Some fish live in shoals and prefer close schooling
so that the nearest neighbour is a short distance
away, e.g. arctic charr Salvelinus alpinus (Jergensen
et al. 1993). In other species whose individuals are
forced to endure a higher density than they would
choose, high stocking densities on fish farms, or in
captivity for other purposes, cause poor welfare (e.g.
Ewing & Ewing 1995 for trout and salmon; Vazzana
et al. 2002 for sea-bass; Montero et al. 1999 for sea-
bream). When high stocking density was combined
with insufficient water flow (Ellis et al. 2002) or with
too much disturbance (Turnbull et al. 2005), welfare
was worse than with any single adverse factor, so the
effects of such problems on welfare is additive or
multiplicative.