A synthesis of the early life history of the anglerfish, Lophius ...

74 J. R. G. Hislop et al.
et al., 1998; Gallego et al., 1999) configured to simulate
the dispersal of anglerfish eggs and larvae. It was
assumed that Lophius egg ribbons are spawned close to
the seabed, and rise slowly towards the surface. To
represent this pattern of vertical distribution in the
model, particles were released at nodes of a regular
15 km spaced horizontal grid covering an area simulating
the environmental spawning range of L. piscatorius
(see below). There are no data on the vertical distribution
of anglerfish eggs and larvae from which to gain
an indication of the likely ascent rate or precise depth in
the surface waters, and we therefore made a best guess at
these parameters, and conducted a sensitivity analysis of
the results (see below), with particular respect to the
ascent rate. The default configuration was that each
particle was initialised at a depth of 1 m above the
seabed, and was programmed to ascend at a fixed rate of
15 m d 1 until attaining a depth of 35 m, at which they
remained for the rest of the model run. Horizontal
positions of particles were updated at 1 h intervals
assuming passive transport, based on the magnitudes of
east–west (u) and north–south (v) velocity components
at surrounding nodes in the hydrodynamic model data,
and a Eulerian integration scheme to estimate velocities
at particle locations. The position of each particle was
saved at one day intervals for post-simulation analysis.
To test the sensitivity of the results to the assumed rate
of ascent, a separate model run was performed with a
rate of 150 m d 1 and the outcome compared with the
default results.
The daily resolution particle tracking model results
were investigated using a system based on ‘‘source’’ and
‘‘target’’ boxes. A source box was a geographical region
delineated by a polygonal shape defined by a series of
latitude and longitude locations, which was used to
subset the particle tracking results according to the start
location of individual particles. A target box was a
region, also delineated by a polygon, used to subset the
particle tracks according to their locations within a
prescribed age interval, age being the number of days
since release.
Modelling the environmental spawning range of
anglerfish
The potential geographical distribution of L. piscatorius
spawning was defined on the basis of two environmental
criteria; depth range and minimum temperature of
spawning. Unpublished data provided by scientific
observers from the Marine Laboratory Aberdeen (L.
Bullough, T. Blasdale) working on Scottish and French
commercial vessels fishing in deep water north and west
of Scotland indicated that mature L. piscatorius are
seldom caught deeper than 1100 m or at temperatures