  We present a new method of identifying protostellar disc fragments in a
simulation based on density derivatives, and analyse our data using this and
the existing CLUMPFIND method, which is based on an ordered search over all
particles in gravitational potential energy. Using smoothed particle
hydrodynamics, we carry out 9 simulations of a $0.25$ M$_{\odot}$ disc around a
1 M$_{\odot}$ star, all of which fragment to form at least 2 bound objects. We
find that when using all particles ordered in gravitational potential space,
only fragments that survive the duration of the simulation are detected. When
we use the density derivative method, all fragments are detected, so the two
methods are complementary, as using the two methods together allows us to
identify all fragments, and to then determine those that are likely to be
destroyed. We find a tentative empirical relationship between the dominant
azimuthal wavenumber in the disc $m$ and the maximum semi-major axis a fragment
may achieve in a simulation, such that $a_{\mathrm{max}}\propto\frac{1}{m}$. We
find the fragment destruction rate to be around half that predicted from
population synthesis models. This is due to fragment-fragment interactions in
the early gas phase of the disc, which can cause scattering and eccentricity
pumping on short timescales, and affects the fragment's internal structure. We
therefore caution that measurements of eccentricity as a function of semi-major
axis may not necessarily constrain the formation mechanism of giant planets and
brown dwarfs.
