The trouble I have regarding the option (a) is that I'm assuming
that sampled trips come from the overall trip population (T), and
from my point of view this is not true; Following the sampling
scheme, trip "t" is randomly drawn from the trip population "T_i"
from the year "i" . Many factors (such as shift in fishing
behaviour and techniques, changes in fish populations, yearly
quotas...),makes me think that trip population can not be assumed to
be the same across years. I expect that using the (a) approach
produce an over-inflation of the random effect sigmas.
ndiscarded~year+ [other fixed effects] + (1|year:trip)+(1|year:trip:haul)
which expands the trip/haul nesting (which would otherwise expand
to trip+trip:haul) to include an interaction with year. If this gives
you trouble you could also do this even more explicitly by adding
a 'yeartrip' variable to your data set which was defined as
interaction(year,trip) and similarly for 'yeartriphaul' ...