Disentangling the effect of sea temperature and stocking density on sea louse abundance using wavelets in a highly infested salmon farming area

Abstract

Sea louse infestations are threatening the sustainability of the salmon farm industry worldwide; the southern marine ecosystem of Chile is one of the most affected areas. A highly infested management area located in the Los Lagos Region (42.5°S) has been regularly affected by Caligus rogercresseyi outbreaks during the spring-summer period since 2012. In this study, the effect of salmon stocking density and sea surface temperature on the temporal dynamics of juveniles and ovigerous females was analyzed at a regional scale using wavelets. Main periodic oscillations were detected for ovigerous female time series centered at periods equal to 52 weeks (1-year cycle) and 107 weeks (2-year cycle), which are associated with, respectively, the annual sea louse outbreaks occurring during the warmest season (spring-summer), and the periodic increase of lice driven by an increase in salmon stocking density as the farming cycle progresses. The 1-year and 2-year cycles explained between 10%–20% and between 25%–52% of the variance of ovigerous females, respectively. Wavelet coherence and phase analysis showed a significant association between sea surface temperature and ovigerous female abundance associated with the 1-year cycle with a time lag (in weeks) very close to the louse generation cycle. Coherence between stocking density and ovigerous female abundance was significant, associated with the 2-year cycle. These results suggest the existence of a continuous year-round effect of temperature on louse development rate which becomes more relevant during spring-summer outbreaks, specially at high stocking density levels. Two main stocking density (SD)-sea surface temperature (SST) scenarios were identified, and the location and size of major outbreaks at the high SD-high SST scenario were well predicted using ovigerous female integrated 1-year and 2-year cycles. Future studies should be focused on the development of a stocking density regulation model, mainly depending on SD and SST periodic variability, in order to prevent maximum levels of the two factors from coinciding, because this condition is inevitably followed by major ovigerous female outbreaks in this area.