Effects of salmon lice on numbers and size distributions of Atlantic salmon populations returning to spawn in Norwegian rivers

Negative effects of salmon lice from salmon farms on wild salmonids have
been a controversial issue for decades, and concerns are expressed from
virtually all areas where salmon farming coexists with important fisheries
for wild salmonids. A key question is to what extent lice infestation from
farms reduces the number of wild mature salmon returning to spawn in the
rivers. Here we study counts and recreational catches of small (< 3
kg), medium-sized (3 – 7 kg), and large mature salmon (> 7 kg)
returning to spawn in Norwegian rivers, in association with lice burdens
on out-migrating post-smolt recruits. The expected number of returning
mature salmon was modelled as a function of theoretical smolt production
in the rivers, river catches one generation back in time, size classes of
returning salmon, year as a factor, a non-linear spatial effect, and
parasite-induced mortality (PIM) of out-migrating post-smolts of salmon.
PIM was attributed to small, medium-sized, and large salmon, assuming they
spend one (1 SW), two (2 SW), or three winters at sea (3 SW),
respectively. There was a significant negative effect of PIM on returns of
1 SW salmon and a negative but non-significant effect on returns of 2 SW
salmon. For 3 SW salmon, the effect of PIM was significantly positive, but
for comparably low numbers, implying an overall negative effect of PIM on
returning salmon. The size-specific effects of PIM were manifested by
decreasing proportions of 1 SW salmon in returning populations with
increasing PIM, from ~0.6 for rivers exposed to low levels of PIM, to
predictions of < 0.2 for rivers exposed to high levels of PIM. This
study presents a quantitative relationship between infestations of
post-smolt recruits and size-structured returns of mature salmon to
Norwegian rivers, suggesting that louse infestation from farms may reduce
returns of spawners and restructure the size distribution of mature river
populations of Atlantic salmon. The presented relationship opens up for a
more targeted approach to obtaining sustainable salmon farming. To
accommodate the Norwegian Government’s goal for sustainable aquaculture,
reductions in lice abundances in farms are necessary.