CONNECTIVITY WITHIN THE SCOTTISH MPA NETWORK
Background
MPAs have the potential to offer a wider ecosystem benefit through the build up of
reproductive mass and spill over of individuals and/or export of offspring. This spill
over and export leads to connectivity among MPAs and neighbouring areas of habitat
and is an important consideration in the location and replication of MPA sites
designed to protect priority marine features (PMF). Hence, it is important to estimate
the scale of connectivity in priority species when designing a MPA network.
For priority species with a planktonic phase, transport into and out of an MPA will
depend on the physical environmental conditions (mainly the current flow regime)
and how these interact with the organisms’ behaviour. In general, too little is known
about the life cycle of PMF species to derive accurate species-specific transport
estimates. The Marine Conservation Zones (MCZ) ecological guidance approached
this problem by proposing that similar protected habitat should be separated, where
possible, by no more than 40-80 km between MPA boundaries, which could be
approximated by ensuring proposed MPAs were well distributed across areas. This
scale was derived from a simple model of larval transport that focussed on residual
tidal flow and the proposal of evenly distributed MPAs across the continental shelf
(Roberts et al., 2010). However, in Scottish water, evenly distributing MPAs across
all regions makes little sense. This is because of the diversity of habitats from the
deep sea in the far west to shallow fjordic sea lochs as well as the substantial
differences in residual current flow around Scotland.
This variation in residual current flow around Scotland can be illustrated by a study of
connectivity between spawning and nursery areas of cod around the North Sea and
west of Scotland. Here a biophysical model of early egg and larval transport was used
to estimate the proportion of offspring moved from one spawning area to another (Fig.
1a). Some areas were found to be quite self-contained (retentive) while others
behaved as sources (net exporters) or sinks (net importers) to other spawning areas
(Fig. 1b). The patterns described above addressed connectivity at fairly large scales
(10s to 100s of km) but the potential for non-uniform dispersal from a source area
also holds true at considerably finer scales. For example, on-going work investigating
the dispersal of sea lice within Scottish inshore waters has shown that flow variability
within a sea loch results in a pattern of dispersal which differs considerably from a
simple radius centred at the release location (Fig. 2).
Proposed approach
Given that the MCZ approach appears inappropriate to the Scottish MPA network,
Marine Scotland Science is developing an alternative means of considering
connectivity for relevant priority species. This will in the first instance look at a range
of benthic species that differ with respect to spawning time and planktonic duration
and will include; Tall sea pen, Northern sea fan, Ocean qualhog, horse and fan
mussels and Flame shell. However, the results will be more widely applicable to
many other benthic priority species. A relatively simple modelling approach that will
account for regional oceanographic variation and some degree of biological realism
will be used. This will involve the following steps:
1. the use of output from an existing hydrodynamic model covering Scottish waters
and the compilation of a climatological flowfield to represent “average” conditions.
2. proposed search locations will be considered as “source” and “target” areas for the
dispersal of individual species (at the relevant life stage for dispersal).
3. species life cycles will be divided into 2 or 3 categories, based on common
biological characteristics that may influence dispersal patterns, such as the duration of
larval phase.
4. simplistic Individual-Based Models that simulate the interplay between physical
transport and biological characteristics (“behaviour”) will be constructed and run.
5. simulation results will be processed to quantify connectivity and export/import out
of/into search locations to assess the most suitably located sites and the replication
needed.
This approach does have limitations in terms of the resolution of the hydrodynamic
models available and the use of average conditions but it is more thorough than the
Roberts et al. (2010) approach. The robustness of results will also be considered in
terms of sensitivity in the use of average hydrographic conditions and behaviour of
priority species. Ongoing modelling developments are likely to alleviate the model
resolution problem in the future and will be considered in the eventual review of the
network.
Figure 1. Connectivity between a) spawning areas (shading) of cod considered in
connectivity study and b) proportion of offspring from one spawning area reaching
the numbered nursery areas (see Heath et al., 2008).
Figure 2. Snapshots of particles released from a single position in Loch Linnhe, at 12
h intervals (h 12-60) based on particle tracking results (Salama and Berit, unpub)
using the output of a POLCOMS model of Loch Linnhe developed by the Scottish
Association of Marine Science.
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Heath, M.R., Kunzlik, P.A., Gallego, A., Holmes, S.J. and Wright, P.J. (2008). A model of metapopulation dynamics for North Sea and West of Scotland cod—The dynamic consequences of natal
fidelity. Fisheries Research 93 (1-2): 92-116.
Roberts, C.M., Hawkins, J.P., Fletcher, J., Hands, S., Raab, K., Ward, S. (2010). Guidance on the size
and spacing of Marine Protected Areas in England. Natural England Commissioned Report NECR037.
Natural England, Sheffield.
Salama, N. and Rabe, B. (pers. comm.), unpublished work from Marine Scotland Science project
AQ004.