, 40,000 individual particles were released from each
site, totalling 17.52 million particles (625 particles × 438 sites × 16
days × 4 months) within the Irish Sea. Particles were allowed to
propagate for 10 days from release, based on the maximum pelagic larval
duration of S. umbilicalis (Keith et al. 2011). During each
10-day simulation, individual particles were advected, and their
locations were recorded hourly.
Given that larvae have been shown to settle passively in areas of low
current velocity (Kendall & Lewis 1986), the particles were
parameterized as passive, neutrally buoyant, and released at the surface
to simulate maximum dispersal. No criteria were applied to account for
habitat type, and larvae were able to settle at any of the 438 sites.
Sexual maturity in S. umbilicalis can occur between 1-2 years of
age (Williams 1964; Bode et al. 1986; Garwood & Kendall 1985), but for
model simplicity we assumed sexual maturity occurred within 1 year. We
did not consider mortality/fecundity of larvae so that our results
represent the maximum dispersal potential.
Pairwise site connectivity for the 12 sampled sites was determined by
calculating the proportion of particles released from a source site that
arrived within a 5 km radius of a settlement site. This was calculated
at each model time-step, between 2-10 days after their release,
producing an overall averaged connectivity. The 5 km radius was set to
minimize overlap between release and settlement areas, but also to
account for dispersal potential over a tidal cycle based on the average
tidal excursion of the Irish Sea (Robins et al. 2013).
Multi-generational connectivity was estimated between all 438 sites by
accounting for the connectivity of each site with all other sites to
quantify the year-on-year spread, over 100 years (Fig. S1). In this way,
the intermediary sites implemented between our 12 sampled sites
facilitated stepping-stone spread along coastal sites of the Irish Sea.
Here, we have focused on potential larval spread via stepping-stone
connectivity and have thus assumed the metapopulation size remains
constant, with population fecundity equalling mortality (also see
Giménez et al. 2020b). Pairwise connectivity estimates were averaged
over the four months that the model was run to create a connectivity
matrix between each pair of sites, and this year-on-year spread was
calculated for 100 years. The matrix used in subsequent analyses was
selected based on the maximum number of years it took for any pair of
sites to become connected, excluding sites that never became connected
over the 100-year timeframe (Fig. S2, Table S3).