Residual circulation from nonlinear interaction of the tidal currents with topography in channels, fjords and bays can be important to represent the distance traveled by suspended materials in the water column (e.g. pathogens). Here the role of the semidiurnal tidal excursion was compared with residual currents, during autumn 2011, to evaluate the connectivity among aquaculture production centers in Caucahue Channel (Chiloe Island, southern Chile), which has been widely occupied by the aquaculture industry. During 2015, around 30 000 t of 3 different species of salmon were harvested in this channel, representing 3.7% of the national production. Along-and cross-channel tidal currents explained around 80 and 40−60% of the total variance, respectively. Thus, the major residual circulation came from the cross-channel component (40−60% of the total variance) caused by tidal asymmetry. We hypothesized that this asymmetry is related to the nonlinear interaction of the oscillatory flow with the Quemchi constriction. The residual advective distances (L adv) were compared with tidal excursion (L exc) and biological diffusivity (L diff) scales, considering the distances between centers (L int), the size of the centers (L cen) and the length of the south arm of the Cauchaue Channel (L = 11 km). For 3 d time scales, the L adv /L exc ratio fluctuated between 1.3 and 8, approximately, implying that, although the advective scale is greater, the tidal flow is still intense enough to favor the retention of pathogens. For larger time scales that consider longer-lived pathogens (e.g. greater than 15 d), this ratio grows 1 order of magnitude. In this scenario and in the event of an outbreak within the channel, pathogens could be exported.

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The Puelo and Petrohué rivers were surveyed between April 2016 and February 2017 to assess the presence and seasonal abundance of Didymosphenia geminata in the wild aquatic systems of these two important rivers in southern Chile. Didymo was reported in the Puelo River in 2012, while it was declared absent from the Petrohué River before this study commenced in the fall of 2016. Results showed that cells of D. geminata were recorded in both rivers, in the Phytobenthos as well as in the water column. However, the classical mucilage which characterizes this plague was only recorded in one of the sampled rivers (Puelo). The mucilage was not recorded in any of the sections sampled in the Petrohué River, which was attributed to the high concentration of phosphorous present in the system. The concentration of phosphorous recorded throughout the study in the Puelo River was low. However, differences in the recorded presence or absence of D. geminata between sampling sites on the Petrohué River was mainly attributable to the availability of incident sunlight. This study records for the first time the presence of D. geminata in the Petrohué River.