New study reveals previous under-estimates of smolt mortality
A lice-infested wild salmonid. Previous studies have shown that losses of wild smolts to sea lice could be as high as 39%, but the paper suggests flaws in the way the survival rates are measured
A newly published scientific paper, which focuses on sea-lice burdens of salmon smolts released from the famous river Vosso in Norway suggests that the impact of salmon lice on wild salmon is larger than has previously been reported, and calls into question the way that the effects of sea lice on wild fish have been analysed in the past, writes Mark Bowler.
The study, published by The Royal Society, involved using 19 years of historic sea lice data, which has been monitored in the fjord outside the Vosso river-mouth since 2000, and focussed on the ‘parasite spill-back’ from farmed salmon in the Vosso fjord. This ‘collateral damage’ can distort the balance between host and parasites in surrounding wildlife, with potential detrimental effects on wild populations, in this case, the Vosso’s own wild population of salmon.
The Vosso’s salmon population crashed around 1990, and since 2000 hatchery produced smolts have been released into the river every year to help increase the Vosso’s spawning abundance. Some of these introduced fish were not released into the Vosso, but were transported in tanks 15 to 105 kilometres out from the river mouth (away from the fish farms, and therefore the sea lice). These such fish showed a higher (albeit still low) survival rate of 0.5 to 4%, compared to those introduced into the river itself, which recorded survival rates of 0 to 0.5%.
Measuring lice burden
The sea lice burden on wild Vosso salmonids has been monitored by trapping wild sea trout in the fjord and performing lice counts on them since 2009. This has been shown to correlate with the infestation pressures from fish farms in the Vosso fjord. For the purposes of this study, these historic sea louse counts were limited to the main salmon smolt migration period.
The report showed that annual variation in survival (returning adult wild fish) could in large part be explained by the levels of lice, as measured on Vosso sea trout in the migration route, during the out-migration of salmon smolt.
The study highlighted a flaw in the way scientists have been measuring the affect sea lice have on smolt survival to returning adult. These have been conducted through trials whereby half of the released, tagged hatchery-reared salmon smolts are treated with an anti-parasitic drug, while the other half are left untreated. These trials have in the past reported, on average, from 11% to a massive 39% reduction in return rates of adult salmon.
However, this approach assumes that the treatment protects the salmon smolt from lice infection and affects neither the physiology nor survival through the marine phase. The study suggests that because sea lice are becoming increasingly resistant to emamectin benzoate, and treatment may also incur negative effects on marine survival, that this is not so, and has therefore led to “an apparent under-estimation of the impact of lice on out-migrating salmon smolts”.
The report suggests that salmon lice can have a large effect on wild salmon populations that is not revealed with the current methods used by science. “This should be better accounted for when considering the impacts of farms on wild salmon populations” it concludes.
