At an interior solution this condition states that the net marginal benefits from an additional
angler-day (the increase in angler welfare minus the increase in variable costs as the fleet-wide
number of trips is increased to accommodate the extra demand) are just offset at each point in
time by the discounted capital value of the induced mortality. However, as broached in the
previous paragraph, such an interior solution does not exist given the necessity of full vessel
employment at the optimal solution. An increase in angler days at sea at a fixed angler density
thus necessitates an increase in the number of vessels and an associated increase in fixed costs.
This is easily demonstrated by simple rearrangement of (9) forμ2 :
N
μ2 D ((rFN z FN )N + rF zF + ψ). (11)
DMAX
The benefit of an additional day of available fishing time is simply the value of the reduction in
vessel capital (i.e. the reduction in fixed costs) required to service demand at current angler
densities. Therefore the third term in (10) reflects the industry-wide increase in fixed costs from
the new vessel capital needed to service an extra angler day within the constraints of available
fishing time provided that extra angler day were spread over all vessels equally (i.e. in a cost
minimizing fashion) holding angler density constant.
The necessary condition for angler landings is:
D*
∫MBL (■) dD - μ = λ * (1 - φ). (12)
0
This condition simply implies that the net marginal benefits of additional landings must be offset
by the full dynamic costs of the extra mortality from doing so. Note that if discards experience
full mortality (φ= 1 ) then there is no dynamic consequence to the allocation of catch between
landings and discards and (12) becomes a static condition. It is possible, however, that catch is
12